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  1. #1
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    Nov 2019
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    Two stage heat pump and two stage heat strip control

    An American Standard 4 ton heat pump with 15 Kw heat strips presents a control problem. The unit has two heat pump stages and two stages of heat strip heating. There appears to be a glitch in control logic of the installed model No. ACONT624AS42DAA thermostat. The installer's guide allows setting of the maximum time that the 1st stage heat pump stage can operate without attaining the target temperature before the transition to the second heat pump stage is activated. That is, if operation of the 1stage of the heat pump compressor does not allow reaching the target room temperature in the set max amount of time, the 2nd stage of the heat pump will be activated. However, in the control scheme there does NOT appear to be a max amount of time that the operation of the 2nd stage of heat pump without reaching the target room set point will trigger the start of the 1stage of heat strip heating to supplant the heating supplied by the 1st and 2nd stages of heat pump compressor operation. Also there does there appear to be a max amount of time that operation of the 1st stage of heat strip heating without the reaching the set point for the room temperature which will trigger the start of the 2nd stage of heat strip operation. The 1st and 2nd stages of heat strip heating can only be triggered by selected deltas of the room temperature from the target set point. Hence, if the 2nd stage of heating from the heat pump compressor is not adequate to reach the room temperature set point, the system will not trigger the turn on of the 1st stage of heating from the heat strips. Similarly, the 2nd stage of heat strips will not be turned on if the 1st stage of heat strip heating does not allow reaching the target room temperature. The 2 stages of heat strip heating are only turned on by larger deltas between the target and actual room temperatures than the deltas which turned on the 1st stage of heat pump triggered. This appears to make no sense. The two heat strip stages should come on only after the heat pump stages fail to reach the target room temperature after a maximum amount of time.

  2. #2
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    Nov 2006
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    That’s a hard block of text to read.

    What point are you making with it?
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  3. #3
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    Mar 2016
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    Dayton Oh
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    Quote Originally Posted by kdean1 View Post
    Thats a hard block of text to read.

    What point are you making with it?
    Basically, he is stating it makes more sense to have the heat stages triggered by time rather than time and deference between room temp and set point.

    OP: that would be a question geared more towards the engineering dept of trane but Ill put in my 2 cents. The reason I ASSUME they dont do it the way you describe is possibly for reasons like setback and efficiency. Electric heat is expensive versus running the hp, so its proves more cost effective to run the hp until it is absolutely necessary to turn on the electric. In a perfect design condition you hp would run 24/7 and keep the house exactly at your desired temp. That scenario proves to be the most comfortable for the occupants. In that case, why bring the electric heat on?

    The electric is more of a supplement to the hp when it starts to get colder outside. So its best to keep it off if we can.


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  4. #4
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    Sep 2007
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    I don't really see a question here. Just some sort of statement of facts.
    Can someone please explain to me -
    Why is there never enough time to do it right the first time, but plenty of time to do it twice?


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  5. #5
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    Nov 2019
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    Thread Starter

    The question is:

    Quote Originally Posted by joemach View Post
    I don't really see a question here. Just some sort of statement of facts.
    The "statement of facts" was provided with a three fold purpose consisting of 3 implied questions: 1. Is there another way that the system could be set up? AND, 2. Is there another (i.e. different) thermostat that would include the ability to switch from the 2nd heat pump stage to the 1st heat strip stage based after a selected a maximum time in the 2nd heat pump stage has passed without reaching the target room temperature? 3. Is there also a thermostat that would allow the switch to the 2nd heat strip stage from the 1st heat pump stage once a stated amount of time in the 1st heat strip stage had passed without reaching the target room temperature?

    It would seem that the problem of the system being "stuck" in the 2nd heat pump stage or the 1st heat strip stage without reaching the target room temperature might have been experienced by others who have found a solution.

  6. #6
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    Aug 2002
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    it sounds like the control is calling for whats needed!

  7. #7
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    Mar 2016
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    Quote Originally Posted by whitetiger01 View Post
    The "statement of facts" was provided with a three fold purpose consisting of 3 implied questions: 1. Is there another way that the system could be set up? AND, 2. Is there another (i.e. different) thermostat that would include the ability to switch from the 2nd heat pump stage to the 1st heat strip stage based after a selected a maximum time in the 2nd heat pump stage has passed without reaching the target room temperature? 3. Is there also a thermostat that would allow the switch to the 2nd heat strip stage from the 1st heat pump stage once a stated amount of time in the 1st heat strip stage had passed without reaching the target room temperature?

    It would seem that the problem of the system being "stuck" in the 2nd heat pump stage or the 1st heat strip stage without reaching the target room temperature might have been experienced by others who have found a solution.
    I dont know the specifics of the stat you have, but i know Honeywell has an option in the installers setup that will run the most efficient stages or one that is most comfortable. Basically, it will bring on the electric heat to ensure the temp never drops from setpoint. But please know, this is not an energy efficient strategy and expect it to reflect in the next electric bill.

  8. #8
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    Nov 2019
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    Thread Starter
    Do you happen to know which Honeywell thermostat has the option to run the most efficient stage or the one that is most comfortable?

  9. #9
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    Sep 2008
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    Quote Originally Posted by whitetiger01 View Post
    The "statement of facts" was provided with a three fold purpose consisting of 3 implied questions: 1. Is there another way that the system could be set up? AND, 2. Is there another (i.e. different) thermostat that would include the ability to switch from the 2nd heat pump stage to the 1st heat strip stage based after a selected a maximum time in the 2nd heat pump stage has passed without reaching the target room temperature? 3. Is there also a thermostat that would allow the switch to the 2nd heat strip stage from the 1st heat pump stage once a stated amount of time in the 1st heat strip stage had passed without reaching the target room temperature?

    It would seem that the problem of the system being "stuck" in the 2nd heat pump stage or the 1st heat strip stage without reaching the target room temperature might have been experienced by others who have found a solution.
    Why would you want to energize electric heat when you don't need it? This request doesn't make much sense. You've reached the balance point of your system. You can set the aux heat Delta staging "on" at the minimum setting.

    So, if the heat pump can't maintain the desired temperature it will kick on. The settings show from 0.5-8. I would suggest reducing it to 0.5 if that's what you really want. Standard for most thermostats is 2.

    I would say the option to drop to 0.5 is pretty good logic if you ask me.
    You do realize that aux heat cost of operation is 250% to 400% more expensive to operate vs. the heat pump??
    "The only true wisdom is in knowing you know nothing" Socrates

  10. #10
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    Mar 2016
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    Dayton Oh
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    Quote Originally Posted by whitetiger01 View Post
    Do you happen to know which Honeywell thermostat has the option to run the most efficient stage or the one that is most comfortable?
    Sounds like your current stat will do the same basic thing according to the previous post.
    Can save money and just change the settings in your current stat.


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