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I live in a 3-family house. I have 3 chimneys and want to eventually remove one of them. One of the furnaces in the basement is gravity venting through the chimney to be removed. Now I need to replace that furnace, and I think it would make sense to put a power vent in instead, so I don't need to go through a chimney and can vent through the wall.

I'm thinking of what happens if more than one appliance is running at one time, though. It could prevent the other chimneys from drafting properly by changing the air pressures.

Would I have to replace all the vents with power vents at the same time? Or is it safe to do just one?

  • Are there any other appliances using that flue? – ThreePhaseEel Nov 6 '19 at 0:00
  • @ThreePhaseEel Nope, no others on that particular flue. I'm just picturing the whole basement as kind of a plenum space and I'm imagining the one power venting appliance pulling fumes out of the other 2 chimneys. – kavisiegel Nov 6 '19 at 13:00
  • Most power vented things are, or can be setup for, sealed combustion, where the combustion air is drawn in directly from outside via a 2nd pipe, instead of using ambient air. Does the new furnace you're looking at support that, or have you not picked out the new hardware yet? – ThreePhaseEel Nov 7 '19 at 1:00
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Sealed combustion is your friend

Just about all power vented gas appliances can be configured as sealed combustion appliances, where they draw their combustion air directly from outside via a 2nd pipe instead of pulling it in from the room. This means that no matter what happens, as long as the vent integrity is good, they can't backdraft any atmospheric vented appliances that happen to be sharing the same room, nor will they have issues with starving for combustion air if the house is sealed up tight.

But...you need to use the right stuff when venting

The one caveat with most power-vented appliances is that the corrosive conditions inside the vent piping (due to the possible presence of liquid condensate from combustion moisture), along with the need to provide proper sealing to prevent combustion exhaust leakage, prevent the use of standard B-vent for venting them. While the appliance manufacturers generally specify the use of PVC pipe as a vent material as it's cheap and easy to get, PVC is actually the wrong stuff to use for venting appliance exhaust, as it's just not rated for a high enough temperature to survive if stack temperatures rise much due to impaired appliance operation, such as if a water heater has a scaled-up heat exchanger or if a furnace filter is in need of changing. In fact, there have been documented close calls, and a reported multi-fatality CO mishap, that were due to the use of PVC venting for condensing gas appliances.

The correct (albeit expensive) thing to use in this case is a UL 1738 rated Special Gas Vent, suitable for Category IV gas appliances. These are made from stainless steel, and thus provide both the necessary heat resistance and the necessary corrosion resistance for this application. While they are proprietary systems, and cannot be mixed, most manufacturers of Special Gas Vent provide comprehensive selections of fittings and sizes, and extensively warrant their systems against premature failure when professionally installed, up to and including some manufacturers offering limited lifetime warranties on their stainless steel special gas vent systems.

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