Thermal issues in a tinyhouse
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 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.
Metric U value for window is 0.5 (W/degree-m^2)
U value for a typical built to code 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 of 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 -5 deg c, the conducted heat loss will be, the fencl walls are 85 m^2 total, so total conductivity of 2.55 w/deg. so 76.5 watts, plus 11.25 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 (ACH), or 0.8 cubic meters per minute (29 cfm), or 1084 grams per minute, so without a heat exchanger, that requires 542 watts!
1 ACH is good ventilation, and we want this to be a good house, so… but standard ventilation rate is only 0.3 ACH, though. You could get by with less ventilation too with a “displacement flow” system.
For latent heat requirements, if the air is, say 90% Relative humidity at ~0 deg. coming in, and 60% RH 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 would be required for humidification. (But that’s 20 liters a day too. Huh. I guess you would need some serious humidification to be comfortable if you did things this way, because 50% RH is the low end of the comfort zone.)
So that would be a total of 87.75 W for walls and windows, and 1075 for airflow. A dickinson marine fireplace, the P9000 presumably, puts out 7,500 BTU per hour, or 2198 Watts, but remember this is with much more ventilation than most people have.
So ventilation is by far the biggest problem. Using a heat exchanger would help, a cheap aluminum core unit, which only affects sensible heating requirements, typically gets 50%-60% efficiency, so it would reduce the requirements by 25%-30% total if it got the rated efficiency. But the efficiency rises (hyperbolically) as the flow rate goes down, and those 50% figures are for much more cfm since they are made for a normal house, so it would be a bit higher, though I would have to check stuff to verify how much….
An “energy exchanger” recovers latent heat too, and typically get a rated ~70%-80% total efficiency. Again it goes up as flow rate goes down. But they are expensive though, a couple thousand for a vapor permeable plastic core kind like the lifebreath, though I couldn’t find exactly how much. It is possible to make a regenerator based one much more cheaply, and stirling tech offers a regenerator one already so that might be a better approach…
So having a heat exchanger can save you a lot of energy, because it can cut the cost of heat needed to heat incoming fresh air. You can also further reduce the energy consumption by ~30% or more of the ventilation requirements using a so-called “displacement flow” (googelable) ventilation system, in which instead of mixing the air in the house, the fresh, slightly colder air just sort of rises from the floor.
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 airtigtness of 0.6, which means you get an air change rate of 0.6 per hour when the house is pressurized to 50 pascals. The leaking of air though walls is why heat exchanging ventilator units 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 instead of the hot indoor air going through the exchanger, heating up incoming air, a lot would escape through the walls/door cracks etc. of the house. The natural leakage caused by wind must be much lower than the test rating from the blower door test, 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. If you are connected to the grid, you may well have 5 or 6 hundred watts of heat being released into the tinyhouse because of these things alone. I 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 dickinson marine fireplace might look cool, but if you invested in a heat exchanger and sealing the tinyhouse well, factored in solar gain, you could never or rarely need any sort of heater at all if you were on the grid, and save some money, and eliminate the bother and fire hazard of propane, too.
It is relatively easy to make a tinyhouse a passive house, in other words.
Explanations of the above concepts: