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Closing off an atmospheric vent opening into chimney and flue

rocketscientist51 | Posted in General Questions on

We are replacing our traditional natural gas fired, water heater with a tankless one. The old one was atmospherically vented into the chimney. The new one will be direct vented through the side of the house.

How should I close off the 4″ diameter opening into the chimney, into which my old eater’s galvanized vent was installed? What materials should I use besides cement/mortar? Will the opening in the clay liner need something specific to close off and seal the hole, so exhaust gasses from my furnace/boiler, which is vented into this chimney, do no leak into the chimney walls, etc.?

What are likely building codes regarding this?

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  1. GBA Editor
    Martin Holladay | | #1

    I've seen two approaches. (1) Install a galvanized 4-inch plug, available at any hardware store. (2) Fill the thimble hole with bricks and mortar.

  2. Expert Member
    Dana Dorsett | | #2

    Is there a reason you're not installing an indirect fired water heater operating as a zone off the boiler?

    The net heating + hot water efficiency would in most cases be higher, and it can serve up a LOT more hot water than a typical power vent or direct vent, given that the boiler is going to have more burner capacity than a standalone water heater.

  3. rocketscientist51 | | #3

    @Dana, Our heating system is steam radiators, the kind without separate return pipes. The boiler is over 15 years old ... in a nearly 100 year old house!! Bottom line: we have other areas of (in)efficiency to worry about first, like the in-wall insulation, the roof, and old double-hung windows.

    That said, I will look into how my new tankless could complement a future heating furnace. Open to hearing all suggestions/ideas..

  4. Expert Member
    Dana Dorsett | | #4

    Do you mean to say, you have already installed a tankless water heater (past tense), not "...are replacing..." (future tense.)??

    An indirect works well with a steam boiler too, whether the system is two-pipe or single piped. Steam boilers have crazy high standby losses just sitting there idling between firings and are typically 3x oversized for the actual space heating load. Giving it more load with the water heater is still gong to be good for the net efficiency- more so than tankless water heater. It's usually a bit less expensive to install than a tankless too, since there is no need to upgrade the gas plumbing.

    Also, a boiler is not a furnace- a furnace delivers the heat with hot air. Boilers deliver the heat with hot water or steam.

  5. rocketscientist51 | | #5

    Dana, I beg to differ with the relative merits you state between an indirect fired water heater and a tankless heater ... at least in my case. As an engineer (an aerospace engineer to boot! The rocketscientist moniker is tongue in cheek, but well earned :)) my calculations and research suggest the tankless is a better trade off for my leaky old house, as I tried to clarify in my earlier post. If you have references (links) you can share that throw additional light on the matter, I’m happy for more learning ... before my next upgrade.

    ... and my tankless, temporally, does not quite warrant a past tense reference, yet; I have bought it, and am in the midst of installing it.

  6. Expert Member
    Dana Dorsett | | #6

    The standby loss of the boiler is still being taken 8 months out of the year, whether the domestic hot water is being heated by the boiler or not. Adding an indirect decreases the standby loss by increasing the active duty cycle. Typical combustion efficiency for newer steam boilers is a hair north of 80%, about the same as a standalone gas fired tank.

    The real world performance of a tankless is well below the labeled EF, despite efforts to modify the EF testing to better reflect the negative impacts of the short-cycling of small hot water draws. The 95%+ efficiency is really "in your dreams", and in the lab, not in your house. 80-85%, sure.

    A heat pump water heater in the boiler room would usually beat either a tankless or an indirect on net efficiency. By harvesting heat from the boiler room it puts some of the standby loss to good use, and by lowering the temperature of the room a degree or two it lowers the actual heat loss. The offset in summer the dehumidification energy use provided by the heat pump water heater also counts in southern New England location.

    [edited to add]

    Unfortunately MassSave won't subsidize a heat pump water heater unless it's replacing an electric water heater. But they DO subsidize indirects and tankless water heaters:

    The fact that the subsidy for the indirect doesn't apply for steam boilers, and is only $400 instead of $700 for a condensing tankles could be an indication that somebody has done the modeling for a "typical" MA house and decided the tankless was, on average, a better net fuel savings from strictly an appliance efficiency point of view. MassSave also doesn't subsidize insulating the basement either (only air sealing + band joist insulation), so the accuracy of their modeling isn't all that good at optimizing bang/buck.

    So, with the subsidy skewing the picture the tankless is probably going to make the best financial sense, even though a heat pump water heater is probably what would deliver the most overall benefit.

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