On Dec 18, 12:16 am, firstname.lastname@example.org wrote:
Not unless the unit is unable to keep up. Otherwise let it do it's
thing. Current heat pumps are still effective below 32f. They heat
the air prior to it's entering the resistive heat stage so even if the
aux strips kick in your are still getting something from the heat pump.
Thanks for your reply.
According to what I have been reading, heat pumps do not work well below 40
degrees, but my own observation of my 3 year old system seems to support
what you said.
At some point the heat pump needs to spend so much time/energy de-icing the
coils that it its efficiency drops below the aux heat. At what temp do you
think the heat pump should no longer be used?
I don't know what that point is and it probably depends on the outside
humidity. If the outside humidity was zero (admittedly not realistic) then
ice would never form. I think I'll carefully watch how the heat pump
performs vs outside temp. to see if it can maintain the house temp without
too much aux heat.
The reason I mentioned turning on the emergency heat is because that turns
off the heat pump which is what the outdoor thermometers are designed to
On Sat, 18 Dec 2010 07:25:40 -0800 (PST), jamesgangnc
The newer heat pumps have adequate sensing and protection in the HP
controls (as opposed to the thermostat) and the HP will call for AUX
heat itself when it goes into a de-ice cycle or if it decides it can't
On Sat, 18 Dec 2010 11:43:15 -0500, email@example.com wrote:
What you read is incorrect. Yes, there is a point at which the
efficiency of the heat pump is less than that of the resistance heat,
but that point is roughly 15F, not 40F. Newer, more efficient heat
pumps might lower this point a little, but the problem is basic
physics, so it won't get much lower.
And as already pointed out, the heat pump might switch to the
resistance heat automatically. This likely depends on age and
manufacturer. Also note that most heat pumps will add the resistance
heat when the inside temperature drops about 5F below the set
temperature, but this is separate from the efficiency cutover. This
auto-add at 5F below is the reason you are always advised not to set
the thermostat down (very much anyway) at night when you have a heat
pump. If you disable the resistance heat (which is a viable course of
action in the southern US), then you can ignore that advice and save
some money, albeit at the tradeoff that it may take quite a while to
bring the temperature back up.
One alternate ploy rather than disabling them entirely is what the
installer of our system did routinely -- he used a thermister inline w/
the emergency heat that kept them out of the control circuit w/ outside
temperatures were above about 20-25F.
Trivial to bypass if want/need to but it's a passive solution and a
device that has extremely long MTBF ratings...
(Which leads to a story of when sold the house, however; the buyer's
inspection wrote up a defect that the emergency heat was non-functional;
the buyer ended up paying to have the good idea removed 'cuz couldn't
fathom the benefit even when explained to him and I refused to pay to
"fix" something that wasn't broke but was a beneficial feature.)
Nonsense. Thermistors are extremely reliable devices as are virtually
all passive devices.
The likelihood of the thermistor being the failure point is multiple
orders of magnitude lower than that of any of the other components it's
associated with, including the power grid itself.
Ours throws in the resistance (aux) heat with only a 2F hill to climb; dumb.
With our house in VT (hydronic system), we'd set-back the thermostat about 6F
(65F down to 59F) at night and weekdays. Here we have to keep it set at
66-67F (no set-back capability and the forced-cold-air system) to be anywhere
close to comfortable.
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