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Thread: Boiler Pressure & T&P Relief Valve

  1. #1

    Default Boiler Pressure & T&P Relief Valve

    I have a Weil-McLain WGO-4 boiler. I am using the T&P Relief Valve that came with the kit; 30 psi, 375,000 BTU. I get about a quart of water out of the T&P every week. I'm wondering if the T&P is marginal or do I have too much pressure in the boiler. I opened the T&P a few days ago for a few seconds to see if it just needed to reseat, and it ran without stopping, putting out about 4 or 5 gallons of water before the flow reduced. Even so, it put out about an additional gallon before it settled back into it's usual output of a quart a week.

    Here is a photo of the T&P gauge on the boiler:



    As you can see, I'm running about 185 degrees and 33 psi at the gauge (photo was taken about 2 minutes after the boiler stopped running). Should I simply lower the temperature? The behavior of my T&P relief valve test makes me think the valve itself could be the issue.

  2. #2
    In the Trades Jerome2877's Avatar
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    The releif valve is doing its job. It is set to release pressure at 30 psi and you are exceding that. Your expansion tank is the problem here. If its an old bladderless tank you can drain it, if not change it out. The pressure should be between 12 and 15 psi.

  3. #3

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    I never thought of the expansion tank. It is an Amtrol Model 30. I went to check the air pressure (12psi) at the bottom of the tank and got air and water. So I assume I need a new tank.

    This is the second expansion tank that I've had to install on this boiler in 10 years. What makes them fail so soon? Or is 5 years a expected life for the expansion tank?
    Last edited by Gary in NJ; 09-20-2011 at 10:03 AM.

  4. #4
    Retired Defense Industry Engineer jadnashua's Avatar
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    Most last a bit longer. But, your symptoms are classic expansion tank failure. I don't have enough experience to recommend one brand over an other, but you might try a different brand of the same size (and are you sure it is the right size - this can affect its life as well). And, try pressurizing it a little closer to the normal operating pressure of your system. That would mean the bladder would expand and contract less and may help it last longer.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  5. #5

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    I installed a new Watts ETX-30 expansion tank last night. I checked the pressure at 12 psi prior to installing it. I checked the boiler this evening (24 hours) and still have some water coming from the T&P RV. It's less then it was, but I was hoping for zero.

    I'm sure this is just a slight adjustment of the air pressure. Any suggestions? BTW, the gauge is still showing 33psi, but I didn't expect the tank to change the pressure in the boiler itself.

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    Do you have an autofill valve? If so, it could either be incorrectly adjusted, or defective. Or, if you don't, the fill valve may not close completely, letting water slowly enter the system. Or, if you have an indirect or heating coil, there's a leak in the heat exchanger. Any of those things can increase the pressure in the system. Also, once the T&P gets old, it can get some mineral deposits and fail to properly close (or more catastrophically, fail to open if needed). But, at 33#, it should open. Further investigation is required to figure out where and why.

    Water out of your expansion tank indicated it was bad, but it appears you have a second problem as well.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  7. #7

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    I believe that this is the autofill valve.



    It appears to have the ability to be adjusted. I assume if I loosen the jam nut and turn the arm clockwise I will decrease the flow into the boiler, thereby lowering the pressure.

    The indirect water heater is a tight as a drum (I did the repair myself just a few weeks ago).

    My gauge is always showing 33psi, whether the boiler is running or not. The T&PRV only seems to open upon shutdown of the heating cycle.

    Should I try an adjustment of the autofill? Is this done in one-turn increments?

  8. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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    First, I'd try this: get your water pressure set to 15#, then turn off the valve below the autofill. See if the pressure stays stable over say a day. If it does, that isolates it to a leaking autofill valve. If it is stable, you can try adjusting it, but it is more likely a candidate for a replacement. See what the pros say, but it's my feeling, that once they stray from their initial setting, readjusting them ususally doesn't work-the seals are probably shot. This would also let you find out if your T&P valve will seal and doesn't also need to be replaced.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  9. #9
    Engineer Furd's Avatar
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    First of all, boilers DO NOT have "T&P" valves, they have safety valves that operate on PRESSURE ONLY! Good practice is to change the safety valve every five years.

    If your gauge is always reading 33 psi then the gauge needs to be replaced. Without a properly functioning gauge you are "flying blind" and don't have a clue as to what your system is doing.

    Depending on the size of the system a "30" size expansion tank may be too small.

    Replace the gauge and replace the safety valve. You may also need to replace the Pressure Reducing Valve (PRV) on the make-up water. Most residential hot water heating systems should run at about 12 to 15 psi cold and less than 20 psi when hot.

  10. #10

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    It didn't occur to me until today that the pressure is stuck at 33 psi. I've never really paid much attention to it until recently, and today I realized that I never saw any other reading.

    I replaced the "30" series expansion tank with another 30 only because that is what has always been there. I used the Watts calculator to determine the correct size an my result was a calculated total tank volume of 2.3 gallons, so a 30 series would be correct (600 to 700 lf of 3/4 pipe/baseboard results in about 17 gallons plus 6 gallons for the indirect and another 14 for the boiler = 38 gallons).

    I will replace the gauge and the safety valve, since they are easy. From there I can make an educated guess if the make-water PRV needs attention. From what I've read, if the boiler isn't leaking or venting, the make-water valve (autofill) could remain in an off position since once filled, the water in the boiler is permanent. I've also read that it is desirable to not have the safety valve blow off as that water needs to be replaced, which introduces oxygen and minerals into the closed loop system, which can cause corrosion.

    I might as well correct this right because in a few weeks the heat will be in use and I'd rather not shut down the boiler mid-winter to address a problem.

  11. #11

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    I installed a new Temp & Pres gauge on my boiler this afternoon. The pressure is right on 20psi; a good indication that it's time to change the relief valve.

    Thanks for the advice guys.

  12. #12

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    I now see that my boiler is running at 28 psi with the new relief valve. In another thread, Jim gave me this advice:

    Quote Originally Posted by jadnashua View Post
    You mentioned the valve at the expansion tank...that air pressure needs to be set when the plumbing system is NOT pressurized. If you opened that valve and didn't restore the pressure, it is not set properly, and you are liable to get an overpressure situation, especially if all zones are calling for heat and you get all of the air out of the system. If you're nominal pressure is 20# (14-16 is more normal, but 20 may be called for depending), then the expansion tank should be just under that pressure as a precharge. Once the system is pressurized, the expansion tank will read the same as the system water pressure....but, you need to set up the precharge properly, or there'll be no room for the water to expand.
    Do I need to remove the pressure from the system before I adjust my precharge in the expansion tank? Assuming that the precharge is 12 psi, how many pounds of pressure should be removed?
    Last edited by Gary in NJ; 10-12-2011 at 05:09 PM.

  13. #13
    Retired Defense Industry Engineer jadnashua's Avatar
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    When the system is pressurized with the water, the air pressure will be exactly the same. The only way to 'blow up that balloon' properly, is when it is not being affected by the water pressure. So, you have to remove all water pressure from the pressure tank's inlet, and then, you can check its precharge. To minimize the deflection in the expansion tank, it works best if the precharge is at or slightly below the system's normal working pressure. Minimizing deflection means less wear on the bladder and can extend its life at least some. Too little pre-charge, and you could be in a situation where you have no room to add new water when it expands, or the pressure will rise too high, and then blow some off through the pressure relief valve.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  14. #14

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    I might also mention, when I checked the pressure on the 29th, I had all 4 zones running. This last week I've only been running the DHW zone. Could this impact the reading? I just want to make sure I'm not chasing squirrels up a tree here.

  15. #15
    Retired Defense Industry Engineer jadnashua's Avatar
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    The whole purpose of the expansion tank is to provide room for the water to expand when heated, and exhaust that water when cooled to maintain the constant pressure in the system. If anything, when all zones are off, the cool water might cause the pressure to drop, not rise, if the expansion tank had essentially emptied its contents.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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