Heating and cooling and off-grid home in SW MT
My wife and I are building an off-grid home in SW Montana (Climate Zone 6). Home will be 2×6 construction with 6 to 8 inches of exterior rigid foam and either blown cellulose or open-cell spray foam between the studs (feel free to comment on that one too). We haven’t done a heat / cooling load calculation (the home is a 2 story, ~1600 total FSF – 800 feet up and down), but my initial thoughts were to heat primarily with radiant in-floor heat supplemented by a wood burning stove. However in reading some recent GBA articles it turns out that both are on Martin’s least favorite things list. Cooling provided by ductless mini-splits on each floor (which I don’t think Martin hates). In our area, we have three options for fuel: wood, propane, or electric (PV solar as we are thousands of feet from the nearest power). Given we have 95+ degree August and -20 degree January and we have yet to lay footings, what is the general consensus on how to heat and cool our home?
As an aside, the heating with wood part is virtually free in terms of fuel source – our property backs up to a very large national forest where we can harvest 4 cords of firewood for $20 per year.
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Replies
Peter,
I'm pretty sure Martin's objection to wood heat only applies to grid-tied houses. The considerations change once you go off-grid. He himself heats with wood.
Some elements to consider:
a large tank of water for thermal storage
wood boiler
air to water heat pump
water cooled, propane powered generator with heat recovery
a "no power" propane wall heater
Radiant heat would work great with wood as a fuel source. Use an outdoor wood boiler, ideally a “gasifier” type for best efficiency. You want a big, insulated storage tank too. The goal is to run the boiler all-out, burn an entire load of wood, and charge up the storage tank like a big hot-water battery. That’s the most efficient way to operate the boiler. You then draw heat off carefully so that the water in the tank STAYS stratified — you don’t want to mix the warmer water at the top with cooler water at the bottom. You can use the same system for your domestic hot water system by using a heat exchanger. You can use heat recovered from a genset to help charge that battery too, but that is more difficult. Thermal differentials don’t always work in your favor.
If you want to run heat from your solar setup, you absolutely want to use some form of heat pump system for best efficiency.
I’d probably use the minisplits as primary, and the wood boiler for auxiliary heat when it’s really cold, or when I want that toasty feeling from radiant heat. There is no reason you can’t use both systems together, and the minisplits have the added advantage of providing air conditioning in the summer.
If you want to save some money by scrounging, it’s common to repurpose old propane tanks that fail their testing as hot water storage tanks. Since the storage tanks in this type of system run at very low pressure, you don’t generally have an issue with the old propane tanks. Have a spray foam contractor cover the tanks with a few inches of closed cell spray foam to insulate them.
You can get used generators with the “city water cooling” option, which is a heat exchanger in place of a radiator. You can use this setup to recover waste heat from the engine. You can also use heat exchangers on “regular” liquid cooled generators. Note that setting this up to operate well is NOT trivial, but isn’t particularly difficult if you have the skills. The biggest issue is arranging things to recover useful heat without causing the engine to run either too hot or too cold.
If you want to look into wood heating and hydronic systems, there is a lot of good info for that kind of thing in the forums at hearth.com.
Bill
See this article: "How to Design an Off-Grid House."
>" Home will be 2×6 construction with 6 to 8 inches of exterior rigid foam and either blown cellulose or open-cell spray foam between the studs (feel free to comment on that one too)."
If the windows are similarly high performance there won't be much of a cush-factor to the radiant floor due to the very low heat load, and hydronic pumps use electricity, which can be in short supply on some days.
A site-built massive bell-stove/rocket-stove with a massive warming bench would be simpler, more comfortable, and a LOT more efficient than a wood boiler with a massive buffer tank, no electricity needed.