Thermostat for separate heating and AC
seiverth
| Posted in Energy, Heating & Insulation on
I have a boiler system with baseboard heat. I just installed a separate AC system (SpacePak). The question I have – a simple one at that – is can I use my current thermostat?
The thermostat has heat and cool options, along with auto and on for the fan. My boiler has two wires. One of these wires (R) will be shared with the (R) from the AC unit. Understanding that this is only 24 volts, are there any concerns with running two separate thermostat wires from two different units to the same thermostat? Or, should I be looking for a better thermostat?
Replies
You probably need to disconnect one of the 24 Vac transformers in either the AC or the boiler.
Worst case they are out of phase, in which case you will burn some wires.
Best case is they are both the same part number or have exactly the same turns ratio, in which case there is a 50% chance they are out of phase, 50% chance OK.
More probable is that they have slightly different turns ratios, and even if in phase, the 24 Vac will be slightly different resulting in overheating.
A single new thermostat would not help unless equiped with isolation circuits and relays.
If you are not sure how to rewire using just one of the 24 Vac transformers in the AC or boiler, best to stay with 2 separate thermostats. Easy to rewire IF you know what you are doing, just needing to ask the question says that you may be best off to stay with the 2 thermostats.
I was leaning towards two. The other problem I was concerned with was the internal settings of the thermostat. Right now, it is set for a boiler. It just seems a little more difficult to try to wire these both together into one unit.
In theory one shouldn't need to disconnect one transformer, since the way the heat/cool thermostats work only one system or the other is connected at a time.I'd really have to look at the wiring layout to say for sure, but I'd lay about 75% odds that it would work, if you didn't totally snafu the wiring. Odds are probably 90% that you could make it work if you checked the wiring first and planned it out.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Typically it can be made to work, but you need to tie the common on t he AC to the common on the heat. But that might not be too logical to do.But many of the digital thermostats have options to split the commons in the thermostat so you wire each individually..
William the Geezer, the sequel to Billy the Kid - Shoe
The only time I would bring the common to the thermostat is to avoid using batteries on some digital stats.
Otherwise your only bringing 1 leg off the transformer to the stat.
"Otherwise your only bringing 1 leg off the transformer to the stat."That leg is COMMON to the heat, cooling (where part of the heating), and the blower manual.But I know that some of the terminal designation on HVAC is kinda weird..
William the Geezer, the sequel to Billy the Kid - Shoe
I thought you were describing eliminating 1 of the transformers.
Then you would tie (jumper) the cool and heat terminals, and you would have to run the 2nd leg of the xformer to the stat or between the units. That #2 leg is typicaly called the common.
I would think if they shared terminals that they would both run at the same time. one trying to heat the other trying to cool. right?
No only share the common. You have a separate heat call and cooling call..
William the Geezer, the sequel to Billy the Kid - Shoe
They only share one terminal.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Boiler has a red and a white only. AC has a red, yellow...and green? Not near it tonight, but for sure, the only shared terminal would be the red.
Even if the red is OK as a shared terminal, what about the programmed cycling of the thermostat? I know that the settings for this are different for a boiler than a furnace.
I forget which is the common terminal on the AC. Basically you'd tie the AC common to one (arbitrarily selected) wire from the boiler.Not sure I know what you mean by "programmed cycling". A programmable thermostat generally has a separate heat/cool switch that decides which side is activated, regardless of the program.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Boiler has a red and a white only.
Red to Rh................ White to white (call to heat)
AC has a red, yellow...and green
Red to Rc........ Yellow to yellow (call to cool) Green to fan or G terminal.
You probably got a micro switch or jumper for Gas or Elec. Furnace. Switch it to Gas position, or fan will come on when heat is called.
They only share one terminal.
Which one is that??
I don't remember the color code. If I were doing it I'd buzz out the wires.
As I stood before the gates I realized that I never want to be as certain about anything as were the people who built this place. --Rabbi Sheila Peltz, on her visit to Auschwitz
Go for it.
Just make sure there is no jumper between Rh and Rc.
It's a common setup for independent units.