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96% Furnace venting

Cooper1957 Member Posts: 4
edited December 2023 in Gas Heating
Hi new here. Although I have extensive HVAC experience (35yrs). It's all been commercial work. I'm replacing my current 18 yr old 90% furnace with a 80000 96% furnace.The existing exhaust vent is 3 inch, approx 20 ft with 4- 3 inch 90's to the outside ( of course).. According to the manual I can use the 3 inch because the exhaust will accommodate either 2 or 3 as long as I make the transition to the 3 inch as close to the furance as possible. I'll be using the basement for combustion air, the basement stays around 50 degrees in the winter. I have read that sometimes if the combustion air is below freezing it can cause noise from the furnace on startup during to the cold air. So I know this is a long question but does anyone see an issue with anything I have stated?.... I'm retired now and boy I don't miss the no heat calls when on standby... because the manager of big box store says it's cold....well 9 times out of 10 it's because the EMS ars are controlling the store and the store saves money when the setpoint is 67.
Thanks for your help...
Forgot the model number


  • pedmec
    pedmec Member Posts: 925
    Always make your transition from 2" to 3" vertical at the furnace. Having existing 3" works great for you as a 100K unit is where you would be required to have 3" pipe on your exhaust. But there are a couple of things you need to check. One being using basement air for combustion air is never a good thing. Depending on your basement you could eventually cause premature failure of your secondary heat exchanger by having dirty combustion air start plugging the heat exchanger. the condensation created in the secondary will cause the dirty air that traveled into the combustion chamber to muck it up.
    Another issue is whether the location you are putting the furnace in meets the requirements for combustion air. In Massachusetts where i'm from you need 50 cubic feet of area for every 1000 btu's. I don't know if your pulling a permit but in the city (Boston) you would fail if you had less than the requirements.
    The last thing is making sure your furnace is allowed by the manufacturer to use basement air for combustion air. Most manufacturers don't allow it now. some of the low end manufacturers will allow it because they want the sale. If they don't and you do it anyway will void the warranty.
    i always take my air from outside. I have seen to many heat exchanger failures from taking basement air. not only dirt but chemicals, lint, etc too. its easy enough to pipe your air intake outside.
    And not to really rain on your install be aware of code changes. no pvc use allowed in my state. although the manufacturer approves its use it is not allow for venting flue gases. this is a whole other topic for another thread.
  • Cooper1957
    Cooper1957 Member Posts: 4
    I appreciate your input. I'm in New Jersey. PVC piping for venting is acceptable according to code. The reason I know this is because even though I have done commercial work some of our big box stores may have split systems and I have seen PVC venting is quite a few stores. My old company always pulled permits, and never had a problem. The reason I'll be pulling combustion air from the basement is actually allowed according to the spec sheet. I do however understand your point of mucking up the HX. I will eventually put the combustion air outside. The problem I have is my outside walls are solid block it took all I had just to get the 3 inch out with a Hilti drill.
    I was much younger back then. Now I have 3 fractured discs in my back that why I retired, So I'm going to try a young man who can do the drilling for me, I don't know how things are where your at, but kids just don't seem to do grunt work in our field.
    I really do appreciate your advice!
  • pedmec
    pedmec Member Posts: 925
    I’m still plugging away. Hard to get kids in the field. 

    Pvc could be existing but here, if you do an upgrade we are required to install FGV pipe. No other options
    EBEBRATT-Ed Member Posts: 15,307
    @cooper 1957

    You can use PVC for the air intake if you go outside.

    Be cautious about PVC for venting and here is the reason. Most of the PVC was never tested or approved for venting. This did not stop the equipment mfgs from saying "PVC is ok for venting so everyone installed PVC

    This is becoming an issue the past 3 years or so.

    As far as I know the only PVC approved for venting is "IPEX" and it is required to be labeled on the pipe "approved for venting gas appliances"

    Just because PVC was installed in the past and permitted and inspected does not automatically mean it can be used or reused now it's only been the last few years this is becoming an issue.

    Most everyone is going to polypropylene now.
  • realliveplumber
    realliveplumber Member Posts: 349
    Be careful where your vent terminates with single pipe exhaust. It needs to be 4' away and/or 4' below from windows, etc.

    With 2 pipe (outdoor combustion air) you only need to be 12" away or below.

    PVC is acceptable, must be solid PVC. Cannot be cellular core.

    Some manufacturers require tees with drains near the unit.

    Where are you in NJ?