Thermostat wiring for separate PTAC cooling and hydronic boiler heat
Combining a PTAC for cooling and a hydronic boiler for heat with limited thermostat wires forces you to manually move wires around depending on whether you want the house hot or cold. This wiring constraint prevents automated system control, leaving homeowners stuck juggling adapters or facing high technician fees.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
Found another way around this?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
- 1Jumping Y to O at the PTAC terminals to force the reversing valve to engage whenever the compressor runs for cooling.Did this help you?
- 2Using a Honeywell wire saver adapter to work around having only a 3-wire cable.Did this help you?
- 3Manually moving wires around between slots depending on whether heating or cooling is neededDid this help you?
- 4Turning the boiler switch off manually at the unit to cut powerDid this help you?
- 5Considering returning the smart thermostat and reinstalling the old oneDid this help you?
- 6Asking AI or online forums for wiring adviceDid this help you?
- 7Calling a professional installerDid this help you?
- 8Setting a larger dead band (gap of more than 3 degrees) between the heating and cooling limitsDid this help you?
- 9Opening a window instead of turning on the ACDid this help you?
- 10Turning down the heat and using warm blankets insteadDid this help you?
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“I just bought a new house that has a boiler and a/c. They are on separate thermostats. Fall in the Midwest means needing heat and a/c on the same day.”source ↗
“In my old house I had a smart thermostat and I had an upper limit and a lower limit and it just turned on whichever needed to run and that was preeeeeettttty convenient.”source ↗
“Have the ptac ONLY run AC with hydronic boiler as ONLY heat source.”source ↗
“I only have a 3 wire going to ptac from tstat so I'm running the Honeywell wire saver adapter utilizing K for Y & G.”source ↗
“Tosot ptac needs the reversing valve engaged during a cooling call. I'm considering jumping Y to O at the ptac terminals.”source ↗
“Since I have added this wire I have to move one of the two wires around depending on if I want to have the house hot or cold.”source ↗
“I want to hire a tech but I don't have $1000.”source ↗
“How can I know for sure? Will Ecobee automatically sense the correct configuration?”source ↗
“The problem is that it’s both freezing and boiling at different times of the day.”source ↗
Where this came up
People with this problem also raised
- 4What is W3 on a heat pump thermostat?
- 2How to set up a thermostat for a 2-stage furnace
- 13How to know if the blue wire is the C wire
- 2How to set a schedule on a mini split remote
- 3Why is there a massive temperature difference between floors?
- 4HVAC contractor installed mismatched heating and cooling parts