There are some applications where a single furnace cannot deliver enough BTU’s or CFM to heat a space. When these applications arise, a simple solution may be to put in 2 furnaces and “twin” them together. This allows both furnaces to operate off of 1 thermostat and be tied into a single duct system.
There are some simple rules that need to be followed to accomplish this:
- When twinning furnaces — you need to have identical twins
- same model, BTU, CFM capacity
- all motor leads must be on the same taps
- all control board settings must be the same
- Boards must be identical part numbers
- or the boards will not communicate properly with each other
- the power to the furnaces must be off the same buss bar in the fuse or circuit breaker panel
- Twinned furnaces must be applied on a common return and supply duct system.
- Isolation relays need to be installed on the “slave” furnace to isolate transformers.
- Only PSC motor equipped furnaces can be twinned
- Furnaces with ECM motors cannot be twinned
One error a lot of people make is assuming that the blower capacity is “additive”. When furnaces are twinned, the combined blower capacity is only 85% of the total CFM produced by the 2 blowers. Where this becomes a problem is when contractors think they have two 5-ton blowers so they can put a 10 ton air conditioner on them. Since the capacity is reduced 15% when twinned, those two 5 ton blowers will only be able to deliver 3400 CFM — enough for a 7 1/2 ton application.
If all of the above “rules” are followed, successful twinning of furnaces will work very well and provide higher heat capacity and blower capacity to heat the space.
On our furnaces, twinning is made easy without any additional controls. On each board is a terminal marked “twin”. By running a single wire between the twin terminal on each board and running a wire between the “C” or common terminal on each board, the furnaces are now twinned and will operate in unison.
You can provide additional comfort by using a 2-stage thermostat and operate the heating separately — keeping in mind that both blowers will always be running on any call from the thermostat. And, if you use 2-stage furnaces, you can use the staging timer to actually provide 4 stages of heating.
This is a very simplified version of furnace twinning but it should give you he basics to assure proper operation. On www.upgnet.com is a Service Tips sheet that goes into more detail and has all the wiring diagrams needed to do multi-staging of the furnaces and for adding air conditioning to the application. It covers both single stage furnaces and 2-stage furnaces.
The most important thing to remember though is the fact that everything be identical and, if you do that, the job should go smoothly.
One last thing to mention — when servicing twinned furnaces and you need to replace a control board — don’t forget to replace the other board at the same time or make sure that the new board is the same as the one you are replacing. Even for service, twinned furnaces must have identical components.