Two pipe furnace, intake not terminated outside

[David Meiland]

A friend sent me this photo of his furnace, in a vented crawl in Berkeley, CA. He temporarily re-tasked the condensate pump to deal with storm water accumulating in the crawl space in the middle of the night, but promises to get a proper sump pump installed ASAP... but I digress. What I noticed is that the intake pipe is not routed to the outside--it's terminated in the crawl. My comment to him was that this certainly was not correct, that the manufacturer probably stipulated that both pipes be terminated outside and close to each other (possibly in the same fitting), and that pressure differentials at the two terminals could be an issue, although probably not a pressing one.

Comments?

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[Erby]

The furnace sucks combustion air from the vented crawl space (which then draws more air in through the vents. The combustion air gets spit back outside the crawl space. I doubt there is that much of a big deal about it. A lot of them are installed IN the house drawing combustion air from inside the house too.

[Robert Jones]

I have seen this in both attic and crawl space cavities. I didn't notice any issue(s) with the operation of the furnace(s).

[Bill Kibbel]

I've read a lot of manufacturers' instructions and I don't think any absolutely require the intake be piped to the exterior - provided there is plenty of air inside for combustion. Many manufacturers strongly recommend it and state the benefits. Check the installation instructions. The manual is supposed to be kept at the furnace.

[mjr6550]

It is permitted according to some manufacturer's manuals. I have seeno where this changes the discharge pipe clearance from 1.5 feet to 4 feet.

[kurt]

Interior air for combustion and dilution is allowed so long as all other requirements for installation are met. Local restrictions may apply.

The crawlspace part is kinda lousy though. It would warrant a comment if I was writing a report, even if there's no fine print prohibiting it.

[plummen]

Im guessing theres lots of humidity in the crawl space which I wouldnt consider good for the internals of the furnace.

But as long as theres enough air available for combustion I dont see it be a code issue,Although it does lower the efficiency rating of the furnace if it is using previously heated air for combustion.

[Mike Lamb]

I agree damp air combustion can't be good for the furnace. I look up the manufacturer's instructions for the furnace in these situations and report what they say. Carrier says no to combustion air supplied from crawl spaces.

I talked to a Carrier tech many years ago and he said with a non-direct vent set up you lose about 2% efficiency.