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Clearance for supply/exhaust


elgato
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I have a client who is concerned about heavy snow restricting air flow and shutting down his furnace. This is a propane high efficiency hot air furnace with PVC ducts to the exterior for combustion air and exhaust. The ducts exit the foundation about 16" above grade on the west side of the home. Drifting snow could easily build up. Any solutions? Can extensions be placed on the PVC piping?

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Hi,

Seems to me, it would be a simple thing to put a couple of elbows and extensions on those guys to extend them above the anticipated drift height.

I'm not so sure about the exhaust melting the snow. Didn't someone post a photo here last year of a huge icicle that had frozen over the end of one of these things?

OT - OF!!!

M.

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All the spec's I've seen say something like "outlet shall terminate 12" above the highest likely snow level", whatever that means.

Either way, it would be easy to fix if it was, in fact, a problem w/snow.

Les, do you remember "Who's Next", the seminal Who album from about 1970?

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Originally posted by kurt

All the spec's I've seen say something like "outlet shall terminate 12" above the highest likely snow level", whatever that means.

Kurt,

I actually ended up researching this a while back, Chicago's greatest recorded snow depth was 29 inches of snow on Jan. 14, 1979, so to meet the 12" rule in Chicago the vent terminal should be at least 41 inches above horizontal external surfaces.

IMO, the problem is not so much "snow" as the accumulation of frozen condensation in wet snow, I've seen this happen when the furnace has been off for a few hours in a "setback" period and exhaust passed through a down-turned elbow above a roof - the blockage was not total, but it was enough to trip the back-pressure safety switch.

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My ASHI chapter had a presentation from a local gas company. They stated that the minimum volume necessary to supply sufficient combustion air is 50 cubic feet per 1000 BTU's. If this is true, I don'y know that I've ever seen sufficient volume provided for combustion without an external source. Can anyone confirm or deny this 50/1000 rule?

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Sure, if they derive their combustion air from the interior of the home and it meets the Btu/Hr requirement, why not? Also, I see furnaces and water heaters installed in large garages that meet the minimum volume requirement. I also see plenty of them in both of these categories that don't meet the minimum volume requirements. It's something that we need to be looking at.

OT - OF!!!

M.

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Originally posted by mthomas1

Originally posted by kurt

All the spec's I've seen say something like "outlet shall terminate 12" above the highest likely snow level", whatever that means.

Kurt,

I actually ended up researching this a while back, Chicago's greatest recorded snow depth was 29 inches of snow on Jan. 14, 1979, so to meet the 12" rule in Chicago the vent terminal should be at least 41 inches above horizontal external surfaces.

That assumes a lack of snow drifts. You know we get 'em 3'-4' high, if not more, easy.

Back as a service manager for a builder, time when the first generation high-efficiency furnaces were installed, I got a call from a neighbor to a house we built that water was coming out of the front door. Turns out the homeowner was in Florida, and didn't know the *low* exterior flue our heating "expert" installed was covered with a snow drift and the furnace couldn't start. That, and the below-zero temps told the story; frozen and burst pipes throughout.

As I recall, the house was a week out of warranty.

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Originally posted by elgato

My ASHI chapter had a presentation from a local gas company. They stated that the minimum volume necessary to supply sufficient combustion air is 50 cubic feet per 1000 BTU's. If this is true, I don'y know that I've ever seen sufficient volume provided for combustion without an external source. Can anyone confirm or deny this 50/1000 rule?

He's correct. It's actually 50 cubic feet of volume per 1000 btus of all appliances in the space if the building is of ordinary tightness. (Whatever that means.) That's not an excessively large space. Let's say you have a 100,000 btu furnace and a 40,000 btu water heater, that's 140,000 btu. Divide 140,000 by 1,000 and multiply the result by 50. That gives you 7,000 cubic feet. If the space has 8-foot tall ceilings, that means you need an 875 square-foot space.

- Jim Katen, Oregon

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Originally posted by Les

Kurt,

Yep, I do seem to recall "Who's Next", along with only 5-6 of the other posters!

I trust you were applying the music to this thread!

Actually, no. I was applying the album cover "art" to your little peeing commentary; the album jacket had a picture of the 4 members of the band all peeing on a large concrete monolith. It was the ne plus ultra album cover for obnoxious little 18 year olds.

Killer album; Baba O'Riley, Won't Get Fooled Again, Love Ain't for Keeping, Behind Blue Eyes, etc., etc.....

Confined vs. Unconfined space is the terminology for this conundrum, no? If the space meets the minimum cubic volume, it's considered unconfined space; if it doesn't, it's confined and additional combustion air source(s) must be provided.

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Nope,

You're confusing flue size formula with Btu/Hr volume requirements. Jim's right; total the input Btu/Hr of all appliances in the space, divide by 1000 and multiply by 50. That gives you the minimum acceptable volume. For folks back east where they have lots of basements and the laundry shares spaces in the basement with a furnace and water heater, this includes any gas dryer installed in the same space.

OT - OF!!!

M.

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The 50 cubic feet per 1K BTU formula is to determine if it is a confined space or not if you have less than this much volume supplying MUA to the appliance(s) you must provide MUA per the code.

Regarding extending the Cat. IV vent terminations: you must check the mfrs. listed instructions. Since elbows cause pressure drop, many mfrs. limit the pvc runs to three 90degree ells along with X number of lineal feet.

Yes, you must allow for snow drifts. If you know one side of the house gets drifts really bad, you may have to terminate on another side of the house. The state of Taxachoosetts now requires a min. 4ft above grade in the wake of a 7 y/o girl's death from snow blockage. Normally, the vacuum switch and hi limit switches would have tripped but the vent within the wall was disconnected so it vented into the wall cavity without tripping the safeties. Same thing can happen with a draft hood equipped water heater sharing the vent with a furnace. The WH can become the vent relief for the furance and it will be perfectly happy venting out the WH draft hood in the event of a blockage. No safeties on most WH draft hoods....

Another problem with making PVC periscopes is freeze-ups. The condensation can totally block the vent termination. Again, check with the mfr. and check your clearances to intakes and windows.

HTH,

Hearthman

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The 50 cubic feet per 1K BTU formula is to determine if it is a confined space or not if you have less than this much volume supplying MUA to the appliance(s) you must provide MUA per the code. Cat. IV furnaces provide their own MUA so this formula does not apply. The sizing of the pvc vent intake is part of the listing by the mfr.

Regarding extending the Cat. IV vent terminations: you must check the mfrs. listed instructions. Since elbows cause pressure drop, many mfrs. limit the pvc runs to three 90degree ells along with X number of lineal feet.

Yes, you must allow for snow drifts. If you know one side of the house gets drifts really bad, you may have to terminate on another side of the house. The state of Taxachoosetts now requires a min. 4ft above grade in the wake of a 7 y/o girl's death from snow blockage. Normally, the vacuum switch and hi limit switches would have tripped but the vent within the wall was disconnected so it vented into the wall cavity without tripping the safeties. Same thing can happen with a draft hood equipped water heater sharing the vent with a furnace. The WH can become the vent relief for the furance and it will be perfectly happy venting out the WH draft hood in the event of a blockage. No safeties on most WH draft hoods....

Another problem with making PVC periscopes is freeze-ups. The condensation can totally block the vent termination. Again, check with the mfr. and check your clearances to intakes and windows.

HTH,

Hearthman

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