hooking up Pellet stove with Relay in dual transformer system

[jpstallings]

Hello sorry if this question has been asked before but I can't find it on the forums.

I am trying to install a thermostat using My Harman P43 as first stage heating and my furnace as the second stage. I am using a 24v relay to close the millivolt circuit that controls the pellet stove.

unfortunately, my HVAC system is a dual transformer system and the furnace transformer has only RH and W contacts. The air conditioning transformer has RC, C, Y, G, and W (W is unused as it isn't connected to furnace). I have attached a wiring diagram. I need to close the W1 circuit to activate the relay but there is no way to close it on the furnace board without turning on the furnace

Can I attach the other end of the relay to W on the HVAC transformer? thanks for your help!

 

[jpstallings]

I've been thinking about it some more and one option I see would be jumping RC and RH. this should allow me to return W1 to the C on the HVAC controller with no issue. the only problem is whether this would short circuit the two transformers. they're both 24vac though.

 

[Former Farmer]

What thermostat are you using? You would need a 2 stage or more. You are going to want one that you can set your differentials between the heating stages.

 

[jpstallings]

I have an ecobee lite 3. It has w1 and w2 hookups so I thought it should handle two stage.

 

[Don2222]

I have an ecobee lite 3. It has w1 and w2 hookups so I thought it should handle two stage.

Hello
Here is my take on this as I have done some research.
In the HVAC world where you have a 2 stage setup. This 2 stage setup was designed for heating large spaces where one furnace does not have enough capacity to heat the entire area. Therefore when more heat is needed stage 2 kicks on the second furnace to give the required heat. This is usually not what most people want with a pellet heater and an HVAC furnace. They want the pellet stove to provide heat and if it fails to use the HVAC furnace as a backup system. To accomplish this there is no need for any physical connection between them. In my case I just set the T-Stat on my Harman to 72 Degrees F and the T-Stat on the furnace to 68 Degrees F.
However if the plan is to use the HVAC furnace heats one are of the home and the pellet stove heats a different part of the home, then their -T-Stats can both be set for a comfortable 70 Degrees. Although many insurance companies will not accept a pellet stove for use as the only Sole heat for any portion of a home.
Just my 2 cents.

 

[Former Farmer]

Did you try connecting to W1 and C? I would try that before jumping RC and RH.

 

[Dataman]

I agree with poster that said just set it. I have mine at 74f and forget about it. On Auto it shuts down when not needed (Harmon XXV). Furnace is at 68f. It won't come on unless I run out of pellets (gone for couple days). Simple. Your making it Hard.

 

[Former Farmer]

Have you thought about running two relays? You could use C and W1 to control the Harman, C and W2 to control the furnace which would have the RH and W wires connected to it from the furnace. This way, you wouldn't intermix the two transformers.

You may have to put a jumper between RH and RC.