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Thread: Issues with Ecobee and boiler short cycling when AC is on

  1. #1
    Junior Member

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    Issues with Ecobee and boiler short cycling when AC is on

    Hi there - I am new to the forum and I apologize if my question is dumb. I am handy, but not an expert on HVAC systems so was hoping someone who is could give me some advice.

    I have the setup in the attached image - this was installed before I bought the house. Two zones, two thermosats, two hydronic air handlers and one shared high efficiency boiler connected via a valve control panel Click image for larger version. 

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    In trying to improve efficiency, I tried installing an ecobee with voice. Because I only had 4 wires, I installed the PEK. The system worked great all winter, zero issues and I love it. Then, I needed to turn on AC...

    I happened to be in the utility room and when the AC was running I could see the taco orange light for zone 1 would periodically start coming on intermittently until eventually zone 1 valve would close and the boiler would be initiated. By the time the boiler started, the taco panel lights would go off. This would then happen again a few minutes later as long as the AC was running.

    Thinking it was the PEK, I removed it and ran a C wire to the air handler. The same issue persisted. I removed the ecobee entirely and backed out the C wire and installed the original relay based thermostat. Guess what - the taco panel is still intermittently calling the boiuler for heat via zone 1 when the AC in zone 1 is running.

    Right now, I have disconnected the zone 1 connections to the Taco panel to stop the boiler being repeatedly cycled. My question(s) is:

    • Has the Ecobee damaged the air handler of Taco panel to cause this?
    • Does the taco panel need to be connected via the air handlers two white wires of can it be connected directly to the thermostat?
    • What could cause the panel to do this, even with a regular relay thermostat?

    It is quite possible this behavior has always happened and I just didn't know about it. The contractor on this house made many bad choices.

    Any help truly appreciated - sorry for the long email!

  2. #2
    Senior Member

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    Connecting TT of the air handler to the Taco's TT doesn't make sense. Does the air handler even have TT terminals?

  3. #3
    Junior Member

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    I apologize. The air handler has two unlabelled white wires which are connected to TT on the taco.

  4. #4
    Senior Member

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    See where the white wires connect in the air handler. One setup could be a relay that is driven off of the W and C but maybe something strange. A more "normal" connection would be to take the two TT wires to the Ecobee and attach to W1 and Rh. Look at Ecobee's boiler diagram at

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