Expansion tank sizing for tankless heater?

seattleCraftsmanMarch 8, 2011

I am slowly getting my act together for a tankless install, looking at the Rinnai RC98HPi.

The Seattle area plumbing code appears to require an expansion tank be installed with all water heater. Looking at online calculators for expansion tank sizing, they don't seem to take tankless heaters into consideration.

While I wait for the inspector to call me back to confirm it is indeed required with a tankless heater, can anyone offer any sizing guidance?

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I'm not a plumber, I don't play one on TV, and I didn't stay at a Holiday Inn last night...

Installing a tankless WH should require permitting and a licensed plumber with a NG or LP ticket who should have the answer or contact your local plumbing inspector and ask him/her.

    Bookmark   March 8, 2011 at 12:57AM
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A tankless heater should have little impact on expansion tank size.

The only time it heats water (causing expansion) is when a faucet is open.

Expansion tanks are needed for closed systems (or if required by law) that heat large volumes of water when NO faucet is open.

The expansion of the water in the tank has no place to go except to raise pressure in a closed system (or force some water back into the distribution lines in an open system).

    Bookmark   March 8, 2011 at 8:27AM
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All true but the codes may not have been changed to recognize that fact regarding tankless water heaters. AFAIK, what the code says gets enforced till the code is changed.

The authority on what is required at the time of permit and inspection is the local code enforcement people.

    Bookmark   March 8, 2011 at 8:55AM
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Just spoke with plumbing inspector - said unless my house has a pressure reducing valve, which we don't, that an expansion tank isn't required for our install.
Permits filed, time to complete my orders and lose the tank.

    Bookmark   March 8, 2011 at 10:56AM
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Make ABSOLUTELY sure you don't have a closed plumbing system. Just because you don't see a PRV inside your house doesn't mean it isn't buried.

    Bookmark   March 8, 2011 at 3:41PM
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"AFAIK, what the code says gets enforced till the code is changed. "

And not all the codes require an expansion tank if the system is not sealed.

Even if they do require one, the smallest one wil be adequate since a tankless does not create hot water except when a faucet is open and that relieves any pressure.

The tankless has no effect on the sizing of an expansion tank.

    Bookmark   March 8, 2011 at 6:29PM
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