I'm already calling out this furnace install but wanted to see what you guys think of this....

New construction, forced air gas furance - gas water heater exhaust joins a couple feet up the furance flue.

First questionable thing is the junction is a 90 degree 'T' and the furnace venting is perfectly vertical, meaning the water heater vent pipe has no slope as it joins the furance exhaust. When I fired the furance it didn't start but the induction fan did. When the water heater firing with this induction fan all (or a good portion) of the exhaust from the water heater was back drafting into the garage. Now, I can't say I've ever checked the drafting of the water heater with ONLY the induction fan running but is this at all normal to have backdrafting, even only temporary (as it would be if the furance actually fired)??

The use of the 'T' fitting really just seems wrong. To me it seems as though the installer planned to just barely meet the standard right from the start. There was plenty of room to elevate the water heater venting more and join the furance vent with a more favorable junction piece. Honestly, I'm not up to date on all of the precise venting codes but from what I have learned this is just not right... Anyway, there's a picture below.... I'd love to hear any thoughts... thanks!

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I have no thoughts on the "T" but the water heater flue pipe should have 12" of vertical rise prior to angling towards the main flue pipe. That could be the cause of the backdrafting at the water heater draft hood.

It's also possible that the tee and the vent from there up needs to be one size larger.

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Jerry Peck, Construction / Litigation Consultant
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )

Thanks for the replies guys.... I've heard the 1 foot vertical thing but for some reason in my area the installers don't do it that way and the city lets it go. I see them like this everyday. I see the 90 degree 'T' fairly often but it's usually when the furance vent is not vertical so the overall venting of the water heater is going up. Are either of you guys aware if it's ever okay to have that backdraft for a few seconds when the induction fan is on but the furance is not? I can't imagine it's okay but I'd hate to hang my whole case on that and be wrong... thanks!

Matt,

I too have had water heaters backdraft when the furnace inducer fan starts up. I have asked HVAC guys about this and the answer that I usually get is the tee is wrong and it should be a wye (when the furnace has an induced draft). However, I have yet to find a specific reference in the installation instructions or the code. The HVAC guys have not been able to give me a code reference either.

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The apostrophe troll.

Thanks, It's just so unfortuante that often times the installers set out to just barely meet the code. The result, inevitably, is us standing there with a micrometer trying to determine if they squeaked by....

There is nothing specific in the codes about using a wye versus a tee (actually, it is a "siamese" but who's counting?). If you look in the design manuals of most B-vent mfrs., you will see a similar configuration shown. Yes, having the water heater enter at a little bit of an angle helps in some cases but.....

The ANSI Stds. allow up to 45 seconds of "spillage". After that, it is termed "backdrafting", which is a legal term, if you get my drift. Yes, the std. allows the pre-purge to pressurize the vent for that short period and yes, you may get CO spillage. The codes figure the small amt. of CO spilled will quickly dilute out to where it isn't a problem. There is a growing concern by some very astute practitioners such as DavidR, an HVAC expert who visits this forum, that fan assisted Cat.I appliances should never be allowed to common vent with water heaters. There are many reasons for this but the first is that the draft hood provides a relied opening for spillage. A double acting barometric damper with a spill switch interlocked to the gas controls is a method gaining popularity, though it may not always be recognized.

The pic shown shows no draft hood connector, B-vent larger than draft hood, no 12" rise, no vent support, plenty of room for more vent rise, and is that common vent oversized?
Bob

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disgusted with some people on this forum. Out of here!

The problem is exactly what you stated...backdrafting. The mechanical draft furnace vent needs to be above the water heater vent. When properly configured, the water heater will draft properly by being drawn by the mechanical draft, not a secondary outlet for the furnace as currently and improperly installed.

The proper configuration can be found in the appendix of the gas code. It needs to be changed as it is a code violation as well as a CO hazard.

Dead men tell no lies. I saved a family's life many years ago who had an improperly vented water heater. Their basement had 3,000 times the max CO limit when the gas co arrived...and they just thought they were overly tired from a hard days work.

Can someone post a rule, law, or manufactures installation instructions on the 12" rise that is brought up so often.

Thanks

Frank, can you post what sections of the code you are quoting from or an illustration?
Thanks,
Bob

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disgusted with some people on this forum. Out of here!

Mike this was recently discussed at nausea. Do a search.

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Dave

The picture in the 'Code Check' series shows a 2' necessary rise so who knows?

I've seen city inspectors in my area pass water heater venting that goes right out the top to a 90 and then 1/4" per foot rise (barely) for 10 feet or more to a chimney. It seems crazy and its certainly far from ideal but they let it fly. I really club my buyer over the head with the CO detector thing and will write up the installation as barely acceptable, etc.. I feel it's all I can really do since the installers and the city inspectors won't budge.

See post "Myth Buster" in plumbing section.

what about 2006 UMC 802.3.4.4? (NFPA 54:12.4.3.4)

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Jerry McCarthy
Construction Consultant
www.expertbuildingconsultants.com

Dan,

I, and others, have posted excerpts from manufacturers installation instructions which state that 12" height.

We've done it until we are blue in the face.

What more do you, and John, want?

We've taken you to the water, we've all but drowned you in it.

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Jerry Peck, Construction / Litigation Consultant
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )

Me? I was just pointing to a previous thread on the topic. I'm innocent, I tell ya!

Oops, then!

I thought you were one of the ones defending those who insist on having a Presidential Declaration before thinking it was so.

My apologies.

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Jerry Peck, Construction / Litigation Consultant
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )

I may be traumatized and in need of extended treatment, but I accept your apologies and will keep you apprised of my progress toward complete healing.
cc Dewey, Cheatum and How, attorneys at law.

Bob,

I have to admit there are some sticky points to my comment. Under the Standard Gas Code 610.3.5 "A gas appliance vented by natural draft shall not be connected into a vent, chimney or vent connector on the discharge side of a mechanical flue exhauster."

The IFGC 503.10.4 - Two or more appliances connected to a single vent states "...Vent connectors serving category I appliances shall not be connected to any portion of a mechanical draft system operating under positive static pressure...".

My first impression of the description of backdrafting led me to think that the mechanical draft was operating under positive static pressure. If the furnace mechanical draft proves to be a negative static pressure the installation would be ok. Then the question would be has the vent above the WH connector been sized properly based on the combined BTU input and vent height.

As to the issue of backdrafting thru the WH draft hood, the vent system needs evaluation and make any necessary repairs.

Actually, it states:

- 503.10.4 Two or more appliances connected to a single vent.

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Jerry Peck, Construction / Litigation Consultant
Construction Litigation Consultants, LLC ( www.ConstructionLitigationConsultants.com )