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Timed switch for thermostat

Discussion in 'Sensors and Actuators' started by Landlubber, Sep 29, 2018.

  1. Landlubber

    Landlubber

    4
    0
    Sep 29, 2018
    In order to reduce the number of on/off cycles for a compressor, I would like to use the current on/off switch (making the switch is ON ) as the input signal to a circuit which will close a relay for a minimum of 5 minuten (adjustable?) , but stays on if the input is still on. Then switches off and stays off for 5 minutes, regardless of the input signal.
    Does something like this already exist?
     
  2. Tha fios agaibh

    Tha fios agaibh

    1,765
    575
    Aug 11, 2014
    Just buy a 24v AC "off-delay" timer so you can wire it into existing hvac circuit.
    Parallel the timed relay contacts with the thermostat contacts. (yellow wire for cooling and common) The thermostat will work as normal but the timer will override it for 5min (keep switch on) even after thermostat turns off.
    You could put this override timer in the thermostat itself, but you'd have more room to place it in the condensing unit (compressor) itself.
     
  3. Landlubber

    Landlubber

    4
    0
    Sep 29, 2018
    Thanks, I will look at this as a solution, although if I understand "off-delay" well, it is going to stay on for a set time after the input goes off. I would like a solution where the relay stays on for the remainder of 5 minutes when the input is up for less than 5 minutes. If the input stays on for more than 5 minutes, the relay should go off when the input goes off. Then there should be at least 5 minutes downtime, regardless of input.
     
  4. Tha fios agaibh

    Tha fios agaibh

    1,765
    575
    Aug 11, 2014
    Sounds like the same thing to me.
    In other words, the off delay timer will stay on for 5min when its input goes off.

    The 5min dwell time begins on the falling edge (when switch turns off) of the input, not the rising edge (turning on)
     
  5. Landlubber

    Landlubber

    4
    0
    Sep 29, 2018
    I added a timing chart to explain more clearly. Hope it helps.
     

    Attached Files:

  6. Tha fios agaibh

    Tha fios agaibh

    1,765
    575
    Aug 11, 2014
    I think I understand what you want although the second output cycle looks longer than 5min.
    You could wire this as I mentioned above with the override "off delay" relay and add a second retentive "on delay" timer inseries with it. If this is for a air conditioner, the 5min off condition you want is not necessary imo. It will take at least that amount of time for the temperature to rise back up to call for cooling. Just the off delay will make the compressor cycle less but you'll have wider temp swings.
    I suggest you also look at thermostats. Some models have cycle rate differentials to achieve what your trying to do.
     
  7. Landlubber

    Landlubber

    4
    0
    Sep 29, 2018
    It is for a heating system, but since it heats 6 separate rooms the input depends on the temperature of each separate room. The heat pump replaces a gas heater that didn't care about heating cycles. The existing thermostat has a setting for maximum number of cycles per hour, but that doesn't work.
    I will try with just an "on delay" timer and see how this works out, but it is not the ideal solution, so if someone else has a good suggestion....
     
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