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Thread: problem?

  1. #1
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    Default problem?

    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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  2. #2
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    Default Re: problem?

    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.


  3. #3
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    Default Re: problem?

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

    Jerry Peck, Construction / Litigation Consultant
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  4. #4
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    Default Re: problem?

    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!


  5. #5
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    Default Re: problem?

    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.

    Department of Redundancy Department
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  6. #6
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    Default Re: problem?

    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....


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    Cool Re: problem?

    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

    Keep the fire in the fireplace.

  8. #8
    Frank Kunselman's Avatar
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    Default Re: problem?

    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.


  9. #9
    Mike Huppi's Avatar
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    Default Re: problem?

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

    Thanks


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    Smile Re: problem?

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

    Keep the fire in the fireplace.

  11. #11
    David Banks's Avatar
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    Default Re: problem?

    Quote Originally Posted by Mike Huppi View Post
    Can someone post a rule, law, or manufactures installation instructions on the 12" rise that is brought up so often.

    Thanks

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


  12. #12
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    Default Re: problem?

    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.


  13. #13
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    Default Re: problem?

    See post "Myth Buster" in plumbing section.

    "There is no exception to the rule that every rule has an exception." -James Thurber, writer and cartoonist (1894-1961)
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  14. #14
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    Thumbs down Re: problem?

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

    Jerry McCarthy
    Building Code/ Construction Consultant

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    Default Re: problem?

    Quote Originally Posted by dan orourke View Post
    I've looked in several manufacutre guidelines - - not there....so where does it come from? I've search the archives , 'lots of discussion about it , but no code posting....sounds like another inspector myth going on here!

    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.

    Jerry Peck, Construction / Litigation Consultant
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  16. #16
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    Default Re: problem?

    Quote Originally Posted by Jerry Peck View Post

    What more do you, and John, want?
    Me? I was just pointing to a previous thread on the topic. I'm innocent, I tell ya!

    "There is no exception to the rule that every rule has an exception." -James Thurber, writer and cartoonist (1894-1961)
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  17. #17
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    Default Re: problem?

    Quote Originally Posted by John Arnold View Post
    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.

    Jerry Peck, Construction / Litigation Consultant
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  18. #18
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    Default Re: problem?

    Quote Originally Posted by Jerry Peck View Post
    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.
    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.

    "There is no exception to the rule that every rule has an exception." -James Thurber, writer and cartoonist (1894-1961)
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  19. #19
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    Default Re: problem?

    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.


  20. #20
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    Default Re: problem?

    Quote Originally Posted by Frank Kunselman View Post
    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...".
    Actually, it states:

    - 503.10.4 Two or more appliances connected to a single vent.
    Where two or more vent connectors enter a common gas vent, chimney flue, or single-wall metal pipe, the smaller connector shall enter at the highest level consistent with the available headroom or clearance to combustible material. Vent connectors serving Category I appliances shall not be connected to any portion of a mechanical draft system operating under positive static pressure, such as those serving Category III or IV appliances.

    Go all the way back and read and review Appendix B, SIZING OF VENTING SYSTEMS SERVING APPLIANCES EQUIPPED WITH DRAFT HOODS, CATEGORY I APPLIANCES, AND APPLIANCES LISTED FOR USE WITH TYPE B VENTS


    That clearly shows, states, and sizes vents for natural draft appliances in combination with fan assisted vent appliances.


    Jerry Peck, Construction / Litigation Consultant
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  21. #21
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    Default Re: problem?

    Frank, just because the furnace's inducer pressurizes the vent on pre or post purge doesn't make it a Cat. III or IV appliance so no, it is not positive vent pressure as far as the codes are concerned. ANSI Stds allow up to 45 seconds of spillage before it is termed "backdrafting". Still a serious problem but I wanted you to be able to keep the terminology and therefore, code references straight.

    As Jerry pointed out, the smaller vent is expected to enter higher than the larger appliance. This is to allow more vent rise so it has built up a head of steam before it tries to enter the common vent.
    HTH

    Keep the fire in the fireplace.

  22. #22
    Frank Kunselman's Avatar
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    Default Re: problem?

    Bob, thanks for the reply...I missed Jerry's post. I agree that the WH vent should enter higher for that reason. Thank you for the ANSI note.


  23. #23
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    Smile Re: problem?

    Hey Frank, that's what we're all here for--to learn from each other. I learn from you guys every day.

    Take care,
    Bob

    Keep the fire in the fireplace.

  24. #24
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    Default Re: problem?

    John,

    I got my fat letter from Dewey, Cheatum and Howe.

    I think they included half of the fee they charged you and told me to 'just go away'.

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

  25. #25
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    Default Re: problem?

    "There is nothing specific in the codes about using a wye versus a tee (actually, it is a "siamese" but who's counting?)."

    Bob, what makes a "tee" a "siamese"?

    Thanks


  26. #26
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    Smile Siamese

    Technically, a wye is where one pipe splits into two while a siamese is two pipes merging into one. This is taught in Fire Service hydraulics. I'm sure if you go into any HVAC supply house, they and their catalogues will refer to a true siamese as a wye. It's in our lexicon that way.

    HTH,
    Bob

    Keep the fire in the fireplace.

  27. #27
    Kevin VanderWarf's Avatar
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    Default Re: problem?

    I certainly don't like "T" fittings in a flu line, and wonder why the make them.
    Drafting may be a problem, but something else may be going on if both the water heater and furnace have induced drafts.
    If the induced draft from water heater is on when the furnace starts it could very well be that the furnace fan is free wheeling just enough to break the pressure switch. Usually during ignition, the control board must detect a break before a make on the pressure switch or else it will lock out the sequence and leave the induced draft fan running.
    I have seen this in the field before, usually a wieghted damper is added.


  28. #28
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    Default Re: problem?

    Quote Originally Posted by Gunnar Alquist View Post
    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.

    Gunnar, the wye versus tee fitting is a bunk argument sadly enough.
    It is an excuse that many of us HVAC guys use when we don't really understand what's going on.

    This type of installation is allowed by code but should be banned IMHO as it is a timebomb waiting to go off.

    Guess there is not much of a concern to just intermittently poison someone, a little CO never hurt anybody right?

    Any installation that allows the inducer fan to effect the water heaters ability to draft should be tagged & shut off IMO as it is unsafe.

    Many inducer fans in furnaces are capable of pulling over an inch of pressure while the pressure switches in that very same furnace are only rated at .30 to .40 inches of water column.

    If the vent cap becomes plugged for any reason that furnace will keep right on running dumping flue products out the drafthood of the water heater.

    Just something to think about.

    Measured Performance more than just a buzzword

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