Geothermal heat pump to preheat propane water heater water?
I have an older house that was built in the early 60s. The first floor has poured concrete foundation walls on the rear and sides of the house. One side and the back of the house is 5′ into a soil… one side is about 1.5′ into the soil and the front is at grade. When we bought the house it had a heat pump that ran the primary cooling and heating needs. It runs forced air through ducts in the attic and down to the first floor using ducting through some closets. This system is on its last leg. It also had a propane boiler that circulated water through 2 zones, an upstairs and downstairs zone. The water is ran through baseboard heaters. It also had a weird wood stove system that forced air through pipes in the flue hood on the first floor for heat. We have replaced this with an energy efficient wood stove.
I have ran some analysis of the existing structure and the future structure as we update insulation. We need to be able to deliver 36,000 btu/hr – first floor and 18,000 btu/ hr – second floor during the coldest days. The second floor demand could go up as well since we have enclosed a porch on the second floor that was above the basement family room. We may decide to heat it during the winter. If we do, it would require an additional 13000 btu/hr (It has a lot of windows).
So right now I need to make a decision on the HVAC equipment that should be installed. My current plan is to install a geothermal heat pump with loops through a large pond on the property. I will run water through this system and into a propane water heater to add the additional heat needed for baseboard heater. Based on the hydronic calcs I ran it looks like water needs to be 160F during the coldest days and 120F on 35F days. I will use the wood stove when possible and turn off that zone when it is running. During the summer the geothermal should perform well since the water at the bottom of the pond is pretty cold… I will send the heated water into the water heater and potable hot water will run through an internal heat exchanger… Is this overly complicated? Or is this a good approach?