How to Stop Heat

 

Watch this video lesson to find out! 

Spoiler alert: it's impossible to stop heat! You can only slow it down. If you want to lose less heat from your house, then the first thing to think about may be the surface area.

Warning: Contains equations.

What is Energy?

 The first lesson. If you want a low energy building, this is the first question you need to ask. 

What is energy? How do you measure it? I thought of a dozen different ways. This shows how confused our language is over the science. 

In politics, language is power, but when it comes to science, power has a different and much more precise meaning. Heat seems like it's hot, but often it's not. In the next video, after explaining that heat and temperature are not the same thing, I described a difference in temperature as a difference in heat. In this video I used the word "precise" when I meant "accurate". I did this while I was talking about the difference between precision and accuracy. Maybe it's not the language that's confused. Maybe it's just me!

Anyway, this has nothing to do with the content of this video. You can read more about the lesson here. 

Please watch the video, and subscribe to the channel. 

How to build a house part 5: What exactly do you need to know about heat

Some people spend six years studying for architecture degrees, and it can take a lifetime to build the perfect house. In fact it's now 90 years after Gaudi was knocked over by a tram, and his is still not finished. Admittedly that wasn't your everyday family house, but I digress.

So, you'd like to build a house in the next year, and you're also going to be busy at work, and spending time looking after your family. What do you really need to know?

If you're trying to build a low energy house, two important areas of knowledge are thermodynamics
and economics. Structural knowledge is essential, but if you are working with professional builders in Japan, they should have all the structural knowledge necessary to keep your building standing, probably even through the strongest earthquake ever.

As well as knowledge of what to do, you need to know how to do it, and procedural knowledge is also important. So you need to know how the design process works, but I'll get to that later. First, here are five things you should know about thermodynamics. In most places in the world, the biggest energy use of buildings is heating and cooling.

1. Heat will leave the building by the easiest route in the winter. And it will get in by the easiest route in summer. Heat is a lazy opportunist. This means that you should be worrying about the parts of your walls, ceilings and floors with the least insulation rather than being impressed by the parts with the most insulation. Be aware of the performance of doors and windows, and anything in your thermal envelope that is poorly insulated. There may be conflicts between the structural desires of the builder and the thermodynamic needs, but it is possible to make buildings that are structural sound and thermally right.

2. There is less heat loss as walls get thicker and areas get larger, and more heat loss as temperature
differences increase. So thicker is better for your walls and roof, and smaller is better for the surface area of your house. When you are designing the house, you can't do anything about the temperature. It will get hot and cold outside, and the people inside will want the temperature to be within their comfort zone. If the building does not deliver that comfort zone, the people in the building will use electricity or other fuel to change the temperature.

3. Heat loss depends on the insulation performance of the material in your wall, roof, floors and foundation. Very broadly, metals are the worst insulators, or the best conductors, followed by earthy things, including stone, concrete and glass. Next come plastics, which we can start to call insulators, then fibres, which include wood. Foams are generally better insulators than fibres. In both cases their
performance comes from the excellent insulation credentials of air, but foams also stop the air from moving, and in some cases can use different gases to air. Other gases are better insulators than air.

This table shows the thicknesses of different materials needed to get the same insulation effect as 10 cm (4 inches) of glass wool. Depending on where you are in Japan, you may need the equivalent of 20 or 30 cm of glass wool to make a low-energy building.

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Krypton (gas)	2 cm	three times better than air
Argon (gas)	4 cm
Phenolic foam	5 cm	twice as good as glass wool
Air	6 cm
Polystyrene, expanded styrofoam	8 cm
Glass, wool Insulation	10 cm	three times better than wood
Cork, re-granulated	11 cm
Hardboard high density	38 cm
Wood, oak	43 cm	three times better than medium concrete
Polycarbonate	48 cm
Concrete, lightweight	50 cm
Polyethylene low density, PEL	83 cm
Concrete, medium	1.4 metres	thirty times better than stainless steel
Concrete, dense	3.5 metres
Stainless Steel	40 metres	twelve times better than aluminium
Brass	270 metres
Aluminum	500 metres	Yes, half a kilometre!

4. There are five to ten litres of moisture in the air inside your house, and given any opportunity it will build up and cause condensation, mold or rot. This happens where air is not moving and there is a cold spot or a sharp temperature difference. It will happen where you are not looking, possibly on your favourite coat. This can be stopped with airtight insulation.

5. Reflective coatings are a good idea, since they will reduce the amount of heat radiated in or out of your house. However, most heat is lost through convection or conduction, so the first priority is to add insulation. Things that look shiny may just look shiny.

Bonus: It may be useful to know how a heat pump works. There's a great explanation here using a rubber band refrigerator.

Note

While 500 metres of aluminum has the same insulating performance as 10 cm of fibreglass, metals are not effective as insulators. As the insulation gets thicker, the outside area of the house also gets bigger, so you will more heat, not less heat, as you put on more layers.

Efficiency of spending and the thermodynamics of happiness

When it comes to money I've mostly been looking at how to spend more when you build a house so that you will save money while you're living there. How do you balance capital expenditure and running costs?

Another consideration with money is how spending it makes you feel. The Guardian asks, is there a proven link between housing and happiness? People often say that money can't buy happiness, but are they just going to the wrong shops?

It depends what you spend money on. A lot of research seems to show that spending money on other people will make you happier, while spending money on yourself will not. There is also little evidence that more money will make people more happy, once people have got above the poverty line. So if you're in the lucky position of having shelter, warmth, food, fresh water, employment and access to education for your children, the best thing you can do to increase global happiness may be to send any spare cash you have to people who are not as fortunate as you.

People sometimes talk about the pursuit of happiness, which I find deeply problematic. Happiness is a journey and not a destination, so the happiness is in the actual pursuit. People may be happy when they receive something, or be happy thinking about getting it, or choosing exactly what to get, or going through other activities that will lead to getting it. They may be deeply unhappy if they don't have something, especially if everyone around them has one. But once they get it, and are used to having it, their level of happiness will quickly revert to the norm. In this context, there must be a steadily increasing flow of material things for them to make us happy, and they must be new, or faster, stronger, bigger or better in some way. I don't know about you, but for me that is a pretty miserable idea!

Last year we bought a new fridge, and a new washing machine. Within a couple of days the novelty
of the new white goods wore off and they just became doors in the wall.

Last year we also bought some plane tickets at a similar cost. I certainly didn't spend the whole flight enjoying the seat, and in fact long distance flights are not terrible comfortable. I do still remember what happened at the other end of the flight, and it's very easy to think about those memories without thinking about the money we spent on the tickets. Those memories of other times, visiting other places, and meeting other people will remain long after I've forgotten about the airlines and hotel bills.

So what does this have to do with thermodynamics? Not a lot. Happiness is just chemicals in the brain.

What does it have to do with house building? Just two things spring to mind: Think about other people when you are building a house, and think about what the building will let them do.   

Economics as Thermodynamics

We tend to think of human activity as something altogether different to thermodynamics, but James Lovelock looked at the amount of energy that people have used and noticed that things start to get culturally interesting when we use more than one watt per square metre. As I wrote before, he linked human activity to the reynolds number, which is a measure of turbulence. 

Tim Garrett of Utah State calculated total human wealth is proportional to the amount of primary energy we consume. One 1990 US dollar is approximately 10 milliwatts.  

It's worth noting that this is talking about power and not energy. Wealth is not proportional to the amount of energy. It is proportional to the rate at which energy is used. So when there is inflation, and the amount of financial wealth increases, there is an increase in the rate we are using energy. In the fossil fuel paradigm, this means an increase in the rate we are using up resources. Inflation is compound, so the increase is exponential.

If we change to renewable sources of energy, we may be able to reduce the rate we are using up resources, but unless we have a zero-carbon economy, we will still be using up those finite resources. If you're heading towards a cliff, slowing down is not going to help: you need to stop or change direction. We don't need an economic paradigm where growth is used as the main metric, and lack of growth met by frowns on the faces of newscasters. 

Free market economics has been an interesting experiment, and we seem to be doing well at the moment, but in terms of experiments, it's a bit like an experiment with drugs. I once asked a friend what speed was like, and he said it was like riding a motorbike. But you had to walk back. The sooner we realise that free market economics, along with fossil fuel use, is an addiction, and that we too are going to have to walk back, the sooner we can start kicking the habit. 

When I talk about costs in terms of energy rather than in terms of money, I've been worrying that I'm just being an idealistic hippy. In fact, the reality may be precisely the opposite. Energy is the real metric, and money is just a loose approximation to it. Energy underlies the motions of the universe, not just in the shine of the sun and the orbits of the planets around it, but in the myriad human activities and their effects on a macroscopic level. It is people who don't see this who are living in a fantasy world. 

It's also worth going further into generalisation and thinking about the future. People spend a lot of time worrying about where to invest their money for the future. The Bible says that love of money is the root of all evil. Not the money itself. Rather than worrying about money, we should be thinking more about how we invest energy. It is now in great abundance, but there is no guarantee that it will be in the future. Money may lose most or all of its value, but energy will always have currency.  

Reference

Garrett, T. J.: No way out? The double-bind in seeking global prosperity alongside mitigated climate change, Earth Syst. Dynam., 3, 1-17, doi:10.5194/esd-3-1-2012, 2012.

Lesson 13: How do air conditioners work?

I made a bit of a miscalculation. There were three lessons left and three student presentations, one of which was on low-energy buildings in hot climates, and another on generating your own power. I had planned to do a lesson on cooling, and had some leftover material on solar electricity, and should really have had these presentations on three different days, then filled in the rest of the class with my information, assuming that the students had not covered it in their presentations. Or if they had, then continue the lesson with discussions on the relevant topics. Unfortunately they both ended up in week 14, so week 13 became empty, and the only material ready for it was the left-overs on cooling and solar electricity. 

If I'd been doing a full lesson on cooling, I would have started with a brainstorm of different ways to stay cool, with my non-exclusive list ready in the wings: windows, insulation, thermal mass, trees, heat-exchange ventilation, fans, air conditioners, de-humidifiers, and ice—preferably large blocks. Another thing that was not on my explicit list, which probably should be, was shading. 

Not wanting to take away too many options for the following week's presentation, and keen to teach some science, I skipped this and went straight into an exposition of the workings of air conditioners. 

Before getting into the details, we needed to understand four concepts relating to gases: volume, pressure, temperature and heat. These are all interrelated. Other things being equal, if a fixed amount of gas is in a smaller volume, it will have a higher pressure. Other things being equal, if the amount of heat in a fixed amount of gas goes up, the temperature will rise. If you don't add any heat to some gas, but squash it in to a smaller volume, then the amount of heat won't change, but the temperature will go up. And if you expand it, the temperature goes down. 

A heat pump sends a fluid in a circuit through a hot area and then a cold area. The Fluid is compressed as it goes into the hot area, which will increase the temperature and allow it to transfer heat to the hotter area. It is then allowed to expand when it goes into the colder area so the temperature will drop and heat well flow from the cold area into the fluid. 

That's the Carnot cycle. Heat pumps are basically trying to defy the second law of thermodynamics, by getting heat from a colder place to a hotter place. We use them in our fridges and air conditioners, and increasingly they are used for space heating and water heating. 

The coefficient of performance is used to measure the efficiency of a heat pump, and it measures the amount of heat that is transferred divided by the amount of energy that goes in. Typical domestic heat pumps have average COPs of 3 to 5, but precise numbers are very difficult to find. 

There are limits to the COP, and as the temperature difference goes up, the COP will go down. If a heat pump is used for generating hot water in the winter, using cheap night-time electricity, the COP can get very low, and the heat pump is not performing much better than an electrical heating element.  

Thermodynamics of the bathtub

We got a cover for the bath tub. It already has a lid, but there's a sizeable air gap between the water and the lid, and the lid is not exactly airtight. As the water is going to be reheated most days, it makes sense to keep it as warm as possible. Putting the lid on makes a big difference, but the heat from the bath is still evaporating away into the air, which is going to be humid and capable of carrying more heat away. They sell bath covers in hardware shops and supermarkets that you can cut to the shape of the bath. They are usually about 4 mm thick plastic foam with one side silvered. I just used a camping mat, essentially the same thing but thicker and presumably better at insulating.

The big question is, of course, do you put the silver side up or down? My initial feeling, probably like yours, is that the silver side should go down, so it is reflecting heat back into the bath. One problem with this, as we experienced in our old house, is that the silver foil can come away from the plastic foam. This was exacerbated when we forgot to take the bath cover off when heating the bath. The bath heater, which was on the outside wall next to the bathroom, sent very hot water back into the bath tub, often bubbling and steaming. The system we now have sends in water at a much more modest temperature.

So here is a plan for finding out, experimentally. After bath time, leave lids and covers in various combinations, for example: lid with no cover; cover, shiny side up with no lid; cover, shiny side down with no lid. Put a thermometer with a data logger into the bath. Repeat a few times to get statistical significance. Observe results and rate of temperature drop, from which we can deduce the heat loss.

For the reliability of the experiment, we need to take a few things into consideration. The volume of water in the bath should be the same, so we may need to add or take out water. The temperature of the bathroom should be the same, so we should be having baths around the same time each day. We should also be measuring the temperature of the bath water in the same place. We have some data loggers attached to thermometers, so it would be easy to put one of these in the bath, but perhaps we need to fix up a rig that can be lowered into the bath when we're not in it. For example a PET bottle with a weight in the bottom, and a thermometer fixed onto its side somewhere.

In the summer we were taking showers rather than running the bath, and also when we did run a bath we wanted to keep the heat in the bath rather than letting it escape into the house, so winter is going too be the best time to carry out this experiment.

We may of course find that any difference is marginal, and that even where there is a difference it is in the steepness of initial temperature drop, and not in the difference of temperatures twenty three hours later when we want to heat up the bath again.

I think we'll find that pointing the shiny side upwards is more effective than pointing it downwards. The shiny side stops radiation, but will have little effect on conduction. Since the bottom of the cover is in contact with the water, heat is mostly going to be conducting, and radiation will make very little difference. At the interface between the top of the cover and the air, there will be less conduction since air can carry something like four thousand times less heat than water. So the radiation is going to make a difference.

The counter-intuitive part is that when it comes to heat, the reflective part is just as good at reflecting heat in as it is as reflecting heat out. Our vision is impairing out judgement.  

Stirling engines, hydrogen energy storage and other perpetual motion engines

The Stirling engine was invented in 1816. It produces power from heat. Since there is heat everywhere, it should be the answer to all of our problems. It doesn't seem to exist in any practical applications on any significant scale, with the possible exception of Swedish submarines. And that is only if you think that eight is a significant number of submarines. Wikipedia has several applications, each hedged by "may", "could", or recording historical experiments. Cryocoolers are cited as a use of stirling engines, but this doesn't really count as they are not being used as engines to convert heat into power, but are using power to create extreme cold. The largest use of Stirling engines is probably in classrooms, to demonstrate how Stirling engines work. This sounds a bit like perpetual motion.

We've all had ideas for generating energy that amount to engines connected to generators. They all fail before we start as we are bound by the second law of thermodynamics, and rather than the energy generating itself, it will fall short as some is sapped away by entropy. There are many ideas that aren't actually perpetual motion, but they are so impractical that they might as well be.

Hydrogen is one of the components of water. There are gazillions* of hydrogen atoms literally floating around in the ocean. They come in pairs bonded with oxygen atoms, so all you need to do to get the hydrogen out is pass a bit of electricity through the water. Wouldn't it be easy to store extra electricity by just converting it to hydrogen?

I used to do this when I was a kid, with a transformer and a couple of carbon electrodes. Hydrogen gas would bubble up from the anode, and oxygen gas from the cathode. A little bit of salt helped the process by making the water more conductive. The build-up of hydrochloric acid was only very slight, but the electrodes deteriorated and discoloured pretty quickly.

I may have been trying to produce enough gas to fill a model airship, but that project was plagued by lack of materials, equipment, knowledge, experienced personnel and time. Without effective storage technology, all of my hydrogen was destined to leak away, or vanish with squeaky pops. 

You can try this as home, but it's not a very practical way of storing electricity. Very little of the electricity going into electrolysis actually liberates hydrogen atoms. Once they have been liberated, the hydrogen molecules need to be stored, probably via a compressor which will use more power. When you have the compressed hydrogen, you need a special engine to use it as a fuel, unless you are just content with a squeaky-pop machine, or an explosion hazard.

The R100 airship started to develop engines that could run on either hydrogen or kerosene. (That was the airship that didn't explode in Picardy, Northern France.) It used hydrogen for buoyancy; the tanks of kerosene would get lighter as the journey went on, and they could then switch to hydrogen, which would make the ship heavier. In the end they didn't have enough time to develop the engines.

If electrolysis were a good way of producing hydrogen, they would be using that to produce hydrogen on a commercial scale. In fact 95% of hydrogen gas is produced by passing high-temperature steam through natural gas.

So don't hold your breath for hydrogen as a way to store all that excess solar energy.

* a million is one with six zeros, a billion has nine zeros, a trillion has twelve. A gazillion has more than that. The word is often used colloquially to describe large numbers that would more accurately be described as trillions, billions, millions, thousand, or even hundreds. 

Lesson 2. What is energy?

In the quest towards low energy buildings, it's pretty important to have a good grasp of what energy is. My key questions for the lesson were: What is energy? How do you measure it? What is the second law of thermodynamics?

I should probably not have been surprised to find that some of my students didn't even know the first law of thermodynamics. So I told them a bit about James Joule, and the crucial link between beer and science that is often forgotten. The combination of the temperature-critical process of brewing, high-precision thermometers, newly developed electrical equipment and the desire to save money on the various industrial processes he was responsible for led Joule to discover and to prove that heat, work and energy were the same thing. Previously heat had been perceived as stuff. This was called caloric theory, and is where we get the unit of calories.

Calories are the normal measurement of energy in food, while we buy electrical energy in kilo watt hours, rate batteries in amp hours, buy oil in litres, gas in cubic metres or BTUs, and measure atomic energies in electron Volts. We're pretty confused about energy.

And as well as thinking of heat as stuff, and as the nights start to draw in, coming out with scientifically untenable concepts such as letting the cold in, we often intuitively confuse the idea of heat and temperature. To highlight this I asked two questions. First:

Which is hotter, a litre of hot air or a litre of hot water?

The answer is, of course, that they are the same. Actually, the question is, what do you mean hot? If we take hot as 80 degrees centigrade, then they are the same.

Next question:

Which has more heat, a litre of hot air or a litre of hot water?

Most people realise the answer is the hot water. Especially when it is rephrased: if you were to take a bottle to warm your bed, would you fill it with hot air or hot water?

Next question: How much hotter? 

I provided the specific heat capacity of Water (4.2 kJ/kgK) and the volumetric heat capacity of air (1.2 kJ/m³K) and they worked out that the bottle of water has over three thousand times more heat than the air.

And so to the first important teaching point: precision and accuracy. I showed them a picture of an analogue clock and a digital clock to demonstrate the difference. The analogue clock said it was five past eleven, when in fact it was twenty past. The digital watch said it was 14:27 and 36 seconds on the first of November. A much more precise answer, but several weeks less accurate.

People often confuse precision and accuracy, providing as many decimal places as possible and being fooled when many decimal places are provided. In all cases it is important to know the accuracy of the figures you put in, since the answer is going to be less accurate than these.

In the case of heat capacities, they change depending on the temperature, so the numbers are only accurate to within a few percent.

The other important thing is to know how accurate your answer needs to be.

In most cases, one significant figure is enough. You need to know whether the answer is 4 or 5. You don't need to know whether it is 4.13 or 4.12.

In the case of low energy buildings, we often need to choose between two alternatives and work out which will use less energy. A calculation to one significant figure will usually tell us. If we're choosing between water and air to transport heat through a pipe, we know that water will move over three thousand times more than air. It doesn't matter if it is three thousand or four thousand. In fact it doesn't matter whether it is three million or three.

And if the numbers are the same to one significant figure, then we may want to do a more accurate calculation. Or, more likely, we will decide that there is not much difference between the two alternatives in terms of energy use, and other factors may be more important, such as cost, or which one looks nicer.

So, it's important to know how accurate the numbers you use are, it's important to know how much accuracy you need, and you should not use more precision in your answers than your accuracy justifies.

After this digression from the meaning of energy, it was time for power.

Watt is the name of the inventor of the steam engine.

You have to say that with rising intonation for it to work properly.

He didn't actually invent the steam engine, but he probably invented global warming when he got one to work on a coal mine where it would pump water out of the mine, allowing them to dig out more coal, keep the pump pumping and so dig out even more.

He also gives his name to the unit of power. Power and energy is another source of confusion, with power the rate of change of energy. The use of kilowatt hours as a unit of energy does not help, although as a unit it's a pretty useful one.
One kilowatt hour is roughly equivalent to:
• leaving on a 100-watt lightbulb for 10 hours.
• 0.1 litres of paraffin　
• 0.1 m³ of gas
• 5 rice balls
• 20 litres of hot water
• 200 mobile phone batteries
• 0.04 milligrammes of uranium 235

(All figures are to one significant figure, except the atomic weight of uranium. Luckily I'm not teaching nuclear physics!)

The Scottish problem

In the process of preparing a presentation about plus-energy housing, I noticed just how many Scots were involved in the whole business of thermodynamics. It's probably no exaggeration to say that James Watt invented global warming when he came up with the idea of burning coal to pump water out of mines so that you could get more coal out. The irony is that making a house more energy-efficient needs the same level of understanding of science that started the problems in the first place. Oh brave new world that has such people in it. 

James Clerk Maxwell was another Scot, discovering the demon better known as the second law of thermodynamics. This is the gambler's ruin theory of heat. And I didn't even get onto James Dewar, who invented the Thermos flask but unfortunately did not file a patent for it. 

It has been alleged that double glazing was invented in Scotland in Victorian times, although it was not commercialised until the 1930s in the US. The inventor of that was probably called James too. In fact another person involved with the whole business was James Joule, but he was a brewer from Manchester. Then there was Lord Kelvin who was not called James and may not have been born in Scotland, but did his work there. 

And he's not usually considered a scientist, but Billy Connolly's line about there being no such thing as bad weather, just the wrong clothes, belies a deep understanding of heat that would serve the building industry well: there's no such thing as a harsh climate, just inappropriate housing.

So given that global warming was all originally the fault of Scots, and since people in the United States are gradually starting to believe in it, it's only a matter of time before the lawyers get hold of the idea and they decide to sue. It therefore seems like a good idea for the English to support devolution. 

And I don't say that because I'm not British deep down. It's true that I usually tell people I'm English but that's more a matter of convenience since people where I live have usually heard of England, and use a similar name in their language, and it takes several minutes to explain that the country is not really England but the United Kingdom. It takes longer still to convince them that the first international football match was between England and Scotland, and that it was indeed an international match. At least they do appreciate the explanation of the Union Jack, until they start asking where the flag for Wales is. 

The point is, when those lawyers in Manhattan start suing Scotland, if it's a separate country from England, then at least some people in Britain will be unaffected. Of course this may come back to bite those south of the border since many of the insurance companies are in London. 

 

Cool jets of air on a hot summer's day

It's very tempting to believe, when you stand next to an open window and feel the breeze on a hot summers day, that it's cooling you down. It may be cool as you stand there, but if the temperature outside is higher than inside, and you happen to be in a well-insulated, airtight house, and it's likely to be over 30 degrees every day for the next month with a chance of a few nights staying above 25 degrees, then you really don't want the windows open when it gets hotter. 

The heat exchanger in the ventilation system will do a much better job than the windows at keeping it cool. If it's 30 degrees outside and 25 degrees inside, the air coming in through the windows will be at 30 degrees, but the air coming in through the ventilation system will be a little over 25. It may be more humid, but that's another issue. Humidity could make it feel one or two degrees warmer, but not five.

Of course, the air coming in through the window may be cooling you down by helping evaporation and blowing heat away from your body, but even if it is, the heat is going into the house and will be there for you later.

Then there's the effect of air at velocity expanding into the room.

I remember this from the day of the first airtightness test, 9th August, 2011. It was a hot one, 31.6 degrees outside, according to the test report. It was 31.3 inside. The house was still being built then, so the windows were usually closed at night and left open during the day. We now do the opposite.

For the airtighness test, the windows had all been closed. They had fixed a fan to one of the windows, then blew a lot of air out until the pressure dropped about 50 pascals below the pressure outside. Then the fan switched off and the machine started to measure the pressure go up as the air leaked in again, and that was our chance to go around the house searching for places where air was getting in.

You could feel little jets of cold air coming in at the corners of some of the windows. I remember wondering at the time why the air should feel cold when it was hot outside, inside, and presumably in the wall between. Now I realise it was the air expanding. The same amount of heat. Bigger volume. Lower temperature.

So the same thing is probably happening, to a lesser extent, when the window is open and air is rushing in to a large room. But when the air stops moving and settles at the ambient pressure, which is very close to the atmospheric pressure, no coolth has been gained. Or rather no heat has been lost, since "coolth" is neither a word in English nor in science.

It would be nice if this effect could be used in some low tech way, with fresh air outside somehow increased in pressure so that it would come inside at a desirable temperature and pressure, and genuinely cool the house. Something more sophisticated than an open window but less than an air conditioner, which in fact uses the same principle but with a coolant rather than air.

Defrosting the ventilation

The Passive House lady was talking about defrosting inside the ventilation system, and this seems like it could be a problem on cold winter days. So far it hasn't actually been a problem, but I'm not sure why.

The problem relates to humidity. As we all know the capacity of air to hold water decreases with temperature. We can see this in the condensation on low spec windows and on glasses of beer as the cooling air deposits its moisture. We can see the opposite effect in hair dryers, as the hot air carries water away.

So in the heat-exchanging ventilation system, on a cold day, the air leaving the house drops from room temperature towards outside temperature, and the humidity will go from 50%, or whatever it is in the house, up to 100% if it drops 10 degrees. As the temperature continues to drop, with 100% relative humidity, the moisture will start to precipitate. Back in December, when the airtightness expert came to show us how the ventilation system worked, we could hear a gurgling sound as the air flow in the ventilation system was turned up, evidencing this jettisoned moisture.

It stopped gurgling when we put a loop in the hose, as it suggests in the Steibel manual, which the expert had apparently not read very carefully.

Anyway, what was worrying the Passive House lady was what happens when it's really cold outside, and the heat exchanger is taking the outgoing air below freezing. In this situation, the precipitating moisture is going to be ice, or perhaps snow, or is going to form icicles inside the pipes. Ice is a crystalline structure, and once it starts, it will keep growing. This could spread to take over the ventilation system, block the exhaust ducts and lead to a lack of fresh air within the building.

To try to get an idea of the size of this problem, we need to think about the amount of water that the air can hold, which goes up exponentially with temperature. Very roughly, the capacity of air to contain moisture is 4 grammes per kg of air at freezing. This halves or doubles with each 10 degree change, so is 2 grammes at -10, 1 gramme at -20. Going the other way, it's 8 grammes at +10, and 16 grammes at +20.

When it's cold outside, it's going to be cold inside the heat exchanger and the air could be precipitating moisture below freezing and ice could form. This may start to happen around -4 degrees (water needs to be below freezing to actually freeze). If it's 20 degrees inside, and the heat exchanger is 80% efficient, that's going to start happening at outside temperature around -8 degrees or something. Even if it's very dry inside, say 25% humidity, by the time the air temperature has dropped 20 degrees, the relative humidity will have doubled to 100%.

The total amount of moisture that air can contain at that -4 C is something like 3 grammes per kilogramme. As it was probably saturated (100% humidity) at 0 C, it was holding 4 grammes and will have precipitated 1 gramme. A kg of air is about 0.8 cubic metres, so if the pump is set to 120 cubic metres per hour, that's 150 grammes of water per hour. If the outside air gets colder, it's going to precipitate more.

Of course the air is moving. If the cross-sectional area is 100 square centimetres, or 0.01 square metres, that's moving at 12 km/h, so some of the precipitation is going to be carried away. That's not a very fast wind, and we've all seen icicles form in windier situations. 

http://lilt.ilstu.edu/jrcarter/geo211/webpage-211/mod5-p4.htm
I've been monitoring the outside temperature, and it's been below -8 only half a dozen days, and then for an hour or so in the morning. It doesn't stay that cold here for very long.

In Central and Northern Europe, where this technology comes from, it does get that cold, and does stay that cold, so there are electric heaters within the system to keep the air above the dew point.

Counter-intuitively, the more efficient the heat exchanger, the bigger the problem, which reduces their efficiency as electrical heating consumes a lot of energy. Perhaps our ventilation system also has one, although there was no evidence of excessive electricity usage on those cold mornings.

The airtightness expert seems to have been completely unconcerned with this problem, perhaps because it is not an issue in the Japanese climate. It may be that he's just not very good with numbers, unless they have yen signs next to them.

Too bloody hot

December and January could be the hottest months in the house. At least, somewhat counterintuitively, they are the months with the highest solar gain. It's not that the sun is hotter in December and January. In fact, the sun is more or less the same temperature all the time, and cares little whether it is winter or summer in the northern hemisphere on Earth, but of course there is a difference in how much of that heat reaches the surface of our planet.

In terms of the radiation from the sun, there is more in the summer than in the winter. There are two reasons for this. First, the days are longer, so there are more hours of sunlight. More hours of sunlight mean more heat. Second, the angle of the sun is higher. This has two benefits. First, more sunlight is going to hit a given area of the earth. If the sun is directly above, a square metre of sunlight is going to hit a square metre of the earth. If the sun is 60 degrees below vertical, 30 degrees above the horizon, a square metre of sunlight will be elongated over two square metres of the earth so the incident radiation is halved. Also, the higher the sun is, the less atmosphere it has to get through, so the rays are stronger when they reach the ground.

The point with a house is that the windows are on the walls, so we aren't really interested in how much sunlight reaches a square metre of the ground. We want to know how much reaches a square metre of window. And this, almost by some divine intervention, means that in the winter, when we may expect it to be coldest outside, we get the most heat coming in through the windows. And when it gets warmer in the summer, less heat comes in. If we are careful with balconies and eaves, then we can try to keep this radiation to a minimum. Reflection is another thing that may lead one to believe that God invented windows, or at least that God was a double glazing salesman. The smaller the angle between solar rays and glass, the more is reflected and the less heat comes in. This means that more of the low winter sun will get through, and more of the high summer sun will be reflected.

So this is why it got up to 28 degrees centigrade in the living room at lunch time on 12th January, even though it was only one degree above freezing outside. The bottom line on this graph of temperatures over the first few weeks of our residence shows outside temperature (green - averaging more than one degree below zero). The highest temperature is inside temperature south (red at the top), and inside temperature upstairs north is pinkish below that, but dancing to the same tune. The others are slab temperatures. The big leap in inside ambient temperature was when we closed the windows and switched on the ventilation system on 23rd December, but you can see the jump in the temperature at the middle of the floor (light blue) as the underfloor heating started working on 26th December three days later. The effect at the bottom of the foundation slab (middle - dark blue) is slower, with about a three-day delay. At the North West corner of the foundation, the temperature change is much slower.   

Obviously it would be churlish to complain about the house being too hot, when all around are pouring gallons of oil into theirs and still freezing, and of course there are a few things that we can do before resorting to opening windows and letting the heat out. According to the thermometer in the upstairs north room, it is significantly cooler there, so if we open the inside windows from the atrium into the bedroom, the heat should go in there. Also we can open the door into the genkan and washitsu, which are to the north and significantly cooler. 

Part of the reason the north side is cooler is that the slab is much cooler there. This is by design. Kind of. The underfloor heating passes from the boiler to the south side of the floor, then to the north side of the floor, then back to the boiler, so the south side is being heated more effectively. Eventually the slab will probably have a constant temperature, but it actually seems like a good idea to have some temperature difference in the house. It would be nice to be able to control it a bit better, and I'm sure there is something we could do with the ventilation system. At the moment we are using a fan to blow air from the cooler northern parts of the house. 

But, going back to emissivity, I can't help feeling that it may have been a good idea to have had a higher emissivity for the floor and the walls so that they would have been absorbing more heat. What I guess is happening is that the radiation is just bouncing around the floor and the walls and getting the air really hot. The white terrace outside is probably helping by reflecting more sun into the house.

We're going to get some blinds soon anyway. I'd really like Venetian blinds with white on one side and black on the other, but I'm not sure if they are available or aesthetically pleasing. 

The Crookes radiometer shows the difference between black and white, invented by the eponymous Victorian chemist, William Crookes, who was pleased with himself for being able to make vacuum tubes. It was supposed to work as a kind of light mill, the white sides of each panel reflecting the sunlight, the black sides not reflecting anything, and spinning accordingly. When it started spinning, it went the wrong way; the black panels going away from the sunlight. The simple explanation is that the black sides get hotter than the white sides, and heat up the air molecules next to them, because actually the vacuum was far from perfect, which push the wheel around. A more detailed and accurate definition can be found 

here on wikipedia, unless the US government has shut it down. The difference the vacuum makes is to greatly reduce the resistance, so the effect of the heat becomes more significant. 

Black bodies and getting my head around emissivity

There's something about emmissivity that doesn't seem to have made sense, and probably should have done much sooner.

My attention to thermodynamics regarding the house has mostly been concentrating on conduction. Of the three ways heat gets around, this is probably the most significant. Obviously convection is important, but I think if you're dealing with a wall with inside on one side and outside on the other, and you treat the air as being at a constant temperature, then you won't be far wrong. What is actually happening, if the wall is hotter than the air next to it is that it is heating up that air, then that air is moving away and is replaced by a new bit of air which needs heating up again.

The resulting heat flow is more or less the same. 

Probably.

Anyway, I haven't really been thinking about radiation. I can remember a physics teacher telling us that radiators don't really radiate, they transfer heat by convection. And that seems to have stuck in my mind. I have, of course, been thinking about solar radiation, as that is how the sun gets its energy to us. We worry about nuclear power and radiation in the hands of humans, but in the case of the sun, it's an important part of life. If you want to see how much radiation nuclear power produces, you just have to look at the sun, as that is basically a big nuclear power station. In fact it's probably not a good idea to look at the sun as it will make you go blind. 

So the solar heat coming into the house is coming in by radiation. The windows are low-e, which everyone who knows about windows will tell you is important, so that you keep the heat in the house. The "e" is for emissivity, and it doesn't really fit into my idea of common sense. As far as I can tell, "low e" just means that it reflects heat. 

Emissivity, on the other hand, is a measure of how much a body radiates. It is a number between 0 and 1, or zero and a hundred percent. For a black body, which I'll get to in a moment, the amount of radiation is proportional to the fourth power of the absolute temperature. This corresponds to emissivity of 100%. You can read more about that here, if you're really interested.

The black body is a theoretical object that will absorb all the radiation that falls on it. What confused me for a while is what exactly this has to do with windows reflecting heat. It seems that three things are going on when radiation hits glass. First, some of the radiation is reflected by the glass, second some it it goes through the glass, and third, the rest of it is absorbed by the glass and warms it up. These seem to be independent ideas. Obviously all the parts are going to add up to 100% otherwise we'd be breaking the law of the conservation of energy, but what does radiation have to do with absorption?

I've given in to what seems a much simpler reality. It just comes down to one number - the emissivity. Bodies are like mirrors. If they absorb a lot of heat, they can also radiate a lot of heat. If they reflect heat well, they radiate heat badly. 

Polished metals have very low emissivity, which is why the lunar module was wrapped in tin foil. Not necessarily what you'd want around your house as it would conduct the heat away, but in space there is no atmosphere, so conduction is not an issue, and heat is lost, or gained if you're in the sun, through radiation. Radiation and emissivity are issues in buildings on earth, as we shall see in the next post. 

Colder than air - beating the second law of thermodynamics

The temperature of the roof seems to go lower than air temperature at night. I noticed this before with a simulation from OM Solar on the temperature of their roof. OM Solar runs on the principal of heating air under the roof, which it uses for space heating in the winter or  heating water, via a heat exchanger in the summer.

It starts to get cold at this time of year, and if you go out on a clear night, it really feels like the heat is being sucked from your face by the starry sky. This is because the heat IS being sucked from your face by the starry sky. Heat transfer by conduction depends on the temperature of the air, and convection keeps making sure that as soon as the air next to your face wams up, it  moves away and will be replaced by some cold air. Wearing a fur lined hood reduces the air flow around your face. Something more is happening on a clear night. Beyond the clouds, that are not there, it's very very cold, around fifty five degrees below freezing.  

But in fact the heat is not really being sucked from you at all. Your body is constantly radiating heat, depending on its absolute temperature and regardless of whether you stand in front of a starry cold night or a burning open fire. Usually, something is radiating the heat back again though. If you're inside, the walls and ceilings. If they're low e, the windows are going to be reflecting your heat back to you, because low emissivity means high reflectivity. If you're outside, the clouds are radiating heat back. Even though they are a long way away, below freezing and have a high emissivity, they're still radiating a lot more heat than the vacuum of space beyond. If it snows, the snow is radiating much more heat than any clouds. It may be below freezing, but that's still a few hundred degrees above absolute zero, where nothing is going on at all. That's why it feels warmer when it snows.

http://www.webcitation.org/6300EDKJi