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Side Wall Venting 80% Gas Furnace

regular_guy Member Posts: 3
I have my furnace in an enclosed space off the side of my balcony, so it’s not inside the living quarters. There is a large opening (with filter) in the wall between my living room and the outside enclosed space.

First question: If there is backdrafting, will CO come into my living room through that filtered opening?

The existing furnace is 20 years old and needs to be replaced. It is side wall vented WITHOUT a power venter. The previous owners were obviously not poisoned by CO during that 20 years.

Next question: Why do I need to add a power venter on the new furnace?

The new furnace is American Standard 80% fan assisted (induced draft blower).

The installation guide says, “IF DESIRED (my caps), a side wall termination can be accomplished through the use of an add-on draft inducer.”

“If desired” doesn’t sound like something that must be done.

Also, the install guide says, “EXCESSIVE COMBUSTION VENT PRESSURE OR FLUE BLOCKAGE. If pressure against the induced draft blower outlet becomes excessive, the pressure switch will shut off the gas valve until acceptable combustion pressure is again available.”

To my understanding, this means that if a backdraft condition occurs, the furnace will not run.

So how does the absence of an add-on power venter create a safety risk? The furnace already has a safety pressure switch.

Wouldn’t the power venter be needed only in the case of the furnace failing to fire because of the existing safety sensor being activated on a regular basis?

I’m not an HVAC guy. I’m just a regular guy finding conflicting information, and I’m trying to understand why some people say the power venter MUST be used with side wall venting.

Seems like it would only be necessary if the furnace keeps shutting down.

Any comments would be much appreciated…


    HVACNUT Member Posts: 5,251
    You won't be able to find an 85% gas furnace that doesnt have induced draft. It's not an "add on", its advancement in technology.
    The American Standard is a fine furnace. Get a service agreement and it will be maintained once a year, which it definitely should. Twice a year if A/C is part of the system.
    20 years ago, they probably didn't pay too much attention to combustion air with an atmospheric furnace. The furnace room needs 1 sq. in. of free air per 1,000 BTU input.
    If the furnace is 100K input, then it needs 100 sq. in. of air transfer. That doesnt equates to a 10×10 grill. It need to be free air space (not counting slats and screens).
    2 grills to cut in the door, or an exterior wall. 1, 12" from the ceiling, 1, 12" inches from the floor.
    Or a Fan in a Can.
    Also look into a steam humidifier for better comfort.
  • lchmb
    lchmb Member Posts: 2,997
    when it says "if desired" that is saying if you wish to side wall vent.. not if you desire to add the powervent unit. Find the install manual for the old unit, it probably required an external unit as well. Although this unit has an internal safety, the last thing you want to do is put your families life in the hands of a plastic switch which can fail...
  • Jack
    Jack Member Posts: 1,046
    In '92 when the Energy Specs were changed, furnace manuf tried to make one furnace for Cat I & III. It was a catastrophe. I won't bother you with the details, but suffice to say that they quickly went back to individual units per category. Lchmb is correct in his reading. For a sidewall termination your new furnace will require an after market power venter. I would suggest that you get a couple good CO detectors for the space regardless of how you vent it.
    HVACNUT Member Posts: 5,251
    You can't penetrate out the roof, or there's a floor above?
  • regular_guy
    regular_guy Member Posts: 3
    Thanks for the comments guys.

    HVAC NUT: can’t go out the roof. There are two floors above me.

    Can someone explain something to me?

    My furnace is outside off the side of my balcony. Inside I have a big vertical return duct in my living room a filter.

    When there is a backdraft, where does the CO in the flue gases go? Does it leak into the surrounding area of the furnace (which in this case is outside my living quarters)? Or does it actually enter the home through the ducts in each room?

    Thanks much…
  • Fred
    Fred Member Posts: 8,541
    Most likely it would backdraft into the area immediately around the furnace but then get picked up, to some degree by the furnace blower and circulated into the house.
  • lchmb
    lchmb Member Posts: 2,997
    in theory the unit should shut down if there is a backdraft..that's the job of the pressure switch. So any combustion issues should be kept to a minimum. With a pv the chance of this is even less likely.. again..that's why the added safeties are there..
  • Sal Santamaura
    Sal Santamaura Member Posts: 503

    ...The previous owners were obviously not poisoned by CO during that 20 years.

    Doesn't seem obvious to me. What evidence is there that they weren't subject to low-level CO during that period? Any UL-rated detector is designed to ignore it so first responders won't be disturbed by less than immediately life threatening conditions.
  • regular_guy
    regular_guy Member Posts: 3
    Ok, I’m all for safety, and I appreciate advice that errs on the side of extra safety. I’m actually leaning towards adding the power vent.

    But I still wonder if the safety issue is exaggerated.

    Low, or high, levels CO could also occur with a roof vent if there is backdrafting. Yet the power vent is not used with a roof scenario. So why is the furnace’s built in pressure switch adequate for a roof vent but not a side vent?

    The power vent adds complexity to the furnace operation and it only has a 1-year warranty (questionable quality, yet an expensive part). Can anyone comment on the reliability of Tjernlund power venters?
    HVACNUT Member Posts: 5,251
    You can easily solve the power vent dilemma by installing a 90+% furnace. They're not too much more complicated and as in with a 85% furnace, it needs yearly maintenance.
    I would make sure the manufacturer approves venting alterations.
  • lchmb
    lchmb Member Posts: 2,997
    Im guessing you cant go 90 plus due to a cold space install. A chimney creates a natural draft and again the pressure switch is still there to protect you. The inducer motor is not designed to over come the vent..it's just to get the flue gasses through the unit. Tjernlund is a a good unit. But like anything it will fail with time.. If you can, put up an asbestos chimney and be done with it... if not.. guess you dont have much choice..
    EBEBRATT-Ed Member Posts: 13,790
    Bottom line is the furnace must be installed and vented in accordance with the install manual. I feel the venter is mandatory with a sidewall vent.

    If the furnace is truly outside the living space you are reasonably safe if you make sure any return ductwork in the furnace room is well sealed and that the furnace has combustion air provided to it coming from the outdoors
  • Bob Harper
    Bob Harper Member Posts: 985
    An 80% furnace must be vented vertically unless it is approved and uses a sidewall power venter that places the entire vent under negative vent pressure. Inducers and power vent aftermarket kits cannot be used unless the venting is listed for use with positive vent pressure. Install a CAT IV furnace in a weatherized closet so the condensate doesn't freeze. Get an unlisted low level CO monitor. UL listed CO alarms are junk.