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Coverting gas to electric water heaters...flue issues?

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  • Coverting gas to electric water heaters...flue issues?

    Is there any flue size issues when you take out a water heater, and only leave a furnace using a flue sized for both? I have been told that the flue may be oversized when this occurs, and not draw correctly.

    Anyone familiar with this theory?

  • #2
    Re: Coverting gas to electric water heaters...flue issues?

    I don't know all the specs around this issue but I have a cousin who replaced a conventional furnace (which had a water heater in the same clay tile flue 8"x10") with a direct vent furnace. The gas company shut him down until the water heater had a smaller diameter flue put in (stainless) due to it now being oversized (and apparantly it wouldn't draw).

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    • #3
      Re: Coverting gas to electric water heaters...flue issues?

      Right, thats kind of what I am talking about. But a furnace can run all night during the winter months, and the water heater might fire up once. So isnt the flue oversized when the furnace is running and the water heater isnt? The flue is designed to handle both running at the same time.

      That does not happen all the time...so what gives?

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      • #4
        Re: Coverting gas to electric water heaters...flue issues?

        Well, what about the flip side of that discussion? From May to Sept the furnace doesn't run in Toronto, just the water heater...so that's ok? I had this discussion with the gas guy. He told me that's the code. Then went on to say cold air (from winter) is heavier (which is true) so more heat needs to go up a larger flue to keep the flue working. The water heater (38,000 BTU) alone wouldn't be able to do that with an oversized flue (and I don't wanna talk about condensation, which will ruin the oversized chimney if it runs cold, which is true).

        We will see what others have to say.....

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        • #5
          Re: Coverting gas to electric water heaters...flue issues?

          btu rating maybe? the furnace is typically going to pull a hell of a lot higher btu's than the water heater.

          As to the water heater running all summer with no furnace... eh screw it, lets just start roughing in 2 flues.
          No, it's not rocket science, it's plumbing and unlike rocket science it requires a license.

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          • #6
            Re: Coverting gas to electric water heaters...flue issues?

            You should still get enough draw for the furnace or the water heater to work, but you will get condesation somewhere in the chimney, this leads to acidic vapors depositing on the inside wall of the flue, and over time can eat through the brick, mortar, etc. I had this problem in a duplex I bought, in the Spring the wall on the first floor apartment would start to bubble and the paint would discolor, found out that the condesation was right in this area, so we had to have a 4 inch and a 5 inch liner installed, one for the furnaces and one for the water heaters. The smaller diameter liner piping seems to draw better, and we will see in the spring if the wall problem comes back.

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            • #7
              Re: Coverting gas to electric water heaters...flue issues?

              i remember on 1 project we were able to connect 3 heaters on the same floor to a common 5'' vent. 3'' into 4'' into 5'' up the wall.

              the pilot light will keep the vent warm to the touch out here.

              oversizing the vent is not legal, but if the furnace was originally a 4'' vent and the water heater was a 3'' the common vent would have been a 5 or 6''.

              how about all the 4'' water heater vents i see with only a 3'' 32,000 btu heater installed. they will still draft once they come up to temperature and that's quick as the pilot is already a warm air flow.

              what's the vent sizes and btu ratings of the w/h and furnace?

              rick.
              phoebe it is

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              • #8
                Re: Coverting gas to electric water heaters...flue issues?

                I do not see how that install could be a problem. The water heaters runs quite infrequently, and the furnace runs at regular intervals 24/7, so the flue has to be correct for the furnace alone.

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                • #9
                  Re: Coverting gas to electric water heaters...flue issues?

                  I talked to a heating guy today. Seems if the flue is on the exterior, then sizing might be an issue...due to condensation, which can rot the flue.

                  Case closed.

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