We are in the midst of designing an off-grid home which will probably have a wood-based heating system (e.g. masonry or wood or pellet stove).
Shawn Bell wrote:
Since you are designing your home this really is not a problem for you. First of all use all the thermal mass you can in your building design, this helps regulate temperature fluctuations.
I should clarify that we have almost completed the design stage but had agreed to isolate-out the heating & electric systems design so those remain up in the air. Our builder is quite confident that our tight" strawbale design will not need more than a small woodstove to heat. I have done some analysis using the areas and R-values and it seems pretty good. My wife and I are in the over-55 set and not comfortable on concrete floors so that source of thermal mas is out. The design does incorporate passive solar elements including solar-south orientation and a relatively large area (135 sq ft) of south-facing double-pane, low- glass. The overall footprint is 1000 sq ft plus a loft and an unheated basement.
Now for the really cool off-grid heating design, solar hot water radiant heat floors! Your solar panels can power the pump to circulate the solar heated water. The thermal mass stores the heat to release it later that night.
Very definitely looking into that. Lacking structural thermal mass I am trying to figure out how much water-mass we might need. We could easily accomodate, say, a 500 gallon insulated storage tank in the basement and my quick calculations suggest we could potentially store a decent amount of heat from a solar thermal system for hydronic floors.
You might also consider a rocket mass heater, it is a lot like a masonry heater only cheaper (I believe).
This is a very interesting technology but doesn't seem suited to long-term unattended usage ?
How about really big windows facing south letting the sunshine warm the thermal mass floors, then automated draperies to insulate at night.
There is also geothermal heating, again I believe the solar panels could power a low energy version of this.
The house faces south across a 2.5 acre pond and we have looked HARD at pond-source geothermal. Unfortunately the numbers just don't seem to add up with a requirement for quite a bit of electricity (even with a COP (efficiency factor) of 3.0 or 4.0) at the time of hear when we are getting the least amounts of sunlight.These systems also have a pretty high upfront cost ($20K plus) and a spotty reputation for performance in some circles... too bad since it would be very cool (no pun intended) to heat our home in the winter from a frozen, snow-covered pond !
Just my thoughts...
Actually, my wife and I are building a home in Central NY and have been struggling with the same issue. It seems to have been competition between immediacy/convenience (I'm going away for about a year this fall) and sustainability. What we've decided on is to go ahead and use propane, with the goal that as soon as I come home we'll begin walking down the path of bio-gas production. Hopefully the propane tank can figure into that system as well.
As you are building your own home.... High mass. High mass connected to the ground, preferably an insulated chunk of ground. All insulation should be on the outside of your high mass walls. Berming is nice but not required. You do not need to keep the house at 72F when you are gone. Absolutely! I have run the thermal models for 50 degrees and it makes quite a difference. Our local "solar thermal and PV" installer says that in our climate it is very tough to get 100% solar thermal-based heat but that 70% can be done. I am asking them if that 70% can be translated from 7-days-solar-3-days propane to 10 days of solar at 50 degrees !? Keep your plumbing away from outside walls. VERY good point. I will make sure this happens. Thanks for pointing it out. Heat the mass not the air. Lots of mass will allow over sizing south facing windows without over heating the house. There are homes in some pretty cold places at high altitude that use no heat all winter and still have 60F plus min. temp. Some are partially buried but others are all above ground. Our crawl space seems to stay at a minimum of 12C (50 -55F) even though it is isolated from the heated areas, but it is connected to the ground 4 ft below the surface. Just an ordinary stick house.