Click here to Find a Contractor in your area.
In fairness to all, we don't discuss pricing on the Wall. Thanks for your cooperation.
Need to contact us? Visit

sharing a flue

johncjohnc Member Posts: 2
I have the opportunity to get a Wm CGi-4 gas fired boiler. This model is the one with a forced draft motor. This boiler is replacing an existing natural draft boiler of basically the same size. The boiler shares a common flue with a gas fired hot water heater. My question is will the forced draft of the boiler back feed into the hot water heater, and if so what can be done to prevent this? Currently the flue size for the hot water is 3in the boiler is 5in and the chimney liner is 6in. The hot water ties into the 5in in a T would a Y be preferred.     thanks for any help john


  • Tim McElwainTim McElwain Member Posts: 3,904
    That boiler can be vented into a chimney or a "B"

    vent as a category I appliance which NFPA 54 National Fuel Gas Code allows Fan Assisted to be vented with a natural draft appliance. I would vent them separately into the common vent.

    That boiler can also be vented side wall which it then becomes a Category III appliance and must be vented by itself
  • Mark EathertonMark Eatherton Member Posts: 5,584
    If it were me....

    I would get rid of the stand alone gas fired tank and place an indirect or reverse indirect DHW heater. If that is not an option, I would install a manual reset spill switch at the draft hood of the water heater and wire it in series with the TT connection going to the boiler. This way, if the flue products start back spilling out of the water heaters draft hood, the bigger of the two appliances is shut down. Probably wouldn't be a bad idea to put one in for the DHW heater as well...

    Regardless of what you decide to do, make SURE you have functional CO detectors throughout the home.

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Tim McElwainTim McElwain Member Posts: 3,904
    I agree with Mark

    it is always a great idea to install a spill switch (blocked vent switch) on the draft hood of any equipment today. That way in the event of a reverse chimney or back-draft all the equipment will be shut down. There are also issues with fan assisted and natural draft equipment being connected into the same horizontal "vent connector" with the fan assisted equipment causing venting problems with the natural draft appliance. A solution in some cases is to vent them separately into the common vent (vertical), but I would still install vent switches.

    Spillage of flue products back into the living space becomes an issue as those products are C02 (carbon dioxide) and hopefully small traces of CO (carbon monoxide) neither of which belong in your living environment. There should be "ZERO" parts per million of CO in your living environment at any time.

    With the problems with Fan Assisted with a natural draft appliance the smaller (typically water heater) has a pressure difference created at its draft hood causing products of combustion to spill back into the room when the fan assisted is running and especially an issue when both appliances are running. There is also an added concern with the new FVIR water heaters which run at very high stack temperatures and have also had spillage issues here locally in New England. Better to go with an indirect and get rid of those problems as Mark suggested.
This discussion has been closed.


It looks like you're new here. If you want to get involved, click one of these buttons!