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Disable aux heat on a heat pump?

Post in 'DIY and General non-hearth advice' started by nola mike, Nov 20, 2012.

  1. nola mike

    nola mike Feeling the Heat

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    Just purchased a cyberstat wifi thermostat to control my heat pump at the vacation house. I keep the temp at 42' when I'm not there just to keep things from freezing. When I get there, I fire up the stove. Depending on outside temps, it can take many hours to get the house comfy. The hope was to pre-heat the house a bit before I got there so that I could convince my wife to come as well...
    Don't want to use the aux heat though, really ever. I don't care how long it takes to heat up, and really if I can get it to the mid/upper 50's even, that makes a big difference. I'm thinking I can just disconnect (or better yet, install a SPST micro switch in line) with the W wire, which from what I've read controls the aux heat. My concern is that this wire also seems involved in the defrost cycle of the heat pump, and I don't want to screw that up. It looks like it's only function in the defrost is as an output from the heat pump--during the defrost cycle, the pump signals the tstat through the W wire to turn on the aux heat during the defrost. In other words, during the defrost cycle, the heat pump isn't heating, so the strip heat is turned on temporarily to keep some heat coming out. If this is its only function, I could disable that as well...

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  2. woodgeek

    woodgeek Minister of Fire

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    If I understand....I think you got it. My unit calls the aux heat during defrost through a W wire (on the defrost controller board in the outdoor unit). Disconnect, and it will blow coldish air during defrosts, and then make up the BTUs later with the compressor. The compressor does not need or care about the aux heat during defrost....its wired that way for your comfort. Aside from the comfort issue, disconnecting the aux on defrost maximizes COP. You could close a switch when you are there for comfort (if needed), open it when you leave.

    FYI, this W bypasses the tstat....the outdoor unit will call the aux on defrost even if the aux is locked out on the stat.

    I installed a 24VAC 'delay on' timer in my W line, so it only calls aux 2 minutes into a defrost cycle. That way, I am not paying to run aux on all those little 2 minute cycles (that I can't feel any way), but those 8 minutes cycles during freezing rain, the aux DOES come on and keep me from freezing.
  3. nola mike

    nola mike Feeling the Heat

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    i think you almost lost me there. what's COP?
    so you're saying that if i disconnect the w wire at the tstat, i won't get aux heat when heating generally, but it will still come on during the defrost cycle? and more importantly, disconnecting the w wire won't interfere with the defrost cycle?
  4. woodgeek

    woodgeek Minister of Fire

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    COP is a measure of heat out versus elec in. For aux COP=1. IF the compressor has COP = 2, then each BTU delivered costs half as much.

    For the rest...yes. There is a W wire in the tstat to control the aux. That one controls the tstat calling the aux. In your app, most tstats would call the aux to do the big warmup when you are about to come out, if you pull the W, it will be forced to use the HP instead to provide all BTUs. The downside....fault protection if the HP fails. With the W attached, the tstat would call the aux as backup (which would presumably only happen during a failure at 42°F).

    Even with the W detached, aux will still get called on defrost (in most systems), via another 'W' or 'W2' wire, on a controller board inside the outdoor unit. IF you disconnect THAT, then it will not call aux on defrost. IN most systems, this will save 10-15% in energy usage.

    And yes, the system does not need aux to run during defrost. IF there was a fault (e.g. stuck in defrost) then the HP might freeze your house, and aux might keep it from doing that (but sticking you with a big bill).

    OF course, a much easier way to do all this is the breaker box. The aux presumably has its own breaker. Flip that when you leave, and the HP will provide all BTUs while you are gone, including the long recovery when you are coming back. When you are there, flip the breaker on so it runs during defrost for 'comfort'. IF the tstat can notify you if the house gets too cold (during a HP fault), you have to drive out there to fix it and flip the breaker back.

    You will want to check that there is no 'compressor lockout' temp on the tstat, or when the outside drops below that, your HP will shut down.
  5. WhitePine

    WhitePine Feeling the Heat

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    As said above, aux heat usually has its own breaker. I just turned mine off when I had an unoccupied house that I kept at 50F. No problems.

    If you have a powered up fridge there, you might want to check and see if it can handle a house at 42F. Some of them spec a minimum indoor temperature of 50F. The danger is slugging the compressor if it turns on for some reason.
  6. nola mike

    nola mike Feeling the Heat

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    Doh! The breaker didn't even cross my mind--assumed it was on the same circuit as the HP. Well, now I'm assuming it's that mystery 30A breaker in my box that doesn't seem to do anything. Certainly the easiest solution. Unfortunately, the Tstat doesn't have an indication on when the aux is on, so not quite sure how to check that that breaker is the correct one...
    As far as the fridge (and the kegerator), well, this is our 4th winter there. So far so good.
  7. woodgeek

    woodgeek Minister of Fire

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    Usually if you crank the tstat by more than a few degree (in cold weather) it will call aux, which is discernible by extra hot (furnace-like) air from the registers, or your meter spinning at an alarming rate! You could then repeat the exp with the 30A breaker off.

    One issue, of course, is that if the outside temp is too low, the house would ever make it up to the tstat setting until you came and flipped the breaker back on. But then you would heat up fast.
  8. WhitePine

    WhitePine Feeling the Heat

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    Turn on Emergency Heat at the thermostat. That will call aux heat and nothing else (the compressor won't run, if everything is wired properly). With the mystery breaker off, you should get no heat whatsoever if it controls aux heat.

    If everything is on a single breaker, it will be huge. Probably in the neighborhood of 100 amps.

    One fly in the ointment might be an inside air handler run entirely on the 30 amp breaker. That would also kill regular heat pump operation with the 30 amp breaker off. It shouldn't be hooked up that way, but you never know.

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