Dual Stage vs Single Stage Heat Pump
Hello all:
New construction in climate zone 5A. The HVAC contractor plans to install a SINGLE stage heat pump (http://www.heil-hvac.com/en/us/products/heat-pumps/15-heat-pump/) and DUAL stage propane furnace as back-up (http://www.heil-hvac.com/en/us/products/gas-furnaces/96-gas-furnace-2/). For an upcharge of about $700, he can install a dual stage heat pump (http://www.heil-hvac.com/en/us/products/heat-pumps/deluxe-16-two-stage-heat-pump/). Note that the two-stage unit has a 10-year warranty on the compressor, coil and heat exchanger, while the single stage unit’s warranty is 5 years.
The house will be fairly tight (taped Zip-R sheathing; double pane windows) but not to passive house levels. It’ll have 3 zones, two on the 1st floor and one on the 2nd floor. The HVAC contractor and an energy rater both performed manual J calcs, the results of which were close to each other. As such, I’m not terribly worried about sizing. Nonetheless, I would love some guidance on whether to go for the two stage heat pump. Our HVAC contractor seemed to think it was unnecessary but was generally indifferent to which unit we went with.
As I understand it (from reading articles here and elsewhere), the primary reasons for using a two stage unit are: for comfort, modest energy savings, less noise (though the decibel rating differences are nominal), as a potential bulwark against oversized equipment, and a longer warranty. Am I missing anything? Would I be foolish to choose one or the other? Thanks.
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Replies
Adam,
I can't resist noting that double-pane windows are nothing special. They are mandated by the building code. Single-pane windows are illegal for new construction.
To know whether it's a good idea to get a two-stage heat pump (it usually is), it's essential to know (a) whether anyone performed a Manual J heating load calculation, and (b) what your design heat load is. If you tell us your design heat load and how many square feet of house you are building, we may be able to get a ballpark idea of whether your design heat load came from a good Manual J calculation or (as often happens) a stab-in-the-dark guess.
-- Martin Holladay
Yes, the HVAC sub and energy rater both performed Manual J calculations. Sub's came to 61,400 btu/h and rater's came to 60,700 btu/h. The house has about 3,200 square feet of livable space. Please let me know if you need more info. Thanks!
I believe the highlighted portion of the attached spec sheet captures what the contractor plans to install, including the 4 ton "Indoor Model" and "Furnace Model" along with, in this case, the 2 stage heat pump. Thanks.
Get the 2 stage, especially if it's only $700. You'll be running in the low stage majority of the time.