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Thread: Boiler getting overpressurized -- reaches 30 PSI and PRV releases water

  1. #1
    DIY Junior Member BrianW's Avatar
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    Default Boiler getting overpressurized -- reaches 30 PSI and PRV releases water

    I just bought a house in August that has a boiler. Iíve never had a boiler before Ė what little I know, Iíve learned from reading the excellent posts on this forum, as well as a few other websites. I have a problem with pressure, and would really appreciate some help.

    Hereís the brief description of my problem: My boiler runs at a constant 16 PSI during the day when the heat isnít on. At night, when the heat kicks on, the boiler runs up to 30 PSI and triggers the pressure relief valve, draining water out. It also makes a fair amount of noise Ė some of the pipes are making loud knocking sounds, and thereís a much quieter knocking coming from inside the boiler. Based on what Iíve read, Iím guessing that most or all of the symptoms Iím experiencing are because the system is becoming overpressurized. Iím trying to figure out why.

    Other information: Itís a Burnham P209. Itís installed in a closet in our finished basement. A note written on the wall in the closet has the date 9/12/88 and indicates that it should be run at 23-25 PSI. I presume 1988 is when it was installed, although I donít know that for sure. The house was expanded significantly around the same time Ė 7/20/88 is when the building permit for the expansion was granted. It would make sense that a new larger boiler would accompany the expansion. The house is 3600 square feet, three floors, and has a four-zone heating system.

    I have an Amtrol Extrol 30 diaphragm expansion tank. From what Iíve read, overpressurization when the water is being heated seems to most often be related to a problem with the expansion tank. I did a simple test on the tank, touching the outside of it while the system was under high pressure. Roughly the top 1/4 of the tank was hot to the touch, and the bottom 3/4 was cool, indicating that the bladder is still holding air. I donít know of any other quick tests I can run on the expansion tank.

    Thereís a pressure and thermometer gauge on the outgoing pipe, right after the water comes out of the boiler and just before it goes through the expansion tank. Thereís also a pressure and thermometer gauge on the left side of the casing around the boiler. These two gauges give very different readings Ė the gauge on the pipe reads higher for temperature, and the gauge on the boiler reads higher for pressure.

    When the boiler heated up tonight, the gauge on the pipe got up to 31 PSI and 210 degrees. The gauge on the boiler reached 38 PSI and 185 degrees. When the gauge on the pipe hits 30 PSI, water starts getting released by the PRV, which is rated 30 PSI, so that seems to confirm the PSI reading from the pipe gauge. I donít know which temperature reading is correct.

    Around 200 degrees on the pipe gauge, the boiler shut off, but the temperature continued rising to 210. After that, it dropped down to about 160, and the boiler went back on, taking the temperature back up to 195 (175 on the other temperature gauge). The pressure again went up to 30 PSI, releasing more water. Itís not a lot of water getting released each time Ė Iíd guess less than a cup.

    There are two things I suspect may be going wrong here:
    1. Primary issue: The system is getting up to 30 PSI, triggering the PRV. My quick touch test seemed to rule out a complete failure of the expansion tank Ė the tank isnít full of water. What could be causing the pressure to rise so much?
    2. Secondary issue: Temperature probably shouldnít be reaching 210, so it seems like thereís a problem there too. Could be that the temperature gauge on the pipe is reading too high, or it could be that I need to lower the high temperature limit on the aquastat. (The aquastat is a Honeywell L8148E.)

    Iíd really appreciate any help on what to try next to diagnose and solve this problem! Thanks a lot!!

  2. #2
    In the Trades Tom Sawyer's Avatar
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    210 is way too high for the limit. Drop it to 180 or 190. Touching your expansion tank means nothing. Tank pressure needs to be tested by dropping the system pressure to 0 and then checking the tank. It should be around 12 - 17 lbs.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Unless you have an unusual situation, most residential boilers run around one atmosphere - 14-16 pounds...running it nearly double, so close to the relief valve setting, is counter productive. IF the expansion tank is intact and sized properly, it should easily keep the pressure within the static, cool pressure range. If your pump is not sized properly, or there's a restriction, it's possible they set it up that high (in pressure) to keep the boiler from flashing to steam because the water isn't passing through the boiler quick enough and getting too hot on the pass. If that's the problem, it needs to be addressed.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4

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    The banging and noise you notice could be a result of the boiler making steam. As mentioned above, lower the temperature to 180.

    Another check of the expansion tank is to tap the top of the tank with a wrench, it should have a dead sound. Hit the bottom of the tank and it should ring just a bit. If you get a dead sound from the bottom part of the tank it means that the tank is full of water and the bladder has failed.

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    DIY Junior Member BrianW's Avatar
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    Thanks a lot for all the replies!

    The aquastat is wedged in between the boiler casing and the wall, so it was hard to get to. With a headlamp and a mirror, I can now see that the aquastat has a high temp setting that can range from 180 to 240 degrees. It was already set at the minimum, 180. Despite that, I've seen the temperature on the gauge read as high as 210, so I don't know if the aquastat or the gauge is faulty.

    I tried the "tap a wrench on the expansion tank" test. The tank has a dead sound on top (as expected), a hollow sound if I tap it on the side near the bottom, and a dead sound if I tap it right on the bottom. Does that mean it's likely that the tank has failed? If so, it sounds like the next step to take the system pressure down to zero and measure the tank pressure? Any other suggestions?

    Thanks again for all the help!

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    Assuming it is a screwp-in temperature gauge, when you have the presure drained off, get a new one and install it. If you take an instant read meat thermometer and wrap some insulation around it to hold it right on a pipe, after a few minutes, it should give you a close approximation of the temp. An IR thermeter would work, but kind of expensive for a one-time use!

    The aquastat may be defective. On the expansion tank, it's probably shot. If you turn the autofill off, then momentarily open the valve on the bottom of the tank, if you get any water out, it's shot. A really shot burst of air won't change things much, but I think you'll probably get some water. Also, after you get the cap off, it may be obvious if it is all rusty underneath, another indication (not always, though).

    There are only a few reasons the pressure would rise on its own: expansion tank shot or non-existant; a defective fill valve; a bad indirect WH or tankless coil. The easy one is probably the expansion tank, and the most common. You can often replace it without tools. All of the other choices require tools and would probably cost more to fix.
    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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    Your tank sounds normal. I just went down and checked my tank (which is only two months old)...

    Top 1/3 I get a dead "clank"
    Bottom 1/3 I get a ring
    Bottom of tank near the air valve I get less of a ring, but a ring none the less

    I think it would be odd to get a ring from the bottom 1/3 and no ring at all at the bottom. A failed bladder is a failed bladder.

    Regarding the zero pressure test, turn off the boiler, close the auto fill ball valve, and then isolate the zones with the ball valves. From there you'll need to open the bleed valves for the zones. You aren't looking to drain the zones, just release pressure. The water will flow for a few seconds at a rapid rate and then drop-off. Close the valve when it drops off. After the pressure is released from the zones the pressure gauge on the boiler should read zero.

    Now you can check the bladder pressure. If you get water from the valve, you've got a bad bladder.

  8. #8
    In the trades Dana's Avatar
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    If the circulator pumps are also circa 1988 it's conceivable that you're not getting sufficient flow to keep that monster-sized burner from taking the boiler over-temp. DO check that out (as jadnashua suggested), or have a pro do it for you. That's a pretty serious 9-plate boiler with an output well north of 200KBUH, and probably 3x oversized for the load (or more), and it doesn't take long for it to hit the steam flashpoint if a pump motor is running half-speed or the impeller is corroded to the nubbins.

    That boiler/system is also old enough to vote (and maybe even smarter than some voters? :-) ) On day-1 it was running a combustion efficiency of ~80%, with an AFUE of ~78%, provided it was no more than 1.6x oversized for the design-condition load. If it's 3x or more oversized it probably started out life in the low-70s, and is hitting around 60-65% now, after decades of corrosion & scale on the fire side and modest liming on the water side of the plates brings it's max steady state efficiency into the low-mid 70s. If properly installed & adjusted, a right-sized modulating-condensing boiler would deliver 90% AFUE or better, reducing the heating bill by 1/3 or more.

    Even an old schoolers "lessee, 3600 foot taims 35 btu a foot give ya 126 thou BTU" type of heat load calcs would put this beast at ~1.7x oversized (~220k/126K) and if the place has any insulation at all and doesn't leak air like a wind tunnel that calculation would be more than 2x oversized. Simple reckoning says that the real load would likely be under 75K, making it more than 3x oversized. It happens a lot, even now, and was even more common 20+ years ago.

    Before pouring money into anything other than getting it though the season, it's probably time to think about replacing it, and using some intelligence to size it correctly. With a mid-winter fuel-use bill pegged to exact dates by which we could look up heating degree-day weather data it would be easy to put a whole-house upper bound on it for boiler sizing. (Alternatively an annual use against typical seasonal HDD would get you close enough.) The 99% outside design temp for Boulder is 0F, and even though it gets colder than that at least a few times a season, a boiler rated EXACTLY for the heat load at 0F would never leave you cold, and at even 10% oversizing you'd be 100% covered down to -8F. Of course you never know, it might hit -190F one of these winters, and you'd be glad to have a 3x oversized boiler in place when it does, eh?

  9. #9
    DIY Junior Member BrianW's Avatar
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    Good news: The expansion tank needs to be replaced. I used a tire gauge to check it, and water squirted out rather than air.

    I thought this might be a cheap job ($100 for parts and $100 for labor, maybe?) so I called two local plumbers. Both quotes were in the $500-600 range. Pricey! For $200 or so, I would have been willing to hire someone, but at $500-600 I'll do it myself. Sounds like it should be pretty easy. Now I need to acquire parts...

    Looks like Lowe's and Home Depot carry Watts expansion tanks rather than the Extrol tanks. I see the Watts ETX-30 on the Lowe's website, sized at 4.7 gallons, for $33. Sounds like the same size as the Extrol 30 -- would I be OK using the Watts tank then? Is Extrol a more reliable brand, or is there any other reason I'd want to go with the Extrol? Best price I see on the Extrol 30 is $56 shipped.

    I also see a Watts temperature/pressure gauge for $19 on Lowe's website. Since the temperature reading from my gauge is suspect, I'm planning to replace this too.

    Seems like that's all I need, plus a wrench?

    One other question: When I depressurized the system to check the expansion tank, I released water from each of the four zones one by one, closing off three zones and opening the drain valve for one. After I was done, two of the valves (one of the drain valves and one of the valves to close a zone) were dripping a little bit of water. After a day or so, the dripping went away. Is this anything to be concerned about? Should I be more careful somehow when I close and open these valves the next time?

    Thanks again for the help!

  10. #10
    DIY Junior Member BrianW's Avatar
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    Replacing the expansion tank was more of an adventure than I expected. On top of the expansion tank are the air purger and air vent, with a small valve between them. I guess the valve was placed there to allow replacing the air vent without having to take the system pressure down to zero? Anyway, that valve has leaked, so the air purger is rusty, and the expansion tank was rusted solid into the air purger. With some help, I managed to break the tank free from the rust and replace it.

    OK, great, now I have a brand-new expansion tank. But I see absolutely no change in the pressure profile of the boiler vs. before, when I had the waterlogged expansion tank. During the day, when the boiler has been off for a while, pressure is 12 PSI. When the boiler fires up again at night, pressure spikes to 30 PSI, the relief valve lets off some water, and then overnight as the boiler is cycling, pressure stabilizes in the range of 22-26 PSI.

    For what it's worth, the boiler cycles pretty quickly overnight Ė itís on for 2 minutes or so before shutting off again.

    So I havenít fixed the pressure problem at all. But I know the expansion tank is good. Whatís the next most likely suspect? From all the replies I received before, it seems that the possibilities are:

    1. Expansion tank is too small. I had an Extrol 30 before, so I replaced it with a new Extrol 30. But I donít know for sure that thatís the right size. How could I find out?
    2. Fill valve is leaky, allowing water in when it shouldnít. This doesnít seem to be the problem Ė pressure is at 12 during the day when the boiler is off, and doesnít rise above that. Seems like the fill valve is letting in just enough water each day to replace what the relief valve leaks overnight, and no more.
    3. Problem with indirect WH or tankless coil. I donít know what these are. How would I diagnose this?
    4. Taco pump undersized or defective. How would I diagnose this?

    Thanks again so much for all the help Iíve received so far. Itís disappointing that simply replacing the expansion tank didnít solve the problem, but I hope with everyoneís help I can get to the bottom of this.

    And thanks Dana for the advice to just do as much as necessary to keep the boiler running this winter and then replace it. That makes sense to me, but when we had a pro up to the house to look at the boiler, he gave an estimate of $15K for replacement! Given my annual gas bill of $1000 or so, even an efficiency improvement from 60% to 90% would take many decades to pay off. So Iíve been of the mindset that I should keep my oversized, old-enough-to-vote boiler running well into middle age if I can. But if Iím thinking about this the wrong way, Iíd really appreciate feedback.

    OK, thanks again, and happy New Year!

  11. #11
    Retired Defense Industry Engineer jadnashua's Avatar
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    Before pressurizing the system, did you check the precharge pressure in the expansion tank? It should be the nominal pressure you fill the system to, which is usually somewhere near 14-16#, dependent on the system design. Most relief valves on domestic boilers are set at 30psi, so running it at 1/2 gives 100% margin. You can't check the expansion tank's pressure when you have water pressure applied, since the air pressure will then equal the water pressure. Check out http://www.watts.com/pages/support/sizing_ET.asp for sizing verification. You'll need to know or be able to calculate the volume of your total heating system's water supply.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    In the Trades Tom Sawyer's Avatar
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    Take a picture of the boiler, front on so that I can see the whole front of the boiler and I will get back to you.

    There are only a few ways a boiler can gain pressure.

    1 - the expansion tank is bad or has no air charge in it
    2 - the auto-feed valve is leaking by
    3 - the high limit temperature is set too high
    4 - you have a tankless coil for making domestic hot water and it has a leak in it allowing house pressure into the boiler.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    DIY Junior Member BrianW's Avatar
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    Thanks for the helpful replies! At Tom's request, I've attached a photo of my boiler. Thanks for the suggestion -- I should have done that a while ago.

    Jim -- Yes, before I installed the new expansion tank, I used a bicycle pump to pump it to 18 PSI. It seems entirely possible that an Extrol 30 might be too small for my house, and I'd love to use the Watts calculator to find out, but I just don't know the volume of water in the system. Is there any way to estimate? From the size of the house and number of zones, you might be able to estimate the total length of the pipes, but you'd also have to know the diameter of the pipes, right? Seems like a lot of guesswork. So short of draining all of the water and measuring it, I don't really know how I would estimate.

    Tom -- Thanks for the clear list of possible suspects. I don't think my boiler is tied to my hot water at all. I have two closets in my laundry room -- one contains the boiler, and the other contains my water heater. They seem totally separate, except for the fill water.

    And the pressure drops to 10-12 PSI during the day when the boiler's not running and stays there, so it seems like I don't have a leaky auto-feed valve.

    You mentioned the high limit temperature being set too high as a possibility. The aquastat is set to the lowest allowed temperature -- 180. But it definitely seems possible that the water is getting too hot and flashing to steam. When the boiler fires up, it only takes two minutes or so for the temperature gauge to jump 20-30 degrees and then the boiler shuts off. Seems possible that the boiler is overshooting and producing steam, due to an undersized or faulty pump, perhaps.

    I'm really enjoying learning about my boiler and debugging this problem, and I'm confident that we'll get it figured out, but I wouldn't have the foggiest idea how to approach it without your help. Thanks so much for all the support!
    Attached Images Attached Images  

  14. #14
    Retired Defense Industry Engineer jadnashua's Avatar
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    231 cubic inches = 1 gallon.

    So, estimate the length of the piping, and use pi*r^2*length (in inches) to get the volume (you may need to do several calculations for estimates of the various diameter piping and rads), then divide by 231 to get gallons. Figure maybe a couple of gallons for the boiler itself (or see if it says in the manual) and you should be close. A larger expansion tank only hurts when you buy it, one too small will give you the symptom if it gets saturated/filled-up. If there is air in the system, when it gets heated, it will expand more than the water...purge the system of air and check your air extraction device's operation, or add one if you don't have one.
    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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    DIY Junior Member BrianW's Avatar
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    Thanks Jim. Here's my quick math:

    House is at most 40' x 60', so perimeter is 200'. Suppose piping for each zone goes all the way around the perimeter of the house. 200' x 4 zones = 800'. Add extra footage for supply and return between the levels of the house, and I estimate 890' = 10,680 in.

    Assuming pipe diameter of 3/4", radius is 3/8", and pipe cross-section area is 3.14 x 0.375 x 0.375 = 0.44 sq in.

    So total volume = 10,680 x 0.44 = 4716 cu in.

    Using 231 cu in per gallon gives 20 gallons. Adding a few for the boiler itself I have maybe 25 gallons at most.

    This is probably a conservative (that is, high) estimate -- each of the zones actually covers about half of a floor, so total pipe length is surely less than 890'.

    The rest of my inputs to the Watts calculator are in the screenshot attached. Calculated total volume is 1.69 gallons and acceptance volume is 0.74 gallons. Extrol 30 yields 4.5 and 2.5, respectively. Given that I'm pretty confident that I overestimated the water volume, it seems like the Extrol 30 should be more than sufficient size. Does that seem right?

    Thanks!
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    Last edited by BrianW; 01-07-2012 at 07:01 PM.

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