help wiring a 2nd thermostat

[Vinced]

Hello,
I just installed heat exchangers in the plentums of the gas furnaces in my house and shop. I want to install a 2nd thermostat to run the blower and heat off the exchanger from my OWB and leave my original thermostat to work normally has a back up. I have a Luxaire G9V 2-stage variable-speed gas furnace in the house and a York 92% efficient single stage burner and single blower speed. I looked into each furnace and if I jump terminals R and W the blower starts up. Can I just install a 2 wire thermostat to these terminals and leave my other thermostats in place? I did a search on this and read something about needing another relay with a built in transformer? I don't understand why I'd need this because terminals r and w are already 24 volt. Please help.
Thank you, Vince

 

[BrownianHeatingTech]

Hello,
I just installed heat exchangers in the plentums of the gas furnaces in my house and shop. I want to install a 2nd thermostat to run the blower and heat off the exchanger from my OWB and leave my original thermostat to work normally has a back up. I have a Luxaire G9V 2-stage variable-speed gas furnace in the house and a York 92% efficient single stage burner and single blower speed. I looked into each furnace and if I jump terminals R and W the blower starts up. Can I just install a 2 wire thermostat to these terminals and leave my other thermostats in place? I did a search on this and read something about needing another relay with a built in transformer? I don't understand why I'd need this because terminals r and w are already 24 volt. Please help.
Thank you, Vince

R is power. Connect R to W, and you will activate heat. Connect R to G and you will activate the fan, but not the heat.

For a small extra cost, it may be worthwhile to get a 2-stage thermostat. That way, backup always comes on a set number of degrees below the wood system, with no chance of both systems being activated accidentally.

Joe

 

[Vinced]

Thanks for the info. Can you recommend a brand and model of thermostat to use. I looked for a thermostat like that locally,but did't find any.
Thank you, Vince

 

[BrownianHeatingTech]

Thanks for the info. Can you recommend a brand and model of thermostat to use. I looked for a thermostat like that locally,but did't find any.
Thank you, Vince

Personally, I use the Honeywell TH6220D for basic two-stage systems.

I'm sure every major thermostat make has at least one two-stage model. Just a matter of finding out what is available locally. I've had good luck with the Honeywell FocusPro (TH6xx0) line, as it is a good combination of features, programmability, and affordability.

Joe

 

[stee6043]

I was just considering the same question here. If both systems were to try to "turn on the fan" at same time would you damage the furnace fan circuit or perhaps fry both thermostats?

 

[BrownianHeatingTech]

I was just considering the same question here. If both systems were to try to "turn on the fan" at same time would you damage the furnace fan circuit or perhaps fry both thermostats?

The thermostat is just a switch, so as long as the systems are using the furnace's transformer, the same power is being applied - closing multiple switches on the same circuit does not affect anything.

If you have different systems sharing a thermostat, it is important to match the power (for example, a 24VAC system and a 5VDC system cannot share the same thermostat directly - an isolation relay would be needed). In many cases, the thermostat has separate heating and cooling circuits, and the fan is often part of the cooling side, so simply removing the jumper between the Rc and Rh terminals would solve the issue (presuming you don't actually intend to use the cooling circuit for cooling). Even if you use 24VAC control throughout, if the heat and fan signals are connecting to different controls served by different transformers, it may cause issues, depending upon the specifics of those transformers - removing the transformers from the controls and installing a single, standalone transformer to power all the systems can solve that issue (however, you should label any system other than the one which supplies switched power to the transformer with something along the lines of "multiple power sources - this switch does not fully power down equipment" so that a service tech does not unwittingly burn out a control by shorting a wire s/he believed to be unpowered).

Joe

 

[Vinced]

Joe,
Thanks for the t-stat numbers. I'll see what I can find locally.
Thank you, Vince

 

[Redox]

A caution if you have central air or a heat pump: Jumping the R/G terminal may bring on your compressor if you have a cheap thermostat that has an AUTO/ON switch. The better programmable stats have separate output relays for the fan and the compressor and won't do this.

Jumping R and G will activate the cooling speed on the blower and this may be too much air for heating. I would look to see if the furnace has a terminal for continuous fan operation. This will be the lowest fan speed available and will still allow for high speed operation if it is needed. It may be a 120V connection but a relay would solve this problem.

Chris

 

[MooseMan]

I've got a similar project I'm looking at --

We just bought a house that has a two-wire t-stat installation in the upstairs and a high-efficiency Amana furnace with two big cold air returns in the basement. There's also a big Timberline wood stove in the basement. The blower would be the best way to get all that hot air upstairs. Otherwise it mostly stays trapped in the basement.

Rather than run a wire (which looks to be a real pain) I'm thinking of installing a basic thermostat in the basement, set to cool with the idea that if the basement gets too hot - it activates the blower and heats the upstairs. If the tstat upstairs were to call for heat during this time the furnace would kick on and the blower switch would already be activated ("closed" I think) and it wouldn't cause any problems. Is that correct?

I need help with the wiring scheme for t-stat 2 on this. Would it be R and G? Assuming everything I get is 24v compatible, are there any risks?

Thanks!