Is there a need for an expansion tank with a tankless heater?

singh

Yes.

DHW, No.

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James Bates

Tankless water heater

Should an expansion tank be installed with a tankless ?

Mad Dog_2

No....becuase.....................

There is no tank maintaining a constant temp...with the constant expansion and contraction....on the tankless subject......Many of the ELECTRIC tankless manufs don't require a relief valve. THAT IS NUTS!!!!!! That's like not having brakes....I don't care WHAT "THEY" say, we put it on....ANY device capable of producing energy NEEDS a safety feature if it goes whacky. Mad Dog

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billtwocase

Tankless

What kind of tankless are you talking Jim?

nugs

you need an expansion tank for both. don't believe me? Plug up your relief valve, by-pass the safety and watch the unit blow itself off the wall. Let's not be callous here.

www.watts.com find the danger explosion lurks vid

nugs

you need an expansion tank for both. don't believe me? Plug up your relief valve, by-pass the safety and watch the unit blow itself off the wall. Let's not be callous here.

www.watts.com find the danger explosion lurks vid

James Bates

The unit in question is a TK-3 Takagi, gas fired for potable only...

JK_3

as singh said

The water expands as it heats and on a tank you are raising the temp of water trapped in the tank because the water continues to heat after the tap is closed but on a tankless the heating stops as the tap is closed( or its suppoesd too ) so you do not need an expansion tank on a tankless. you do need a releif valve so that if the heating does not stop and the water expands it can expand through the releif valve letting you know that the unit is malfunctioning.

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Jack

No

Feel free to correct my numbers, but if I recall correctly, from 32-212F water will grow in vol. by about 1/23 or a bit under 4%. Given that the tap is always open when the unit is firing and the burner shuts off immediately upon the stoppage of flow,if anything, you will actually see a contraction of vol. as the water cools. As the overwhelming majority of dhw systems suffer from a lack of pipe insulation this will happen in very short order.

Having said that, in my experience, and I only get invited to the jobs with problems, every system has a personality. Over the years I have had, I think 3-4 jobs where an exp tank solved a problem. That is out of a total of a few tens of thousnads of units, so I'll say you don't need one...until you do;)

Mad Dog_2

And why would one plug the relief valve?????

If the relief valve is plugged, and there is a runaway condition and expansion tank is not going to help out for long...Mad Dog

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Mad Dog_2

Couldn't agree with you more on differing personalities

Sometimes things just don't add up, but the problem stops and the call backs with them. Mad Dog

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Gary Kirk

Local code here doe sstill require the tank ...but local reps beg to differ. BUT some tankless like the navien A model has a buffer tank that stores water...so yes to the navien.

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Gary Kirk

ummm ...even if you had an expasion tank, under those conditions it would still blow.

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Adayton_3

Thermal expansion STRESS

and potential system "KNOCK" (banging heard in pipe system) I always thought was a key reason behind the purpose of expansion tanks... With regular hot water TANK, the faucet is CLOSED and thermal expansion can occur. With tankless DHW the faucet is OPEN flowing HOT water until CLOSED after which the captivated HOT water COOLS therefore creating a REVERSE presure FORCE..... nevertheless still a FORCE of pressure [vacuum]. This FORCE regardless of direction places stress on system components and faucet seal/gaskets. Therefore from a pragmatic viewpoint is an expansion tank as ABSOLUTELY necessary in a tankless system? Probably not. However It certainly would be desireable.

Gary Kirk

Expansion tanks are for expansion of the hot water(domestic)..because.... the water utilities started to require check valves at the service points. So expansion could no longer be pushed back into the water lines. A T&P valve (tempature/pressure) is a safety device, and are still required on tankless heaters


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Jack

I once had a call

for "phantom operation" of a Rinnai. With no draw on the system the unit would start and stop. Very brief, but it would spin up to operation and shut down immediately. It turned out that there was no building back-flow prevention and upon heavy draws on the town main running down the street it would pull a back pressure on the buildings system and when the heavy draw stopped, re-pressurize the buildings system and the "phantom flow" would occur.

Also, so not use a t&p on a tankless. Pressure only. I've seen T&P's leak like crazy. Totally dependent upon the piping arrangement and the systems "personality" but it happens regularly.