Tankless WH - Expansion tank?

ChrisJ

I'm curious, is an expansion tank required with a tankless water heater when a RPZ is used?

If not required, is it beneficial?

pecmsg

Doesn't hurt.

hot_rod

I think all the plumbing codes require a thermal expansion “device” on water heaters

A small tank like this works for tankless and combis with small water content 

ethicalpaul

My municipality doesn't require a backflow preventer on the supply into the house. The city water system is my expansion device.

ChrisJ

ethicalpaul said:

My municipality doesn't require a backflow preventer on the supply into the house. The city water system is my expansion device.

Ok.......

ethicalpaul

Laugh if you want , but I believe this is commonplace!

ChrisJ

Time to dig out the Good Grief meme again.

Larry Weingarten

Hi, Thinking about the physics going on with a tankless around a hot water draw and afterwards, I think it would be a fun project to actually test pressure just before, during and for about 1/2 hour after the draw in a "closed" system. If anything, the pressure might drop as plumbing and heater cool down. In that case, more water would enter the system. I'm not seeing a way for high pressure to develop or become a problem. I almost never see expansion tanks used with tankless and never hear of over-pressure problems. Maybe I live in Shangri La?

Yours, Larry

Jamie Hall

Larry Weingarten said:

Hi, Thinking about the physics going on with a tankless around a hot water draw and afterwards, I think it would be a fun project to actually test pressure just before, during and for about 1/2 hour after the draw in a "closed" system. If anything, the pressure might drop as plumbing and heater cool down. In that case, more water would enter the system. I'm not seeing a way for high pressure to develop or become a problem. I almost never see expansion tanks used with tankless and never hear of over-pressure problems. Maybe I live in Shangri La?

Yours, Larry

I tend to agree with you, @Larry Weingarten -- but the question isn't really the physics, it's the code and authority having jurisdiction. Isn't it always?

leonz

I think the new 2023 very expensive Bradford White propane water heater I have has a check valve in it already to prevent ghost flow

ChrisJ

leonz said:

I think the new 2023 very expensive Bradford White propane water heater I have has a check valve in it already to prevent ghost flow

This is about expanding with temperature, not ghost flow.

leonz

I understand that, I mentioned the check valve in my Bradford White Water heater simply because the water flow stops there when the demand stops.

ChrisJ

leonz said:

I understand that, I mentioned the check valve in my Bradford White Water heater simply because the water flow stops there when the demand stops.

Very good point.....