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Relief valve dripping: Advice appreciated.

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dbessey
dbessey Member Posts: 12
Hello
New member here
Have a problem with a dripping relief valve. (or, I have a symptom of a dripping relief valve and my problem is likely something else)
The heating system is hot water boiler (Burnham) with cast iron rads. The home is a rather tall Victorian, all piping and rads are original to the home as far as I can tell. The circulator was replaced a couple years back. Was a B&G, now a Grundfos. The system is equipped with both a diaphragm expansion tank and a steel compression tank hanging in the rafters above the boiler. I have no idea if the diaphragm is working as it should. Have not read that far ahead to know how to diagnose it or check it....

When the relief valve started dripping, I emptied the compression tank and let the system refill. The pressure in the system immediately went down to 20 psi and everything seemed fine for about 12 hours. Came home from work the next day to find the system sitting at 25 psi and there was evidence it was higher at some point as there was water at the outlet of the relief valve (set to 30 psi). Normal operating temperature is 140 F. None of the rads have auto air release on them. I went around to each one checking for air. Found none. We usually get a bunch of air in two of the rads each year, once I let that air out in the early fall, I usually do not see air in the rads for the rest of the season.

My compression tank has no gauge, so I do not know if it's water logged again already or if something else is happening here. When I empty it, I can hear the air going in and I isolate the rest of the system from it, so II am fairly sure I developed the maximum air cushion that I could in the tank volume provided.

To add to the mystery, the pipes are making a weird noise. The normal clicks and pops of the pipes expanding as they warm up in the morning has now been joined by a rapid hammering type noise that occurs a few times a day. Have not been able to track it's origin. The last time it happened, I was at the boiler and the relief valve was NOT releasing at the time of the noise. I have lived with this system for 15 years, so I am familiar with the noises it makes. This latest one (rapid tapping/hammering) is not normal for this system.

Any advice on how to get the system pressure under control first, would be greatly appreciated. Once I get that under control, maybe the weird noise will go away, maybe not.

Thanks

Darren



Comments

  • Jamie Hall
    Jamie Hall Member Posts: 23,443
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    Is there a domestic hot water coil in this boiler?

    And how is the system filled? Pressure reducing valve, manual valve?

    The diaphragm type tank should have a Schrader valve on the top. To check it, isolate it from the system and drain it, then measure the air pressure. Should read no more than 20 psi. Shouldn't read much less, either. If water comes out, it's toast.

    That said, having both a diaphragm tank and a steel compression tank doesn't really work that well. You want an air separator on a system with the diaphragm tank, but you don't want one on a system with a compression tank -- it will gradually take all the air out of the tank.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • dbessey
    dbessey Member Posts: 12
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    Hi Jamie

    Thanks for responding so quickly.

    There is no coil for domestic ht water in the boiler. The water heater is a totally seperate appliance

    The system is filled by a line equipped with a pressure reducer (Watts 158) followed by a pressure relief . I assume the relief is there in case the pressure reducer fails.

    I cannot easily isolate the diaphragm expansion tank, but i grabbed it and it feels essentially empty. It seems to have no weight to it whatsoever. There is a strange looking device that it threads into.



    Not sure if this will work. Or not, but here’s my attempt at including a picture of the device....Is that an air seperator?

    Thanks

    Darren

  • Jamie Hall
    Jamie Hall Member Posts: 23,443
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    It is. Whether it's working or not is another question. Perhaps more to the point in this setting, there is no easy way to isolate that tank, so the usual ways of checking the air charge aren't going to work.

    The air separator, if it's working, will pretty well ensure that the big overhead tank will waterlog remarkably quickly. I'm deeply suspicious that more than one person has worked on this system, and not always quite understanding what's there.

    For the moment, though, is there a way in which you can manually shut off the feed to the boiler? If so, this weekend -- or sometime when you can keep a really close eye on the system, close the manual shut off and drain off just enough water to bring the pressure down to 20 psi. Then watch the pressure through several heat and cool down cycles. It should rise a few pounds -- perhaps as much as to 25 psi -- when it's fully hot, and should drop back to where it began when it cools off. If it rises more than that, chances are the diaphragm pressure tank isn't doing anything and the overhead tank is waterlogged. If it does behave properly, then the possibility is that your pressure reducing valve isn't set properly -- or is leaking.

    Among other possibilities...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • dbessey
    dbessey Member Posts: 12
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    Hello Jamie

    I'll give that a try...thanks

    As an aside, I suspect the pressure reducing valve is set to 15 psi. After I had drained the compression tank and opened the system back to the town water feed, the pressure in the system initally dove down to 10 psi and then rose to 15 psi. At that point the sound of water flowing into the pipes grew silent. Unfortunately, I did not let it sit for some time after that. Instead, I just fired up the boiler and let the system go..

    I'll try your approach this weekend and see what happens.

    In the meantime, I'll read up on air separators and why they waterlog the compression tank...

    Perhaps I should be adding a gauge to the compression tank to see if waterlogging is occurring..... (or just remove the air seperator next summer)..or both.

    Thanks

    Darren





  • dbessey
    dbessey Member Posts: 12
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    Jamie

    Decided to have a very exciting evening, pulled up a chair and watched the boiler. Just discovered that the relief valve starts to drip (just a couple) when it hits 25 psi (according to the gauge on the boiler) So, either my gauge needs changing or my relief valve does........neither of which is convenient now, but I guess that might explain the relief valve drip (if the relief is going off at a lower pressure than it should)

    will still try to determine if the pressure reducing valve is leaking by.......

    Darren
  • delta T
    delta T Member Posts: 884
    edited December 2018
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    Relief valves are designed to fully open at 30 PSI, and in my experience will start dripping at around 25-27 PSI.

    Another factor to consider is the location of the pump and the point at which the expansion tank (or in your case tanks) connect to the system. There is a difference between the new grundfos pump (steep pump curve) and the old Bell and Gossett (flat curve) that could be affecting system pressure and either overpressuring the boiler, or causing the the pressure reducing valve to sense a lower pressure than it should and adding water when it shouldn't.

    If you could give us more information on the system it would be helpful.

    Are there multiple zones or is the whole house run off of one thermostat?

    Can you give us a picture that incorporates the boiler and the piping near it in one shot?

    What is the model number of the grundfos pump currently installed?

    What is the btu/hr input rating of the boiler?

    How many radiators do you have? Can we see a picture of a typical one?

    Your problem sounds like it has to do with an undersized expansion tank, and if you have a bunch of cast iron radiators connected with large piping and a large boiler, I would bet that an Extrol 30 expansion tank is not enough. You do have the compression tank as well, but with the air separator installed that will always eventually waterlog, though how long this takes, depends on a ton of different factors. Did this problem start after the pump was replaced?

    I wouldn't worry too much about changing the gauge, the new one won't be too much more accurate than the one you have, it sounds like its pretty close to reality. They are mostly +/- 2-3 degrees when new in my experience. I would plan on replacing the relief valve though, once they have been leaking a while, the seat can wear and cause it to not seal.

    Another thing to check that I see occasionally on that specific type of air separator is the connection to the expansion tank gets blocked with corrosion and rust. Make sure to stick something up into the air separator (I usually use a screwdriver) and make sure the opening is clear. You won't damage anything in the air separator (there are no innards on that one, it is just a hunk of steel) so make sure that connection is open. Check this when you check the expansion tank as Jaime suggested, if you want to. Your system may be hard to do this on though if you do not have any isolation valves.

    I see this happen especially on a system that has leaks (I notice the air vent on the air separator appears to be leaking). Any time you lose water, the fill valve will add it back. This fresh water carries with it dissolved oxygen which will in time corrode any ferrous component of your system, which in your case is most of it. It is critical for the long term health of a heating system to repair even minor leaks as soon as is feasible. Corrosion is cumulative.....
  • dbessey
    dbessey Member Posts: 12
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    Hi Delta;

    The system is controlled by a single thermostat.
    The boiler has a rated output of 164,000 btu/hr
    The rds are a mix of tubiuular and column
    I counted up the sections
    8.75” wide by 38” tall 4 tube design 43 sections
    9” wide column rad, 3 column, 22 “ tall. 174 sections
    7.5” wide column 38” tall 2 column 5 sections

    Grundfos pump is a US 15-58 FC

    The boiler is a natural gas fired Burnham. It’s roughly forty years old. Inspector told me heat exchanger was “surprisingly clean”


    I will draw up a sketch of the piping around the boiler. Pictures are tough to take such that piping orientation is clear

    Thanks so much for your interest.


    Darren
  • Zman
    Zman Member Posts: 7,573
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    Having and expansion tank and compression tank is a very odd combo.
    I suspect that the expansion tank has failed. The compression tank won't hold air because of the autovents.
    If the expansion tank has failed, it will usually sound waterlogged when you tap on it.
    If this is the case, your valve is working perfectly, and as you said "it is the symptom".
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • mikeg2015
    mikeg2015 Member Posts: 1,194
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    In the picture you posted, that’s not a PRV. That’s an automatic air vent. Those can get crude up and leak. Might be best to just replace it. Or take it off and try cleaning it out.

    However, you may still have pressure issues to check.
  • dbessey
    dbessey Member Posts: 12
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    Ok...so the pressure seems to be somewhat under control. The pressure relief still drips now and then when the system hits 25 psi....but i can live with that.

    Now, this has not solved the “hammering noise” i get when things start to warm up.

    This is not the regular noises of pipe expansion (though i guess it could be pipe expansion) but rather. A rhythmic hammering noise..lower in tone than the clicks of an expanding pipe or radiator.

    It almost sounds like someone in the house. Hammering quickly on the pipes with a rubbber mallet....

    Where do i start my search? Any advice appreciated

    darren