Heating and Cooling

Here's a lesson on heating and cooling. 

More about the lesson here.

And more about air conditioners here.

Lesson 5: How do you feel?

When I started planning this lesson, it was going to be about cooling, since it's starting to get a bit cold and would be nice to talk about the summer. And I wanted a break from calculation-heavy thermodynamics. Of course, I have made it clear that if we are going to save the planet, we have to do some mathematics.

Also I wanted to try to integrate the class a bit. After initial success in my language policy of dividing the class into people who want to speak English and people who want to speak Japanese, it was rapidly turning into segregation on national lines, where all the Japanese students were on one side of the class and all the non-Japanese on the other. I thought it would be good to get mixed groups of tropical and temperate-dwellers, along with my permanent goal of getting my students to talk to as many other students as possible. While I try to promote classroom democracy and learner autonomy, as the teacher it is in my power to go around the class and tell students where to sit, so I did!

The title of the lesson was originally cooling, then cooling and heating, but in fact the best title is my first question to the class: How do you feel? I wanted them to choose an answer between Hot and Cold, which I then converted to a number between +3 and -3. 

Hot -3

Warm -2

A bit warm -1

Neutral 0

A bit cool 1

Cool 2

Cold 3

I took the average, which is called the predicted mean vote, after research by Fanger in the 1970s. The average was 0.9. There is a relationship between the predicted mean vote, and the predicted percentage dissatisfied, so if the mean is zero, you can expect fewer than 5% to feel uncomfortable. There will always be some people who are dissatisfied—an important lesson for life! If the mean vote is between plus and minus 0.5 you can expect 90% to be satisfied. At plus or minus one, 28% will be dissatisfied, so our number looks like a quarter of the class were feeling uncomfortable. This is not a terribly good figure, since uncomfortable students are less likely to learn anything, but my immediate goal is in educating about thermodynamic. Solving the effects of thermodynamics on education will take a little longer!

The next question, inevitably, was why they felt hot or cold. I wanted them to brainstorm for a list, and I also gave them the big question: heating makes us feel hotter; true or false?

They wrote down all of my prepared answers, like air temperature and clothing, and a few others such as being with people who you like, eating, and drinking hot drinks. They also mentioned snow, which actually I think makes you feel warmer, but I need to do some more research on this for a definitive answer.

There were four groups and I got one of them to explain each of radiation, humidity, air movement and activity.  

The group talking about activity had some theory about blood going through the veins more quickly and creating more friction. I asked politely whether it might not be because we are burning sugar when we exercise and if we exercise more we burn more sugar and create more heat. I even drew the chemical formula on the board: 

Sugar + O₂ -> H₂O + CO₂

And further elaborated sugar to C_xH_yO_z, which impressed upon them my chemistry inability. 

The group discussing air movement correctly observed that moving air will take away more heat from our bodies, and the humidity group noted that we lose heat when moisture evaporates from our skin, and more humidity means less evaporation so we lose less heat. I later told them that 10% extra humidity feels about 1 degree celsius warmer. 

The radiation group was talking about radiation making the air around us warmer, which I also had to question. They did appreciate that heat was being radiated from many things around us. I later showed them the relation between air temperatures, surrounding surface temperatures and the temperature we feel. For example if the air temperature and surface temperatures are both 20 degrees, it will feel as hot as the surface temperatures being 17 degrees and the air temperature 23 degrees. 

I also told them about radial symmetry, and how it will feel uncomfortable if there are different temperatures between head and feet, or from different directions. 

We all began to agree that heating does not make us hotter. We are warm-blooded, and we make ourselves hot, in fact putting out something like 100 watts. In fact, heating makes us lose less heat.  

So what about the summer? 

Does insulation make buildings hotter in the summer? 

Does insulation work the same for keeping buildings cool in hot climates?

Are there some buildings that don't need insulation?

To which my answers are no, no and probably not. 

Insulation does not make anything hotter, it just stops heat moving so fast. If it's hotter outside, then insulation will stop heat getting into a building. There are two important differences between the way insulation works for hot and cold climates.

First, many things in buildings produce heat, including people, electrical appliances, cooking and hot water. In a cold climate, these are all helping to keep the house warm. In a hot climate, these are extra things we must fight to keel the house cool.

Second, cold places are a lot colder than hot places are hot. For example Yakutsk in Russia has average winter temperatures of minus 34. Around that temperature it doesn't make much difference whether you are talking about fahrenheit or centigrade. Meanwhile the average summer temperature in Kuwait is 38 degrees C. These look strangely symmetrical around the freezing point of water. Or perhaps not so strange considering our environment is as dependent upon the temperature stabilising effect of water phase change as a glass of gin and tonic. However, when we remember that humans favour temperatures between twenty and twenty five degrees, we can see that the temperature difference we are working against is four times more in the cold climate. 

I gave the example of Australia, considered to be a hot country, where more people die of cold weather than hot weather. Additionally, more people die of cold in Australia than they do in Sweden, which is most definitely a cold country. Why is this? Insulation of course. 

There may be somewhere in the world where insulation is unnecessary, but in cold climates it will help to keep you warm, in hot climates it will help to keep you cool, and in many temperate climates it will do both at different times of year. 

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.)

Winter's coming, but not too close!

The temperatures are starting to drop below zero every night, and the other day it was trying to snow outside.  The garden is often tinted with frost as we look out over breakfast. On clear evenings the moon reflects on the snowy tops of the mountains, under a starry sky that seems to suck heat from your face as you look up.

November has come and gone and winter is definitely here, destined to get colder and colder for the next couple of months. Meanwhile, inside the house, we haven't put the heating on yet but the low temperatures are still a degree or two over 20. When the sun shines, it gets into the high 20s around midday with all that solar radiation.

On the last day of November, we put away the thin blankets we've been using since summer and got out the winter futons. In the old house, the stoves came out and the thick futons went on at the beginning of October. By now we would probably have added a blanket on top, and hot water bottles inside. We'd be facing icy corridors, and a real struggle to get out of our warm cocoons in the morning. In the new house, the kids sometimes jump out of bed before the sun even comes up. 

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. 

The temperature is being logged

Some data loggers have just arrived for the thermometers we put into the slab at a couple of levels. The recording all started back in the middle of winter at the bottom of the foundation, when we added sensors into the rebar.

Then we had all these sensor plugs growing like flowers from the wet concrete.

Most of them survived the layer of aggregate, then another sensor was added in the metal grid for the screed floor. I tried to get them to move the sensors as far as possible from the underfloor heating pipes, outlined in red.

Hopefully the sensor in the middle is around this position, although it may have moved when the concrete was poured.

After the screed floor, there's a few centimetres of wire to the plug. This is at the back of the house where the store room floor is a few centimetres lower and the slab a few centimetres thinner.

To avoid another decapitation, the carpenters very quickly made little boxes to cover the protruding sockets.

Tsukanaka-san arrived from T&D, and fixed the one we broke when the aggregate was poured in. I had tried to fix it, and stripped back the cover of the wire only to find three identical wires inside. I tried to fix them together, hoping for the best, but when Tsukanaka-san and the company president Morizumi-san plugged in the sensor, there was no reading. The president pulled my work apart, and reconnected two of the wires. One is apparently a dummy. He got the right two wires first time!

When the data loggers arrived they had to make bigger boxes.

You can see from the readings that there's already over 2 degrees difference between the temperature at the top of the slab to the bottom. There was also a difference from the west side of the slab, which gets some sunlight in the evening, while the other parts are in the shade. We can track the temperature over the next few months and get some idea of its reaction time, which should help when we start operating the underfloor heating. I'm hoping to some extent we can build up heat at the end of summer, and then release it over the winter so the slab is as cool as possible when summer hits. Probably not enough thermal inertia though. 

Each logger has an index number from 1 to 10, so we have started in the middle of the house with number 1 at the bottom of the foundation, inside the insulation, and number 2 in the concrete floor. Each logger can store 16,000 readings, or around 110 days' worth of data. The batteries last six months, so we need to fix the loggers where we can get to them. They have extension cables, but three pairs are positioned in or under cupboards, one is under the stairs and the other is in the storeroom. Maybe the cupboards should have removable floors or something. The data can be collected by a radio collector, which can then put the information into a computer.

There are fifteen channels, so we can record the room temperature and humidity in a few places, possibly outside as well. Also I was wondering about recording the temperature in the air channel under the solar panels, which I think is going to get quite hot and impair the power generation.

How warm is it going to be?

It should be warm, but to find out we're putting thermometers in each corner and in the middle, in the bottom slab and in the floor slab, a few centimetres higher.