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Thread: 2 GSHP 3 stages w/ damper 1 thermostat

  1. #1
    Junior Member

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    2 GSHP 3 stages w/ damper 1 thermostat

    I have been looking at this forum and have come up with some basic ideas for my build but now it is time to put the pieces together.
    I have two GSHP (geothermal- ground source heat pumps) that are different sizes. One is a 6 Ton with 2100 cfm and the other is a 3.5 Ton with 1400 cfm. I want to control both with one thermostat and get 3 stages of heat and whatever on the AC.
    If this was an HVAC forum I would give more detail on the build but for now it's the thermostat.

    My thermostat is a Braeburn 7205 that I purchased with my heat-pumps. My 6 T is a two stage and the 3.5 T is a single stage. Because of the different size blowers I am using a NC spring damper (24VAC) attached to the 3.5 T. It my desire to have the 6T connected to the stages 1 & 2 of the thermostat and the 3.5T come on for the 3 rd stage attached to the AUX (w3?). Also when the 3.5T comes on then the damper will open to allow the blower air to enter the plenum. I expect to use a separate transformer to activate the damper.

    I would appreciate some guidance on the relays.
    Thanks in advance.

  2. #2

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    If this was an HVAC forum I'm thinking they would say what you are doing is not recommended. When combining systems they need to have matched blower sizes otherwise the lower speed one may be ineffective or even have backwards flow. When the damper is closed it could generate turbulence to reduce the effectiveness of the prime system.

    That said for what you are doing my concern would be to not overload the thermostat and issues if the system transformers are out of phase. Get a multi-relay board such as so all connections to 2nd system are isolated. It also helps organizing wires and has LEDs showing what is going on for 2nd system. Relay 1: thermostat O/B connects sys 2's O/B and R. Relay 2: thermostat's W3 connects sys 2's R to Y and G. Relay 3: thermostat's W3 to damper.

  3. #3
    Junior Member

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    Thank you for the reply. You are correct about the different blower sizes, this is not an ideal situation but for now it is what it is. I am using the damper to keep sys 2 blower from spinning backwards when sys 1 is on without sys 2. I have hopes that sys 2 will add just the little bit extra I need during extreme low temperatures here in the Seattle area.

    That is great information about the relays at Bravo Controls. I will order a couple of 4s so I have an extra. I am still a little challenged about relays but we will see how it goes. Thanks Again..

  4. #4
    Junior Member

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    I installed the relays yesterday and blew a fuse...... I screwed up because I did not realize that the relay input gets the 24VAC from the control wire, in this case the w3 from system #1 and the common also from system #1. My thinking was that it needed red + and common -, so poof. I had to get more fuses... Thanks to Amazon, one day delivery I am up and running and could not be happier with my geothermal system and thermostat.
    Thanks for the advice and the lead on the Brovo relays... what a great relay and soooo inexpensive.Click image for larger version. 

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    Last edited by rbento; 01-15-2020 at 04:27 PM. Reason: added photo

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