Return to the Plumbing Forum | Post a Follow-Up

Expansion tank sizing for tankless heater?

Posted by seattleCraftsman (My Page) on
Tue, Mar 8, 11 at 0:28

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?

Follow-Up Postings:

RE: Expansion tank sizing for tankless heater?

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.

RE: Expansion tank sizing for tankless heater?

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).

RE: Expansion tank sizing for tankless heater?


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.

RE: Expansion tank sizing for tankless heater?

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.

RE: Expansion tank sizing for tankless heater?

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.

RE: Expansion tank sizing for tankless heater?

"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.

 o Post a Follow-Up

Please Note: Only registered members are able to post messages to this forum.

    If you are a member, please log in.

    If you aren't yet a member, join now!

Return to the Plumbing Forum

Information about Posting

  • You must be logged in to post a message. Once you are logged in, a posting window will appear at the bottom of the messages. If you are not a member, please register for an account.
  • Please review our Rules of Play before posting.
  • Posting is a two-step process. Once you have composed your message, you will be taken to the preview page. You will then have a chance to review your post, make changes and upload photos.
  • After posting your message, you may need to refresh the forum page in order to see it.
  • Before posting copyrighted material, please read about Copyright and Fair Use.
  • We have a strict no-advertising policy!
  • If you would like to practice posting or uploading photos, please visit our Test forum.
  • If you need assistance, please Contact Us and we will be happy to help.

Learn more about in-text links on this page here