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Tankless water heater – attic or crawlspace?

onacurve | Posted in General Questions on

Climate zone 5 — Indianapolis, Indiana. I live in a very small house, 1000 sq ft., so I want to free up the large closet that currently holds the furnace and tanked water heater, both natural gas. I’m chucking the ducted hvac system in favor of a ductless mini-split and converting the tank water heater to tankless. I want to put the new water heater in either the attic or crawlspace. Is one location preferrable? Do I need to purchase a unit designed for outdoor use?

My goal is to move the laundry out of a bedroom and into that large closet space.

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  1. GBA Editor
    Martin Holladay | | #1

    Here in Vermont, it's very easy for plumbing pipes to freeze if they are located in a vented attic or a vented crawl space. Of course, if you are talking about an unvented conditioned attic, or an unvented conditioned crawl space, either location is protected from freezing.

    I don't know if plumbing pipes are susceptible to freezing in your area (Indianapolis) if they are located in a vented attic or crawl space -- but I suspect that they are.

    If both locations are vented, it usually costs less to create an unvented conditioned crawl space than it does to create an unvented conditioned attic.

  2. davidmeiland | | #2

    We had a propane tankless in our vented attic for a number of years. Unprotected pipes do freeze here, but I put the unit near the floor of the attic and the piping went straight down through the insulation and into the house, so very little was exposed. A lot of them have electric freeze protection built in, at least for the unit itself.

    Seems like part of your question is about which location will be easier to run gas to, and run the vent from. In our case the unit had Cat. 3 stainless flue, and the attic made for an easy vertical vent with few pieces (they are expensive).

    You might also put yours in the closet with the laundry, up near the ceiling, out of the way. I see that done fairly often. No issue with cold pipes if you do that.

  3. onacurve | | #3

    Thanks for that Martin, and David.

    Your answer raises a few more questions.

    After spending half the day trying to figure out what's best, I was leaning toward going with the attic because access is easier, which is good because tankless models require more maintenance. The attic is vented so there will be a freeze concern. Some of the units I've looked at have built-in freeze protection either by using an electric warmer or by lighting a small amount of gas to keep the temperature above freezing. (1)

    I'm trying to figure out if an outdoor model would be ok to put in a vented attic. Can you install outdoor models in freezing climates? They must have freeze protection since they are installed outdoors, right? I don't really get what outdoor models are used for – hot tubs on a deck?

    Or maybe I should go with the crawlspace since I'm converting it from vented to an unvented one by insulating the block walls and rim joist with rigid polyiso, which I bought for next to nothing, recycled because I can't afford spray foam. I'm also putting 8 mil plastic sheeting on the dirt floor. I'm sealing up the vents and thinking of eliminating the small crawlspace access door that opens into the yard in favor of one that opens up inside the house. So that would make my crawlspace unvented. But since I'm chucking the ducts and putting in a ductless mini-split, I'm wondering if that will make my crawlspace no longer conditioned. You know -- like no leaky ducts down there to condition it. Either way it seems the freezing risk would still be lower in the crawlspace. In case of a water leak, less damage would occur if it was in the crawlspace.

    My utility closet, where I'd like to put it if it I had to put it in the living space, is in the center of the house, not connected to an exterior wall, so that causes a problem with venting. I want to use a direct vent model.

    It seems like prices have come down a lot since I last looked into tankles water heaters a year or so ago. I don't know why. The fad has worn off? Or maybe the Chinese government is subsidizing, them as with solar pv. Or has Martin's 4-6-2012 article “Are Tankless Water Heaters a Waste of Money?” suppressed demand enough to force suppliers to liquidate them?


  4. GBA Editor
    Martin Holladay | | #4

    Outdoor-rated tankless water heaters are installed on an exterior wall in a relatively warm climate. In that location, you don't have to worry (much) that the cold water supply pipe will freeze.

    If you install such a water heater in a vented attic in Indianapolis, you don't have to worry much about the water heater unit freezing. The worry is the cold water supply pipe, which is very likely to freeze. I wouldn't install the water heater there if I were you.

    Installing the unit in your crawl space makes more sense.

  5. Expert Member
    Dana Dorsett | | #5

    Seal and insulate the crawlspace- it's huge heat loss even WITHOUT ducts, and a conditioned crawlspace would be by FAR the better location for the tankless than the attic. Just be sure the exterior vent termination is protected from any snow-drift potential, assuming you side-vent it.

    BTW: It's worth the extra few hundred to go for a heating & cooling mini-split rather than cooling-only, even with cheap natural gas. The heating efficiency of those beasties at 40F+ is so high that it's cheaper to heat with the mini-split during the shoulder seasons. Anything with an HSPF >9 is pretty good, and there are even some relatively low-cost options with SEER>20 and HSPF>11. Unlike hot-air furnaces, inverter drive ductless mini-splits maintain very stable room temps.

  6. onacurve | | #6

    Many thanks for that, Martin and Dana.

  7. davidmeiland | | #7

    Before you buy one to install in the crawl space, make sure you read up on the clearances required, especially headroom above the unit.

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