75g Water heater not venting properly

Discussion in 'Water Heater Forum, Tanks' started by masswhh, May 30, 2021.

  1. masswhh

    masswhh New Member

    Joined:
    May 30, 2021
    Location:
    Boston
    Hello,

    6 months ago I had my old 40 gallon water heater replaced with a new 75 gallon water heater. About 6 months after the install I bought a device called AWAIR Element and plugged it in the basement. To my shock the device is showing spikes of Co2 to 3800 PPM and TVOCs at 4800 PPB.

    The HVAC company that did the install came out and found small leak in the cone shaped vent that sits directly over the WH. They replaced this but the problem persisted. At this point they advised I did not have adequate air movement in the basement for the WH to properly vent.

    They presented a solution to install a "Fan in the Can" system, ultimately its 30 feet of pipe with an in-line fan that will turn on whenever my WH turns on to help it vent properly. They also said they would upgrade the exhaust pipe from the 3" it is currently to 4".

    I'd like to get some expert opinions on this situation. Does everything outlined above sound legitimate?

    As an aside I've done a bit of googling and from what I've read it seems 75 gallon water heaters should exhaust to a 4" pipe. If thats true, I'm a little bothered because they did not replace the exhaust pipe when they installed the new water heater, it was simply added under the existing 3" exhaust. If what I've outlined is correct, I'm curious to get an expert's opinion on whether the "fan in can" system is really needed, or if I just need to have the exhaust pipe upgraded to 4".

    Thank you in advance for your response!
     
  2. Terry

    Terry Administrator Staff Member

    Joined:
    Aug 17, 2004
    Occupation:
    Plumber
    Location:
    Bothell, Washington
    The 40 gallon gas water heater was about 38,000 BTU and the 75 gallon gas is 75,000 BTU
    The 40 gallon was using a 3" chimney and the 75 requires a 4". I think I see a problem here. Are you finding that the family is getting more headaches now?

    Pipe area
    3" = 7.069 square inches for 38,000 BTU
    4" = 12.57 square inches for 75,000 BTU

    I would either install a new 4" chimney or find another water heater.

    Is this what they would be installing for air supply?
    https://www.fieldcontrols.com/fan-in-a-can-cas-3-oil-and-cas-4-gas/
     
    Last edited: May 30, 2021
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  4. phog

    phog Active Member

    Joined:
    Jul 29, 2017
    Location:
    Rochester NY
    First, absolutely upgrade to 4". That's definitely a big problem, flue undersizing will cause flue spillage, which is exactly what you're seeing. And 4" is definitely minimum required for your appliance.

    Second, regarding the make-up air duct + fan. It sounds like you're describing something that will force-assist inside air from upstairs down into the basement. I'm skeptical that anything like that will be needed.

    I would start with a 4" B-vent chimney install (which can be pricey) and see if that doesn't fix the problem before taking any further steps. Or as Terry mentioned above get a water heater with a much smaller burner.

    I'm surprised this isn't something your plumber discussed with you before installing that water heater in the first place. Most plumbers around here would quote the 4" chimney as required part of the install and flat refuse to do the job if you asked them to skip the chimney upgrade and plug & chug only the water heater. (And you are now finding out why unfortunately)
     
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  5. fitter30

    fitter30 Well-Known Member

    Joined:
    Feb 2, 2020
    Occupation:
    Retired service tech
    Location:
    Peace valley missouri
    Take a pick of the water heater flue connection, and termination at chimney or flue. Does the flue pipe run uphill from the heater?
     
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  6. Master Plumber Mark

    Master Plumber Mark Sensitivity trainer and plumber of mens souls

    Joined:
    Feb 6, 2005
    Occupation:
    Sensitivity trainer.. and plumber of mens souls
    Location:
    indianapolis indiana - land of the free, home of
    Terry has probably named the game here already.... they threw in the 75 and
    it only has a flu pipe for smaller units...

    If the plumber who did the original 75 gallon heater tied it into a 3 inch chimmney,
    they should be held responsible for this mess....

    Sometimes you cannot upgrade the flu pipe to accomidate the larger size heater...
    You might have to force them to install a 75 gallon POWER VENTED heater and capp
    off that old line....
     
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  7. masswhh

    masswhh New Member

    Joined:
    May 30, 2021
    Location:
    Boston
    Hi Terry!

    Thank you for the thoughtful response.

    Yes, the device you linked is exactly what is being recommended for an air supply.
     
  8. masswhh

    masswhh New Member

    Joined:
    May 30, 2021
    Location:
    Boston
    Thank you for the response. A chimney upgrade was never mentioned by the HVAC company.
     
  9. WorthFlorida

    WorthFlorida The wife is still training me.

    Joined:
    Oct 28, 2009
    Occupation:
    Retired
    Location:
    Orlando, Florida
    In the mean time I would crack open a window or two in the basement to allow fresh air to enter. What is isn't quite clear is it 3" all the way to the roof or just 3" to a brick chimney or metal chimney?
     
  10. Clog

    Clog Member

    Joined:
    Jan 28, 2019
    Location:
    California
    I would insist on the entire 3" vent being replaced. It is likely corroded inside anyway, and due for replacement even if new water heater was the same size as the original.

    But given that the new water heater has twice the BTU output, to heat twice the number of gallons of water in the tank, it was inappropriate for the installer to reuse the existing 3" flue.

    Does the water heater share any upper portion of it's flue with a furnace? Does the existing 3" flue stub or Y into a 6" flue for both appliances? If so, what is the angle of entry?

    When the water heater flue is replaced to 4", a best practice is to have the first foot of flue venting above the vent cap / draft hood be straight vertical. "Provide at least 12" vertical inches of flue vent between the draft hood outlet and the first elbow or sloping flue vent connector."

    Since bends (such as elbows) restrict flow, having a bend immediately above the draft hood has some effect on the flow volume/velocity of exhaust gasses, so water heater manufacturers recommend that the first 12" above the draft hood be straight up, and thus non restrictive, for the best draft effect. I see a lot of professional plumbers miss this detail, and while the 12" rise may or may not be written in the Plumbing Code of the authority having jurisdiction, all codes require that a gas appliance be installed in accordance with the appliance manufacturer's instructions, and I have yet to read a water heater instruction manual that didn't have the 12" rise recommendation.
     
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