Thermal issues in a tinyhouse
EDIT: turns out the mfgrs of the heat exchangers lied about the efficiency. See the air conditioning post for details. Also, the amount of ventilation is a bit high.
Some people might already know this, but reading the “heating a tinyhouse” thread on the tinyhouse forum, and the post mentioned there on the Tinyhousedesign blog, it seems like people are assuming that they need far more heat than they really do.
I do love wikipedia, you know there is this cool thing in firefox where you can add a “keyword” to a text box, simply right click the text box and click the “add a keyword for this search”, I simply type “wik x” in the address bar when I want to look something up. Check the bottom of this post if you want to know more about the concepts here.
U value for window is 2.8 (EDIT some months later: this is wrong, the wikipedia article was wrong at the time. Huh. That’s the first time I have caught wikipedia being seriously wrong on something. Looks like the U value for an air filled double glazed window is 0.5.)
U value for a typical wall according to wikipedia is 0.03 (the numbers often given for this seem to be inconsistent sometimes though, and that seems pretty low)
fencl is 8 (240 cm) by 19 (570 cm) by 12 (360 cm), let’s just ignore the roof pitch and assume it’s a box and that the roof and floor are insulated the same as the walls. Suppose there is a total of 0.75 sq meters windows.
vapor pressure at 0 deg 4.85/m^3, at 25 23/m^3
heat of vaporization of water 2257 kJ/kg
Heat capacity of air 1.012 J/g(k)
Suppose the indoor temperature is a toasty 25 deg c, and the outside is 0 deg c, the conducted heat loss will be, the fencl walls are 85 m^2 total, so total conductivity of 2.1 w/deg. so 64 watts, plus 52 watts for the windows.
Then there is the loss resulting from the airflow, supposing the tinyhouse is totally sealed, except a ventilator exchanges about 1 air change per hour, or 0.8 cubic meters per minute (29 cfm), or 1084 grams, so without a heat exchanger, that requires about 25000 joules per minute, or 416 watts!
If the air is, say 90% Relative humidity at 0 deg. coming in, and 60% at 25 deg. going out, that’s 14.5 grams of water every minute leaving as water vapor, or 32 kj per minute, or 533 watts! (but that’s 20 liters a day, I guess you would need some serious humidification, because 50% RH is the low end of the comfort zone. I guess if you didn’t have that much disposable water you would have to get used to lower humidity! You could get by with less ventilation too, with a laminar flow system and lower exchange rates and an energy exchanger, I think you could have it down to 0.5 liters a day… hm, there’s a use for all that urine water….well, hypothetically)
As you can see, having a heat exchanger can save you a lot of energy, because it can cut the cost of the ventilation by about 90% (and save almost 100% of the water EDIT on 2010/09/12: that’s wrong, can’t be that much, it is almost 100% of the sensible heat that gets recovered, but some of the latent heat usually escapes) if done correctly. You can cut that again by almost half by using a laminar flow ventilation system, not because that reduced the amount of heat required per unit of airflow, but because you get the same indoor air quality with half the airflow.
However, it is also clearly very important to seal the tinyhouse well, or you will get that much un-heat-exchangerified ventilation anyway whether you want it or not. Of course there will always be some leakage, apparently in a passivehause they assume there will be an airtightness of 0.6, which means you get an air change rate of 0.6 per hour when the house is pressurized to 50 pascals, I think. 50 pascals could only be caused by a ventilation system, though, which was blowing air into the house on purpose, which is why ventilators have 2 blowers, one to draw air in, one to pull it out. If you had only one blower blowing in, and depended on the air flowing through the heat exchanger on the outward path due to the air pressure in the house being higher, you would be very disappointed, because the heat exchanger cannot work well when the air is escaping through cracks in the walls rather than going through the exchanger. The natural leakage caused by wind must be much lower, however in a tinyhouse, the surface area to volume ratio is much higher, so it will have a tendency to be leakier on an air change per hour basis just from that. So I guess careful sealing is the order of the day.
However, a lot of people don’t realize that all the energy you use for powering the lights, etc, and your body (a human outputs something like 150 watts resting, I think) is being turned into heat. You may well have 5 or 6 hundred watts of heat being released into the tinyhouse because of these things alone. A assume a tinyhouse will be a much higher power density situation than a regular house, because the computer, fridge, etc. is all contained in a very small volume, unlike a regular house.
In other words, a dickenson fireplace might look cool, but if you invested in a heat exchanger and sealing the tinyhouse well, you would probably never or very rarely need any sort of heater at all, and save some money, and eliminate the bother and fire hazard of propane, too.
It is very easy to make a tinyhouse a passive house, in other words. You don’t even need solar gain or anything, though that would further add to the heating…. Actually, come to the think of it, I think the only time you would ever need a heater is if it was -30 out or something, or you had left everything or nearly everything turned off, and had come home in the early morning, after the house had been losing heat all night long. Even then, the thermal capacity of the house may be enough, especially if the ventilation system is shut off. Especially if the water tanks were inside the thermal envelope, because they will be a big thermal mass.
So, consider the following pages,