Geothermal Sizing

[dginid]

My HVAC contractor is soon to install a Bryant/Climatemaster Geothermal heat pump. He says I can go with either a 5 or 6 ton size because I'm between sizing.

Calculated Heating load = 59,500 BTU
Calculated Cooling Load = 42,500 BTU

According to the attached chart I could use a 6 ton for the heating load, but only need a 4 ton for cooling. I called Climatemaster and was told that for good heating I should go w/ the 6 ton, but that in the summer the unit would short cycle and cause higher humidity. I was thinking of compromising and doing a 5 ton but don't want to be under sized and kick on the electric back-up strips. Our cold & hot months are about equal here in Southern Idaho.

[beenthere]

Accually, a 4 ton is appropriate for your house for cooling.

[fsq4cw]

FFirst of all, I hope your contractor did the proper Manual J load calculations. Sounds like the folks at Climatemaster would like you to spend a lot of money. The answer to your question is quite simple; just stick your hands in your pants and ask yourself, ‘how deep are my pockets?’

If your pockets are not very deep, go for the 4-ton unit. Your home should be very comfortable during the summer (providing someone did the calcs!) and during the winter perhaps the backup may come on a little more often than with the bigger units – probably no big deal.

If on the other hand, if your pockets run deep, you can install the 5 or 6-ton HP with a 2-speed compressor and variable speed blower. That way you’ll still be comfortable in the summer (proper dehumidification) and during the winter the backup will either come on very little or not at all. Either way you’ll be comfortable year round.

Some things to keep in mind, first of all, along with the higher price for the larger, ‘fancier’ HP will come a ‘fancier’ price for the accompanying/matching ground loop – expect more drilling – a lot more! You can also expect more HDPE pipe, more grouting, more antifreeze, more trenching, larger flow center (pump), more labor etc; you get the idea. Not to discourage you, that’s the bad news. The good news is that because the larger units will (should) have a 2-speed compressor and variable speed blower, should you ever want to add a modest addition to your home, you will likely have the capacity. Conversely, as you tighten it up with insulation, better windows & doors, more efficient appliances etc. you won’t short cycle and be over sized as the HP will just likely never ramp up to its full capacity.

So…

SR

[dginid]

Thanks so much for the replies. This unit is a Water Source heat pump from the well, so extra cost in piping etc is not an issue. The only issue is the size if the unit. The difference in cost between the units is really not much at all. My big concern is just the over sizing of the unit for Summer.

This is new construction and we are doing good insulation, windows, etc. We are putting in dual compressors w/ variable speed. Maybe I don't need to worry about oversizing and short cycling in the summer???

[beenthere]

If the first stage is bigger or almost as much as your cooling load. 2 stage doesn't matter.

[dginid]

"If the first stage is bigger or almost as much as your cooling load. 2 stage doesn't matter."

Then I guess I'm quite oversized for cooling w/ either a 5 or 6 ton...

So if my required cooling load = 42,000 BTUh

and 1st stage cooling on a 5-ton= 51,900 BTUh
and 1st stage cooling on a 6-ton= 59,800 BTUh

Then both are overkill. Does anyone think the short cycling would be much different between the 5 or 6 ton????

So maybe I'd be better off with the 6 ton since it will be a safer bet for my heating load, and I'll just have to deal w/ some short cycling in the summer.

[beenthere]

Some short cycling?
How about a lot.
Like when its 80 out, the unit run for 3 minutes, and the Indoor humidity is 80%.

Better get a whole house dehumidifier if your going to over size like that.
Its not just the cost of the unit.
More water flow.
Bigger ducts, more registers to move all that excess air.

Its your house, its your money, do what you want.

[a0128958]

... The only issue is the size if the unit. The difference in cost between the units is really not much at all. My big concern is just the over sizing of the unit for Summer. ... Maybe I don't need to worry about oversizing and short cycling in the summer???

I would think you'd want to at least match your earth heat exchanger (loop field) to whatever size HP unit(s) you purchase. I.e., you'd want a loop field that can adequately reject 6 tons of heat into the earth if you purchase a 6 ton unit. You certainly want to avoid having a loop field that's only sized for rejecting 4 tons of heat with a 6 ton unit - it won't work well.

Best regards,

Bill

[dginid]

Actually, the system is an open loop off of the well.