Our off-grid house building is on schedule. The goal by the end of this year was to have the structure and outside completely finished, and we reached that this fall. We've also gotten the inside insulated using spray-on insulation, and the last step this year was to install hoses for in-floor radiant heat.
A lot of time has been spent thinking about how we're going to heat the house. We will be off-grid, but will have solar and propane and wood.
With the philosophy that too much is better than too little, we’ve gotten plenty of solar panels for our electricity needs. We built the house with the roof slanted rather than peaked so that they will be at a south-facing angle. There should be lots of power and then some as long as the sun shines. The excess power will be put to good use after the batteries have been fully charged. It will be diverted to heat salt water in a 2,000-gallon insulated milk tank (that we happened to have laying around the farm, a great way to repurpose!). This tank will be used as a heat exchanger, which will have pipes running through it, heating the water running through them. The electrical elements running from the solar panels will keep the water between 180 and 200 degrees. We'll also build a wood burning unit into the tank to heat the water if the sun doesn't shine for several days.
We did some experiments with salt water, and found that in addition to freezing at a much lower temperature, it retains heat better and longer than just plain water, and has a slightly higher boiling temperature. Because the tank will be completely enclosed and constantly heated, the convection motion of the water will keep it agitated so we're hoping the salt won't crystalize. We’re also going to need to buy a lot of salt to get it as saturated as possible.
The main heat for the house will come from antifreeze running through the hose in the floor, which will coil through the heat exchanger. This radiant heat should keep the house toasty, but we also have a wood-burning cookstove for a backup. The house is 850 square feet, which isn't a huge amount of space to heat.
Installing Pex hose into the subfloor was a big project. The recommendation is to leave around 12 inches between each row of hose, but we put our hose rows 2 feet apart, hoping that it would do the job. We figured that if it didn't produce enough heat, we would add more hose between our existing rows. (Needless to say, my arms and legs got REALLY sore from drilling and shoving and pulling pipe through the floors, and I really didn’t want to have to install more.) My partner built a heat exchanger stove for his shop that burns used farm equipment oil. So we connected the hot water hose that was heating the shop to our new house to test our handiwork. The water was 140 to 160 F as it entered the house. It heated the house to over 90 degrees overnight when the outside temperatures were in the 20s. Granted, when we put the subfloor and flooring down, the heat isn’t going to be radiating so strongly, but it should produce a nice even heat with the flooring, and we don't need it to be 90 degrees in there anyway! In addition, the water coming in from the heat exchanger will be closer to 200 degrees. So it seems that the 2-foot spacing between the hose rows should work fine.
The hot water for the house will also be heated with the heat exchanger tank. A hot water heater with a regulator will be used as a holding tank, we don’t want to run 200-degree household water. In addition, we will have a hot tub outside that will be filled and maybe circulated with water that runs through the heat exchanger as well.
It’s been pretty exciting to talk about ideas and create designs from scratch, and then move to the process of putting it together and making it happen. We are both creative, big outside-of-the-box thinkers and good at following through, so creating these plans from scratch and building them from the ground up together has been a great process for both of us. The house won't be finished until next fall. We’re taking a break this winter and heading south to spend the coldest months in the desert in Arizona.