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
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.
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
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.
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
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
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