One of the two biggest challenges in reducing the climate impact of our off-grid home is heating (the other being transportation). For heating, most off-grid homes rely exclusively on burning fuel, whether fossil fuel (propane) or renewable (wood). Burning propane releases CO2, and burning wood, while not contributing as much to climate change, creates smoke that we cannot tolerate.
The renewable, climate-friendly, clean-air options are
Solar thermal heating: water circulates through panels on the roof, gaining heat from the sunlight, then circulates through the floor or radiators to heat the house.
Air source heat pump: pulls heat from the exterior air and transfers it either directly to the indoor air (via a "mini-split") or to water that circulates through the floor or radiators to heat the house. We don't want forced air heating, so in our case, it would be an air-water heat pump.
Ground source heat pump: pulls heat from the ground by circulating water through a closed ground loop and then circulates the water through the floor or radiators to heat the house.
Over the past two years, we've learned a lot about the trade-offs in these approaches.
Solar thermal heating is more complicated than it seems, involving many valves, controllers, and water storage tanks. Because of the complexity, it requires an expert to design, install, and maintain the system. After soliciting several bids, we found that the system required to heat even a small, well-insulated house like ours exceeded our budget. Because solar thermal systems are difficult to maintain, they have a poor reputation in the real estate market. The money invested in them may not be reflected in an increase in the house's value, even though it offsets the cost of heating it. They also generate excess heat in the summer, which is wasted and can overheat the equipment.
Air source heat pumps work well at low elevations and moderate temperatures. Since they pull heat out of the exterior air, their efficiency decreases at high elevations and cold temperatures. Units that perform well in cold temperatures have only come on the market recently. We had trouble finding contractors with experience designing and installing these relatively new systems, and they couldn't guarantee that they would work well to offset our heating needs. The exterior unit also needs to be mounted above the level of the drifting snow, which is not an easy requirement to meet where we live and adds to the expense. We concluded that it was difficult to tell whether an air-source heat pump could run on the amount of electric energy we had to spare and offset our heating needs enough to justify the expense and complication of the system.
Ground-source heat pumps have the advantage of not being affected by cold air temperatures, and the technology is well-established, having been in use for decades. They maintain their efficiency at high altitudes as well. The downside is that building the ground loop requires either excavating a large area to a depth of 5-6 feet (not something we would be willing to do) or drilling a well similar to our water well at a significant cost. Having spoken to several contractors, we identified a reputable one who gave us an affordable bid, considering the 40% federal+state rebates.
The 2 ground source wells were drilled to a depth of 300 ft by a team of 2 young petroleum engineers who quit the fossil fuel industry to drill geothermal wells instead and their dad who sold his farm to finance the company. In addition to drilling the wells (which took about a week because of the granite underneath our house), the crew had to trench from the wells to the garage where the heat pump and other equipment will be installed. The earth work involved truckloads of heavy equipment and created quite a mess, but it was all in areas that are either in the driveway or under where a flagstone patio will be, so no harm done.
The interior equipment (heat pump, circulation pumps, thermostats and hot water storage tanks) will be installed next.