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tying two furnaces together

I am looking at a job to replace a 160,000 BTU output gas fired furnace. I did the heat loss and this is the correct size that needs to go back in. The trouble I am having is finding a furnace that size. I was wondering if I put 2 80,000 BTU furnaces together, would that work as well? I know you can twin two furnaces together, but I have never done this.

How would I connect the duct work? I am thinking I will need to split the supply and return, so that the same volume of air flows through each furnace at the same time. I know to wire them, there is a terminal on the mother board for twinning, and my electrician should know how to do this.

If anybody knows somewhere I could find information about this, please let me know.

Thanks for your help.

R. Griggs


  • Mark A. CustisMark A. Custis Member Posts: 247
    Twinning is easy

    these days. Get two 80's and set them side by side. The sheet metal out may need to be a pair of pants, and the sheet metal in (return air side) may need to get split and set to the outside intake of each of the twins. Most all furnaces come with Twinning plugs on the IFC (intergated furnace control). Just hook 'em up. (Read the installation insructions, your furnace may vary from what I typed.)

    Now you can be a hero and offer two stage heating and two stage cooling. While you are stroking the twins consider Zoning, unless the hole building needs to be the same temp all the time, all at once.


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  • harveyharvey Member Posts: 153
    no problem

    WE twin furnaces all the time and it works fine. You will have to return the furnaces from the bottom. We connect the supply plenums separate to the main trunk. Another neat way to use two furnaces is to run them 1st stage/2nd stage. wiring is different but is worth it. WE've been doing the 1st/2nd stage thing for seven years and have never had a problem. Of course use a two stage stat
  • Mike T., Swampeast MOMike T., Swampeast MO Member Posts: 6,928
    Dumb question

    When you twin a furnace like that do you always run BOTH fans? If you don't wouldn't it affect the "throw" from the registers significantly?
  • JimGPEJimGPE Member Posts: 22
    Not so dumb....

    If you run only one fan, not only will the throw be affected, but you might back-wheel the fan in the other furnace. You'd need a tight backdraft damper to keep this from happening.

    Even when running both at the same time, they'd better start together, or the first will get the second spinning backwards. When the second one starts, it will either keep running backwards, or will draw a LOT of amps (heat up a lot) trying to stop the backwheeling and get going in the right direction.
  • harveyharvey Member Posts: 153

    the first system we installed in 1996 I was afaid of the wheel spinning backwards and possible problems. We have about 25 1st/2nd stage installations since then and not one problem with the blowers. HSI yes blower motors no
  • robert griggsrobert griggs Member Posts: 65
    sizing furnaces for twinning

    I was just wondering, do the furnaces have to be the same size as each other, or could one be say 100,000 and the other 60,000? Would it work just as well?

    For sizing the air conditioning, would I split the load just like for heating? Would I use 2- 2 1/2 ton systems to equal a 5 ton output?

    Thanks for everybody's responses. They have been helpful.
  • Mark A. CustisMark A. Custis Member Posts: 247
    Never done it, but why? or

    why not.

    I am being taught to move BTUs with water by these guys, my "tin card" tells me you could mix and match, but to what end. Sounds to me that the job needs a very hard look at zoning.

    If you have a 400 HP motor in your car in NYC, what are your chances of seeing it red line?

    Chances are if you need to twin forced air equipement to satisfy a need you can zone and save the money in fuel.


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  • clammyclammy Member Posts: 1,995

    i've done this before usally i'll make a duct 10 x30 heavy gauge sit the units on it with internal angle supports and pull return from the bottom then just bring the existing return into usally work well because the new units are only 40 inches high i also do it for all my furnaces with over a 4 ton blower.i've also used 2 a coils and staged the cooling .never make the duct less then 10 inches
    your indoor fan may go off on temp.this also makes sure your unit gets enough return on larger unit direction tell to cut out both returns.the return base is alot easier and keeps the job neat
  • Twinning Furnaces

    When you are setting up furnaces for heating and the BTU demand is higher than one furnace can satisfy you have to twin them up. Example 200,000 BTU's replace with two 100,000 BTU units. They must be same company same controls. If you hook them up you use one thermostat and wire it to furnace #1 you then use an isolation relay to bring in furnace # 2. There are twinning connections on the Electronic Fan Timers or Integrated Furnace Controls. On some they are labeled Z1 and Z2, they must be connected together so that the system blowers will come on together. That will prevent running one backwards.

    It is an option when it is necessary to do this that you can stage the units. Furnace one becomes stage #1 R to W1 on thermostat and then furnace two becomes stage # 2 R to W2 which brings in the isolation relay for furnace two.

    I have a number of manuals and guides which outline very clearly how this is done and also the controls involved.

    The printed circuit boards used on these units are not however interchangeable. If I were a homeowner I would have a couple of spare boards hanging around for future use.
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