Single Thermostat Needed for Seperate Heating/AC Units

[Butrbal]

I have quite an old heating/AC system with two individual units/thermostats.

The AC unit is all contained outside the home (blower within), with the large inlet/outlet ducts (~12" in diameter each).
The Heating unit (electric) is a separate unit all contained inside the home (also with its own blower).

I currently have two thermostats on the wall, one of them for the AC system, the other for the heating system. They are both within a few feet of their respective return registers.
It would be fairly easy for me to get the thermostat wires from the heating system to the current location of the AC thermostat.

I need to know of there is a single thermostat on the market that will operate both systems so I don't have to fiddle with two thermostats on days when heat is needed at night and AC is needed during the day.
Yes I live in the SOUTH. lol

 

[LS6 Tommy]

You can use pretty much any heat/cool T-stat, but you will need to use an isolation relay on one of the two units. You can't run one stat for both. The transformers will "buck" eachother.

Tommy

 

[brewchief]

How many wires to each stat? Lots of stats have a jumper between rh and rc for this very issue but the heat unit will need to yse only r and w for it to work, if the heat unit also needs a green wire to bring on the fan it will take some creative wiring to make it work.

Sent from my SM-G900V using Tapatalk

 

[Butrbal]

brewchief,
I'll get some pictures of the wires going to each thermostat and post them this afternoon.

 

[LS6 Tommy]

How many wires to each stat? Lots of stats have a jumper between rh and rc for this very issue but the heat unit will need to yse only r and w for it to work, if the heat unit also needs a green wire to bring on the fan it will take some creative wiring to make it work.

Sent from my SM-G900V using Tapatalk

That's why I mentioned the isolation relay. Both units have blowers. You can have two xformers on one stat without one, but many times you have "bucking". Basically, you wire the heat side of the t-stat to energize the coil of a relay that makes the "W" circuit on the heating unit. That way only the A/C unit xformer is running through the t-stat.

Tommy

 

[Rockhead261]

1. No isolation relays are needed.

2. Be sure your electric heat thermostat is not a line voltage thermostat. You need to have 24 VAC control wiring in order to combine the systems on one stat.

3. Provided the electric heat has a 24 VAC control circuit, go out and purchase any thermostat that has separate Rh and Rc terminals AND permits configuration for electric heat. Most heatpump thermostats will accommodate what you want. THIS is a good example of what you need.

4. If you're not perfectly comfortable wiring it, hire a tech to install it.

 

[nehog]

1. No isolation relays are needed.

2. Be sure your electric heat thermostat is not a line voltage thermostat. You need to have 24 VAC control wiring in order to combine the systems on one stat.

3. Provided the electric heat has a 24 VAC control circuit, go out and purchase any thermostat that has separate Rh and Rc terminals AND permits configuration for electric heat. Most heatpump thermostats will accommodate what you want. THIS is a good example of what you need.

4. If you're not perfectly comfortable wiring it, hire a tech to install it.

*this*

A common configuration, but you do need to make sure that it is installed correctly.

 

[Butrbal]

Well I cant upload my pictures because they are larger than the allowable file size, so I'll just list the colors of the wires.

Heater Thermostat:
1 white wire
1 red wire

AC Thermostat
1 white wire on same terminal as 1 green wire
1 red wire

I didn't think about checking the voltages until recently reading some of the previous comments.

Can I check the voltage of the heating system at the thermostat or will I need to go to the unit to do that?

 

[Rockhead261]

Well I cant upload my pictures because they are larger than the allowable file size, so I'll just list the colors of the wires.

Heater Thermostat:
1 white wire
1 red wire

AC Thermostat
1 white wire on same terminal as 1 green wire
1 red wire

I didn't think about checking the voltages until recently reading some of the previous comments.

Can I check the voltage of the heating system at the thermostat or will I need to go to the unit to do that?

What size are the heat wires?

 

[zmaxmotorsports]

You can use pretty much any heat/cool T-stat, but you will need to use an isolation relay on one of the two units. You can't run one stat for both. The transformers will "buck" eachother.

Tommy

Id eliminate the transformer on the outdoor unit,and use the indoor stat to control the contactor like a newer unit.
The 2 transformer systems were common back in the 70s,too many moving parts to break down.

 

[LS6 Tommy]

Id eliminate the transformer on the outdoor unit,and use the indoor stat to control the contactor like a newer unit.
The 2 transformer systems were common back in the 70s,too many moving parts to break down.

That works, too, as long as the xformer VA is enough. Might have to upsize.

Tommy

 

[75gmck25]

I have a Buderus gas boiler for my radiators and a Bryant central A/C unit to cool the house. They are both connected to the same Honeywell thermostat, and its just a standard setback model from Home Depot. There are separate thermostat wires running to each unit, but no isolation relays or other devices to make it work.

Bruce

 

[Butrbal]

1. No isolation relays are needed.

2. Be sure your electric heat thermostat is not a line voltage thermostat. You need to have 24 VAC control wiring in order to combine the systems on one stat.

3. Provided the electric heat has a 24 VAC control circuit, go out and purchase any thermostat that has separate Rh and Rc terminals AND permits configuration for electric heat. Most heatpump thermostats will accommodate what you want. THIS is a good example of what you need.

4. If you're not perfectly comfortable wiring it, hire a tech to install it.

How do I verify that the heating system is 24volts?
Is it as simple as checking the voltage between the red and white wires at the heating thermostat?
Do I also need to verify the cooling system is also 24volts?

 

[Butrbal]

What size are the heat wires?

Without measuring the diameter of the conductor, I would estimate the wire including insulation is approx. the diameter of the lead in a No 2 pencil.

 

[Butrbal]

You can use pretty much any heat/cool T-stat, but you will need to use an isolation relay on one of the two units. You can't run one stat for both. The transformers will "buck" eachother.

Tommy

How many wires to each stat? Lots of stats have a jumper between rh and rc for this very issue but the heat unit will need to yse only r and w for it to work, if the heat unit also needs a green wire to bring on the fan it will take some creative wiring to make it work.

Sent from my SM-G900V using Tapatalk

1. No isolation relays are needed.

2. Be sure your electric heat thermostat is not a line voltage thermostat. You need to have 24 VAC control wiring in order to combine the systems on one stat.

3. Provided the electric heat has a 24 VAC control circuit, go out and purchase any thermostat that has separate Rh and Rc terminals AND permits configuration for electric heat. Most heatpump thermostats will accommodate what you want. THIS is a good example of what you need.

4. If you're not perfectly comfortable wiring it, hire a tech to install it.

Alright guys, I have verified the voltages at both thermostats.

I'm getting 27 VAC at the heating thermostat & 30 VAC at the cooling thermostat.

My research has lead me to the following conclusion.

I need a thermostat that I can install in a 5 wire configuration,
with the following terminals: Rh - heating transformer / W - heating relay / Rc - cooling transformer / Y - cooling relay / G - cooling fan

With my wires connected as follows:
Heating - red wire to Rh / white wire to W
Cooling - red wire to Rc / white wire to Y / green wire to G
What do ya think?

 

[Butrbal]

can anyone provide some assistance?

 

[LS6 Tommy]

Alright guys, I have verified the voltages at both thermostats.

I'm getting 27 VAC at the heating thermostat & 30 VAC at the cooling thermostat.

My research has lead me to the following conclusion.

I need a thermostat that I can install in a 5 wire configuration,
with the following terminals: Rh - heating transformer / W - heating relay / Rc - cooling transformer / Y - cooling relay / G - cooling fan

With my wires connected as follows:
Heating - red wire to Rh / white wire to W
Cooling - red wire to Rc / white wire to Y / green wire to G
What do ya think?

Your wiring sounds technically correct. Normally you would use the yellow wire for the cooling "Y" terminal, but if all you have is 3 conductor R/W/G, it's fine. The machine doesn't know what color the wire is...

Tommy

 

[Rockhead261]

Your voltages are good... just wanted to be sure you weren't fiddling with line voltage.

Here's your wire colors from your earlier post:

Heater Thermostat:
1 white wire
1 red wire

AC Thermostat
1 white wire on same terminal as 1 green wire
1 red wire

It breaks down like this:

Your red heat wire is Rh (heat [power)
Your white heat wire is W (call for heat)
Your red A/C wire is Rc (A/C power)
Your white A/C wire is Y (call for A/C)
Your green A/C wire is G (fan)

Sounds like you have it under control. Just be sure to remove any jumper that might be in place between the Rc and Rh terminals.