# Pressure reducing regulator question...

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Thanks...

#### Brian Ferri

##### New Member
Turns out that the Expansion Tank is shot, that's being replaced. Also, the PRV is being replaced...

##### Retired Defense Industry Engineer xxx
The reason why sizing an ET is so important is that having a small change in the acceptance volume due to expansion will have VERY little affect on the overall water pressure. When the tank is smaller, that means that when the water expands, it must compress a much bigger volume of the air in the tank, and this will raise the pressure much more steeply.

Think about pushing on a balloon...the bigger the balloon, the further you can press it in with the same pressure...now, think of that as water expanding into/against the balloon...the same volume change as in a smaller one, but to get that, you have to push harder on the small one, raising the pressure. The whole goal is to minimize the pressure changes, and sizing the ET properly is important as is setting it up properly.

If your pressure gauge has a tattle tale hand...leave it on for say 24-hours, and report back the peak and average pressures...I think you'll find that the pressure rise with the ET is quite small now that you have a functioning ET.

According to Watts, one maker of ETs, you want the precharge in the ET to be the same as your nominal water pressure.

, not higher or lower as some advocate. If it's higher, the system water pressure must jump to that value before the ET can begin to accept any water...if it's lower, the bladder is pushed further, stretching it more than 'normal', and wearing it out sooner.

The underlying principle here is Boyle's law. This calculator will let you put some numbers in. Boyle's Law Calculator (omnicalculator.com) For example, if you had about a 20l (about five gallon) ET, and it needed to accept 4l (a bit over a gallon) of water due to expansion, the end pressure if you started at 50psi would be about 62.5psi...but, if the tank was a 10l one with the same 4l of expansion, the end pressure would be about 83psi, so size does matter. The online calculators will tell you how much expansion the water will have based on the amount of water you are heating, the starting temperature, and the ending temp. You may not regularly empty your WH, so using the full volume of the tank would be the worst case situation...and, in the summer, the incoming water will be (maybe) much warmer, so the expansion will be much less...so, the expanded volume will be much less. For a more normal situation, where you're not emptying the WH of all the hot water, and need to refill the whole thing from the cold tap, the amount of expansion will be much less, keeping the pressure rise quite small. Take that 10l tank, but only use enough hot to cause expansion to be around 2l versus four, and the pressure rise will drop to 62psi, or about the same as the larger tank with the maximum amount of expansion the example.

Science helps explain things if you're willing to listen and learn.

Your goal should be to keep the worst case pressure below 80psi, regardless of how much of the WH capacity you should use. Size your tank accordingly.

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#### Reach4

##### Well-Known Member
Your goal should be to keep the worst case pressure below 80psi, regardless of how much of the WH capacity you should use. Size your tank accordingly.
Bull. You are messing up. You should get a pressure gauge, as Brian did, and if you have a check valve feeding your personal unit, find that you most likely don't meet your own criterion.

Play with the Watts calculator at http://tools.watts.com/ETP/, and you will be able to deduce they have built 135 psi in as their max number.

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##### Retired Defense Industry Engineer xxx
To minimize the wear and tear on hoses, seals, valves, cartridges, and to help prevent blasting a glass out of your hand when filling it after the ET is filled to max, size it so that your max pressure doesn't exceed the plumbing code's value. Yes, the WH is tested to 300psi, and the T&P valve should open at 150, but having your pressure high, for what could be most of the day after everyone showers and then leaves home for work and/or school, is not ideal. The goal of using a PRV is to limit the max pressure to a code maximum value...a larger ET is relatively cheap, and by minimizing the amount of flex it has to do, will also extend its life, saving money in the long term.

FWIW, the OP's ET was determined to be defective, and he's replacing it...this should minimize the peak pressures seen.

#### Reach4

##### Well-Known Member
To minimize the wear and tear on hoses, seals, valves, cartridges, and to help prevent blasting a glass out of your hand when filling it after the ET is filled to max, size it so that your max pressure doesn't exceed the plumbing code's value. Yes, the WH is tested to 300psi, and the T&P valve should open at 150, but having your pressure high, for what could be most of the day after everyone showers and then leaves home for work and/or school, is not ideal. The goal of using a PRV is to limit the max pressure to a code maximum value...a larger ET is relatively cheap, and by minimizing the amount of flex it has to do, will also extend its life, saving money in the long term.

FWIW, the OP's ET was determined to be defective, and he's replacing it...this should minimize the peak pressures seen.
Are you backing away from claiming that the thermal expansion tank should keep the expanded water pressure to 80 or less, or are you moving to something else?

##### Retired Defense Industry Engineer xxx
Are you backing away from claiming that the thermal expansion tank should keep the expanded water pressure to 80 or less, or are you moving to something else?
No, ideally, you'd keep the pressure below the code maximum under all conditions. The fact that systems can exceed that does not change the fact that code wants it less than 80, or the fact that seals, valves, hoses, etc. do not like more than that. As I said, worst case, with everyone showering, then going off to school or work...the excessive pressure could be sitting in the system for many hours...that is not ideal..

A wire coat hanger doesn't break the first time you bend it, but keep doing it and it eventually does. Skewing the pressure in the system maybe 50-60 pounds is just as bad for hoses, and even the WH, and anything else in the plumbing system except probably the copper pipes which can withstand a couple thousand pounds of pressure before they'd start to deform.

In the long run, you'll save money by using a larger ET, since the pressure swings will be smaller, and the wear and tear on things, including the ET, will be less.

Yes, plumbing products are tested to higher pressures than 80psi, but that doesn't mean that they won't wear out sooner if you regularly exceed that, and having it vary, is probably worse than sitting at a static pressure.

#### Reach4

##### Well-Known Member
No, ideally, you'd keep the pressure below the code maximum under all conditions. The fact that systems can exceed that does not change the fact that code wants it less than 80, or the fact that seals, valves, hoses, etc. do not like more than that. As I said, worst case, with everyone showering, then going off to school or work...the excessive pressure could be sitting in the system for many hours...that is not ideal..
If you were to make your own spreadsheet, using Boyles law, one of the factors would have to be the max pressure to design to. If you presume non-expanded water pressure was 70, and precharge was 70, with a 50 gallon wh, you would compute the need a much larger pressure tank than the recommendation tables ever call for.

If you care to test my claim that the Watts calculator sets their design expansion pressure max to 135 if the relief valve value is 150, it is easy enough to test. Just say that the incoming water pressure is 134. Set relief valve to 150. Default starting and final temps to 90 and 140F and the calculator calculates the need for a 144 gallon tank accepting 0.55 gallons of water. Say it is 135, and the required tank size is "infinity".

So dropping to 70 psi incoming, the required tank comes to only 1.26 gallon tank, still accepting 0.55 gallons. Clearly, this calculator expect the pressure to rise way above 80.

Amtrol does not provide a calculator. It just provides tables. They do provide this:

They don't recommend a particular design max, but they do provide table 2, with examples of 100, 125, and 150 max.

No thermal expansion tank maker or plumbing code says that the tank should be sized to limit the pressure of the thermally expanded water to 80 psi. I understand that you will interpret their words to say that, and you understand that I say that if they meant that they would say that, and the consequence would be much bigger thermal expansion tanks than are recommended by the selection tables.

Further, I know you will object to https://www.sharkbite.com/resources/blog/thermal-expansion-relief-valves-replace-expansion-tanks

##### Retired Defense Industry Engineer xxx
In the age of water shortages in more and more locations both in the US and around the world...does it really make any sense to dump water away that provides no useful purpose? That's what the expansion valve device does instead of an ET. My local plumbing inspector would not approve! Yes, it works to maintain pressure limits. No, it's not a great idea except maybe as an emergency safety device like the T&P valve already is on your WH.

Whether you can keep the expanded water pressure below 80 or not, it should be your goal to minimize the rise. That's done in a hydronic system all of the time. The pressure on my boiler only varies a couple of pounds between room temp and max hot. The relief valve on it is 30-psi, so anywhere the range of what they specify for potable WH use would need a much differently sized ET.

Amtrol gives you all of the information you need to calculate the size of a tank to maintain any maximum pressure.

So, take a typical 40-gallon WH, the expansion factor is 0.015. From the chart, multiply 40*0.015=0.6g. They provide a formula to calculate the design pressure factor. Let's take it to be 80...so that becomes (80+14.7)/(80-50) (assumes you've set the PRV to 50psi) and you get a factor of 3.16.

Then, they say to multiply the expansion * the design pressure factor, and you get a required tank size of 0.6*3.16=1.9g. So, in this example, your pressure would rise to NGT 80psi if the tank's acceptance value is at least 1.98g...a larger tank would limit the pressure rise. Note, that's acceptance value, not the physical size of the tank. IOW, it's easy to keep your pressure within the code maximums if you size things prudently.

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Hey, wait a minute.

This is awkward, but...

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