How cold is too cold for my heat pump??

kjboggsNovember 19, 2008

I just built my home and finished it this summer. I had a third party do a manual J for it, and had a reputable installer install a 2 ton 14 seer and 2.5 ton 14 seer goodman heatpumps for my 2 story 3516 sq ft. home. The home is insulated well with damp blown cellulose in the walls, and every penetration sealed as well as possible and other energy efficient features.

The balance point calculated was about 30 degrees, but last night it got down to 20 just outside of Charlotte, NC and the pumps seemed to be working fine. I dont think the units are set to turn on the emergency heat on their own, I have to do it manually with the programmable thermostats.

I noticed that they did run for a while before kicking off, but kept the home at 71* all night.

I was just curious is this could damage the heat pumps in any way running them when it is this cold out, or if I know if it is going to be in the low 20's, switching to emergency heat before we go to bed??

The cooling season was great, and i much prefer the heatpumps to any system that I have had previously.

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You can't really hurt the compressor.

Every system has a balance point based on unit capability to reach desired temp indoors with a given outdoor temp, efficiency based on electric costs, and some other factors such as cost to run indoor fan while heatpump is trying to reach indoor setpoint.

I can share that in NY on my home, with an American Standard 15 SEER, variable speed indoor fan, the balance point for me was around 29 degrees

    Bookmark   November 19, 2008 at 10:00AM
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I also live in Southeast about an 1 1/2 hrs down 85 from Charlotte and have a dual fuel system-HP with nat gas furnace backup.

I don't mean to be rude but it is clear from your post that you and many other homeowners who have heat pumps don't understand its operation.

First you never move your thermostat to "emergency heat" unless your outside HP condenser is not operating due to some mechanical problem and your home needs some heat until outside condenser can be repaired.

What mdl thermostat do you have? Most HP thermostats have an indicator that shows auxilliary heat engaged with the outside condenser also operating. HPs provide some heat even at low outside temperatures. When the HP condenser can not maintain home's inside thermostat setting by itself, the aux heat will come on. Aux heat is nothing more than heat strips located at your air handler. Heat strips operating by themselves without the outside HP condenser running is called emergency heat. Do you know the size of the heat strip in each system? I would suspect a 7.5KW or even a 10 KW. Each KW produces about 3400 BTUs of heat. These heat strips will also come on when your HP goes into the defrost cycle which is nothing more than running your AC. The heat strips will temper your supply air so you don't have several minutes of cold air coming into your home during the defrost cycle. I personally prefer electronic demand defrost over the timed defrost method that most HPs have.

Hope this is helpful and has answered your questions.


    Bookmark   November 19, 2008 at 11:23AM
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Tiger is right the HP will produce some heat even at low temps, the reasoning is the amount of heat produced at these temps is not enough, so your auxillary system kicks in. In most setups this point is around 26 to 30 degress depending on the system design.

    Bookmark   November 19, 2008 at 12:02PM
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Thanks for the info, I was getting the emergency and auxillary heat terms confused. I have honeywell TS29 thermostats, and the 2 ton handler has 5 KW of auxillary heat, and the 2.5 ton unit has 7 KW.

I just was not sure whether or not the auxillary heat kicked in by itself when the pumps could not keep up or not? Since the balance point was around 30*, I was curious since it got close to 20* last night and was able to keep up fine, and I have not seen any light or anything on the thermostat to indicate that the auxilliary heat was on.

    Bookmark   November 19, 2008 at 1:19PM
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Keep in mind, keeping up and being wasteful can go hand and hand.

Perhaps your HP is large enough to satisfy room requirements running alot, but not the most efficient use of energy, in this case electricity with air handler combined.

    Bookmark   November 19, 2008 at 2:45PM
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Not sure I read in careful enough detail to not be redundant, but here's what I know about the question.

In general, an air-to-air HP efficiency goes from its maximum COP, perhaps as much as 4, to the minimum COP of 1, resistive heat efficiency somewhere over a range of 0 to 50 degrees F. Thus, even at 10 degrees the HP is producing heat more efficiently than is resistive (emergency/back-up) heat. My geothermal HP,an old unit of about 15 years old, has an inside control that is sophisticated enough to cut in back-up resistive heat if it take "too" long for the HP to reach the demand temperature. Mine has two stages, cuts in 5KWatt first and 5KWatt additional for maximum heat. At maximum the delta Temp across the HP is about 60 degrees, i.e., if room at 60 degrees, the HP outpug is 120 degrees. When in the HP mode the delta temp is between 20 and 25 degrees (I have a two speed unit).

The balance point is the outside temperature at which the HP can maintain a given inside temperature, usually 70 degrees I think, we never run that warm. My system has a balance point of 0 degrees, and I live in north-central NJ. Again, mine is geothermal, so the balance point is not affected by the air temp beyond what is lost through the house exterior, walls, ceiling, doors, windows. Wit air-to-air the HP gets less efficient as the temperature drops, so I'd guess my compressor, etc,. would have a much higher balance point outside temperature if it were operated with an air-to-air exchanger.

    Bookmark   November 20, 2008 at 2:53PM
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I have a Carrier 3 ton heat pump and a carrier oil furnace (80% efficiency). Where can I find the "COP curve" for the heat pump (it is the Carrier 16 seer infinity)? The heat pump is two stage piston and the furnace blower is constant on/variable speed. What I would like to do is to set the temperature auxillary heat vs heat pump use within the Carrier Edge thermostat to that at which oil is less costly than heat pump heating. I live near Pittsburgh, Pa. All I can tell so far is that the two stage heat pump can maintain 72 degrees in the house at 28 degrees outside temperature cycling between 1 nd 2 stages in the heat pump. Register air temperature runs about 87 degrees, or about a 15 degree rise of house temperature air going thru the furnace with outside tempeartature at about 30 degrees. This is at room registers not at the furnace. Somehow I would think that this data should allow me to calculate the COP.
My goal is to minimize winter heating costs knowing the price of oil and KWH electric costs.

    Bookmark   November 21, 2008 at 7:29AM
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Just had a 13 seer Frigidaire heatpump installed, replaced my 10 seer setup. It's 22 degress outside and I just tested the air at the first duct after the air handler, which is the bedroom, and it's putting out about 82 degrees. Thermostat is located in the family room and is set at 70m and themostat is showing it's 69 in there. Is 82 degress off the first supply link in the bedroom ok for 22 degrees outside with temp set at 70? Just want peace of mind.

    Bookmark   February 25, 2009 at 1:22AM
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