H/P efficiency
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Thread: H/P efficiency

  1. #1
    Join Date
    Oct 2005
    Posts
    21

    Question

    At what temp does a heat pump become inefficient? I have a Carrier Infinity 13 duel fuel package unit w/the thermidistat. I am trying to figure out the ballance setpoint when to shut off the H/P and switchover to gas. The guy that installed it set it @ 40*. I thought that was too high and spoke to someone else @ the dealership that installed it and he said set it @ 20*. That sounds too low to me. I know there are formulas & calculations that I could do with energy costs & factors, but all I want is a good ballpark #. Thanks.

  2. #2
    Join Date
    Apr 2005
    Location
    NE PA
    Posts
    698
    If you are talking efficiency, a modern heat pump will deliver heat more efficiently than straight electric resistance heat when the outside temp is in the low single digits. This does not take into account defrost cycles which will actually increase the heat loss (since you are in A/C mode while defrosting). Most units will deliver 2 watts of heat per watt of electricity when the outside temp is in the low teens.

    The problem with heat pumps is that the capacity, or number of BTUs that it can deliver, drops as the outside temperature drops. So a 36,000 BTU (3 ton) heat pump may be able to deliver 36,000 BTUs at 45 deg F, but at 15 deg F it will only deliver 9,000 or 10,000 BTU's. Unfortunately, as it gets colder out, the heat pump's capacity drops, and the heat loss from the structure INCREASES. Where the heat pump output matches the building's heat loss is called the balance point (temperature).

    With a heat pump with electric strip auxiliary heat, the heat pump and the electric heat can operate at the same time. This allows the heat pump to provide some of the heat efficiently and the additional heat needed is by the not so efficient electric strip heaters.

    In a dual fuel system, the heat pump and furnace can not operate at the same time (due to having the heat pump coil on the warm air outlet of the furnace and the associated thermodynamic operating problems with the heat pump), therefore the furnace can not substitute, only replace the heat pump for heat. So it is at, or above the balance point where the changeover is set. Also defrost with a dual fuel is a bit trickier to handle, so this may be why the temp is set at 40. You can try lowering the changeover temp and see how the comfort is neat the changeover point.

    The optimal dual fuel system is a hot water furnace that has a heat exchanger coil in the heat pump air handler, so the furnace and heat pump can operate together, like with resistance heat. It is not often seen due to additional complexity and cost. Most I have seen were houses with hot water radiant floor heat and a heat pump. Adding a zone and heat exchanger to the air handler makes a very good system.

    paul

  3. #3
    Join Date
    Apr 2004
    Posts
    7,680
    Personally I dont use a changover stat, I let second stage do the switching for me.

  4. #4
    Join Date
    Oct 2005
    Posts
    21
    Personally I dont use a changover stat, I let second stage do the switching for me.

    So you let the H/P run until it looses enought degrees to kick in the aux? Doesn't this get to a point where the H/P runs constantly and blowing out only slightly warm air?

  5. #5
    Join Date
    Apr 2004
    Posts
    7,680
    Yep, thats the way they are supposed to work. If it gets cold enough that the HP cant keep up, the gas takes over. If you shut a heat pump off before it runs constantly then you left money on the table

  6. #6
    Join Date
    Oct 2005
    Posts
    21
    Thanks docholiday, I'll try that and try and catch what outside temp the aux kicks in or when the house gets uncomfortable and have it switch over just before then.

  7. #7
    Join Date
    May 2004
    Location
    Rapid City, SD
    Posts
    7,414
    Doing exactly that is probably the most precise way you'll ever figure out where to set the shut off point.

    You can crunch a ton of numbers to find out, but there's all sorts of variables that could throw that calculated number off.

    If I ever have the oppertunity to setup a heat pump like you have I'd do the same. Do some experimenting, find out what temp outside it starts loosing loosing temp on the stat inside. Maybe at 3 or 5 degree's to that number and be done with it.

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