PRV Pressure

[MikeG]

Hey guys!

I'm about to install the thermal expansion tank (Watts DET-12-M1) this weekend. Last week my plumber replaced the 20 year old PRV. The pressure on the lines before we replaced it was about 75 - 80 psi

With the new PRV the pressure is around 50 -55 psi. I'd like to increase the pressure. Based on the installation sheet, it is basically loosening the lock nut and turning the bolt clockwise (in). It is a Wilkins #70.

What is a good household pressure to use? I was thinking about 60 - 65 psi. Are there any adverse issues with increasing the pressure that much?

Thanks, Mike

 

[Dunbar Plumbing]

60-65 is what I set all PRV's to during installation. You need to set the Expansion tank REMOVED from the water system to the same working pressure of the PRV. If anyone tells you different, look at the packing slip that came with the expansion tank, it will give exact instruction. You must match the working pressure. If you do not have the document, go to watts.com or call them, they will mimic the same sentiment. A good competent plumber should of done that upon the replacement of the PRV. If not, this causes premature wear of the expansion tank bladder which in turn will cause premature failure. Same theory follows under/overinflating a car tire, it does not have good results.

 

[Sponsor]

 

[MikeG]

Rugged - Since I have not installed the tank I was going to set the pressure on the PRV first then set the tank to match and install the tank.

Thanks, Mike

 

[Dunbar Plumbing]

Cool Beans!

 

[hj]

tank

An expansion tank is not rolling down the road so any pressure imbalance is immaterial up to a point. Unless it is so low that the tank fills with water, or so high that no water can enter the tank, then anything in between is adequate for the amount of movement that the diaphragm will make, and its movement will be over a very limited, and repetitive, range no matter what the initial charge is. Once the system is activated and some water enters the tank the air will be at the system's pressure.

 

[Master Plumber Mark]

hj is correct

If I read you right Hj you are totally correct...

no wcorrect me if I am wrong here......

I usually like to set the thermal exp tanks
about 10 -20 lbs higher than the prv valve.seting.....


the thermal expansion tank
is there for HIGHER PRSSURES than the sysytem is set up
for through the PRV valve....

if you set it at the same pressure ,
wont the thermal expansion tank become sort
of filled with water inside the chamber because their
is no higher air pressure on the other side of the bladder wall
to keep the water at bay ????

over time the exp tank will become water logged or inneffictive??


anyway , its worked good for me kicking it up a tad bit..

 

[JK60]

It would seem to me that if the tank is pre-charged to a lower air pressure then the supply water, then water will enter the expansion tank and compress the bladder. This compression causes higher “air pressure†inside the tank since now you have the same amount of air filling a smaller volume of space within the expansion tank. When the bladder is compressed enough by the water to reach the same air pressure as the water supply, an equilibrium state is reached, i.e. no more water will enter the expansion tank. In this scenario the expansion tank is never fully evacuated. There is always water inside the expansion tank. I don’t know if that is a problem or not.

 

[Bob NH]

I usually like to set the thermal exp tanks about 10 -20 lbs higher than the prv valve.seting.....

I prefer to set the initial air pressure just below the PRV pressure.

CASE 1 - If the PRV is set at 55 and the air in the expansion tank is 75, then when the water expands by perhaps 0.4 gallon in the hot water heater, the pressure will increase until it reaches 75 psi. When it exceeds 75 psi, the water will enter the tank and further increase the pressure. The resulting decreased volume of air will cause the pressure to increase to about 85 psi if you have a 4 gallon expansion tank.

Now when you open a valve, the pressure will decrease from 85 psi to 75 psi as 0.4 gallon of water is used, and then almost instantly drop to 55 psi. That is not something that you usually want to see.

CASE 2 - Now if the PRV is set at 65 psi, and the air in the tank is set at 62 psi-gauge (77 psia) when empty, the water will enter the tank to increase the pressure to 65 psi-gauge (80 psia). The amount of water in the tank will be about 4% of the volume of the tank.

When the water heater expands the water trapped in the system, it will increase the volume by about 1% (0.4 gallon in a 40 gallon heater) which could force that much water into the expansion tank. If you have a 4 gallon tank, that will increase the pressure by about 11% of the initial absolute pressure, or about 9 psi.

When you draw water, the pressure will drop back to 65 psi while you use the 0.4 gallon of water, and will then hold steady.

COMPARISON - In the first case you have a pressure change of 30 psi including an instant drop of 20 psi in mid flow, but in the second case you have a pressure change of only 9 psi.

I prefer the second system where the initial pressure in the expansion tank is just a little below the PRV setting; not above it. It also gives you a higher steady state pressure and a lower peak pressure.

 

[Master Plumber Mark]

the debate begins.....

well, I beg to differ.....

if you make the exp tank lower than the incomming pressure

the exp tank is sure to just to fill up with water and become basically

useless, actually it becomes a "storage tank" for extra water


read the post that HJ made just before mine earlier...

I agree with his all around idea here.......

their is a "happy middle ground" not too low.....and not too high...

just right



I would prefer letting the "higher powers that be" on this site


decide my fate... am I right or wrong..

 

[Bob NH]

"An expansion tank is not rolling down the road so any pressure imbalance is immaterial up to a point. Unless it is so low that the tank fills with water, or so high that no water can enter the tank, then anything in between is adequate for the amount of movement that the diaphragm will make, and its movement will be over a very limited, and repetitive, range no matter what the initial charge is. Once the system is activated and some water enters the tank the air will be at the system's pressure." hj"

Mark,
Your suggested charging method is actually inconsistent with the system described by hj. His suggested conditions that the pressure not be so high as to prevent water from entering the tank, and not "so high that no water enters the tank" is contrary to your idea of precharging the tank: "I usually like to set the thermal exp tanks about 10 -20 lbs higher than the prv valve.seting....." mark

Water is virtually incompressible compared to air. With a regulator that prevents backflow, any increase in volume caused by expansion of the water will cause the pressure to increase enough to deal with the water. It will either go into the expansion tank or out the relief valve. That means that an increase in water volume will ALWAYS cause the pressure to increase above the air setting.

I was explaining how the tank works if charged the way that is implied by his description (CASE 2 of my post) compared to how it will work if charged the way you suggested (CASE 1 of my post).

"Higher powers that be" don't trump science, and hj's description is consistent with what I posted.

 

[Dunbar Plumbing]

When an expansion tank is not connected to a water system the bladder is extended downwards towards the threaded port where it connects to the system. They come preset @ 40 pounds. The reason the manufacture states matching pressures is so that the bladder isn't extended one way or another in it's normal most common use when no water is being used, until thermal expansion occurs. That means the bladder is flat inside of the tank if compressed air is 65, water pressure is 65. The max on an ST-5 is 85 pounds on one of those tanks for recommended air pressure. That is why you see so many expansion tanks blown on homes without PRV's. Don't know why they enforce tanks when it isn't a closed system but 80 to 120 pounds of pressure along with a tank with a preset of 40 gives one meaning to the inside of the tank, severe distortion.

There is a good reason the mfg. is stating the equalized pressure from both opposing forces of air and water with a bladder in between. Case 2 sounds logical and without any premature wear on the tank. Inspectors are cracking down on tanks being installed incorrectly these days. Thought it was ridiculous to hear of it but apparently there is a good reason the mfg. states in the paperwork, on the website, how to use their product. I would think they would know alot more than the users/installers of their product.

Doesn't matter to me, I just pick beans for a living.

 

[hj]

tank

well, I beg to differ.....

if you make the exp tank lower than the incomming pressure
the exp tank is sure to just to fill up with water and become basically
useless, actually it becomes a "storage tank" for extra water

That could only happen if the air in the tank could compress to zero volume, which would be impossible in the first place, and require an immense pressure in the second place. The air will compress only to the point where its pressure equals the system's water pressure. That will happen at any point between 5psi, which means there is some air in the tank with a maximum amount of water, and slightly less the system water pressure which means that there would be mostly air, but some water, in the tank. At zero pressure the entire tank would be full of water, unless the diaphragm was trapping some air in which case that air would then become pressurize. At system pressure, or above, NO water can enter the tank so it contains only air. The same principles apply to well storage tanks.

 

[MikeG]

2 more questions

Two more questions....

1. Today I picked up the Watts expansion tank DET-12. The instructions say to match the pressure of the tank to be the same as the cold water line with a faucet open. That would be a few pounds lower than with the faucets closed. I thought it would be the same as the pressure with the faucets closed. Should I set it to be the pressure with a faucet open?

2. My plan on installation is to run a T from a verticle cold line going into the heater. Then to a 90. Then to the tank so that the tank mounts verticle. The Watts diagrams all show without using a 90 and mounting wither verticle or horizontal. Will a 90 have any affect on the performance of the tank?

Thanks, Mike

 

[Jadnashua]

The 90's won't affect operation. Mount it where it is easy to service and support. If it is vertical, it is easier to remove without messing up the threaded connection and dropping the 4 or so gallons of water it will hold when the bladder eventually fails.

 

[JK60]

Two more questions....

1. Today I picked up the Watts expansion tank DET-12. The instructions say to match the pressure of the tank to be the same as the cold water line with a faucet open. That would be a few pounds lower than with the faucets closed. I thought it would be the same as the pressure with the faucets closed. Should I set it to be the pressure with a faucet open?

My interpretation of the instructions were that first you open the faucet to equalize the system pressure. Then close the faucet, install the pressure gauge, and read the pressure with the faucet open. This was my assumption even though the instructions do not specifically tell you to turn the faucet off and then back on after gauge installation. I believed that it is a given and the only reason you initially let the water run is to make sure that you are not reading higher system pressure as would occur after the water heater kicks in.