Can A Heat Pump Run Too Much?
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  1. #1
    Join Date
    Jan 2013
    Posts
    12

    Question Can A Heat Pump Run Too Much?

    I've decided to purchase a single stage HP/dual stage NG furnace duel fuel system and while waiting for my install date I've been reading through the Lennox Wifi manual. It appears that the thermostat allows me to set the stages by either temperature based, timer based or a combination of temp and time. It also allows me to enter the furnace lockout temp and a compressor lockout temp. Upon initial read through I'd think that these points should align roughly with the cost balance point and thermal balance point.

    Let's say that the heat pump is cost effective to run until 20 degrees but the thermal balance point where the heat loss of the house matches the heat output of the HP is at 35 degrees. If I were to run the HP between those two stages the house would not be warming up, but also not cooling as fast as with it not running. This should prevent the furnace from cycling as often and save some money, is this correct? I'd imagine if this is the case, and that the thermostat was using temperature based stages, that the HP would be running nearly constantly until the furnace kicks in. Would this be harmful to the HP? Would it be better to set the thermostat to use a temperature/timer based stage logic instead of a temperature only logic?

  2. #2
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    I think your on the right track. If it's cost effective ot run to 20F, then you want the HP lockout set to 20F. Then you set the furnace lockout to a tmeprature that about equal to you thermal balance point... which in most situation is somewhere between 35-45F depending on your climate and sizing. You may need to use observation to see what works. You will still want ot limit setbacks. It takes a lot more capacity to recover within 2-3 hours than it does to maintain temperature. This is still a opportunity with dual fuel to save energy overnight, but you can't get too aggressive.

    Do you have the modulating furnace or 2 stage? What size of each. Thsi matters, because usually once it's switched to the back-up fuel source when your between the lockout temperatures, it will remain on that source until it needs ot downstage. So if you have a modulating furnace that's sized correctly, below 30F it might never downstage back to the heat pump, and will just keep running on the furnace. Thsi is where hydroincs or electric strips have an advantage. It's also why seomtiems for effciency with dual fuel, you want a furance whos lowest capacity is still higher than you heat loss down to you economic balance point.

    If you have naturla gas, it's less criitical, since your balance points are usually a lot higher.

    Finally, at some point, I would focus on comfort over energy savings. Let the heat pump be an extra stage of heat and save you money over 40F, but below that, enjoy the comfort of the furnace, especially if it's a modulating unit. Below about 38F, the heat pump will be in the window there depending on outdoor dewpoint, it may start to need defrost. Expect it to stage up to the furnace when this happens.

  3. #3
    Join Date
    Jan 2013
    Posts
    12
    Thanks motoguy128. I will have a 70Kbtu 80% two stage variable fan speed furnace matched with a 2.5 ton XP17 H/P.

    It sounds like I'll need to play around with the comfort/economic balance points for my household and see what works best.

  4. #4
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,694
    The heat pump can run24/7 without harm to it.
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    How many times must one fix something before it is fixed?

  5. #5
    Join Date
    Jul 2010
    Location
    nebraska
    Posts
    1,628
    That thermostat will give you a goofy error code if the heat pump runs more than so many hours at a time. I don't remember the exact number of hours but it's somewhere around 15. Just posting a FYI,it hardly ever happens.

  6. #6
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,607
    your salesman should be able to use your gas and electric prices and load calculation to calculate the best temp to change over .unless gas is really high and electric is cheap i would set it about 40.
    We really need change now

  7. #7
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    +1. 40 is the threshold where you see a step shift in efficiency since the outdoor coil temps will approach 32f.

    With an 70k 80%r and 2.5 tons, you might try 35f. But i wouldn't run the furnace pump above 45f... Unless elec rates area really high. The furnace will have fairly short cycles say that temp too... Sup more comfort on heat pump.


    In dual fuel the heat pump is the primary heart source so it should keep trying to shortage to it even if you set the balance point to 20f and it fell behind.


    Sent from my DROID RAZR using Tapatalk 2

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