# Boiler Pressure Relieve Valve



## Mmaul (Nov 28, 2007)

I have a Weil Maclane Gas boiler in my house. I have noticed the the pressure relief valve has been blowing when I'm not home This is the first time I've had a problem with it. I installed a new digital thermostat upstairs, I have not drained the system for two years.I have priced a new valve is $28.00. Could the problem be the new thermostat, rust that has caused the valve not to seat, or just age. Also the new valve Btu rating is 550,000 btu the old one is 650,00 btu, does this make a difference? Thanks in advance.


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## Nofossil (Nov 28, 2007)

I'll hope for someone more knowledgeable than me to chime in....

but before I did anything else, I'd check that the expansion tank was not waterlogged. It should be only partially full of water, and if you relieve the pressure in the system, it should be empty. If it's full, it needs to be recharged with air at a minimum, and might need to be replaced.


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## Mmaul (Nov 28, 2007)

Would rechargeing it be something that I could do or would it have to be a paid professional.

Also how do you check to see if its waterlogged?


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## Eric Johnson (Nov 28, 2007)

I believe the BTU rating is the max, and you're probably nowhere near that with a domestic boiler.

When you say the valve "blows" does it leak or do you come home to find water all over the place around the valve outlet? Does it only leak/blow when it's up to temp, or all the time?

Are there times when it doesn't leak? If so, you can discount the rust theory, and probably the bad valve theory.

I think nofo is probably right about the expansion tank. It's probably an Extrol 60 (gray). If so, they cost about $100 to replace, if you DIY. If you do that, depresurize the system, isolate the tank if possible (should be a ball or gate valve between it and the rest of the system). If not, then you need to isolate what you can (zones, etc.) and drain the system down below the tank before removing it.

Expansion tanks have a rubber membrane inside with system water on one side of the membrane and air on the other side. They're factory set for 12 psi and you can charge them up with a bicycle pump and check the pressure with a bicycle pump gauge. If you push in the little pin in the valve and water comes out, then your tank is toast, because it means that system water has breached the rubber membrane.

If you're not sure about the status of your tank, I'd take it off and take it down to a plumbing supply house and have them check it out. They'll also sell you a new tank if you need one, probably for about the same price you'd pay anywhere else. Doubt they have anything like that at Lowe's or Home Depot, though I have seen smaller tanks at Lowe's. Turn the boiler off when you've isolated and/or removed the tank, or your pressure relief valve will blow again when it heats back up. The heated, expanded water volume has no place to go in the absence of a working tank, except out the pressure relief valve.


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## Mmaul (Nov 28, 2007)

I knew this was the place to ask. It does seem to drip a little amount of water when the boiler is off. It does have a grey Extol exspantion chamber on it. Did you say I could check this with out taking it apart?


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## Mmaul (Nov 28, 2007)

While we are on this subject the water inlet valve, the valve that lets fresh water in is always on, when I bought the house this is what the plumber told me to do, is this right? I would think this would stay in the off position.


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## Eric Johnson (Nov 28, 2007)

If your pressure relief valve leaks at all, then you should replace it. They cost about $15. But sure to get one designed for a boiler and set for 30 psi.

I don't know if you can really check out the tank without removing it. Like I said, if you push in the little pin in the fill valve (located at the bottom of the Extrol tank) and water comes out, then your tank is bad. But if air comes out, don't keep pressing it, or you let out too much pressure. Just check it.

I'm still not clear on what your problem is. If the valve is actually blowing, it will dump water all over the floor, and then it's probably the expansion tank. If it's just dripping a little water, then the valve is either dirty or bad, and should probably be replaced. You can open it up to try to flush it out. You can bang on it with a wrench or hammer while holding it open to try to dislodge anything that might be sitting on the valve seat.

If you don't know the answer to this question, put a bucket under the valve (should be a copper pipe going almost to the floor as a code requirement and safety measure) and see how much water collects in it over the course of a day. If it's just a little, then you've got a leaky valve. If the bucket is full and the floor is wet, then the valve is probably blowing and probably because of a bad expansion tank. Though not necessarily. Your boiler overheating would produce the same result.

As far as the water feed goes, that's a subject open to debate. I always leave mine open, on the theory that a catastrophic loss of water and system pressure wouldn't necessarily result in all my pumps burning up, at least not before I have time to correct the problem. And I don't have to worry about a leak in my system somewhere eventually doing the same thing. On the other hand, every time that valve opens and lets fresh water into your system, it's also introducing oxygen, which is a bad thing over time. Sometimes if I get paranoid I'll go down and close the valve. But then I get paranoid and go back down and open it back up. Your call.

The presence of glycol changes the arguments, but not the fundamental debate.

One more thought: If your fresh water supply valve is bad, then that could boost your system pressure and cause the pressure relief valve to open periodically. Ditto for an in-tank domestic hot water coil. A failure in either will put house water pressure into your heating system, which is designed for lower pressure. So you could try shutting off both of those things (assuming you have a DHW coil and you can shut it off) and see if that fixes the problem. Then you've narrowed it down.


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## Mmaul (Nov 28, 2007)

It seems that the valve blows when operating only because of how much water is on the floor. But I have noticed it leaking a little bit of water even when its off. When I get home tonight I will check the expansion tank and I will try to dislodge the relief valve. I will also put a bucket of water there to see exactly how much is being lost. The store I was going to by the valve from quoted me a price of $28.00 is this to much? I can shut the system down just about any where there are shut off valves all over the boiler.


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## Eric Johnson (Nov 28, 2007)

The last one I bought was only $15, but they may have a fancier model. The one on my Weil-McLain gas boiler looks like it cost more than $15, so it's probably in the ballpark.

If you can take the expansion tank off without a lot of trouble, then try that and see if it's full of water, as in heavy. It shouldn't weigh more than a few pounds if it's working properly. Otherwise, it's probably got about five gallons of water in it.

Make damn sure you isolate it before removing it. And probably best not to try it if the boiler water is hot. If your shut-off isn't working, you could get scalded. Just be careful and spin it off slowly.


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## Mmaul (Nov 28, 2007)

If I spin if off will that release the pressure, or does it not work like that.


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## Eric Johnson (Nov 28, 2007)

If your shut-off valve is to the pressure tank is working, then you won't have a problem. My point is that you really don't know for sure if the shut-off is working properly until you try to unscrew the tank. If it leaks, then you're going to get wet, hopefully not scalded. You should proceed with caution just in case. A small amount of pressure from within the tank should be expelled when you unscrew it, but not very much. If you get down to the last couple of threads and the thing starts squirting water and won't let up, then screw it back in.


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## Mmaul (Nov 28, 2007)

Thanks Eric I will try relieve valve tonight if that doesnt work I will check the expansion tank if that seems to be a problem I will remove it on friday, I will also make sure I have the boiler off. Thanks for clearing up alot of my questions, I have had some of these questions for two years now, but I am a firm believer in if it isn't broke dont fix it. I will post my findings tomorrow.


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## BrownianHeatingTech (Nov 28, 2007)

The fancy way to test an expansion tank is to tap the side with a screwdriver handle.  You should definitely be able to tell the difference between the "thud" near the pipe inlet end, where there is water, and the "ring" near the opposite end where there is air.  If that's the case, it's good.

If the water from the relief valve is more of a trickle, then it's likely just a failed valve.  The rubber seat gets old and leaks.

Get a Watts 374A 30PSI valve.  There are a couple other manufacturers, but the Watts valves seem to last longer than the others.

Also, is the "shaft" of the valve horizontal or vertical?  These valves are designed to be installed with the shaft vertical, but sometimes folks install them sideways, so that the threaded discharge points down (to save on a 90 in the pipe, apparently).  If the piping to the valve is set up for it to be the wrong direction, spend a couple bucks for a 90 and install the valve correctly.  That keeps sediment from building up along the edge of the seat (certain sediments can corrode the rubber).

The water inlet should run through a backflow preventer (keeps heating water from backing up into your domestic water) and then a pressure reducing valve.  The most popular is the Watts 911S, which has both units connected together, one after the other.  Second behind that would be an individual Watts 9D backflow preventer and 1156F pressure reducing valve.  Different brands will obviously use different numbers.  The PRV feeds water into the boiler until it reaches the setpoint (12-15PSI, factory), then closes.  The isolation valves before and after this (sometimes only before, if the installer was slacking) should be left on.  Under normal circumstances, a system will lose a small amount of water each year, mostly due to steam flashes and oxidation.  The PRV keeps it topped off, so it won't run dry.

Joe Brown
Brownian Heating Technology
www.brownianheating.com


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## Eric Johnson (Nov 28, 2007)

Thanks Joe. I was getting in a little over my head on this one. I know what I would do in his situation, but it's hard to explain and probably best not to give advice if you can't be more clear.


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## Mmaul (Nov 28, 2007)

Joe thanks alot the valve in question is a Watts 174A model M3 but I believe you are right about the way this is mounted it is mounted with the valve facing down this is probably my culprit. Thank's Joe and Eric.


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## heaterman (Nov 29, 2007)

What pressure does your boiler gauge read when the water temp is at its highest point? Turn your thermostat(s) up to make the burner fire, sit down there and watch the Temp/Pressure readings. If you have a waterlogged/failed expansion tank you will see the pressure rise from normal (12-15PSI) to 30 PSI at which point your pressure relief will indeed relieve itself. IF.......your gauge is functioning, and you see no indicated rise in pressure as the boiler temp increases, the problem is just a failed valve. BE AWARE THAT THE VALVE MAY RELIEVE VERY SUDDENLY.  

Note: ASME boiler code, section 4, prohibits the installation of a shutoff valve anywhere between the boiler and the tank. This is to preclude the possibility of someone isolating the expansion tank from the system while in operation causing the problem you have or worse. Is it always done that way? No. Many installers place a valve ahead of the tank for easy replacement when the tank fails.


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## Eric Johnson (Nov 29, 2007)

Thanks for the code clarification, heaterman. I like being able to isolate things, but that makes sense.


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## Mmaul (Nov 29, 2007)

Last night I went home and tapped on the expansion tank and it only made a thud at the top, very different sounds from the top and bottom. When I sat down there and watched the valve it only leaked it never blew, but the psi did hit 20, does this sound like a problem?


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## Eric Johnson (Nov 29, 2007)

It shouldn't open up until the pressure gets around 30 psi. Sounds to me like you've got a bad pressure relief valve.

That's good news.


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## Mmaul (Nov 29, 2007)

From the amount of water on the floor I just assumed it was blowing, but I sat down there for about an hour and it just leaked. Do you think the pressure is to high? When I had it checked out, when I bought the house the furnace guy said it was running 18psi, thats what its about right now.


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## Eric Johnson (Nov 29, 2007)

Anything under 30 psi is OK. Mine usually sits on 15, but it will rise and fall a few pounds from time to time. The valve shouldn't leak at anything under 30. You've got a bad valve, that's all.


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## Mmaul (Nov 29, 2007)

I can replace that, it was starting to sound like maybe this was going to be over my head. Thanks to everyone that has given me advice, if I have any more question I will be sure to email.


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## Mmaul (Dec 3, 2007)

Thanks to everyone who helped me with my questions the new pressure relieve vavle did the trick it worked fine all weekend while I extended my hearth. The basement is dry again. Thanks


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