Thursday, May 31, 2012
Getting Closer to smaller living: Stage 1.2 - Insulation and Heat
Figuring on lower numbers, assuming I cant get 1.5" thick board so I have to only use 3" in the walls and 5" in the ceiling and floor. R13 wall and R22 ceiling, 1 door and 4 windows. a typical Micro home will lose about 5000 Btu per hour. A person at rest puts off 222 Btu per hour, yes you are a heat source, so we can remove that from the 5000 Btu/h number for heating. So I need to make up 4800Btu/h to maintain the temperature in the home, this is assuming 70 degree inside and 10 degree outside temperature. So how do we heat the house? Well a typical 800 watt electric heater will deliver about 2,750Btu/h and a 1500 watt electric will produce about 5,100Btu/h. You can easily heat a 180 sq foot micro home with a 1500 watt oil filled portable heater. If you had electrical service then assuming $0.10 per Kwh cost. running your heater 12 hours a day drawing 1.5Kw means it will cost you $1.80 a day to heat your home when it is 10 degrees out and you are not getting any heat from the sun coming in windows or hitting the house. and remember, the sleeping loft will be warmer than the floor, so night temps can be set lower to save energy.
So my idea of a 12,000Btu/h RV furnace is really overkill, but probably not a bad idea. Excess capacity means it will run less, and when solar power is considered, I would rather heat from a Propane source than an electric source.
Air conditioning is the opposite, 5000Btu/h heat gain rate from outside and add in the 222btu/h per person in the home and you need to get rid of 5222btu/h. luckily tiny AC units are huge. The smallest one I can find is a 5000btu/h AC unit. but if you have friends over you can quickly overwhelm it by the additional heat, plus heat from cooking also adds to the load.
So how much propane? well, Propane has about 21,500 Btu per pound. so a 5 pound grilling cylinder has about 100,000 btu in it. (rounding down for losses) that means that I can run that 12,000 BTU furnace for 8 hours 20 minutes before the 5 pound tank (if it has 5 pounds in it) is empty. That is total run time. If I need to make up 4800btu per hour, that means I have heat for 20 hours 30 minutes. A 20 pound grilling cylinder gives me 89 hours of heat (again assuming a full 20 pounds) that means in the dead of winter you need to get a 20 pound grill tank refilled every 3 and a half days. This also means that the tiny coleman 5 pound cylinders can be used as an emergency gas source for 8 hours of heat if you needed to. This is assuming only Furnace use. this does not account for hot water heater or cooking. RV water heaters use as much as the furnaces, they just dont run as often or as long.
This is where insulation really kicks in. If you were to increase insulation and reduce heat loss by only 500btu/h That 20 pound tank will last an additional day plus 4 hours. All of this is assuming worst case. dead of winter in the arctic circle with no sunlight at all. in reality it heavily depends on the weather and your location. sunny days reduce heat loss significantly, even when it's 10degrees F out the sun can give you 1300btu/h through a typical 32" by 32" window. also heat gain through the wall as the house heats up from solar radiation, etc.. plus if it's only 40 degrees out, your heat loss is a lot lower than the calculated, etc...
By my estimates, a 20 pound grill tank can easily heat your home to a normal 65-68 degrees for at least 7 days in the winter. Spring and fall are looking at a monthly cylinder fill. Worst possible case, $80.00 a month heating bill in the arctic circle using only R13 walls and R24 ceiling. if you were to add storm windows, triple pane instead of double pane and higher insulation, you can do a lot better.
Again, this is assuming that the home is off the grid. no electric except for solar, and you have to cart in your own propane in your car/jeep on your own. If you will be in a location that has full hook ups or you can have electrical service ran, these issues are not a bother to you. But it's good to know what the effects are, remember your monthly expenses in living are tied to the decisions you make before you build. spending more when you build means you spend less while you live there.
In my off the grid case, the loft will be a lot warmer on it's own and if I was to run 12V outlets up there to use a 12 volt heated blanket even more gas can be saved. However, they use about 80 watts of power. So now that is a higher load on the solar storage system. 80 watts over 8 to 10 hours is about 800 watt hours used out of the storage system. Everything in off grid living is a trade off. And if you want to be portable, you can not use alternative systems like wood heat. I have seen tiny wood stoves, but honestly I cant see them being used safely in a micro home. the only small ones are the old cast iron pot belly type and they utterly scare me. If someone made a tiny modern type I may change my mind.