Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Flue Problem

I installed a new water heater and used a 3 to 4 inch adapter to connect it to the existing 4 inch flue. The existing 4 inch flue has an ameri-therm automatic damper. This worked fine.

The next day, I decided it would be better to replace the 4 inch flue with all 3 inch one. So, I got rid of the damper and the adapter and made the whole flue 3 inches in diameter. When I turned the gas on, the exhaust and a whole lot of heat backed up into the room and melted the silicone grommet around the the input water pipe on the top of the unit..

Is my problem removing the damper or is my problem reducing the size of the flue to 3 inches? The new heater has a 3 inch exhaust.

I don't know how weird this is, but in my house the water heater flue ties into a stand pipe that is about 2 feet in diameter. The stand pipe goes up and out through the roof. I am wondering if the large diameter causes some overpressure in the flue forcing the exhaust from the water heater back into the house.

Any thoughts?



  • Ironman
    Ironman Member Posts: 6,868
    What is connected...

    To the large flue? How many Btu's is it?

    I can't imagine anything in a single family house that would require a flue that large. In fact, most commercial bldgs. don't have any thing that big.

    I would put it back the way it was - with the damper. It worked that way and a short run of 4" isn't going to make a difference on a water heater.

    You might want to have a knowledgeable heating contractor check your entire vent system.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • Charlie from wmass
    Charlie from wmass Member Posts: 4,216
    edited October 2010
    FLue is too large

    There is a minimum input as well as a maximum for chimneys. Get a professional in there before we are reading the CO section about you please.
    Cost is what you spend , value is what you get.

    cell # 413-841-6726
  • John_Moffett
    John_Moffett Member Posts: 4
    Flue Problem

    BTU is 38,000.

    I attached a picture of the flue as it is now. From the heater, there is a 3"-4" adapter, a damper, then a run up to the chimney. I will take your advice and get a professional to come look.

  • John_Moffett
    John_Moffett Member Posts: 4

    The way it is in the picture is the way I have it now and it is working well.
  • John_Moffett
    John_Moffett Member Posts: 4
    It's the chimney size

    I think that my water heater was orphaned at some point and left to be the only thing venting into the chimney. The chimney has about a 94 sq in cross section which is too big for a 3 in flue to tie into. This explains why my 4 in flue works and the 3 in does not. I think that I should abandon the chimney altogether and make my water heater flue go strait up and out the roof.

    At the following site:http://www.askthebuilder.com/205_Water_Heater_Venting.shtml

    I wound this guide:

    "You can do a quick check of your chimney to see if it meets an old rule of thumb and some of the current codes. Measure the diameter of the exhaust pipe leaving your hot water heater. If it is a 3 inch diameter pipe, the inside cross sectional area of the chimney flue liner should not exceed 49 square inches. If the exhaust pipe is 4 inches in diameter, the flue liner cross sectional area should not be larger than 88 square inches."
This discussion has been closed.