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  1. #1

    Cycle per Hours, Humidity, and Magic

    I'm getting the impression that some of these thermostats are like automatic cameras. You never know what the heck they are doing or why they made the choice they did. For example, why did the camera increase the ISO rather than increasing F-stop?

    I've got a couple (basic?) questions that I'd appreciate help with. I've read posts here, but they didn't get down to the (what might be very simple) specifics I was looking for.

    Regarding CPH (cycles per hour)... A cycle includes the on time and the off time. If CPH = 3, then a cycle will be 20 minutes. But how much of that cycle must be on and how much must be off?

    Is it possible/common that a cycle is 100% on and 0% off? For CPH=3, is 20 minutes of on time possible? Is there any requirement that the system must be off during a cycle?

    How does a thermostat like the Trane/Honeywell 803 with humidity control effect CPH? Which has higher priority? CPS or reducing humidity? For example, will the thermostat change CPH on the fly to deal with humidity (perhaps go from CPH=3 to CPH=2)? Or is CPH fixed (at whatever setting the installer specifies) and humidity control just changes on long the "on" portion of a cycle will be?

    The manuals don't address this type of info, as far as I can tell.

    Also, how does the thermostat's "wait" (compressor safety) play with all this? If the compressor must stay off for at least 5 minutes once turned off, how does that effect CPH? Here is an example. Assume CPH=3. Cycle starts, AC runs for 18 minutes and then turns off. In 2 minutes the first cycle is over (18+2=20). But the 5 minute compressor timer still has 3 minutes remaining. So will the second cycle start but the AC will remain off until 3 minutes into the second cycle?

    People say the most efficient system will run constantly, 24 hours a day. How can this occur when, for example, CPH=3 and there is (apparently) required "off" time (plus the compressor safety timer).

    Sorry for all the questions. I'd really like to understand this better --- and this seems like the right place! Don't worry about correcting me if I've made an incorrect assumption anywhere.

    Thanks!

    ...MM

  2. #2
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,376
    As a photographer - most of the time I drive my camera manually. I care about my photos, so I want control over what is selected to make the shot.

    As for CPH - if the room temperature remains above setpoint for an entire hour or more, the system will continue running until setpoint is met. Just like old mechanical thermostats did. CPH is maximum cycles per hour, not minimum. It comes into play whenever the space is below a design extreme, such as when it's not 110 outdoors or 5 below zero.

    The compressor delay restart only kicks in after the thermostat sees a power loss. It is to prevent short cycling of the compressor; many reciprocating compressors do not have start assist kits beyond the standard PSC configuration, so due to unequalized pressures in the system the compressor stalls and trips on internal overload if not enough time has elapsed to let the system equalize pressures.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  3. #3
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,501
    For humidity my thermostat has a "dehum" terminal that goes low when dehumidification is required. This is used to reduce the blower speed of the furnace therefore increase latent capacity. My setup does this with a simple SPDT relay, but other systems may work differently.

  4. #4
    Join Date
    Jan 2004
    Location
    PA
    Posts
    68,758
    On Honeywell its temp, humidity, then CPH for priority.

    CPH is only accurate at the middle load range.
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  5. #5
    Thanks Shop, 54 and beenthere. Very useful info.

    What does it mean to have CPH for stage 1 and stage2?

    For example, what would the thermostat do if CPHs1 = 3 but CPHs2 = 2?

    I thought the system, for cooling, would always start with stage 1 and move to stage 2 if needed.

    I was surprised there doesn't seem to be some parameter to control how long stage 1 stays active. How is this typically decided?

    ... Altan

  6. #6
    Join Date
    Jun 2001
    Location
    Moore, Oklahoma, United States
    Posts
    4,501
    Some of the older installation manuals will explain the programming and how it decides how many CPH to run. If stage 1 cannot keep up it will stay on 100% and the thermostat will cycle stage 2 to maintain setpoint. The CPH is the LIMIT of how many times the thermostat will cycle per hour. Higher CPH means typically means more accurate temperature control, lower CPH typically gives better energy efficiency.

    Some thermostats use a "fixed point" temperature along with minimum on/off timers. One thermostat I've owned worked like this in cooling:

    Building gets up to 75, thermostat kicks on and starts 2 timers, a minimum run time (could be 5 minutes) and a minimum "next start" time. If CPH is 4 then wit will not start the next cycle until at least 15minutes has passed. The thermostat checks the temp after 5 minutes, if it's below the setpoint it shuts the system off and will "try again" in 10 minutes. If in 10 minutes the temp is above setpoint it starts another cycle.

    1:00 start cooling
    1:05 check to see if it's below setpoint, shut off if it is
    1:15 check to tempature to see if system needs to run again. repeat as required

  7. #7
    Join Date
    Jan 2004
    Location
    PA
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    68,758
    Depends which model thermostat. The IAQ will bring on second stage when it determines first stage is at 90% of its capacity.

    Other models will bring on second stage when the temp drops 1 degree.

    Stage 2 CPH will determine how long second stage is on. If the outdoor temp is not at design.
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  8. #8
    Thanks again guys.

    It's a TCONT803 thermostat. Any knowledgeable about that one?

    Either way, thanks for your help.

  9. #9
    Join Date
    Jan 2004
    Location
    PA
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    68,758
    Its equivalent to the Honeywell TH8321. 3 stage heat, 2 stage, and can over cool to dehumidify.
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