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  1. #1
    Join Date
    May 2008
    Posts
    5

    Heat Pump & Temperature Sensor?

    I will be replacing my current A/C unit with a Trane XR13 heat pump. The installation includes a Honeywell VisionPro thermostat. This will be my first experience with heat pumps. I live in Central Indiana.

    I have been given conflicting advice from two HVAC sales professionals regarding using an outside temperature sensor to switch between the heat pump and my gas furnace.

    1) Salesperson 1 said I need a sensor to switch back and forth between the heat pump and my gas furnace based on the outside temperature. This is the traditional method used in heat pump applications.

    2) Salesperson 2 said I should go without a sensor. The explanation was that this way EVERY time the thermostat calls for heat, it will first go to the heat pump to try and heat the house. If it is unable to heat the house and the temperature continues to drop, the Adaptive Intelligence feature of the thermostat will switch over to the gas furnace to heat the house up to the required temperature setting. This way I would get more use out of the heat pump on days where the temperature might be below the traditional 30-38 degree F switchover point. As an example, he pointed out that on a 20-25 degree F, clear sunny day where the house was getting some “solar heating”, it might be sufficient to use the heat pump to warm the house back up to the set temperature. In other words, he said that the outside temperature was only one factor determining the heat required to warm the house; others included clouds, sun, wind/air infiltration into the house, etc. He also said the heat pump is designed to work down to minus 30 degrees F.

    The second scenario appears to make sense if indeed the thermostat is smart enough to know when to switch back over to the gas furnace based on the inside temperature continuing to drop after the heat pump has been activated.

    Any thoughts or suggestion would be appreciated on whether Scenario 2 makes sense and is a viable scenario with the Honeywell VisionPro thermostat.

    Thanks!

  2. #2
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,089
    The Vision Pro is either gas or HP but not both. It will not bring on backup if the pump doesn't keep up. To do what #2 is suggesting, you would use the VP IAQ as it will bring on backup if the house temp drops 2° below setpoint.

    I personally don't like that idea, most people don't like the "cool" blowing of the heat pump in cold weather. That's why the dual fuel is becoming so popular. We are in the same area and I set the stats to shut off at 30° or 35° out, depending upon brand of heat pump. NO complaints yet and sure save the money on heat bills.

    Actually I switch my gas on manually as I want more control. But since it is just me and nobody else will be uncomfortable, it works.

  3. #3
    Join Date
    Jan 2004
    Location
    Lancaster PA
    Posts
    67,913
    Baldie.

    Read page 11 again, the OD sensor is optional on dual fuel on the 8320 and 8321.

    To the OP.
    Letting the thermostat decide when to go to aux heat by temp drop is teh most efficient way.
    But as Baldie pointed out, you can have times were the air may feel to cool before it changes over to aux heat.

    You need to decide if you want the highest efficiency, or more comfort.
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  4. #4
    Join Date
    Mar 2005
    Location
    Indianapolis, IN
    Posts
    241
    Quote Originally Posted by beenthere View Post
    Read page 11 again, the OD sensor is optional on dual fuel on the 8320 and 8321.
    Uhh, but i would not be going for this setup... W/o t sens tstat will have no idea that it is too cold to use HP and will keep blower running until staging. Besides t sensor is not that expansive anyway.

    I would do what Baldie said: request IAQ with OAS. They use same temp sens as 8320(1) tstats.

  5. #5
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,089
    Page 11 talks about wiring & mounting the unit.

    Page 33 talks about dual fuel and it says:

    "When the outdoor temperature is above the selected balance point temperature, only the compressor operates and the fan G terminal energizes when the thermostat calls for heat."

    "When the outdoor temperature is below the selected balance point temperature, only the fossil fuel operates ... when the thermostat calls for heat."

    It says nothing about bringing on backup or running without an outdoor sensor. If you'll note on the IAQ, it has the option (#345 & #346) of how to handle backup heat in dual fuel. If the VP could do backup, why doesn't it give you these same options?

  6. #6
    Join Date
    May 2008
    Posts
    5
    Thanks everyone for the input.

    1) What is the lowest temperature set point the switchover sensor can be set at? Perhaps I would be comfortable with the heat pump output at a setpoint of 20-25 degrees F whereas someone else might want their aux. fuel to come on at 30-35 degrees F.

    2) Is the temperature sensor switchover point easy for me to set from the thermostat, heat pump or sensor itself?

    Thanks!

  7. #7
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
    Posts
    34,089
    Changeover depends upon several factors

    Biggest is temp setting. I keep it 66° or so in the winter so can get by with 20°something changeover.

    Depends upon size of HP vs needs. Normally you size for cooling load so around our area, 30-35° is best changeover.

    Depends upon output of HP. For A-S/Trane units, I set 5° warmer to compensate for their low heat output. Other brands put out more heat for the same cooling capacity and can work longer.

    This is changed on the thermostat with the VP or IAQ.

  8. #8
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    67,913
    This is from page 11 of the install instructions downloaded from Honetwell for the 8000 series.



    Heat Pump Temperature Lockout (with fossil-fuel backup): If the thermostat is installed with an optional outdoor sensor, you can select a compressor lockout temperature (Function 0350). When the outdoor temperature is below the lockout temperature, only the auxiliary heat operates. When the outdoor temperature is above the lockout temperature, only the compressor operates.


    Again its a choice for those that want the highest operating efficiency from there system. Not comfort.
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  9. #9
    Join Date
    May 2008
    Posts
    5
    Quote Originally Posted by beenthere View Post
    This is from page 11 of the install instructions downloaded from Honetwell for the 8000 series.

    Heat Pump Temperature Lockout (with fossil-fuel backup): If the thermostat is installed with an optional outdoor sensor, you can select a compressor lockout temperature (Function 0350). When the outdoor temperature is below the lockout temperature, only the auxiliary heat operates. When the outdoor temperature is above the lockout temperature, only the compressor operates...
    Can you provide the link to this download - I can not seem to find this information on anything I download from the Honeywell site.

    Thanks!

  10. #10
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    Baldie, is providing the link ok?
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  11. #11
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
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    Sure, I'd love to see it too. Never seen anywhere that outdoor sensor and changeover based on outdoor temp is an option.

    Found this on page 16 of

    http://www.hotfreshcool.com/Product/...0(68-0280).pdf

    Option 210, "no external fossil fuel kit is controlling heat pump
    backup heat. This thermostat controls the dual fuel. Must
    install outdoor sensor and set Installer Setup Number
    0340 to number 2
    ."

  12. #12
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    Jan 2004
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    Lancaster PA
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  13. #13
    Join Date
    May 2000
    Location
    Indianapolis, IN, USA
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    34,089
    I certainly see where you are coming from but Page 7 of the same manual contradicts that and repeats what I found where option 210 set so the VP is the dual fuel kit says "outdoor sensor required".

    I'll give Honeywell a ring tomorrow. Maybe I can find someone who speaks English and can answer

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