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  1. #1
    Join Date
    Dec 2008
    Location
    Tacoma, WA
    Posts
    90

    CPH for dual fuel system

    A variety of threads here have said that the CPH for both a HP and the auxiliary fossil fuel backup should not be set too high with 3 and 4 CPH being bandied about as good places to be.

    I have a few questions about CPH:

    1. One of the reasons stated is efficiency, that the system is more efficient when it runs continuously. If that is the case, why have multiple cycles in the first place? In the old days, a thermostat was essentially a switch that turned the heat on until the T-stat was satisfied.....it didn't cycle at all, right?
    2. How long is an "off" cycle? Does the system just cycle down very briefly, or are the off cycles as lengthy as the on cycles?
    3. With the IAQ, one can set a different CPH for each heat/cooling stage; are there advantages to having a different setting for different heating stages? (I have 2 stages of HP, 2 of aux. head [fossil])

  2. #2
    Join Date
    Jan 2009
    Location
    Keokuk, IA
    Posts
    5,520
    I'd also like to better understand how CPH is used by the thermostat. Is it primarily an indication of how tight it will control the temperature? Or does it mainly affect staging?

    What I mean is that if a lower CPH value is used, is the thermostat going ot overshoot the temperature to get a longer runtime on the furnace or HP when heat loss is lower than capacity.

    Does CHP represents a constant target, or a minimum or maximum CPH? FOr example, with my 4 stage system between 26-35F, the HP or furnace should be running almost constantly in one stage or another to maintain an equilibrium between home heat loss and system output. That would be a CPH of 1 correct? Or does a shift from 1st to 2nd stage on the HP or to a stage of auxillary count as one cycle?

    I'll see more cycling at around 8 - 25F and 36-55F and again from -10F - 7F.

    I'm really interested ot see how this thermostat responds. After a few months i could probably answer these questions myself.

  3. #3
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,795
    CPH controls duration of cycle.
    1 cycle is 1 on time plus 1 off time.
    Set too low, it will over shoot temp, and leave the home get too cool before running the system again.

    So it also effects personal comfort.

    CPH, is actually set for 50% load conditions. At 10% load conditions, it will only cycle once an hour.
    At 90% load conditions it will only cycle one.

    In the old days, thermostats had heat, and cooling anticipators that were set for the equipment it controlled.
    With Digital stats, CPH replaces those anticipators, and gives more control for comfort and efficiency.

    Length of the off and on time, will vary as the load varies.

    There are advantages with using different CPH sttings for the different stages.

    Depends how you want your system to operate.

    Quote Originally Posted by rcamner View Post
    A variety of threads here have said that the CPH for both a HP and the auxiliary fossil fuel backup should not be set too high with 3 and 4 CPH being bandied about as good places to be.

    I have a few questions about CPH:

    1. One of the reasons stated is efficiency, that the system is more efficient when it runs continuously. If that is the case, why have multiple cycles in the first place? In the old days, a thermostat was essentially a switch that turned the heat on until the T-stat was satisfied.....it didn't cycle at all, right?
    2. How long is an "off" cycle? Does the system just cycle down very briefly, or are the off cycles as lengthy as the on cycles?
    3. With the IAQ, one can set a different CPH for each heat/cooling stage; are there advantages to having a different setting for different heating stages? (I have 2 stages of HP, 2 of aux. head [fossil])
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  4. #4
    Join Date
    Dec 2008
    Location
    Tacoma, WA
    Posts
    90
    Quote Originally Posted by beenthere View Post
    CPH controls duration of cycle.
    1 cycle is 1 on time plus 1 off time.
    Set too low, it will over shoot temp, and leave the home get too cool before running the system again.

    So it also effects personal comfort.
    This makes sense.
    CPH, is actually set for 50% load conditions. At 10% load conditions, it will only cycle once an hour.
    At 90% load conditions it will only cycle one.
    This confuses me....it sounds as if you are saying that at either 10% or 90% load conditions, it will cycle once per hour. Is that right? Or are you saying that at 90% load conditions, "it will only cycle once" meaning it will stay on until T-stat is satisfied?
    In the old days, thermostats had heat, and cooling anticipators that were set for the equipment it controlled.
    With Digital stats, CPH replaces those anticipators, and gives more control for comfort and efficiency.
    Ah, yes. Makes sense. CPH replaces anticipators.
    Length of the off and on time, will vary as the load varies.

    There are advantages with using different CPH sttings for the different stages.
    What are those advantages, if they're easy to explain, about using different CPH settings for different stages?

    Thanks very much for your help.

  5. #5
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,795
    Once as in stay on until stat is satisfied.

    AT 90% load conditions, it might run for 55 minutes, shut off for 5 minutes and then come back on.

    If it tried to maintain 3 cycles per hour at 90% load. It would be restarting the system every minute or 2. Which is hard on equipment.

    Setting the second stage CPH lower then first stage. Tends to make the stat satify the call using second stage(when outdoor temps are cold/hot enough to bring on second stage), instead of running continuous in first stage, and constantly cycling the second stage on and off.

    Higher CPH on second stage, causes the opposite effect.
    And can give a more consistant heat or cooling, by causing the first stage to run 24/7 when outdoor temps are too hot/cold for first stage to handle the load by itself.
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