I have been reviewing three different quotes on a geothermal system, but I'm not sure that any of them make sense for my house.
We currently have a 3 ton A/C unit (3 1/2 ton coil) and a 110,000 btuh input propane furnace (80% efficient). The A/C seems sized appropriately as it just keeps up on our warmest days. I suspect the furnace is oversized.
All three contractors took plans for our house and did load calculations. There were slight variations, but the numbers were approximately 56,000 btuh for heat and 36,000 btuh for cooling. We live in western Michigan. Our supply and return plenums are 10" x 24".
All three contractors suggested two stage units. Two of them recommended 4 ton units and one recommended 5 ton. The most common unit suggested was the ClimateMaster Tranquility 27 (or equivalent rebranded). When I look at the specs on this unit, I don't see how it fits well in our home. From the ClimateMaster manual (closed loop numbers):
Full: 50,000 btuh cooling, 37,500 btuh heating
Part: 39,600 btuh cooling, 31,2000 btuh heating
Full: 64,800 btuh cooling, 48,000 btuh heating
Part: 49,800 btuh cooling, 37,500 btuh heating
Two contractors are suggesting the 4 ton unit. This seems consistent with our duct sizes, but I don't see how 37,500 btuh is close enough to our heat loss to avoid lots of time on the backup electric heat during the middle of winter. I suspect the cooling will be very reasonable at this size, but never use second stage.
The third contractor suggested a 5 ton unit. I feel more comfortable with this from a heating perspective. However, I am concerned about the air flow through our ducts. I thought that 10" x 24" was appropriate for 1400cfm. I see second stage air flows of 1750, 2050, and even 2280 cfm in the ClimateMaster manual (depending on setting). Also, looking at the cooling numbers above, it seems like we will be dramatically oversized for cooling even on the first stage which concerns me for dehumidification.
Anyway, sorry for the long post, but am I missing something here? It seems like this discussion really illustrates the importance of two stage units in geothermal. However, the difference between first and second stage with Copeland scroll compressors does not seem to handle my particular situation very well (not just ClimateMaster, but I looked at other brands as well). It seems like there needs to be a more dramatic difference for the 5 ton solution to work, but even then my airflow concern remains.