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Gas Vent Chimney Changes

Chuck_17 Member Posts: 138
To make sure I am clear:
If, for example, a boiler is removed from a chimney and a water heater remains on the chimney:
If the chimney area is greater than 7X the area of the water heater vent, a correctly sized chimney liner should be installed.
Is that the short answer?

An example where this comes up commonly, say a boiler in a church is replaced with a direct vent (taken off the chimney), and all that remains on the chimney is a residential water heater. Old churches tend to have large chimneys.


  • STEVEusaPA
    STEVEusaPA Member Posts: 6,373
    The short answer is any orphaned water heater is going to need a liner as the chimney will be way oversized if it was originally serving 2 appliances.
    EBEBRATT-Ed Member Posts: 14,495
    Yes you need a liner as @STEVEusaPA said
  • captainco
    captainco Member Posts: 768
    Code says you need a liner but science says you don't! Don't tell the water heater the boiler left it alone.
  • MikeL_2
    MikeL_2 Member Posts: 470
    I've found many cases of chimneys being oversized during warm months, usually resulting in periods of flue gas spillage.
  • icy78
    icy78 Member Posts: 404
    Think about this.....for all those years, what modulated the chimney size when the boiler wasn't running but the water heater was???

    Why do you now have to change the chimney size?
  • MikeL_2
    MikeL_2 Member Posts: 470
    My theory is the quest for energy efficiency and motivation to tighten the envelope affected natural draft. Homes weren't as tight in the past and chimneys acted as a thermal syphon drawing in and exhaling abundant infiltration.
  • MikeL_2
    MikeL_2 Member Posts: 470
    And, there's the dynamic of fossil fueled equipment competing for combustion air with mechanical devices that need make - up air. My mothers kitchen never had a ducted, exhausted vent hood and neither did her bathroom.
  • captainco
    captainco Member Posts: 768
    Now the real problem is exposed, competing for combustion air. But that has always been a problem that chimneys have been blamed for. Making chimneys smaller does not correct the problem. It might hide it but not correct it.
    Why does the 7X rule only apply to two pieces of equipment common vented but not three or more?
    Why does the smallest equipment must exit first rule apply to only two pieces of equipment common vented but not three or more?
  • Chuck_17
    Chuck_17 Member Posts: 138
    Thanks for the comments.

    I also want to know of any detrimental effects to the chimney - e.g. condensation in the chimney.

    The point about the water heater always having operated when the boilers were not operating is a good one.
    On the other hand, these chimneys were made for large old boilers 100 years ago and there was no 40MBH water heater connected to it then.

    Combustion air is not an issue. The boilers are now direct vent and it is a big old boiler room.
    Sort of related - I am involved with two projects, literally right now, that have atmospheric fired water heaters in a small laundry room (with a dryer - in one case is gas fired).
  • captainco
    captainco Member Posts: 768
    Draft allows equipment to vent. Draft is created by the temperature difference of the flue gases versus outside temperature. Draft is also created by pressure difference inside and out (positive goes to negative). Unfortunately Code in most cases has us making the mechanical room negative therefore we have to depend on the temperature difference.
    I realized 40 years ago, equipment with drafthoods are not connected to chimneys, just near by. Also, drafthoods reduce the flue gas temperatures which make them harder to vent and cause condensation. In the past 40 years I have recommended to my customers and students to replace the drafthood with a double acting gas barometric and a safety spill switch and they were amazed how well that worked. No liner ever needed unless the chimney was in bad shape. Plus we added safety that wasn't there.
    Chimneys are just like ducts, the bigger, the better things flow through them.