hooking up Pellet stove with Relay in dual transformer system

jpstallings

New Member
Jun 13, 2020
3
davenport, ny
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!
wiring thermostats.png
 

jpstallings

New Member
Jun 13, 2020
3
davenport, ny
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

Minister of Fire
Apr 12, 2008
520
NE Wisconsin
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.
 

Don2222

Minister of Fire
Feb 1, 2010
8,590
Salem NH
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

Minister of Fire
Apr 12, 2008
520
NE Wisconsin
Did you try connecting to W1 and C? I would try that before jumping RC and RH.
 

Dataman

Minister of Fire
Sep 10, 2018
735
Newport, Wa
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.
 
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Former Farmer

Minister of Fire
Apr 12, 2008
520
NE Wisconsin
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.