Pressure relief valve leaking


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Old 06-28-10, 08:49 AM
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Pressure relief valve leaking

I realize there are multiple threads on this topic, and I have tried to follow the steps outlined in them.

I have an older oil boiler with baseboard heaters. It also heats our domestic hot water. The pressure relief valve is leaking water every time the system heats. It is because when the system heats, the pressure goes above 30 psi. When the system isn't heating, the pressure maintains a bit under 30 psi. The temp on the system seems to be correct. It is set at 170 - 190.

I checked the expansion tank. It is one of the older style steel tanks. I closed the supply to the expansion tank,drained it and reopened the supply. The system pressure dropped to the proper level of 12-15 psi. But when the system heated, the pressure raised above 30 psi and the relief valve started to leak again. So, I don't think the problem is the expansion tank.

The relief valve itself seems to be operating correctly since it leaks when the pressure goes above 30 psi.

From reading other threads, it looks like the problem may be the pressure reducing valve. How do I test this?

Also, I have read that the problem may be a leaking heating coil. How do I test this?

What else do I need to look at?

Thanks in advance for your help.
 

Last edited by clhereistian; 06-28-10 at 09:05 AM.
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Old 06-28-10, 10:34 AM
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The problem is the expansion tank. To change the pressure that much when the boiler is firing it must be an expansion tank issue. When you drained it how much water did you get out? The tank is probably 15 or 30 gallons. Does that sound like the amount of water drained out? I thing maybe the vacuum was not broken.
Here is a link to draining the tanks.
Technical Menu
Look at the last link on the right column.
 
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Old 06-28-10, 01:11 PM
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Thank you for the information. I'm not sure on the tank size, but it looks to be smaller than 30 gallons. It has "8#" printed on the side, which I took to mean it is eight gallons.

When I drained the tank out, it seemed I got about 5 gallons of water. I didn't measure it.

The procedure I used to drain the tank was very similar to what you posted, but it is possible that the vacuum wasn't broken. All I did was close off the water supply to the tank, attach a hose to it, and open the drain valve. I didn't do anything to let air back in. I'll try it again when I get home and let you know.
 
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Old 06-28-10, 03:05 PM
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Exactly what did you drain ?

About 5 gallons might be the capacity of the expansion tank, but I'm not sure that's what you drained the water from. The newer expansion tanks don't usually have a drain valve. They are usually made with a thin sheet metal walls around one foot in diameter (the length varies) and have a single 1/2" pipe fitting on one end and often a schraeder valve (tire air valve) on the other end. Inside is a synthetic rubber bladder. The rubber bladder may be pre-pressurized to 15 psi.

[Once upon a time there were larger (bladderless) steel tanks, and there were schemes for drawing air into them to keep them recharged, but leaving the air cusion in contact with the water had the disadvantage of providing an continuing oxygen source, which shortens the life of the system.]

Ideally, when the system is cold, the (auto-)fill valve will, if necessary, add water to bring the system to something like 15psi. When the burner fires, the the water contained in the system heats up and expands a few percent, filling the expansion tank, compressing the air cushion and raising the system pressure proportionally. If the expansion tank is too small or defective, the system pressure will rise above the limit set for the pressure relief valve, which will open releasing water to prevent any further pressure rise.

Water leaking from the relief valve may be due to a defect in any of the three components.

The fill valve may be over-filling the system when cold. Turn off the boiler, manually shut off the line to the fill valve and allow the system to cool. Note the pressure when cold. If the system pressure is greater than the filll valve setting (or if unknown), attach a hose and slowly drain the boiler to reach a system pressure around 10psi, then close the boiler drain valve. Open the manual shut off for the fill valve and note the pressure after the automatic fill valve has stopped adding water to the system. If the system pressure is now above 15psi, try adjusting the fill valve, and repeat, or replace the valve. (NB: In the unlikely even that the highest radiator or air bleed in the building is more than 30 vertical feet above the fill valve, the fill valve may intentionally be set to a pressure greater than 15psi to avoid pulling a (relative) vacuum at the high point.)

The expansion tank may have lost its air cusion. In a modern tank this will only happen if the bladder has failed. To test this component, shut the fill valve and cool the system as above and drain to reduce the system pressure. Check the pressure at the scraeder valve. Eventually the bladder fills the entire tank and the bladder air pressure will remain at 12PSI or so even as the system pressure drops lower. If the system pressure drops below 10psi and so does the bladder pressure, the bladder has failed, replace the expansion tank. If you can't measure this reliably without letting the air out of the baldder while checking it, then drop the system to 5psi and only check one time and pump it back up if it was good but you let out half the air checking it.

The pressure relief valve can develop a weak spring and dump system pressure prematurely, or get some particulate stuck ion the valve and remain open or weep. As before shut off the system. This time hold down the auto fill and force it to slowly overfill the system. the relief valve should open (usually at 30psi). Stop over-filling the system The relief valve should (soon) stop relieving the excess pressure. As above, slowly drain the system to remove the excess feeedwater introduced for this test.

If none of the components are defective, but the system pressure still increases too much and the pressure relief valve opens when the system is hot, then the expansion tank may be too small for the system. Replace it with a larger one. I've never heard of a problem caused by an expansion tank that was too large, but someone else probably has.
 
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Old 06-28-10, 03:38 PM
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Poly, he said this:

It is one of the older style steel tanks.
Mebbee you didn't see that part?

While it's unlikely that too large a tank would ever cause a problem, there is potential for troubles... you do actually WANT the pressure to increase somewhat when the water is heated. This is because heated water becomes less dense, and in order to keep the pump's NPSH in 'range' and not have the pumps cavitate, you might need a little more pressure. But again, it's unlikely to be a problem unless the system is marginal to start with.
 
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Old 06-28-10, 05:23 PM
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I would like to see some photos - of the tank and all the piping around it and the near-boiler piping. Assuming this is a conventional steel expansion tank (no rubber bladder), the air cushion should last indefinitely (assuming that somebody hasn't installed an air-elimination vent). So, after solving the immediate problem, we need to wonder how the tank lost its air.

I would expect a conventional tank to be larger than 8 gal. Pictures are badly needed - to make sure we know what we're talking about.
 
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Old 06-28-10, 05:32 PM
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Originally Posted by polychrome View Post
Once upon a time there were larger (bladderless) steel tanks, and there were schemes for drawing air into them to keep them recharged, but leaving the air cushion in contact with the water had the disadvantage of providing an continuing oxygen source, which shortens the life of the system.
The "schemes" included a very popular and effective B&G Airtrol system. They didn't exactly "draw air into them," as much as just continually extract any air in the circulating water and inject it back into the tank (from whence it came). My system is 60 years old, and there is no problem with corrosion of the tank, piping, or boiler.

No longer very popular, perhaps, but "once upon a time" is a bit misleading - the equipment is still manufactured by Bell & Gossett. And, in my opinion, more reliable and trouble-free than modern (and less expensive) bladder-type tanks that typically spring leaks within 10-20 years.
 
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Old 06-28-10, 05:34 PM
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A rule of thumb for the old steel residential tanks is measure the length and the rated capacity is half the length. Example a 30" long tank would be rated as a 15 gallon tank.
 
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Old 06-28-10, 07:31 PM
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I'm back with pics and updated info. First the pics:

Here is the expansion tank:


Here is the connection between the boiler and the tank. The tank is directly to the left of the pic:


Here is the side of the boiler. I assume that is the pressure reducing valve at the bottom:


Here is what I assume is the reducing valve:


Here is what the tag on the valve says:


Here is the boiler from the front:


Here is readout from the guage:


Now for the updated info:

I drained the expansion tank again. This time, I am sure I got all the water out of it. After draining the tank, the pressure was in the correct range (about 15 psi). I waited a few hours, and every time the boiler heated, the pressure went up till it hit about 30 psi.

I haven't tried testing the pressure reducing valve yet as polychrome instructed, but I'm on my way to do that now.
 
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Old 06-29-10, 05:41 AM
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One more clarification. It is a 30 gallon tank.
 
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Old 06-29-10, 08:13 AM
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Your second photo shows a silver-colored air eliminator vent installed. That is a big no-no with a conventional expansion tank and undoubtedly accounts for the loss of air cushion. Get rid of that. You'll want to depressurize the system, remove the vent, and install a plug in its place. Temporarily, if there is a valve cap on the vent, you can screw in down tight to defeat the operation of the vent. Check to be sure there isn't another air eliminator installed anywhere in your system.

See the green device that the air eliminator screws into? That is Taco's version of a B&G Airtrol fitting which diverts air into the expansion tank. The line from the Airtrol fitting to the tank must be sloped upwards toward the tank. I can't tell for sure from the perspective of the photos, but if it doesn't slope up toward the tank, that must be corrected.

You may have a leaking pressure reducing valve, too (which would cause the pressure to increase, although not cause the expansion tank to become waterlogged). But there is an isolation valve (with a red handwheel) ahead of the PRV in the connection to the city water supply. Shut it, adjust the cold pressure to 12 psi, and try again - if the pressure doesn't rise, that will tell you for sure that the PRV is leaking past the seat. If it is leaking, then replace it with a new PRV.

Correcting a couple of these possible problems will require depressurizing and draining the boiler - you want to do that just one time. While you're at it, I would install a new pressure-temp gauge (tridicator). And if there are any leaks in the system, fix them.

Your circulator pumps are on the boiler return and the expansion tank is on the boiler supply. This isn't in accordance with the modern theory and practice, but it's the way countless systems were successfully installed for many decades (and the arrangement formerly recommended by B&G). Don't worry about it.
 

Last edited by Mike Speed 30; 06-29-10 at 08:36 AM.
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Old 06-29-10, 10:02 AM
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I am at work now, but I will screw tight the air-eliminator valve when I get home and see if that corrects the problem.

Also, I tried testing the pressure reducing valve, but I'm not sure if I did it correctly. Here is what I did. I turned off the boiler and let it cool overnight. Then I closed the line before the pressure reducing valve. Then I drained the boiler till the pressure was 10 psi. Then I opened the line before the pressure reducing valve. Over the next two hours, the pressure slowly increased till the gauge read 28 psi. The boiler was off the whole time. Does this mean the pressure reducing valve is leaking? The pressure reducing valve should have stopped at 12 psi, correct?
 
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Old 06-29-10, 10:10 AM
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I would conclude that your PRV is leaking. Until you can replace it, it would be OK to run with the isolation valve from the city water supply shut.

Getting rid of or disabling the air eliminator vent won't solve any problem immediately - but it will avoid water logging the expansion tank over a period of months. For the same reason, it's important to make sure that line up to the tank is sloped upwards. When things are set up correctly, the tank's air cushion should last indefinitely (years).
 
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Old 06-29-10, 11:16 AM
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Thank you for you response.

Is the pressure reducing valve something I can replace myself?

Also, I don't know what the isolation valve from the city water supply is.
 
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Old 06-29-10, 12:03 PM
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Originally Posted by clhereistian View Post
Is the pressure reducing valve something I can replace myself?

Also, I don't know what the isolation valve from the city water supply is.
Replacing the PRV is a typical job tackled by reasonably experienced DIYers. You'll need to be handy and have pipe wrenches. I can't tell from your photo where unions might be to simplify the amount of piping disassembly required. If you don't feel confident tearing into plumbing, best to call a plumber.

The isolation valve is the one with the red handwheel in your fourth picture.
 
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Old 06-29-10, 05:39 PM
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Thank you everyone for the help. I think I will give it a shot replacing the valve. Thanks again!
 
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Old 06-30-10, 04:28 PM
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I wanted to tell everyone that the boiler has been fixed, thanks to your help. I bought a new pressure relief valve, and replaced the old one. Now the pressure is holding steady and the relief valve is no longer leaking. thanks!
 
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Old 06-30-10, 04:48 PM
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bought a new pressure relief valve,
Typo? I think you meant Pressure REDUCING Valve, correct?
 
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Old 06-30-10, 06:05 PM
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If the problem was the pressure relief valve, then your pressure gauge is seriously wrong.
 
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Old 07-04-10, 05:59 AM
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Yeah, I meant the problem was the pressure reducing valve. Anyhow, thanks for the analysis, everyone. I learned quite a bit in the process.
 
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Old 07-04-10, 10:04 AM
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The terms "pressure reducing valve" and "pressure relief valve," both abbreviated "PRV," are confusing. Personally, I like to call the former an automatic fill valve.
 
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Old 07-05-10, 12:07 PM
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Yeah... I try to not use the acronyms at all, and rather spell it out. But AFV works too... "America's Funkiest Valve"
 
 

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