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You are close at the "50 degree" estimate. The seasonal average air temperature of Green Bay, Wisconsin is 45 degrees. So that is also going to be the mean ground temperature 30 feet down. At a practical depth for horizontal loops, the ground temperature fluctuates seasonally about the mean temp of 45 degrees. As you pump heat out of the ground, the loop temperature will drop, but with a reasonable sized loop it won't get below freezing. The heat pump size is chosen based on the peak heat load of the house, which in turn is determined through a Manual-J calculation based on the size of the house, insulation, doors and windows, construction quality etc, plus the ASHRAE 97.5 percentile design temp for your location. (If you have accurate data of your past propane usage, you can instead combine this with historic weather data to derive the heat load.) You will have to have a source of supplemental heat (like your current propane furnace) to cover the (few) days it is really, really cold (like below 0 degrees (F)). Statistically, over the full heating season, 90 plus percent of your heating will come from the heat pump, with the supplemental heat kicking in only on the coldest days. This avoids the need to badly oversize the unit to cover the rare extremes of weather. The supplemental heat comes on, yes, but not very often. Your annual climate (8000 heating degree days) is cold, sure, but ground source heat pumps are succeeding in Alaska, Canada, and other even colder places. The size of the ground loop will be a function of the heat pump size and the heat load of the house. Typical rule-of-thumb would be about 400 feet of tubing per ton of capacity. I suspect you'll need a 3 or 4 ton unit but that depends on the size of your house. Figure roughly 1200 to 1600 feet of horizontal loop tubing. This can be laid out in trenches 10 feet apart in a "slinky" arrangement -- it doesn't need to use all that much land area. One retrofit obstacle is the sizing of your existing ductwork (unless you happen to have central AC) Furnaces produce hotter air than do heat pumps, so they don't move as much air per BTU, thus the ductwork tends to be undersized for a heat pump. A lot depends on the layout of your specific house. The main obstacle to saving money with ground source heat pumps is that they are still a boutique product. The local specialty HVAC contractors typically tack a huge markup onto what's a relatively simple unit. This stands in contrast to the more robust competition for air source heat pumps or conventionally furnaces. This may make it difficult for the savings to pencil out for you, even at the current high cost of propane. To give you an idea of what is possible, I heat and cool my 6000 square foot house using a 4 ton capacity ground source heat pump. It costs me around $60 per month in electricity for the coldest and warmest months. The total electricity usage for the entire heating season runs about $250. Now, this is in Northwest Oregon with an average winter of 4700 heating degree days (not as cold as Wisconsin). The mean ground temperature is around 53 degrees (not 45). 1/3 of my house is an underground basement with very little heat loss. I designed the ductwork, sized the heat pump, and trenched in the ground loop coils myself, so I took more care with these steps than a typical contractor would. The results have been spot on to the theoretical numbers based on actual weather, and I have been quite pleased with them.
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