Insteon Thermostat - separate heat and AC units


Old 01-05-17, 09:41 AM
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Insteon Thermostat - separate heat and AC units

Hi Everyone,

I hope someone can really point me in the right direction here since I feel like this can't be solved

Existing System
- Separate boiler for heat (forced hot water)
- Separate AC unit
- Old thermostat had RH, RC, W, Y, G and was battery powered.
- From boiler for heat RH and W were connected to thermostat
- From AC -> RC, Y and G were connected

New Thermostat is Insteon smart thermostat
- This only has R, C, W, Y, G

When I initially installed the Insteon thermostat I hooked
- RH -> R
- W -> W
- RC -> C
- Y -> Y
- G -> G

but nothing on the thermostat lit up. In reading the thermostat needs 24V continuous (common) and I originally did not have that.

In reading the Insteon forums I found this thread where it says to connect the Reds together and the commons (blue) together

Smarthome Forum - wiring question for insteon thermostat

From my boiler I noticed I had a common so I hooked that up (blue) and attached the two Rh and Rc together to make one wire (red)

This made the thermostat work and I had what appeared to be everything working. I tested heat and that worked. I tested the AC fan and that turned on. (Did not try AC since too cold outside)

See attached images for old thermostat wiring and new thermostat wiring

I then proceeded (week later) to replace the upstairs thermostat the same way. In doing so I flipped the circuit breakers numerous times to cut power as I wired things up. When I went back to the downstairs thermostat I noticed heat still worked but the AC fan would not turn on.

Here is my dilemma and questions to the group
- I think I blew my transformer on the AC unit.
- The new thermostat only has R and not Rh and Rc like my old thermostat. Is combining the power from both boiler and AC ok or did that blow the transformer in the AC
- Did I potentially blow my transformer from flipping the circuit breakers too many times (did it at least 10 times). I don't think this AC air handler has a fuse that I have read can protect the transformer

Any help will be greatly appreciated. I fear I might need to return the thermostats since it was not designed to work with two separate heat and AC units.

Thank you,
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Old 01-05-17, 09:55 AM
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The blue wire should not be landed on C. It not a common. The blue wire is power from the AC unit. Flipping the breaker on and off 10 times will not damage the transformer.
For your situation I would send that stat back and go with a Honeywell.
Old 01-06-17, 07:45 AM
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Honeywells are the gold standard in compatibility and reliability but Insteons should be fine for what you are doing. Did you combine blue wires on upstairs thermostat? That could cause a problem as you should never connect both the R's and C's from different systems, just the R's.

Try connecting the Rc and G wires directly to see if the fan comes on.

Does the heat and blower work from the 2nd floor? You need to get a multi-meter to check voltages.
Old 01-06-17, 09:30 AM
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That Insteon thermostat is designed for a single system. It cannot be used for your two systems.
Old 01-07-17, 01:41 PM
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The Insteon does not have separate Rh and Rc terminals but that does not mean that it can not be used. With the R's connected together it makes no difference to the thermostat if the transformers of the systems are in phase. If out of phase the Insteon would see a voltage potential of 48v between Y and W with no heating or cooling call but it would still work.

To make it clean add an isolation relay - C and W from the thermostat would drive the coil while the contacts would bridge the two wires from the boiler.
Old 01-07-17, 02:10 PM
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I would get a different thermostat that has separate RH and R(C) terminals.
Old 01-07-17, 07:32 PM
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I have to agree with Houston. Allowing two systems to be combined usually causes problems.
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