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  1. #1
    Join Date
    Feb 2007
    Location
    Bellingham, WA
    Posts
    115

    Operating time before HP switches to gas furnace

    I have the following system York 3-Ton Affinity HP and gas furnace with a Honeywell 8321 thermostat.

    I noticed on the thermostat had the following readings: 72F indoors and 40F outdoors. The HP turns on because I had the heat rating at 72F. The heat pump stays on for ~2 hours before it stops. During the entire 2 hours the HP was operating, the indoor temperature reading was 72F indoors. I was surprised that the HP was operating for such a long time and did not give up. I was wondering how and when does the thermostat determine that the HP cannot meet the heating demand of the house and start up the gas furnace to meet the demand of the home?

    Is it normal to have the thermostat read 72F indoor temperature with the thermostat set to turn on the heating cycle at 72F and have the HP run for more than 1 hour before the system recognizes that it can't keep up with the heat demand?

  2. #2
    Join Date
    Sep 2002
    Location
    I don't know
    Posts
    2,897
    If the indoor temperature isn't dropping, it's keeping up regardless of run time. An intelligent thermostat will only switch to gas when the indoor temperature starts falling.

    Long cycles result in less wear, greater comfort, and more efficient operation.

    Furnaces are sized for the coldest night of the year, and most are oversized; you can't expect a heatpump to cycle like any gas/oil fired or electric-resistance unit.

    Also note that much like heatpumps, properly sized A/C units can run for several hours at a time. (It's the same type of system)

  3. #3
    Join Date
    Apr 2004
    Posts
    7,680
    as AMD pointed out, HP's in cooler weather will run longer. The capacity balance point is defined as the point at which your heat pumps capacity meets the heat loss of your home. At that point the HP will run all the time. That doesnt mean $$$ are flying out of your pocket. At 40 degrees outside you are heading toward that balance point, then again it could be as low as 20 degrees, only your load calculation and equipment can determine that size.

    There is a economic balacne point also to be considered but you need all the above information plus your utility costs as well.

  4. #4
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    33,398
    The 8321 never "gives up". It won't bring on the backup no matter how cold it gets. Won't switch to gas til the outdoor temp drops below the changeover setpoint.

  5. #5
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    66,807
    If you don't like the way its operating after you get your next utility bills.
    They can switch control over to the Affinities duel fuel defrost board. They'll have to add a bonnet sensor.
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  6. #6
    Join Date
    Nov 2007
    Location
    Saint Charles, MO
    Posts
    3

    My Affinity 15 SEER doing the same.. Mine has switched to fossil fuel..

    Default was 35 degrees so I know I wasn't paying much attention when I put it in my own house during the summer. I remember seeing it and doing nothing. I'm lowering the LTCO to 25 degrees to see how it does in lieu of running a heat load on my house to see what I'm actually at. I run the fan on circ anyway. Have the variable speed 90 plus with the Honeywell touch screen. 3 fuel/2 cool.

    Been running now on heat pump with no problems for the last half hour (decided to hold off hitting post till I was done.) keeping the house at 73 degrees. Kicked in and seems to be doing very well. Will beef up the ceiling insulation and kick it down to 20 degrees next.

  7. #7
    Join Date
    Nov 2007
    Location
    Saint Charles, MO
    Posts
    3

    Cool My Affinity is 2 stg. 15 SEER doing the same.. Mine has switched to fossil fuel..

    Logged out and didn't know this posted automatically when I re-logged in. Erased the duplicate message...

  8. #8
    Join Date
    Nov 2007
    Location
    Saint Charles, MO
    Posts
    3
    Do you have a two stage HP or a single stage. Plus you didn't tell us your outside temp or the wind that's with it. Might be just keeping equilibrium. If it's the first stage and it's cold it might not shut off till it warms up and never causes you to use your gas furnace which is what I'm aiming for. Using a first stage is just plain cheap to operate.
    Last edited by Guvner; 11-23-2007 at 08:30 PM.

  9. #9
    Join Date
    Jul 2004
    Location
    Massachusetts
    Posts
    6,826
    Any heating or cooling appliance is designed to run non-stop, 24/7 when necessary. This does not necessarily point to a problem in the system bur more than likely just indicates that the unit is operating at some design temperature. So if your heat pump is producing its maximum heating Btu's at 40-degrees OAT and your home is just staying at 72 with the heat pump running, then you now know that 40-degrees is the maximum temperature at which your heat pump can maintain 72 indoors at 40 OAT. That's not a problem, it's just a fact. Now if it couldn't maintain 72, if the OAT dropped to 39*F for example, then we'd expect the HP to turn off and the fossil fuel unit to turn on.

    Much of the operation of the system these days is determined by the "Thermostat". I put that in quotation marks because the most sophisticated units do much, much more than just maintain a temperature setting. Some will even "bump" the heating/cooling set points up or down so that the changeover from heating to cooling or back again maintains a single temperature rather than having to manually adjust it. In fact, many system controllers have basic and advanced/service levels where it requires a service tech to access the innermost settings of the unit to prevent unwanted issues from cropping up in the system. It's not that HO's are stupid, just that most lack the depth of understanding needed to properly set the equipment up.
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