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Community and Q&A

Instantaneous gas water heater with small tank to meet PHIUS+ hot water performance requirement

lucyna99 | Posted in PassivHaus on

We are well into the construction now, and don’t have a lot of flexibility. For example, have 3” venting pipes in place for what was meant to be Eternal condensing hybrid gas hot water heater with 2 gallon tank. The small tank helped to reach PHIUS+ requirements of not wasting more than 0.6 gallon of water before the water temperature at the furthers fixture reaches 75F. (We also designed very small diameter hot water pipes to help that goal). Alas, the company making Eternal went out of business and we are in search for a replacement that might have similar characteristics to Eternal GU195 unit. Online search reveals several products, but we’d also like some PHIUS+ experts opinion, if possible.

Other options (don’t seem to have a buffer tank and have a minimum flow


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  1. GBA Editor
    Martin Holladay | | #1

    It seems to me that the diameter of the tubing is a more important factor than the presence of a buffer tank. It's true that an "instantaneous" water heater isn't literally instantaneous -- but what test method, if any, is used to determine the 0.6 gallon rule?

    This is the first time I've heard of the 0.6 gallon rule.

  2. GBA Editor
    Martin Holladay | | #2

    One possible solution is to install a tiny electric on-demand water heater (or a 5-gallon electric tank-style water heater) in the distant bathroom -- and to disable the electric heater after the certification paperwork is complete.

  3. lucyna99 | | #3

    Hi Martin,
    The distances to the bathrooms are not great, and the pipe size is such that about 0.2 gallon of water is "stored" in the pipes between the planned heater and the faucets. The testing method is simple: open the faucet and capture water in the bucket until the thermometer inserted in the stream shows 75F.
    The problem with instantaneous hot water heaters without any buffer tank seem to be the speed with which they are able to ramp up to produce that 75F water, given how tight the requirement is. Any ideas which heater might work here? Thanks.

  4. GBA Editor
    Martin Holladay | | #4

    My point about the test is this: Do designers know enough about the performance of these heaters to anticipate the results of the test at the planning stage? Or are PHIUS designers using a "trial and error" method to specify water heaters?

  5. lucyna99 | | #5

    This requirement has been in place since 2012 or so, at the time when PHIUS combined Passive House principles with Energy Star and Zero Energy Ready Home (under the auspices of DOE) - hence I'm looking to any Passive House US experts to share what they used...

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