Geothermal – Sizing Issues – 4 Ton or 5 Ton?
I live in Cincinnati, OH and I am zeroing in on purchasing a geothermal heat pump for my 5800 SF (conditioned) home. My home is all electric is currently heated by an 8 year old 4 ton air source heat pump with 15Kw electric backup. My issue is the two geothermal companies I have contacted to quote the job disagree on heating load of my home. Both companies did Manual J calculations, but are coming up with different numbers. One company has the heat load at 55k BTU — says a 4 ton is the way to go. The other company has heat load at 68k BTU – says go with a 5 ton unit. All calculations were done based on 6 deg. outside air temperature and inside set point of 68 deg.
I am trying to figure out which size is best for my home. The current air source unit performs well. I have closely monitored it’s performance over the last couple of years and it appears the balance point for the unit, the point where it starts needing help from the electric back up, is 28 – 30 deg. At this outside temperature, at night, when there is no solar gain; the unit will run consistently and never turn off, but it does not engage the electric backup. Based on the TDS of the current unit, at 30 deg. outside air the unit is outputting 32,500 BTUs. I feel very confident in the analysis of my current system’s performance. Is it possible to use the above information about the performance of the current unit to help decide which company’s sizing is correct?
Based on my own calculations from above, at 30 deg. the heat load is 855 BTU/deg. 32,500/38 (the difference between the set point of 68 deg. and the outside air). So based on this, at the 6 deg. outside air temperature that the geothermal companies used, the heat load for my home is 53000 BTU. This puts the sizing squarely at 4 tons.
By biggest question is about my sizing methodology — does it work using by current system performance to establish BTU load per deg.
Thanks for your help GBA friends,