Heat pump auxilary heat

Discussion in 'Electrical Forum discussion & Blog' started by Giles, Feb 10, 2010.

  1. Giles

    Giles Retired tool & Die and Mechanic

    Messages:
    112
    Location:
    N.W. Alabama--Florence--
    I have a Carrier heat pump that is about 7 years old. On the thermostat, when auxilary heat indicator is blinking, it is drawing 30amps. Zero reading when not in aux. heat. This was taken with ampmeter around one wire at the inside unit disconnect. The outside unit was running. This is a split unit with separate feed to each.
    Am I correct in assuming that the auxilary heat is only drawing 30 amps or have I checked it wrong? I have checked meter and it seems to be accurate.
  2. nukeman

    nukeman Nuclear Engineer

    Messages:
    711
    Location:
    VA
    30A x 220V = 6600 W
    30A x 240V = 7200 W

    Usually, the resistance heaters are rated at 240v, but your supply might be more like 220V. The numbers sound about right if you have 7 kW of aux. heat. There is a wide range of options for aux. heat and emergency heat, so it is hard to say if this number is "correct" unless we know what you have.

    The aux. heat helps out the heat pump during cold weather to keep up with demand. There is normally an emergency heat option as well. This runs the heat strips, but not the compressor. This is so that you still have heat if the compressor fails.

    Often, the aux. heat rating can be less than the emergency heat rating. For instance, you might have 10kW of aux. heat and 15kW of emergency heat. The actual values depend on the system and heating loads. A given model of heat pump can have a wide range of heat strips installed. You would have to see what is installed in yours as just having the model number won't tell you much.

    You are lucky. I have an electric furnace (no heat pump) and that thing draws 106A+ whenever it runs. It is the only heat source, so it runs a lot. :)

    The good thing about resistance heat is there is very little that can go wrong with it.
  3. Giles

    Giles Retired tool & Die and Mechanic

    Messages:
    112
    Location:
    N.W. Alabama--Florence--
    nukeman--Thank you very much for your explaination. My system has three breakers, one 60 amp for the outside unit, two separate 60 amp breakers for the inside unit. You have explained what even the company that installed the system couldn't. My assumption was the same as your explaination, that one 60 Amp breaker was for the auxilary heat and the other for emergency heat.
    I was beginning to think that something was wrong in my wording of the question because I have posted this question in two electrical forums and one HVAC forum and you are the only one who had , or offered, an explaination.
    Sounds like you know what your are talking about!
  4. nukeman

    nukeman Nuclear Engineer

    Messages:
    711
    Location:
    VA
    No problem. If you have the installation paperwork, that might list the ratings of your aux heat and emergency heat. You other option is to kill the power and remove the door on the air handler. There should be something inside that say what you have. you can also count the number of heater elements to help get an idea. What I have (electric furnace) is basically an air handler with the heat strips. I have a coil for A/C, but no heat pump installed. In my case, I have 4 elements that are 6kW each (24kW total). They come on in stages using sequencers. This is to prevent a large current draw by kicking them all on at once. My inside unit is fed through a 150A breaker. My ouside unit (A/C only) is 30A.

    It is hard to know what you have without further informatiion. It could be that only part of your aux heat is working. For instance, say you have 2-7kW elements (14kW total). You could have a bad sequencer, bad element, or wiring that is preventing the other element from kicking on. It is hard to say.

    Basically, the heat pump is sized on your heat load. This depends on your location, size of house, number of windows, insulation, etc. With a heat pump, the performance drops as it gets colder outside. The heat loss then increases with colder temperatures. At some point (say 35*), the heat pump cannot keep up, so the aux heat kicks on to make up the difference. How much aux heat is needed depends on how cold it gets, the size/efficiency of the heat pump, etc. This is why you can buy say a 5-ton heat pump, but have a wide range of aux heat choices (say 0 kW to 20 kW).

    Do you think there is a problem? Is it not heating the house enough? If you are worried that there may be a problem with your aux heat, see if you can get more info on how many elements you have and what size. Then maybe with some pictures of the setup, I can help you do some further testing.
  5. Giles

    Giles Retired tool & Die and Mechanic

    Messages:
    112
    Location:
    N.W. Alabama--Florence--
    No I am not having a problem. I have this 5 ton 13 sear central unit and also a separate central natural gas furnace. Also, I have three unvented natural gas fireplaces that I occasionally run on low. I live in N.W. Alabama so cold is not extreme---except for the last two months--. My gas bill last month was greater then my electric.
    I am just trying to figure my least expensive source of heat.
    I might add that while checking the amperage, I had my wife turn the thermostat to maximum heat and it was still pulling 30 amps. It was 25 outside and the thermostat was satisfied in a reasonable length of time. This was with one gas logs set to low at the other end of house, 3200 sq.ft. I have a pretty clear understanding of how a heat pump works and I am not peronoid about heating.
    I have only lived hear for about three months and all of this was in place.
    The previous owner is supposed to send me all the paperwork for the unit and that will be helpful.
    I plan to do as you suggested and remove the cover for closer analysis.
    You have helped me and I appreciate it.
  6. nukeman

    nukeman Nuclear Engineer

    Messages:
    711
    Location:
    VA
    Sounds good. Let me know what you find out. The aux heat may come on in stages, so if the house was being close to being warmed up, it might only kick on 1 element. If the demand is greater, it may kick on additional resistance heat.

    Usually heat pump is cheapest if the weather is mild. Where gas becomes better all depends on the local rates. The papers should also give a table of COP (and BTU output) for different temperatures. There is a thread on here (HVAC section, I think) where someone made some assumptions and used an Excel spreadsheet to determine when is the best switch-over point between heat pump and gas. You might want to check that out.
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