Hi, new to geo and these forums. I'm considering replacing my aging air source heat pump with propane backup. I've gotten a few quotes and 2 companies who performed load calculations. Both heating load calcs were performed by geo only firms and came to ~38k Btu/hr for 2300 sq/ft home. Firm A recommends a 3 ton system and firm B recommends 4 ton. They're both pretty adamant that they are "right". I've followed up with both of them to understand their justifications. Here's the summary Firm A: 3 ton system is sized for 95% of the heating days/year. 4 ton is oversized and too big for cooling load. Firm B: 4 ton system is sized for ~100% of heating days/year and the aux should only be needed for emergencies for extreme cold weather. How do I make an informed decision? What other info do I need?
Hi and welcome. You need firm A to provide you with the anticipated electric usage to cover the 5% of heating load per year, add that to your install cost and compare to the cost of covering 100% of the load from company B. Covering 100% of the load with geo is often not cost effective when compared to even 10% coverage by auxillery heat. It is a choice not a right or wrong scenario. In my world choices are fueled by economic impact, not emotion. Hope this helps Eric
An informed decision depends on your strategy. I like sizing for the cooling load and then add the projected heating load with my second cheapest fuel. I am not sure why A and B are sizing to heat. Mark
If your 38 Kbtu/h is correct, firm A is correct. You only drop down in the single digits a few times a year, it does not make sense to install that capacity when you use it only a few days per year. A 3 ton usually runs more efficient, you need lesser pumping power and despite some electric heat, in your case about $20-30, you will save about $100 annually in operating costs. In addition, you save upfront installation costs. Variable speed technology changes those dynamics a bit, what equipment are they providing?
Sizing to avoid auxiliary use is often folly in that you can actually spend more running a larger compressor all the time vs a little auxiliary once in awhile. I would submit that the individual recommending a 4 ton is unevolved in our field. with that load in MI I would ask the following question: What can we do to get it down to 2 tons or 2.5 tons? Start by seeing if their is an opportunity to reduce load at your home, the cheapest ton is the one you don't buy.
I second the motion to go no higher than 3 tons and investigate ways to drive load down from there. Clients not in MI can reduce load, too. In approximate order of importance deal with: 1) infiltration (air leaks in and out of home) 2) Ensuring ducts are within thermal and pressure envelopes, or, failing that, very well insulated and substantially leak-free. 3) ceiling / attic insulation 4) Fenestration performance (windows and doors) 5) wall insulation None of this cares a whit about system type, size or efficiency. Echoing Joe, the cheapest Ton is the one (or two) you don't need, buy, install, operate or maintain.
