Wire so heat pump can only turn on with Heat/Cool call

Discussion in 'General Discussions' started by samheidy, Sep 19, 2018.

  1. samheidy

    samheidy New Member

    Has anyone ever somehow wired a system so the heat pump can only turn on when there is a Heat/Cool call? I've noticed my heat pump will kick in sometimes after long periods of no use and run much shorter than normal.

    What I found was during the last call, the buffer tank was within a degree or two of the temp limit and the zone was satisfied.

    Not that its a huge deal, but wouldn't it make more sense to only allow the heat pump to run if there's been a call for heat/cool? Once there's been a call the heat pump would then run until the opposite set point is hit.
  2. urthbuoy

    urthbuoy Well-Known Member Industry Professional Forum Leader

    Your water-water (radiant) heat pump right? Most fancy controls only allow the heat pump to come on with two calls: a zone call and a setpoint call (like a tank). I'm guessing you are using an aquastat or built in control? You could probably figure out the same thing using some relays.
  3. geoxne

    geoxne Active Member Forum Leader

    Your current system appears to be a "Running Control". This is the most simple control and maintains the buffer tank to setpoint by aquastat whether the house needs it or not. This type of control is appropriate when a consistent demand is expected.

    It sounds like you would prefer a "Demand Control". In this case the HP will only run if there is a demand from the thermostat AND the buffer tank is below setpoint. Urthbuoy is correct. You can convert your system to a "Demand Control" with a isolating relay that is energized with a thermostat call and switches power on to the aquastat.

    A third control option is "Outdoor Reset Control". ODR controllers are available that will control the temperature of the buffer tank based on outdoor temperature. Most ODR controllers have terminals to identify a demand from the system and allow HP operation. They also have a Warm Weather Shut Down feature that will not allow HP operation above a specified outdoor temperature.
  4. samheidy

    samheidy New Member

    Thanks for the detail! I will look through my 'control module' book to see if its just a setting. Most likely a setting you need software for being its a module.

    I know I can do it with a relay but I'd like to see if the software does it first. Anyone have TTherm GeoLogic software?
  5. samheidy

    samheidy New Member

    Thinking on this a bit more....

    If I were to add a relay to make it demand control and the fan coil calls for heat and gets the to the buffer tank set point, heat pump turns on, thermostat is satisfied 15 seconds later, I just made myself a good way to short cycle my heat pump, right?

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