Can an 80% Furnace be Side Wall Vented?

The answer is  YES and NO! An 80% efficient furnace CAN be side wall vented per National Fuel as Code 7.3.4 with the use of a “mechanical draft system of either forced or induced draft design”.

So what does this mean?  You CANNOT just take the vent off an 80% efficient furnace and run it out a side wall.  You need to use an auxiliary power inducer. Both Field Control and Tjernlund make these kits.Which ever you choose to use, it must include the following features:

  1. a pressure switch, in order to prove the inducer is operating before allowing the furnace to operate.
  2. A barometric damper must be included, this is necessary for 2 reasons:
    1. The combustion air flow through the furnace must be controlled or its efficiency will decline.  The barometric damper will prevent pulling additional excess air through the furnace.
    2. The other reason for installing the barometric damper is to allow dilution air to be pulled into the vent.  This additional air helps prevent condensation from forming in the B-Vent.
  3. The entire vent system must be run in B-Vent from the furnace outlet to the side wall induced draft fan.
  4. An inducer post purge cycle must also be included to allow venting of all the products of combustion at the end of the heating cycle.

In applying auxiliary power inducers be sure that these 4 features are included and exactly follow the manufacturer’s application and installation instructions.

In a lot of case, where a vertical vent is not possible, the use of the side wall auxiliary kit will work.  Also, keep in mind, that all 90+% efficient furnace can be side wall vented with just PVC pipe and the need for all of the above is not necessary.  In some cases, it is more economical to upgrade to a 90+% efficient furnace than to add all of the equipment and B-vent needed to side wall vent an 80% efficient furnace.

For some information you can go to:  or


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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4 Responses to Can an 80% Furnace be Side Wall Vented?

  1. Bob says:

    Just a clarification here: under no circumstances may the vent be under positive flue gas pressure before the termination. CAT I venting is strictly negative vent pressure. B-vent is listed only for negative vent pressure. There are aftermarket kits that mount on a CAT I appliance to blow the exhaust out under positive vent pressure but these are illegal. The appliance is not listed for this application nor is the venting, which must be listed for positive vent pressure. Only sidewall mounted fans that generate negative vent pressure that suck the exhaust out are legal if the above requirements are met. HTH,

  2. ROD VIAR says:

    how is modine doing it if it is not legal?

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