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airheatman, HVAC Technician
Category: HVAC
Satisfied Customers: 8383
Experience:  Furnace, A/C and heat pump specialist.
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I have a very similar question to I am trying to run 2

Customer Question

I have a very similar question to I am trying to run 2 furnaces off one thermostat. I have one side working but the other won't fire up. I am a 500 sqft coffee shop that used to be two 250 sqft units. I used to have two separate thermostats but I want to combine so the furnances are controlled together. The space is so small, I could probably get away with one of these giant furnaces but would have to connect the ducts (which is feasible, if it were the right solution). Just wondering how to approach/fix my problem. Two MagicAire Model 60-bvw/bvx-a. The run off the apartments buildings cold/hot water. Every fall we switch from air conditioning to heat and vice versa in the spring.
Submitted: 1 year ago.
Category: HVAC
Expert:  airheatman replied 1 year ago.

It is not possible to run two furnaces using one thermostat unless you have a twinning kit to do this.The reason is your thermostat gets its power from the transformer in one furnace.

Customer: replied 1 year ago.
do you have a recommendation as to what twinning kit to buy
Expert:  airheatman replied 1 year ago.

Let me see if I can find any information on this that would be a feasible solution.Notice in drawing below the 24v transformer supplies voltage to the contactor and breaks one leg with thermostat.

Expert:  airheatman replied 1 year ago.

OK it does not appear there is a "twinning kit"designed for this unit.

However here is a solution that will work.

Each furnace has a transformer that provides low voltage to controls. Currently each furnace is independent.

So to control both units with one thermostat, we will remove the transformer from one unit and replace the other transformer with a larger 80VA transformer capable of controlling relays in both units.

I have made you a drawing explaining this.

From new larger transformer , run a common to all relays Diagram shows the fan relay.If you have other relays such as solenoid valves to open valves for heat or cool, they will need the same common from this new transformer.

Red (24 volt HOT) will go to R at thermostat.

Then from G (blower) Green will go to relay in both units.

This way on call for unit,we get voltage to relays in both units to bring on both units.

Let me know your thoughts and any questions

Expert:  airheatman replied 1 year ago.

Oops that was first pic. Here is the new pic showing how to do this