“Twinning” Furnaces

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.


About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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22 Responses to “Twinning” Furnaces

  1. Andre says:

    What happens when you twin four DX fan coils together? Is each blower only going to provide 60% of rated CFM?

  2. David says:

    Great article; some (newbie) questions:

    1. Just got an estimate that seems to make the exact mistake you mention:
    “Replace two 5 Ton Furnaces with twinning kit…
    10 Ton Condenser 13 SEER 3 phase, 208/230 R410A
    2- 5 Ton twinned indoor evap. coils R-410A
    2- 5 Ton twinned 80% furnaces
    (no mention of
    What can go wrong if they install this? poor airflow >> icing on coils? Or…?
    (any chance you can explain/show why the combined airflow is less?)

    2. Do I understand correctly that using a 2-stage thermostat to “operate the heating separately” means it will fire the burners separately, but both fans will run, so the airflow will be the same? Is that because if only the firing burner’s fan was on, you might get slippage between sides in the AHU and overheating/high limit trips? or…? Seems inefficient to be delivering the same velocity of air at only modest deltaT as at full load…?

    3. Why can’t ECM furnaces be twinned?

    Thanks in advance for your help!

  3. David says:

    I believe your ~twinning-york-furnaces-rc.pdf answers #2; and the answer to #3 is just because (most) burners’ gas flow can’t follow the variable air flow of an ECM…right?

    Still interested in the answer to #1, and why two contractors would propose something that’s wrong/dangerous?

    • that is something you would have to ask them. My only thought is they just didn’t know

      • David says:

        thanks…but what can go wrong if it’s installed as proposed?

        Surely there’s no rule that you can’t install equal (or more) cooling capacity than heating capacity, or what would happen, e.g., across the southern US states, where cooling demands are clearly higher? Seems it’s just a matter of setting the different fan speeds for heating and cooling correctly, regardless of single or twinned setup…or am I missing something obvious?

      • because 2 separate fans are running simultaneously, there is some ‘turbulance” of the air in the plenum which is why there is reduced capacity. This is why you lose 15% 0f the rated cfm. This can cause problems with coils icing due to lack of air. It also reduces the TSP the unit can handle

  4. R.J. says:


  5. Richard says:

    Thoughts on twinning: Four (4) gas furnaces, 100,000 BTU 2000 CFM with 5 TON A/Cs for a total of 20 TON A/C. Question? What are the pros and cons of installation?

    • it won’t work. (1). twinning means 2 not 4. (2). when furnaces are twinned, they lose 15 percent of their cfm so even though the blower is a 5 ton blower, when the furnaces are tweinned you do not get 4000 CFM you get 3400 cfm.

  6. Frank Blair says:


  7. Jehad Ochana says:

    1- Furnaces shall be the identical, ok, but what about A/C’s, can we twin two identical furnaces with 4 ton and 5 tone units.?
    2- if one burner only is On, does it effect whole system performance since the supply temperature will be less.?
    3- if one air conditioner only is On, does it effect the humidity in cooling mode.?
    4- Code in Ontario ask furnace motor to be ECM, in this case we can’t twin furnaces in Ontario

    • 1. No — A/c must be the same since you are using both blowers at the same timm. 2. if using a 2 stage t-stat and only 1 furnace is running, both blowers will be still running. Air temp will be lower but will function ok. 3. it could affect the humidity control since you are moving more air with both blowers running. 4.you are correct — ECM motors CANNOTbe twinnned.

  8. Mike Blue says:

    When twinning the units do you have to use just one ac unit?

    • Yes, with the coil mounted above both furnaces with enough room for the air to mix and enter the coil. If you need staging, use a 2-stage air conditioner. Remember when furnaces are twinned, any call from the thermostat brings on both blowers simultaneously

  9. Jose says:

    I am working on twined York furnaces . With Johnson automated controls . Two of the twined five ton systems work the other two give me twin error code. When I rest power they run but when cycle they won’t come back on. I think they at twined wrong. Opened control panel and they have all heat wires all cool fans and hot together only one wire goes to terminal post one of the 24volts need help

    • I strongly suggest you contact your local distributor for assistance. When furnaces are twinned, they must be “identical” twins. Even the power to the furnaces needs to be off the same buss bar in the fues/circuit breaker panel. You could look at the furnaces that are working, and if they have the same controls, compare the wiring between those and the ones giving you trouble.

    • Jose says:

      Fixed all wiring in control panel still have a 9 flash error code wrong twinning or bad grounding.Do I need relays to twin this York furnaces 80% 100000 btu.Tried wiring to twin spade from furnace to furnace and common to common Got it to turn on but slave furnaces blower motor slows down and ramps up and I can hear aclicking noise coming from that furnace control board. Tried calling tech support but left message no call back.Did find one furnace is on a different breaker panel than the other 3 ?

      • As I said last time, all twinning must be identical. If the twinning error is on the furnace on the different panel, try getting it on to the same buss bar as the others. Are all the furnaces the EXACT same model number?
        The furnace with the ” clucking” noise and the motor ramping could be a bad motor/board. All blowers should start and be set for the same speed.

  10. I have a customer that has twinned furnaces for 40 years and has replaced one furnace at a time such that one is a belt drive and one is a direct drive . these are hardly identical twins and have never had a problem.

    • John, I have no doubt about your comments. 40 years ago, the twinning of furnaces was done either with electromagnetic relays or a separate twinning kit that only used current sensing relays to monitor the blowers. Today’s furnaces use integrated circuit boards with the twinning capability built in. As a result, twinned units must be identical. Also, using 1 or 2 a/c units does not make a difference even today , that is a matter of choice and/or application.
      Thank you for your comment.

  11. Also one has ac and one does not. oNe has a standing pilot and one is intermittent pilot. Several zones as well.

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