Expansion tank needed for water heater?

When purchasing my new TT, the dealer supplied a water pressure regulator and mentioned the importance of using it to avoid possible damage to the plumbing system in the event a particular campground has very high water pressure. I believe it regulates the pressure to about 35-40psi, which seems very reasonable to me. The owner's manual also mentions using a pressure regulator.

So, seems simple enough, use the regulator, no problem. Right? Well, not so fast. Just so happens water expands as it's heated. I noticed the first time I fired up the water heater that when I first opened the faucet after it had brought the water up to temp, there was a short blast of obviously much higher pressure. No surprise there, 6 gallons went from 60 degrees to over 100. As it expanded and had nowhere to escape, the pressure increased in the entire water system.

Some might say well don't worry about it, however I had an experience several years ago with my house that might suggest that's a bad plan. I came home one day to find water all over the garage floor and dripping (almost running) out of the bottom of my water heater. It was less than 4 years old at the time. Plumber said, well you don't have an expansion tank so every time the water heater runs the pressure builds up excessively and over time will damage the water heater along with possibly other parts of the plumbing in your home. He also said that although it wasn't required by code when my house was built, current code does require an expansion tank. So along with a new water heater, I got an expansion tank and haven't had any problems since.

My thinking is that if it's a good idea for the stick house, it's a good idea for the RV as well. After all they make a big deal about using the pressure regulator, yet without an expansion tank, the benefits of the regulator are completely negated after the water heater runs.

I know I've seen threads here where people have installed either the small Shureflo accumulator or even a tank like I now have in the house, but those threads are always about reducing pump noise, not about dealing with water pressure build up from the water heater. Just curious what the opinion is here regarding this issue?

Thanks for the input!

11 Replies

tpi
Explorer
Apr 29, 2015
smkettner wrote:
I recommend an expansion tank for all closed systems. I had a pesky leak that took almost three years to find while looking off and on. Dealer could not find it. Leak was a bad fitting that just leaked when the heater went full cold to full hot without using water along with the spike in pressure.

Same exact here. My leak in former trailer was in city water inlet/regulator. Since it was located next to shower, I was convinced the shower was leaking-because showers were frequently timed with heat up of water. It was dripping under trailer-near shower. Finally I pulled off a panel and checked water inlet/regulator. Sure enough..drip drip drip when water was heating, but no drips under normal water pressure. Led me to pressure check what was going on.

Most people don't troubleshoot because there is no indication of problem. But a hint... if the initial opening of a faucet after a full water heatup leads to a short spike of extra flow, be suspicious :)
tpi
Explorer
Apr 29, 2015
I'm also in the crack a faucet camp when initially warming water. I've struggled with this air pocket thing for the last three RVs. My former trailer would go over 100 PSI even with the air pocket freshly established (pressure gauge), and then worsen from there. I finally installed expansion tank to thoroughly solve the problem. My current motorhome seems to need the air pocket reestablished on most trips. Since I haven't got around to installing expansion tank, I just let a faucet drip slowly when heating water. The water can be reused if necessary.

Suspect those of us who change elevation frequently have more issue with this.

Lots of people wouldn't be without a regulator on the city water, but IMO this is more of a real concern.
Old-Biscuit
Explorer III
Apr 28, 2015
I never crack the hot facet when heating water.....not necessary. That is what air pocket is for.

Initial fill of water heater I turn on water supply, open cold/hot sides of faucets until air in system is bleed off and water flows. Done.

The hot water out dip tube allows for air pocket to remain in tank.
Dr_Quick
Explorer II
Apr 28, 2015
I agree with Doug E. The first time I start out with cold water, I just crack the hot water faucet it a bit. Works fine. If the water is warm it is not a problem.
NinerBikes
Explorer
Apr 28, 2015
RDMueller wrote:
Well that answers that! Air pocket at the top of the water heater - I had no idea. So of course when I first filled and sanitized the water system, I made sure to do everything possible to get all the air out of the water heater, including opening the t/p relief valve. Had no idea I was defeating the purpose, so glad I asked! I may still add an accumulator for pump noise, but in the mean time, now I know how to easily fix this issue. Thanks!

No, you need to fill up the water tank to full capacity before turning the water heater on, to prevent damage to the heater. Fill it up as much as you can, allowing for the natural airspace in the design, and not a drop less.
DougE
Explorer
Apr 28, 2015
Easy solution. When I first fire up the water heater I crack a hot water valve just a hair to bleed off the expansion pressure until it gets up to temperature.
RDMueller
Explorer
Apr 28, 2015
Well that answers that! Air pocket at the top of the water heater - I had no idea. So of course when I first filled and sanitized the water system, I made sure to do everything possible to get all the air out of the water heater, including opening the t/p relief valve. Had no idea I was defeating the purpose, so glad I asked! I may still add an accumulator for pump noise, but in the mean time, now I know how to easily fix this issue. Thanks!
RJsfishin
Explorer
Apr 28, 2015
When you open the hot faucet, what you think is hi pressure, is really air coming out, making you think its super hi pressure. Mine does the same thing.
Also if no check valves in the cold water inlet to the heater, any expansion will back up into the accumulator tank. And add to that, what do you suppose the pressure relief valve is for ? Ever see it dripping while the water is heating ? That's when there is no place else for the heat expansion to go.
So, how many places is there for the heat expansion to go ?
1) air pocket at top of heater
2) the pressure relief valve drip
3) w/ no check valves, it can back up into the accumulator tank
IMO, expansion tank was not needed for 50 years, and is not needed now.
Old-Biscuit
Explorer III
Apr 28, 2015
Correct water swells when heated........Air Pocket is designed into the water heaters. It normally is established when filling WH tank but can be forced out and can also get absorbed into the water over time.

As for accumulator tank......it is installed on pump discharge not WH tank outlet. It does help minimize water pump run/cycle times
time2roll
Nomad
Apr 28, 2015
I recommend an expansion tank for all closed systems. I had a pesky leak that took almost three years to find while looking off and on. Dealer could not find it. Leak was a bad fitting that just leaked when the heater went full cold to full hot without using water along with the spike in pressure.