# Expansion Tank vs Static Pressure

### Users who are viewing this thread

#### Kistle

##### New Member
Ok, water heater drain confirmed at 105 p.s.i with cold and hot water dribbling. Can we say with relatively certainty that I have a failed PRV?

#### Reach4

##### Well-Known Member
Ok, water heater drain confirmed at 105 p.s.i with cold and hot water dribbling. Can we say with relatively certainty that I have a failed PRV?
Since you have a PRV, yes.

##### Retired Defense Industry Engineer xxx
Say you have a 50G WH. Your inlet water temp is an average 50F, and you raise the temp to 140F. The water will expand about 0.75G. (see https://www.watts.com/resources/planning/etp for this calculation ). Now, you may not dump the entire WH volume, so the total expansion may often be less than that maximum and the pressure wouldn't rise as much. LIve with teenagers that exhaust the WH tank, you'll see that, though.

With the Watts PLT-5 ET, the smallest they recommend and the boyles law calculator, if you started out with 60PSI, it should rise to 96PSI, but if you bump it up to the next larger Watts tank, the PLT-12, the same expansion occurs because the temperature rise is identical, the pressure would only rise to 72PSI. As I said, 115psi is too much. Valves and seals are designed for 80PSI max. Shower in the morning, leave for the day, the system could be sitting at the high point all day until someone uses some water to relieve it. You install a PRV to limit your water pressure. A properly sized ET lets you achieve that. (I rounded off a gallon to = four liters, which isn't accurate, but is a close approximation for demonstration purposes 1g=3.79 liters if you want a more accurate estimate)

So, yes, the pressure will rise while you're heating water with no flow, but it doesn't have to rise all that much if you size the ET appropriately. The science doesn't lie. Now, will your system survive? Yes, but hoses, seals, will fail more quickly and you'll be putting more stress on your WH. Keep in mind that the typical WH tank is glass lined thin steel. Constantly raising and lowering the pressure along with the heat (especially if it's a gas WH), will stress the lining even more. Keeping the pressure within a smaller range has advantages that, in the end will more than offset the cost of a slightly larger ET in both its life, and other things in your home.

#### Kistle

##### New Member
Thanks for all the guidance, guys! I will try to remember to report back here after getting the PRV rebuilt or replaced.

I did buy the Watts PLT-5 ET. Should be fine after I get the dang static pressure down within range.

#### John Gayewski

##### In the Trades
System pressure can't get above 80. If you have an expansion tank that doesn't allow enough water in (during expansion), thus rising your pressure above 80, then it's too small. Despite the manufacturers sizing guides. I rarely go only by manufacturer guidelines without checking good practice recommendations. Manufacturers are in the business of manufacturing not keeping a system healthy.

#### Reach4

##### Well-Known Member
System pressure can't get above 80. If you have an expansion tank that doesn't allow enough water in (during expansion), thus rising your pressure above 80, then it's too small. Despite the manufacturers sizing guides. I rarely go only by manufacturer guidelines without checking good practice recommendations. Manufacturers are in the business of manufacturing not keeping a system healthy.
If you put a peak reading gauge on a home with a 50 gallon WH and recommended size tank, I expect the water pressure to peak out substantially over 80.

https://terrylove.com/forums/index....g-regulator-question.94022/page-2#post-677156 is one of my refutations of the jadnashua claims. And I guess now you too.

##### Retired Defense Industry Engineer xxx
If you put a peak reading gauge on a home with a 50 gallon WH and recommended size tank, I expect the water pressure to peak out substantially over 80.

https://terrylove.com/forums/index....g-regulator-question.94022/page-2#post-677156 is one of my refutations of the jadnashua claims. And I guess now you too.
Which is exactly my point, and the information shown using the calculators I linked to and you regularly disregard. Bumping up the size of the ET is cheap insurance and should keep the pressure within code limits all of the time, not just when someone uses water AFTER the pressure has risen during the heating process.

#### John Gayewski

##### In the Trades
If you put a peak reading gauge on a home with a 50 gallon WH and recommended size tank, I expect the water pressure to peak out substantially over 80.

https://terrylove.com/forums/index....g-regulator-question.94022/page-2#post-677156 is one of my refutations of the jadnashua claims. And I guess now you too.
The shop I worked at was in an odd spot in town and had over 90 psig. We were constantly fixing things. The limit of what an expansion tank can be stretched to isn't the issue. Flexing a water heater will cause early failure, flush valves get rattled into pieces, vaccum breakersdon't work worth a shit.

Staying below what every other fixture is manufactured to hold and operate through its life, is a probably go to be tough to argue against.

Here's how to calculate water expansion.

Delta T x volume (gal) x 0.00023

The problem isn't the size of the tanks it's the size of the air chamber.

#### Reach4

##### Well-Known Member
Good point on less flex being good. I wonder if the pressure was not only higher, but more variable. Thermal expansion tanks is a place where bigger is better.
The problem isn't the size of the tanks it's the size of the air chamber.
The size of the air chamber is pretty much the size of the tank if the precharge has been ideally set.
Staying below what every other fixture is manufactured to hold and operate through its life, is a probably go to be tough to argue against.
And yet I try. The literature from thermal expansion tanks provide tables or calculators for sizing that identify tank sizing that cannot limit the expanded pressure to 80 psi. https://www.amtrol.com/wp-content/uploads/2019/06/9017-112-03_19-MC10007-Sizing-TXT.pdf table 2 on page 2 has 3 rows to help calculation where they allow to go to 100, 125, and 150. The Watts calculator sets 130 psi as a fixed max.

Remember the T&P valve set point is 150 psi.

Last edited:

##### Retired Defense Industry Engineer xxx
One has to consider that Watts is in the business of selling ETs, along with other things. A bigger tank will last longer and limit the maximum pressure rise, meaning less business for them on new tanks.

Take an old steel coat hanger...it doens't break the first time you bend it back and forth...it takes a while. Your water heater doesn't like it, either, along with other things, but seals tend to be more resilient than a steel-lined tank.

Replies
3
Views
593
Replies
3
Views
333
Replies
23
Views
1K
Replies
25
Views
1K
Replies
18
Views
442
Hey, wait a minute.

This is awkward, but...

It looks like you're using an ad blocker. We get it, but (1) terrylove.com can't live without ads, and (2) ad blockers can cause issues with videos and comments. If you'd like to support the site, please allow ads.

If any particular ad is your REASON for blocking ads, please let us know. We might be able to do something about it. Thanks.