Gas water heater venting question...

woodgeek Posted By woodgeek, Dec 14, 2008 at 12:25 PM

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  1. woodgeek

    Minister of Fire 2.

    Jan 27, 2008
    SE PA
    I'm the the middle of an involved HVAC remodel to a 1960s split level that started with oil-fired hydronic heat, no central air, no hot water tank (tankless coil in boiler instead) and no NG service. My goal is to lose the oil.

    I had a air source heat pump installed with attic ducting, sized to carry demand down to typical january temps (4 tons for 2200 sq ft in a relatively mild climate). For space heating/cooling, very pleased with system.

    The next step is clearly the DHW, which is killing me. I have estimated ~200 gals oil/yr (half for water, half for standby losses) to get a system with a max output of ~2.5 gpm (e.g. you can shower as long as you like, but we have to 'take turns' with the usage). In other words $600-700/yr for a lousy demand DHW service.

    Original plan was to pull the boiler, and sit an electric tank DHW heater in the same spot in my finished basement laundry room.

    But, since I might run NG service to the house for other appliances, (running NG service turned out to be much cheaper than I expected) I am now wondering if I can use a NG DHW tank for lower operating cost, faster recovery, etc.

    (Not interested in any demand DHW systems, btw)

    So, my current boiler is vented through a 25' interior tile flue, which appears to have a 5.5" SS flex liner the whole way. When I pull the boiler, this vent will be unused. So a major issue is if I can use the existing, orphaned vent for a new, high-eff DHW heater. Power venting out the wall would be a pain, given the layout of the space. Can I direct or power vent out the chimney? Will I have condensation problems in the chimney? Drop a small liner down inside? Right now, the flex is mortared in 60" off the floor, so I would to bump the height anyway, or buy a shorty heater.

    Am I barking up the wrong tree here? Any advice would be welcome.
  2. jtp10181

    Minister of Fire 2.

    Feb 26, 2007
    Madison, WI
    A non-power vented DHW normally uses a 4" B-Vent I think, possibly 3 or 5. I am just guessing on size from seeing the vents. Anyway... I have seen plenty of them on old houses dumped into old clay flues shared with the furnace. Might just have to reduce the liner with an aluminum flex liner.

    Can someone that knows more about this back me up?
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