Tankless Water Heaters

KeenGreen

When installing a Tankless water heater, like a Navien, etc, do you include a small potable expansion tank as well in your installation? On the inlet side?
I have heard that it is recommended, but not required.
Thanks
Jeff Green

Jamie Hall

If there is any sort of check valve or pressure reducing valve on the domestic water supply -- they come in a variety of situations -- you have to have an expansion tank on the hot water line.

KeenGreen

Yes. On the domestic water supply is a PRV.

Jamie Hall

KeenGreen said:

Yes. On the domestic water supply is a PRV.

Then you must have an expansion tank.

mikeg2015

IF there’s a flow check on hot cold supply, high you should normally have, then you must have a expansion tank.

rick in Alaska

I don't know why you would have to have an expansion tank on a tankless. On a conventional water hear, the water is heated up in a tank and is not going anywhere, so the pressure can build up. But, on a tankless heater, the water is flowing the whole time the burner is firing, so it doesn't have the ability to build any excess pressure.
RICK

Jamie Hall

I'd still want one, I think -- not so much when it is heating, but what happens when you turn the hot water off? How much of a pressure drop will there be as the water cools? Just don't know!

rick in Alaska

But if the water is cooling, it will not be expanding. And, the heat exchanger capacity is so small, it will cool off quicker, thus taking any residual pressure down faster. But, I still don't see it building any pressure to warrant a tank.
Rick

Jack

While I understand the "requirement" I think it is one of those code issues that were made before we ever saw a tankless water heater. That really in not uncommon. When I was on the NFPA 31 Committee we used to regularly have the "Where did this requirement come from?" When water is heated from 32*-212* it expands approx 1/23 or 4% of its volume. In a 40,50 or 80 gallon tank that represents some energy. A tankless will have in the area of 50-80* temp rise and therefore much reduced expansion. Rick is correct on his description of the way the water behaves on a tankless. There is not enough water volume that is contained in the system and when the tap is closed you will not be building pressure as the unit is off. The way I always handled it was to tell a contractor that if he was worried about it to put in the adapter and plug it. If it ever came up as an issue he could then simply pull the plug and install the tank. I supporting several tens of thousand tankless installs I haven't seen it be an issue. AND...prior to hitting the enter button. If it is a high temp commercial system I would always put the expansion tank in. Frequently the system characteristics are the same, but in high temp installs I'm kind of a belt and suspenders guy.

kcopp

They make these cute little ST-1 ones for tankless water heaters....

http://www.amtrol.com/wp-content/uploads/2017/11/MC4090-09_17-ThermXtrol-Brochure.pdf

metrorental

what about the practical need for a T & P valve on a small electric tankless. ie. Stiebel Eltron 223422 240V, 1 Phase, 50/60 Hz, 20 kW Tempra . the manual says it's not needed unless required by code. (or is that the answer )

bob eck

F W Webb stocks the Amtrol ST1
If local code says you need it you need it.
To me the only time the water is expanding is when a faucet is open and the TGWH is firing.

Jamie Hall

rick in Alaska said:

But if the water is cooling, it will not be expanding. And, the heat exchanger capacity is so small, it will cool off quicker, thus taking any residual pressure down faster. But, I still don't see it building any pressure to warrant a tank.
Rick

Quite true, Rick. What I can see it doing, though, would be rather extreme -- if the inlet somehow got shut off. Pulling enough of a vacuum to collapse something. I suppose that is rare enough to not worry about, though...

Gordy

Biggest thing with tank style water heaters, and the lack of using the municipal side of the service for expansion is collapsing the flue.

As been said with a tankless I think that's pretty much a non existent condition.