How do you heat a passive house?

Some people may think the answer is "you don't," but Passive houses do need some heating. 

The real answer is that you don't need very much, so it's not so important how you heat it. 

A comprehensive discussion can be found here from Zehnder Passive House, listing the pros and cons of each approach.

My personal favourites at the moment are underfloor heating and air-source heat pumps, athough I have not always thought so, and cannot guarantee that as a final answer. 

I wrote about underfloor heating before, highlighting the advantages: saving space, evenly distributing heat and increasing thermal mass; and also warning of the problems that can come from incorrect design and installation. One disadvantage that I forgot to mention is that it's very difficult to get to the underfloor pipes if something goes wrong with them, although they are just pipes and it's very unlikely that anything will go wrong.

Air conditioning units are becoming standard installations in Japanese houses, and their COP is getting better all the time. As well as cooling, they can reverse the circuit to heat air. In a regular house using these for heating can be uncomfortable since they are only heating the air, while the building itself stays cold and the temperature is not balanced. In addition, the hot air can rise giving you cold feet and a hot head when you stand up. This may also be an expensive way of heating your house, and it may even be both uncomfortable and expensive. 

Since a passive house has such low heating demands, modest air conditioning units can easily provide the heating needs of a building. The peak heating load of a Passive house should be around around 10 Watts per square metre, so for a 100-square-metre house, you need 1 kW. Units are typically rated at several times this, so one air conditioner can produce all the heating or cooling you will need for a whole house, although you may have to think carefully about where to put it. It can go on a wall or ceiling so, similar to underfloor heating, it won't take up any floor space. 

An additional advantage of an air conditioning unit is that it can also cool the house, and may also have a dehumidifier. The units are stand-alone, rather than incorporated into the ventilation system, so they may be less complicated. And depending on where you are, you may get something that comes with a guarantee, and can be easily be maintained or replaced.  

And by the way, if you're in Japan trying to work out how to read the symbols on the remote control so you can use the air conditioner this post from Surviving Japan may be useful.

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.  

Is it a fair COP?

Just looking at the spec for our eco cute in search of hard data on the temperature performance of the heat pump, it reveals very little. 

The spec is for two models, the CHP-H4619AT, recommended for "Region III" and the CHP-H4619ATK, recommended for Region I. The regions corresponds to the Next Generation Energy Efficiency standards, where Region I is Hokkaido, the coldest part of Japan, and Region III is Miyagi, Yamagata, Tochigi and Niigata in the lower North East of Japan, and the warmer parts of Nagano, which is central but mostly high in the mountains. 

Then it shows annual hot water efficiency of 3.1, which apparently is the ratio of the heat coming out in hot water to the electricity going in. So for a kWh of electricity, you get 3.1 kWh of heat. The note next to this says it is the situation using low energy mode, under certain conditions referred to in another note. The other note says that this is the average performance between Tokyo and Osaka, which are neither in Region I nor III but in Region IV.

Further down the spec, the mid-range rating for the heat pump COP is given as 4.5, which has no notes attached, but there are several notes to the numbers around it, which show the mid-range output heat and mid-range power consumption. "Mid-range" means an ambient temperature of 16 degrees (12 degrees wet bulb) and mains water temperature of 17 degrees with an output water temperature of 65 degrees. The electric power consumption is 1.33 kW and the hot water output is 6 kW; a ratio of 4.51. It also gives a summer figure with ambient temperature 25 degrees (21 degrees wet bulb) and mains water temperature of 24 degrees. The power consumption is then 0.97 kW and hot water output is 4.5 kW; a slightly higher ratio of 4.63). For the winter it gives ambient temperature 7 degrees (wet bulb 6 degrees) and mains water temperature 9 degrees with hot water at 90 degrees. The electric power in the winter is then 1.99 kW producing 6 kW of hot water; a significantly lower ratio of 3.0.

Two things leap out of this data when you actually look at the small print. The mid-range performance is suspiciously close to the summer performance. The winter performance is much worse. Since hot water use is going to be less in the summer, and more in the winter when the heating is on, the performance at the middle of the actual range is going to be between what it calls the mid-range performance and the winter performance. Also, the ambient temperature for the winter performance is about 5 or 10 degrees warmer than the actual ambient temperature we get in the winter. That's in Region III, let alone in Hokkaido in Region I.

It does give another note that under cold conditions, the performance will be lower. 

So there you go. 

The COP is 4.5...

Or 3.1.

Or some other number...

Heat pumps from cold night air

The bath talks to us.

We press a button or set the timer, and a little later it tells us when the bath is ready. If the bath's empty it's easy. It knows how much water is in the tank, it knows how hot it is, and it knows how much needs to go into the bath. If the bath has some water in it, then it must decide how much water needs adding and how much to heat up the water. Either it can add water from the tank, which will be relatively hot, or it can send water from the bath through the tank, from which it will take heat, in accordance with the second law of thermodynamics. Adding hot water is a lot more effective than circulating the existing water since the exchange has inefficiencies and there are heat losses as the water goes through the pipes between bath and boiler, even though we insulated them and kept them as short as possible. This is where the problems start.

Sometimes, there is not really enough heat in the tank to effectively heat up the bath. The bath brain probably starts off by adding hot water to the bath, then thinks that it's still not hot enough, so it starts circulating the heat to get it warmer. What it doesn't realise is that the heat from the boiler isn't going to get to the bath, or the part of the tank where the pipes from the bath are circulating may not be as hot as the part with the thermometer in. Rather than heat going into the bath, it may start leaving the bath, and dissipating into the larger thermal system following the inevitable fate of entropy. Then the bath's brain thinks "sod it", and gives up, without saying anything.

The bath talks to us, but it doesn't know how to say sorry.

This lukewarm bath situation is most likely to happen after a very cold night, when the heat pump is so inefficient that the energy would have been better spent drilling for oil in the Japan Sea. Obviously, I could just switch the boiler to be on all the time, or to "o-makase" (trust me) mode and let it switch on when it's warmer in the day time, but I don't really trust it. 

I would like to change the heating system so that the atmospheric heat source for the boiler is not night-time air, but the air flowing under the solar panels in the daytime.

This is the situation on a couple of clear, cold days in winter, midnight at the beginning of 13th January to midnight at the end of 14th. The black line is the temperature outside. The air temperature drops around 9 below zero on the first night, then up to around 2 degrees in the heat of the next day's sun. Then it plummets to -14 the following night and up to about 2 degrees again the following day. This leaves pretty cold temperatures for the Eco Cute to squeeze heat out of, leading to great inefficiency. 

The blue line is the temperature of air flowing through the channel under the solar panels on the roof. At night it gets just as cold as outside temperature. In fact on the night of the 13th, it was over 2 degrees colder because of the effect of the roof radiating heat to the stratosphere on a cloudless night. In the daytime, it gets over 30 degrees. If we could use the air from the channels in the middle of the day, rather than the air outside in the middle of the night, the air temperature could be 40 or 50 degrees higher. At this higher temperature, the heat pump would be much more efficient. 

The coldest nights and the warmest days are when it is clear, and of course it's not sunny every day. For example, this is the temperature on the panels the day it snowed. You can see the 14th and 15th January here, and how the temperature below the panels is just above freezing. In this case we have nothing to lose. The following day, with some snow still left on the roof, the temperature still got up above 10 degrees in the day time. You can see the temperatures from 12th January to 17th February below, perhaps averaging 25 or 30 degrees under the panels in the hottest part of the day, compared to minus 3 or 4 degrees outside at night. 

The main use of hot water is probably the bath, which we usually run at night time, so another advantage of switching from night time heat pumping to daytime heat pumping would be that the hot water would have less time to cool down, so we would need to use less of it. In terms of economics, this would mean a switch from using bought night time electricity to using our own generated electricity. Since we're buying off-peak electricity at 9 yen, but selling our electricity at 48 yen, the system would have to be 5 times more efficient to be worth changing. However, we only have a 10 year contract for selling our electricity at 48 yen, and no idea what may happen after that. 

This graph may give us an idea of how much energy we would save. Heat pumps are rated by COP, coefficient of performance, rather than efficiency. This corresponds to the amount of heat coming out compared to the amount of energy put in. So if you used 1 kW of electricity and got 5 kW of heat, that would be a COP of 5. The COP is affected by the temperatures inside and outside, so if you're trying to heat up luke warm water with hot air, you're going to get a much higher COP than trying to make hot water very hot from cold air. I've seen that Eco Cutes have a COP of 3.8, but this is a meaningless figure, unless you know precisely what operating conditions that was under. In the real world, not the world of advertising and eco-posturing, the COP for a particular heat pump is on a curve, dependent on the temperature of condensation, which happens outside, and the temperature of evaporation, which happens inside. Many heat pump manufacturers do not publish COP charts. From the graph above I estimate that if the temperature outside is 20 degrees higher, the COP will double. 

(The COP graph was stolen from Science Direct's website, from a paper which I did not pay $36 to read, by Forrest Meggers and Hansjurg Leibundgut of  ETH Zurich, Faculty of Architecture, Institute of Technology in Architecture, Building Systems Group, "The potential of wastewater heat and exergy: Decentralized high-temperature recovery with a heat pump". I hope they don't mind.)

Extractor fan hot water units

The more I think about it, the more sensible seems the idea of pumping heat out of extracted air into hot water tanks. Given a reasonably well sealed thermal envelope, the places you want to extract air from a house are kitchens, bathrooms and toilets. These are also places where hot water is used. 

And if you don't have a well-sealed thermal envelope, then extracting air is not an issue.

If you extracted 50 cubic metres and dropped the temperature by 20 degrees, 1,300 kJ would be available. If you did this every hour, you'd get about one kWh every three hours, 8 kWh per day. According to Without Hot Air by David Kay, in Sustainability without the hot air, a bath takes about 5kWh and a shower 1.4 kWh. He estimates 12 kWh of hot water per day per person, although he seems to include cooking, refrigerating and freezing in his sums. 

The problems, of course, are in economies of scale and system complexity.

In the summer, rather than cooling the air going out, you would want to cool the air coming in, but you probably wouldn't want to be drawing air into the house via the kitchen, bathroom and toilet! 

Air conditioners are now pretty much standard fittings in Japanese houses and models are available that heat water as they cool the air, but these are not widespread, and in installation they work out more expensive than buying separate units for heating water and cooling air, and since the air conditioner is not on for most of the year, another means of water heating is necessary anyway.

Useful physical characteristics of air: 
Air holds 1 kJ per kg per degree change in temperature. 
In cubic metres, that's about 1.3 kJ per cubic metre kelvin. 

Post-Promethian Society

Burning stuff has been a pretty important part of humanity for a while now. Fire has been around in nature a long time, and we have to say that it was discovered and harnessed by humans rather than invented. According to Greek myth, Prometheus stole it from the gods to give to man. Other mythologies share this theme of theft

Perhaps in the distant future there will be myths of how the god Watt stole coal from the ground and turned it into thick air, or how divine Einstein found electricity in rays of sunlight. Wikipedia mentions the hero Mātariśvan in the Rig Veda (3:9.5), recovering fire, which had been hidden from mankind. In Cherokee myths and those of some Pacific Northwest tribes, fire was variously stolen, or almost stolen but ultimately handed over to humans by Possum, Buzzard, Grandmother Spider with her web, Coyote, Beaver or Dog while among some Yukon First Nations people, Crow stole fire from a volcano in the middle of the water. According to the Creek Indians, Rabbit stole fire from the Weasels.

It sounds far-fetched, but remember the ancient four elements—fire, air, water, earth—are in fact four states. Plasma, gas, liquid and solid, liquid.

I watched the Day After Tomorrow the day before yesterday. The basic plot is that drastic climate change happens, but not by a couple of degrees over a few score years, but by scores of degrees over a couple of days. The science is hardly that rigid. It seems that changes in ocean currents cause a massive hurricane-like storm system over the northern hemisphere. I suppose that much is possible, although it's unlikely as hurricanes hardly ever happen high in the arctic, possibly
due to the Coriolis effect, which is largest in the tropics and sub-tropics.

The eyes of these storms brought down cold air from the troposphere, where the temperatures are very low, and froze everything in sight. I think the problem with this is that temperatures are very low in the troposphere because pressures are very low. We'll find out more of this when we consider how heat pumps work, but basically as the pressure drops, the temperature drops and as the pressure rises the temperature rises. You can feel this with a bicycle pump. Generally a rise of 100 metres will lead to a drop of one degree (although less if the air is humid) and a fall of 100 metres will lead to an increase of one degree. This causes the Foehn effect in alpine climates, where humid wind blows up one side of an alp, dropping in temperature slowly and shedding its humidity as rain. It then heads down the other side of the alp dry, gaining temperature as it falls leading to a very hot day in the valley on the other side.

If the eyes of these storms were making holes in the atmosphere where there was no air at all, then there would have been no pressure either, and rather than freeaing, people would probably have boiled, and their eyes popped out. However, I digress from Prometheus. That's sounding more like Tantalus.

I suppose the movie was trying to advocate action against global warming, although a lot of the time it felt like it was just nostalgia for those disaster movies of the 80's. The biggest problem was the reaction to this storm, which was for them to burn as much as they could. The hero was holed up in a library with his septicemic girlfriend, an aging gentleman of the road and a couple of librarians,
and their solution was to start burning books. It would have been much more sensible for them to line the books around the walls for more insulation and to reduce the size of the room, and start burning the furniture and shelves, or the guy who was clutching the bible. The only allusion to this was the gentleman of the road tearing bits out of a book and stuffing them in his clothes. The hero, his two sidekicks and the romantic adversary were all supposed to be academic decathletes, but the bum seemed to know more about thermodynamics than they did, and more than the people who made the movie for that matter.

So the moral of the story was... global warming's coming but you'll be OK if you burn lots of stuff.

Exhaust air and heat pumps

As well as being a highly insulated, highly airtight and wind-tight structure, the house has an active ventilation system with a heat exchanger. It's a good idea to have active ventilation with an
airtight house as it will stop us from suffocating. It's a good idea having a heat exchanger because this will mean we lose less heat in the winter, and gain less heat in the summer. It's a good idea having
an airtight house with active ventilation because this means the air goes in and out through the heat exchanger. Try sucking through a straw with holes in it, and you'll see what I mean.

The heat exchanger is over 90% efficient, so most of the heat will be recovered from the exhaust heat, and transferred to the fresh air coming in. This means if it's 20 degrees inside, and zero outside, the air coming into the house will be 18 degrees and the air going out will be 2 degrees above freezing. In the summer, if it's 20 degrees inside and 30 degrees outside, air will come in at 21 degrees. There is an over-ride so, for example on a summer night, if it's 25 degrees inside and 20 degrees outside, rather than trying to exchange heat, it will just get rid of the hot air and bring in the cool air.

There is a heat pump on the roof which is used by the "Eco Cute" water heating system. This takes heat out of the cold air and pumps it into hot water in a way that is worthy of another post, if you're not careful. The ventilation system is in the loft and will be sucking air in from the East wall and blowing out of the north wall. I was quite seriously suggesting that the exhaust air should be directed straight towards the heat pump. In the winter, exhaust air is going to be a couple of degrees above ambient, which will make it slightly more efficient, and less likely to be below freezing. The heat pump is set to run at night time, using cheap electricity, and in the summer, when the ventilation system is in over-ride, the air being pumped out is also going to be hotter than ambient. Even though it can exceed 35 degrees in the day time, it's usually below 25 degrees at night. In 1983 there were two days when it stayed above 25 degrees all night, and that was a record.

The only time it is likely to be warmer than ambient is in summer daytime, when we're least likely to be making hot water.