Gas boiler exceeding pressure release valve's psi rating ... why? 3

[cadmium]

Here in the northeastern US, I performed some routine annual maintainance on my domestic natural gas furnace/boiler by removing and cleaning out the burners and testing the pressure release valve. Water came out of the valve -- so far, so good. But then it wouldn't STOP coming out because crud had accreted to the valve gasket so that once opened, it no longer formed a tight seal when shut. So I replaced the entire pressure release valve. Prior to replacing it (back when it still formed a tight seal and didn't leak) I never had problem with water leaking out this valve, presumably because boiler pressure never built up enough to exceed the release valve's psi rating (30 psi). (Either that, or I did have problem that I didn't know about because the release valve was, unbeknownst to me, not functioning and just remaining shut tight regardless of the boiler pressure. I'd rather not think about that.) But now, when I run the boiler, pressure builds to 30 psi without fail, causing my new release valve to operate and begin releasing water. So, my question is why is the boiler doing this now? The only difference between what I did this year and in past years is that I tested and replaced the release valve. Cleaning the burners can't have caused this problem. Or is this pressure situation likely to be unrelated to anything I've done? I lack the expertise needed to diagnose the reason for this malfunction, and I hope one of you knows the answer.
Other potentially relevant info: this bioler also has a diaphragm-type expansion tank, and between the tank and the boiler is float-type air valve (45 psi), which has been emitting a very small amount of water, kind an almost impercetible seep. Honestly, I can't really tell you whether the behavior of the air valve is new or not, but I suspect it's not new, given the green/aquamarine staining on it's copper exterior, which suggests that's been going for a while. The regulator on the cold water intake is of the 12 psi variety.

We're enjoying mild November weather at the moment, so I have the entire system shut off. If for some reason you conclude that I ought not to turn it back on under the present circumstances, that I'm sitting on a powder keg that's ready to pop, that the situation is dire, and that, in that light what I'm doing now could be called "fiddling while Rome burns," to an extent that makes you want to scream, "Call a heating professional to your house ASAP to avert disaster," then that info would be very welcome.

Thanks.

 

[TBP]

The biggest mistakes I see with situations like yours, is many people calmly screw a plug into the relief valve outlet. I've even seen pictures of a second relief screwed into the outlet of the first. There's the less common practice of installating a relief with a much higher setting, or simply removing the relief altogether. "These stupid things always leak..."

Check the expansion tank - the air pressure should be checked with the tank valved out of the system. The pressure should be maybe 5 PSI higher than the fill pressure of the system. If you depress the little air valve with a screwdriver, and you get water & air, or just water coming out, then the diaphragm is shot. Another problem is the make-up water PRV is passing or set too high. Does this boiler have a domestic hot water coil in it? A leak in this coil will let the higher pressure domestic water leak into the boiler side.

The air separator, like the relief valve, is just doing it's job. Every time the relief valve burps out some water when the system heats up, the little make-up valve feeds water in, when the system cools down. The dissolved air in this cold make-up water is what's being discharged by the air vent.

 

[cadmium]

TBP --
Thanks for a very informative post. You may be right about the expansion tank; I'm getting only water out of the air vent.
I don't believe the boiler has a hot water coil in it for the simple reason that this same boiler and a separate hot water heater came with the house.
If you don't mind my pestering you for more info ... I haven't examined the expansion tank too closely yet; do I need a special-purpose meter to test the air pressure in the tank? And, assuming the tank is shot, should I put doing it myself out of the question?

 

[TBP]

Assuming there's an isolation valve for the expansion tank, you can likely change it yourself. The air connection is exactly the same as on a car or bicycle tire. Check the the pressure of your system cold (it only needs to have enough pressure in it to fill the system to the highest elevation, plus a few PSI to make sure the air is pushed out), then charge the air side of the tank to the same pressure before you install it, or valve it in.

If you're interested, there's an inexpensive soft cover book "Pumping Away" by Dan Holohan. It's written so it can be understood by almost anyone, and provides an excellent description of residential hot water heating systems, and their components.

 

[chicopee]

If your relief valve is set for 30 psig, than your boiler working pressure should not exceed 20 psig per NBIC.
The build up of mineral on the valve seat was probably preventing the relief valve from lifting. This mineral build up also suggest that the relief valve may have weeped for a period of time prior to manually testing this valve.

 

[GenB]

I understand you ave a closed system! since you operate below 30psi,
look at the fill valve, it may be leaking thus pressurizing the system beyoond 30 psi (City water can be over 75 psi at times),
also the circulating circuit valves could have been shut closed or half closed or are obtructed and possible pressurizing the system, rusty pipes could be a reason of blokage.
genb

 

[cadmium]

TBP, chicopee and GenB:
Thanks much. Since I am already scheduled to spend considerable time in the basement this evening on other matters, I will follow up on your suggestions.

 

[cadmium]

TBP, chicopee and GenB, and anyone else:

Thanks a lot for your advice thus far. I have had the expansion tank replaced as it was, indeed, shot. (TBP, although there was no isolation valve for the old exp. tank, there is one now for the new tank, so I'll be able to switch it out myself in the future. Thanks for tip.) I also replaced the air vent, and I have installed another new pressure release valve because -- even after installation of the new expansion tank 2 days ago -- the release valve I installed 2 WEEKS ago was behaving as if either (1) a new chunk of precipitated mineral (or other piece of boiler-generated crapola) had lodged itself in the valve seat, or (2) there was overpressure. I.e., it was discharging water. (I failed to check the pressure guage, but subsequent events related below indicate that boiler was probably exceeding 30 psi.) So, yesterday, in went another release valve, certified as crap-free. And yet, this AM it was discharging water again. More mineral ...? That's what I thought until I checked the pressure guage: it was sitting at 31-32 psi. Surprising because after installing the current release valve, my heating pro babysat this boiler yesterday PM as she ran up to the temp limit, and during that time she didn't get up above 20 psi, let alone disharge from the relief valve. What a difference the absence of professional authority from the premises makes, so it seems. Is this a variant of the car always making the horrible sound of gears self-masticating, except when you take it to the mechanic?

If I may trouble you for your opinion again ... have I narrowed the problem down to the fill valve, or the circulating circuit valves (see GenB's 20 Nov 05 post)? GenB, re: same post, are you thinking rusty pipes may have resulted in a blockage of the circulating circuit valves in particular, or that they could be the cause of an independent blockage? Water from the release valve is clear, not rust-colored, and water out of the radiators themselves (when I vent them) contains small black specks and smells a bit stale, but I don't see rust particles or rust-colored water.
Is a bad replacement expansion tank possible?(not charged?)
Any advice is welcome.

- Cadmium

 

[TBP]

How are the system components arranged? To work the best, it should be boiler, with the circ pump pulling water from the boiler outlet, discharging it to the system. Between the boiler & the pump, is where the air separator and expansion tank should be located. The expansion tank pressure rules the system - it's the highest pressure. Since the inlet of the circ pump will be the lowest pressure, and just after the boiler (water is the highest temp) that's where the air separator should go, because that's where the water is the closest to the boiling point in the system and will dissolved air will be most prone to popping out of solution. The location of the make-up water connection matters as well. It should feed into the short section of pipe between the system and the expansion tank. If it's located in other spots, the circ pump can fool it into feeding water when it's not necessary.

You can just try shutting off the make-up water for a while, and see what happens. There are two schools of thought on the make-up: one says it should remain valved-in, and ready to go, and the other says shut it off, and leave it off. There are merits to both. It's sure not the wrong thing if your boiler has a low water cut-out.

Dan Holohan is an ex- Bell & Gossett guy, and his book "Pumping Away" explains all of this much better than I can. Soft cover, inexpensive, and plainly worded with simple diagrams.

http://www.heatinghelp.com

under "Books & More".

 

[cadmium]

TBP -
Thanks again. That's sure to be useful info. I haven't forgotten your earlier plug for Pumping Away; I'll order it myself if it doesn't show up in my stocking. In meantime, my heating pro is reasonable and seems to know his stuff (he knows of Mr. Holohan, at least), so I'll have him in and share your post with him. Best if he tackles this instead of me attempting it. But I want the book regardless.

 

[TBP]

If you've got a heating guy on the scene, and he's familiar with Holohan's book, you'll no doubt find the problem shortly. Good luck.