Heat pump question

• posted on December 25, 2007, 12:07 am
What is the operating temperature range for a heat pump? Mainly the low end. I know heat pumps work by heat transfer, even from cold air, but what is the lowest outside temperature before their efficiency starts to drop off?
Thanks
Steve
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• posted on December 25, 2007, 1:00 pm

Presume you mean an 'air' heat pump? Not one that takes its heat from a loop of liquid in the ground? Or from water in a well or pond/stream?
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• posted on December 25, 2007, 2:59 pm
terry wrote:

The exact answer to the question as asked is the efficiency "starts to drop off" w/ every degree. That, presumably, isn't the question you wanted answered. :)
The point at which an air-exchange heat pump becomes cost-neutral w/ alternate heating sources depends on a number of factors, the source temperature being only one. Among the others is the type and cost of the alternate heat source, etc., etc., ...
As a general rule of thumb, somewhere in the mid-20s (F) is about the normal point most consider the breakeven. The answer is available for a particular installation from a competent installer who can run the heat balance calculations for a particular unit, house, and typical heating-days load for a given area with its energy costs. Newer units are more efficient and can lower the break-even point significantly as compared to early units, particularly in areas where utility companies have special rates for heat pump use including the "emergency" heat.
There is no single answer, but you can figure if it gets below 20F for any extended period, you'll want to consider the cost of the aux heat source carefully for air-air heat pumps.
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• posted on December 25, 2007, 3:50 pm
A very general rule of thumb that was given at a York seminar I attended about 11 yrs ago was that at about 30-32 OD, there should be about 20deg temp rise across the indoor coil. For every 3 deg up or down from this, the indoor TD will change by about 1 degree. This should probably apply to most all HPs, not just York. Newer higher efficiency/ R410A units may do somewhat better. Larry
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• posted on December 25, 2007, 4:56 pm
lp13-30 wrote:

Yes, it's these later incarnations that I was referring to for the 20s F numbers in conjunction w/ the rate relief many utilities are providing. W/O the latter it is probably only a few degrees lower owing only to the higher efficiency/charge differences.
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• posted on December 25, 2007, 5:40 pm
wrote:

Presume you mean an 'air' heat pump? Not one that takes its heat from a loop of liquid in the ground? Or from water in a well or pond/stream?
Yes, an air heat pump.
Steve
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• posted on December 25, 2007, 2:55 pm
On Mon, 24 Dec 2007 16:07:14 -0800, "SteveB"

There are lots of experts who will undoubtedly reply to this. I am simply a VA homeowner with a fairly new heat 16i pump. It depends on many things. But, theoretically, I guess it is at or near 32 degrees. Do you have strip heaters as part of the installation? When they come on the cost to operate the system really goes up. I have alternative heat available. I watch current temp, as well as the forecast temp, before turning one off and the other on. I usually do this about 35 degrees; no science behind it - I just sort of decided on that as close enough. I have been told that ti is possible to compute it for a given heat pump, in a given location, but that is beyond me. Also, my boiler can heat the house faster than the heat pump, so I frequently turn it on in the a.m. and let it bring the house up to operating temperature, before turning on the heat pump. I work online, starting about 5 a.m., and I appreciate the quick warm up, since I turn all heat off at night. This also allows the outside temp to warm up, meaning it operates more efficiently