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  1. #1
    Join Date
    Jan 2008
    Location
    Mystic, CT
    Posts
    6

    Heat Pump vs. Emergency Heat

    My 2-year-old Trane heat pump is connected to a 20-year-old Amana air handler with resistance emergency heat elements. The system is heating and cooling a 1600 sq. ft. townhouse condo. The serviceman suggested that I manually switch to emergency heat when it's "very cold" to shut down the heat pump and go resistance-only, on the theory that the heat pump is working so hard that resistance heat would be more efficient. However, he could not say what "very cold" was--he speculated that it was in the teens. I know that when it's that cold, my heat pump tends to run more than half the time, with very low heat rise at the registers, but I have not seen it kick in the emergency heat automatically.

    Is there a way I can estimate the break-point outside temperature for heat pump vs. resistance heating? Or is there a "rule of thumb"?

    Thanks for any suggestions.

  2. #2
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,837
    Unfortunately you've got a very mismatched HP system. I'm surprised the new HP works at all in the heating mode with such an old air handler. Did they change the indoor coil when they changed the HP? With a mismatched system, there are no statistics available to determine the COP to determine at what temperature the HP is less efficient than resistance heat. As the outdoor temp goes down, the ability of the HP to extract heat from the outside air diminishes. So just when you need more heat, the HP is producing less for each unit of electrical energy you purchase. So if the HP actually runs about 1/2 the time when the temp is in the teens, then I'd suspect you'd be good down to the mid-single digits before the resistance heat would be more cost efficient.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  3. #3
    Join Date
    Jan 2008
    Location
    Mystic, CT
    Posts
    6
    Thanks, Skip... I bought the new Trane from very reputable company when I moved in. They said I'd gain a lot from the new HP (compared to the 20-yr-old Amana), but a new air handler wouldn't improve things enough to justify it. The system seems to work as advertised for heat and AC.

    Your reply suggests that some percentage of time operating might indicate a breakeven point. I haven't been timing the system--would you guesstimate that if the compressor runs something like 75% of the time, I've reached that point? Can I calculate it from some amperage ratings? (...which I suspect are on the HP enclosure and somewhere on the air handler--I'm not at home right now.)

    Thanks again...

  4. #4
    Join Date
    Jan 2008
    Location
    Hells Kitchen, Phoenix Arizona
    Posts
    318
    If the air handler has a txv then it might work "sort-of" if its a piston type or cap tube then you are doomed...

  5. #5

    s/w

    Isn't there supposed to be a low temp/pressure switch that engages secondary heat?

  6. #6
    Join Date
    Nov 2006
    Location
    Oregon
    Posts
    853
    Quote Originally Posted by bak2lasvegas View Post
    Isn't there supposed to be a low temp/pressure switch that engages secondary heat?

    Secondary heat can be triggered by:

    1. The thermostat if it's set more then a few degrees over indoor temp or if the thermostat detects that insignificant heat rise. (depends on the thermostat).
    2. During a defrost cycle if the HP is wired to ask for auxilary heat during defrost.
    3. If outdoor ambient is below a preset threshold (assuming an optional low ambient relay is installed)

  7. #7
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,837
    Quote Originally Posted by dcb View Post
    Thanks, Skip... I bought the new Trane from very reputable company when I moved in. They said I'd gain a lot from the new HP (compared to the 20-yr-old Amana), but a new air handler wouldn't improve things enough to justify it. The system seems to work as advertised for heat and AC.

    Your reply suggests that some percentage of time operating might indicate a breakeven point. I haven't been timing the system--would you guesstimate that if the compressor runs something like 75% of the time, I've reached that point? Can I calculate it from some amperage ratings? (...which I suspect are on the HP enclosure and somewhere on the air handler--I'm not at home right now.)

    Thanks again...
    Too bad about the mismatch. I'd disagree with the installing company but I guess that's water under the bridge, unless your inclined to invite them back to 'complete' the job. I don't know what model Trane you got but I would have recommended a nice variagle speed indoor unit. A variable speed motor uses a fraction of the electricity of a conventional motor so IMO, you'd have gained a lot. Afterall, isn't that the whole reason for this thread, to cut your electrical consumption?

    When I spoke of the length of heat cycles, I was alluding to the capacity in Btu's of the HP. When it's running continuously, then it's definitely giving all the Btu's it can but it's also using more electricity to bring them into your home than you'd get by just running the "toasters". But again, not knowing the actual COP's for the combination you have, there's just no clear way to know exactly where the legs of the 'X' actually cross on the performance graph. So guesstimating is all that's left. For that I'd say, very roughly, that when the HP is running 75% or 45 minutes out of every heating hour, then yes, I'd make a switch to the resistance heat. To make that more automatic, you might want to have a thermostat installed on the outdoor unit that will turn it off and require the use of the aux heaters below that temperature.
    If YOU want change, YOU have to first change.

    If you are waiting for the 'other guy' to change first, just remember, you're the 'other guy's' other guy. To continue to expect real change when you keep acting the same way as always, is folly. Won't happen. Real change will only happen when a majority of the people change the way they vote!

  8. #8

    s/w

    I was wondering because I was sure that there is usually a second source of heat on heat pump systmz. Was pretty sure that there may be a switch possibly bad on said system, makes me wonder as I am trying to get better on my troubleshooting skills. Thank You!!

  9. #9
    Join Date
    Jan 2008
    Location
    Mystic, CT
    Posts
    6
    Quote Originally Posted by davefr View Post
    Secondary heat can be triggered by:

    1. The thermostat if it's set more then a few degrees over indoor temp or if the thermostat detects that insignificant heat rise. (depends on the thermostat).
    2. During a defrost cycle if the HP is wired to ask for auxilary heat during defrost.
    3. If outdoor ambient is below a preset threshold (assuming an optional low ambient relay is installed)
    #1 is definitely true--I never raise the thermostat more than 2* at a time to keep the emergency heat from kicking in. I don't know about #2 on my system, and doubt that I have #3.

    Skip: I'll definitely talk to the company (and a competitor) about the air handler and see what the amp difference is for the new technology. Apparently I should assume I have an AC motor. Thanks for your help.

  10. #10
    Join Date
    Aug 2004
    Location
    Northern Indiana
    Posts
    31
    Please correct me if im wrong, the cost of running electric element strips should be more efficiant than HP below 25-30 degrees ambiant temp....right?
    Just when I thought i knew something,I came to this site and realized , I KNOW NOTHING....

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