Energy Efficiency of Water-to-Water Heat Pump vs. Water-to-Air Heat Pump
Short question I have is: how should I think about the trade-off in energy efficiency between the dual speed WaterFurnace NDW120 water-to-water heat pump on the one hand, and 2 x WaterFurnace Series 7 water-to-air heat pump on the other?
Longer version/more details below
I live in an area of New York with massive rebates for geothermal systems and am considering replacing the 2x 25 year old furnaces and 2x 10-15 year old AC compressors in our pretty large (4000 sq above ground and 2000 sq ft finished basement) with a ground source heat pump system. The geothermal bids I have gotten have been comparable to or less than the bids for traditional systems.
Currently each above ground floor is heated by an atmospheric natural gas furnace that is on its last leg. The basement has no ducts/venting, and is heated by a boiler (~20 years old) and radiators. One room on the first floor (sunroom) also has not vents-just radiant heated floor that runs off the boiler.
I have gotten a couple of bids for water-to-air systems using WaterFurnace Series 7 heat pumps (2x 4 ton units), but these don’t do anything for the basement or sunroom (heating or cooling), so I would need to leave the boiler in pace and/or use mini splits in these areas.
I recently also got a substantially cheaper bid for a water-to-water system using a WaterFurnace NDW120 (10 ton) that would also be used to heat the basement and sunroom through the radiators and radiant floor, respectively. This bid also calls for some wall vents for cooling in the basement (which doesn’t need much cooling) and in the sunroom.
The NDW120 is less efficient than the Series 7 options (3.2 COP and 19.0 EER for the NDW120 and 4.5 COP and 31.4 EER for the Series 7) and has a dual speed compressor as opposed to the variable speed compressor in the series 7 units. I understand that, in overly-simplistic terms, the better COP suggests 1.3 additional units of heat is generated per unit of energy used, but am trying to understand (if possible) how that is likely to translate into energy usage from a practical standpoint and whether the dual speed v. variable speed compressors might magnify the additional energy usage of the NDW120.
As I am sure you can tell, I have done enough research on geothermal to be dangerous, but still have a lot to learn. So, any suggestions and/or thoughts on how I should think about this trade-off would be greatly appreciated!
Thanks.
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Replies
Geothermal system efficiency has too many parts to effectively answer your question. A well setup 2 stage geo will cost less to run an improperly set up variable speed unit. Besides the heat pump units, also make sure that all water pumps are ECM. The operating cost of oversized low efficiency pumps can easily equal the COP difference between the units.
Either way you go, I would first double check that you really need 8 to 10 tons of heat. I would run through the numbers here:
https://www.greenbuildingadvisor.com/article/out-with-the-old-in-with-the-new
Properly sizing the setup would probably save you enough on drilling and equipment costs to pay for a higher efficiency system. Lot of times it is even worth it to undersize the geo slightly and use a bit of resistance heat on those polar vortex days.
Thanks. I will take a look. Strangely, the rebate structure (perhaps in addition to contractor preference) encourages increasing size. The rebates are high and on a per ton basis. Increasing the size of the unit by 1 ton is way less costly than the increase in the rebate associated with that 1 ton increase in unit size. I got a couple bids for even larger systems, but have removed them because the size of those systems are clearly unnecessary.
Oversizing on a water to water system is generally not a problem (except for the cost, which might be moot in your case), but you do have to watch sizing with water to air system.
A modulating system does buy you some flexibility (they have about a 2:1 turndown, so not great) but not enough to cover a huge oversize. You want the unit to run in its modulating range most of the time for best comfort and to avoid short cycling, so it is important to size it correctly.