question

[Ally68]

I have a closed system PRV no bypass and no expansion tank my water is about 50psi goes up to about 85 during heating cycle then back down to 50.

Here is my question I was going to work on a line down from water heater so I shut the inlet at the water heater but did not star on pipe for A while. When I went down stairs the heater was cycling with the inlet valve shut and the prssure rose fast to 120 I opened the inllet and the pressure went down.

Why would the pressure build so much faster this way? Does the absence of the cold water pressure cause the tank to not release the pressure in the rest of the system or does the cold inlet coo down the line more or something?

Just for my info I wont shut it off agin with out turning off heater just curious what is happening.

 

[Clayton]

Prv

Most PRVs do have a built in bypass for thermal expansion and will allow water pressure to be released back into the supply when the pressure on the regulated side is higher than that of the supply. Unless the supply pressure is very high such as 150psi. then it will not allow the bypass to work and your t&p valve on the water heater would be discharging at that pressure anyway.

The water in you hot water tank is going to expand the same amount for a given amount of temp. rise if the cold supply is open or not. By closing the cold water supply you have now isolated the expansion of the hot water to only within the hot water pipes, which i am sure is less than half of the total space of all your water line in your house. With the cold supply valve open the expansion of the water heating can spread through out your hot and cold pipes. So now that it has less than half the space to expand in with the valve closed it is going to reach higher pressures faster

Install an expansion tank, depending on size, they are about $30.

 

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[Ally68]

my PRV has no bypass wilkins br4 already checked and when my last one failed my psi was about 110psi so not sure about outside psi but at least 110. I dont really need an expansion tank because my normal rise from expansion is less than 90 psi and it goes back down to 50 after the tank heats. And the cheapest tank I have seen is about 50.00 bucks and another 50 for the plumber to install it. I dont get leaks from my T&P valve at all. I was just curious why it expanded so rapidly and high with the cold inlet off

 

[Gary Slusser]

I think you do need an expansion tank and I'm sure of it 'if' you have any water softener or filter in the plumbing or in the fridge. The increased pressure causes hot water to exit the tank into the cold feed line and in certain cases a softener etc. will allow tha thopt water into them and that weakens the distributor tube, tank and/or husing which if the DT is involved, they eventually break allowing resin/mineral/gravel into the plumbing. To fix that takes days and costs much more than the expansion tank and installation.

Gary
Quality Water Associates

 

[Ally68]

My softner says it is safe up to 120 deg water going in. my heater is only set to 125.

 

[Jadnashua]

I think you are missing the point. Pressure cycling wears on things. An expansion tank evens things out. It is the pressure, not the temp Gary was talking about.

 

[hj]

expansion

Once expansion occurs, it does not "return to nomal when the heating stops". It returns to normal when something drains the excess pressure off. You probably have a leaking cold water faucet, hose faucet, or toilet that bleeds the pressure. When you shut the tank off, that source of relief was cut off and so normal expansion occurred.

 

[Ally68]

Ok I am going to call the plumber and get the tank thing done it should be set at my water pressure correct my guage reads normal 50 psi untill heater kicks on so I should set the air charge to 50 right?

 

[Gary Slusser]

My softner says it is safe up to 120 deg water going in. my heater is only set to 125.

And you believe that your hot water is not over 120f... so get your meat/candy thermometer and check it just after the heater shuts off but... if the weakening of the distributor tube due to that heat finally allows it to break (they usually split vertically and wilt) that allows the resin and any maybe gravel underbed into your plumbing, and you'll wish you'd listened.

The above is in case you balk at the $200 the plumber wants to do the job. lol

Gary
Quality Water Associates