Boiler Pressure relief valve (T&P) still leaking. Please help.

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My 7year old Burnham MPO147 boiler was absolutely fine for years. But it s tarted leaking water out of the pressure relief valve (aka T&P valve) last week. A slow trickle would start whenever the boiler would fire. I quickl y noticed the expansion tank had rusted through on the air bladder side (yi kes!) so I drained the system and replaced the expansion tank with the same make/model as the old one and refilled/purged the lines. So it's all func tional again... but, the T&P valve still leaks when the boiler fires now, o nly just less now. Since it only leaks when the boiler fires, I can't imag ine there's crud in the T&P valve keeping it open. There have been no othe r changes to the water supply or anything else. Did I somehow damage the T &P valve by causing it to leak? Any ideas or suggestions are appreciated! Regards, Theodore.
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On Wed, 23 Oct 2013 22:32:35 -0700 (PDT), snipped-for-privacy@yahoo.com wrote:

Very common for the valves to leak once they get started. Over the years, I've replaced a couple of them on my old boiler and they would be good for another 5 to 8 years.
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I usually figure if they have once discharged, they WILL need replacement. That's why I never "test" one by manually opening it. Boiler, Water heater, it doesn't matter. They are a "safety" item, not a "convenience" item. More of a "fuse" than a "breaker" in concept.
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On Thursday, October 24, 2013 6:43:58 AM UTC-5, snipped-for-privacy@snyder.on.ca wrote: That's why I never "test" one by manually opening it.

All T&P's in a commercial application are required to be tested (I believe semi-annually). At home I would/do test them once a year...a fast release usually will blow-out any crap so it won't leak. Just my .02
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On Thu, 24 Oct 2013 05:40:20 -0700 (PDT), Bob_Villa

has discharged for any reason, it requires replacement within months, at best. Commercial stuff might be better quality than the crap on domestic equipment. On my water heater it reccomends the valve be tested once a year "to ensure the waterway is open" It also says if water does not flow, or the flow does not stop, shut off the water heater and call a plumber IMMEDIATELY. Sounds like they kinda expect problems, no??
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On 10/24/2013 04:39 PM, snipped-for-privacy@snyder.on.ca wrote:

not really, they're pretty reliable (unless you have hard water/mineral build up, then they will leak unless you jiggle them just so) but it would be a huge safety issue if the overflow pipe were blocked off for some reason and the tank burner didn't shut off. water heater explosions are no joke but fortunately rare.
nate
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On 10/24/2013 5:18 PM, Nate Nagel wrote:

I can imagine a HO screwing a cap onto the relief valve pipe, after having it leak repeatedly. That could make for a very dangerous over pressure blast. Mythbusters exploded a couple water heaters,
http://www.youtube.com/watch?v
U-I2ZiML0 and this was one.
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wrote:

Virtually impossible in the "normal" city water supply situation as the pressure can never get higher than the mains pressure - it just backs the volume increase back into the mains. If you have a whole house backflow preventer it could happen, but I've never seen or heard of such a thing. Even ressure regulators on the mains are virtually unheard of around here (Ontario Canada).
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pressure regulators are very common here (northern VA) and as far as I know every one acts like a backflow preventer as well... several people told me my pressure was too high at last house & I should install one but showers @ 70 PSI were fantastic :)
nate
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On 10/24/2013 01:32 AM, snipped-for-privacy@yahoo.com wrote:

The spring inside the T&P valve can get weaker with age. That combined with minerals in the water can cause it to "burp" every now and then and then stick open. Just go to the store and get a new T&P valve before you start freaking out.
I fought with one at my last house for a couple years trying to flush it and "exercise" it occasionally so it wouldn't leak when it "burped" but eventually I just replaced it. In retrospect I should have replaced it first because it gave absolutely no trouble after that.
nate
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So much misinformation in this thread.
If your boiler T&p valve is leaking that's almost always a sign that there is some thing wrong with the boiler. Springs in T&P valves do not get weak with age. Just replacing the T&P valve is NOT a solution. You must have the entire boiler diagnosed and serviced by a licensed technician.
These things blow up all the time and KILL people.
Your boiler PRV may be malfunctioning, the thermal expansion tank may be bad, the boiler could be over fired, there could be a flow issue because of a bad re-circ pump or a restriction, zone valves could be leaking etc, etc.....
No matter how many YouTube videos you watch on how to DIY fix your boiler it will never match the experience of a technician that works on these every day.
Install a faucet if you want to be a DIY'er don't work on something that can kill you if you don't do things right.
Boiler explosion:
https://www.youtube.com/watch?v
ej2OQSKnY
This is a commercial boiler explosion but residential explosions are just as bad.
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On 10/23/2014 3:20 PM, snipped-for-privacy@centralplumbingnm.com wrote:

I'll admit, I've never heard of a residential boiler explosion. How many occur in the US each year? Any proof of this?
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On Thursday, October 23, 2014 7:42:48 PM UTC-4, Stormin Mormon wrote:

Yes, that's a good question, isn't it? I'm sure they occur, but I've never seen, heard of, or even read about one, so I'd bet they are relatively rare.
Plus, if he read the thread, the OP identified what was wrong with the boiler. The expansion tank had failed and it's been fixed. Finally, like others here have posted, I've seen TPR valves that once opened, won't fully close again and have a small leak. Over years, they get slightly corroded, crud in them, etc. And if it's dripping, I don't see the harm in replacing it with a new one and see what happens. If the leak stops, it's the valve. If it continues, then either diagnose further yourself, or call a plumber. How exactly is a boiler going to blow up if it has a new TPR valve on it?
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On 10/24/2014 7:40 AM, trader_4 wrote:

Myth busters did an exploding hot water heater, years ago. I can't remember which episode. They changed the TPR with a plug, and watched from a safe distance, behind layers of plexiglass. It did make a big blooey. If distant memory serves, it blew about 300 PSIG or more. Wasn't easy feat to accomplish.
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On Friday, October 24, 2014 5:41:02 AM UTC-6, trader_4 wrote:

Yes, Pressure Relief valves are designed to open and close as needed. If yo u have particularly hard water then calcium or what ever can build up and s top a PRV from seating properly again and it should be replaced.
How is a boiler going to blow up if it has a new PRV? Boilers pump water ar ound what is essentially a closed loop. Included in this loop are things li ke the heat exchanger, air scoop, thermal expansion tank and Pressure Relie f valve. This loop should have a maximum of 20 pounds of pressure in it. Your PRV op ens at 30 PSI. Air can enter this loop in a number of ways and ideally it is removed by th e air scoop. Lets say you have a leak and air is entering the system. Over a number of days air builds up but is not removed because of a faulty air s coop. This air slowly makes its way around the loop causing pockets of air that grow in size as they come together. A pocket of this air hits the heat exchanger and 200,000 BTUs of heat energy from the burner causes the air t o expand very quickly causing a pressure spike. Ideally your PRV burps and releases this pressure and your thermal expansion tank absorbs some as well . Lets say some of this mineral buildup that clogs PRV valves gets lodged in the PRV causing a small restriction. Air hits heat exchanger again and caus es TPR to burp again causing some more mineral buildup. A few days later th e air bubble has grown some more. Its cold and the boiler has been running at 100% for a few hours. The large air bubble hits the heat exchanger again and expands very quickly. Now at this point there does not have to be a cataclysmic explosion where h alf the house blows up. You might just hear a small ping as the heat exchan ger cracks. All that instant pressure has to go somewhere and scolding hot steam and water shoots out as Mrs Jones walks by carrying her laundry. Maybe nobody is around and the boiler purges steam, the high limit safety s witch malfunctions and the burner does not turn off causing a fire.
This is just one possible scenario, there are many others.
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On 10/24/2014 7:40 AM, trader_4 wrote:

Neglect and lack of maintenance. The TPR is designed that it can take away enough overheated water/steam to prevent an explosion. What can happen though, a lot of crud in the boiler can block the outlet.
My dealings are mostly with industrial high pressure boiler. I'm a licensed boiler operator in Massachusetts. I have to take some classes to keep my license current and they are more about safety than normal operation. There are plenty of war stories from the insurance inspectors and instructors about things that just can't happen but did.
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On 10/24/2014 5:03 PM, Ed Pawlowski wrote:

When I worked as a factory nurse, I had to take care of three guys after a steam pipe burst....they had been on a ledge next to the pipe when it let go and all had bruises from landing hard in their hasty retreat. Must have had healthy hearts, because it took a bit for them to unwind, thinkin' how it might have ended. Shoot, there were select departments, like punch press, that I never wanted to hear from. Eventually, all were represented but not as bad as it could have been. Only amputation I ever had to care for was the tip of a finger, and I worried more about the supervisors who gathered around having a heart attack.
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On 10/24/2014 7:40 AM, trader_4 wrote:

Could not easily find numbers, but this has some photos of what has happened to a couple of them. http://www.northeastgas.org/pdf/p_pirro_preventing.pdf
I bet the lawyers know though http://www.explosionvictimresourcecenter.org/explosions/residential-gas/boiler-and-furnace-explosions/
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On Friday, October 24, 2014 5:50:33 PM UTC-4, Ed Pawlowski wrote:

Those are some great pics of it happening, that's for sure. The remaining question is how often and I still think it's very rare for a residential boiler.
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On 10/25/2014 7:31 AM, trader_4 wrote:

It is rare for a couple of reasons. Most residential boilers are water, not steam. For a steam boiler, both the PRV and water feed have to fail as well as the high limit pressure switch and low water cut off for it to overheat
With a residential water boiler, you have a steady supply of water at 50PSI or so to feed and keep the temperature below 212.
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