Boiler Continues to blow 30 psi pressure relief valve

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Old 02-10-13, 07:15 PM
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Boiler Continues to blow 30 psi pressure relief valve

Hi everyone,

I have read hundreds of threads trying to solve this problem, so any help would be much appreciated.

My house has an older (50's vintage) American Radiator and Standard Sanitary natural gas hot water boiler with 6 cast iron radiators. (House is about 1300 sq. foot, 2 stories plus basement).

When the boiler is on, the pressure rises too high and the relief valve drips water.

The cold pressure is at 12 psi, the relief valve is set at 30psi.

I have already changed the expansion tank (and made sure that it was pre-charged with 12 psi air pressure), and replaced both the auto fill valve and the pressure relief valve. I have bled all air from the radiators (3 times to be sure, and there was not much there to begin with).

Any ideas why the problem continues? I think I have done everything obvious.

FYI, the new expansion tank is a watts ext 30, which replaced the "old style" large tank that hung in the rafters. Water temperature seems to be reasonable, maxing out around 150 degrees F. Pump seems to run fine.

Please let me know if you need more information. Any help would be great as I am sick of spending $$ on repairs and still having to empty a bucket of water.

Patrick
 
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Old 02-10-13, 08:00 PM
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Patrick, it sounds as if the 30 size tank is simply too small for your system.

You've got cast iron radiators. They hold a lot of water...

The more water that is in the system, the bigger the expansion tank that is required.

Think about it for a sec, how could that little bitty 30 size expansion tank do the same job as that big ole one?

Why did you replace the tank anyway? Couldn't you simply have drained the existing waterlogged one? I mean, if it worked for 50 years, couldn't it again?
 
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Old 02-10-13, 08:03 PM
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By the way, how do you know that your pressure gauge is accurate?
 
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Old 02-10-13, 08:29 PM
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To be honest, I'm not 100% sure of its accuracy. What I do know is that with the new valves installed, the gauge on the boiler read 13psi cold (with a 12 psi fill valve) and when the relief valve blows, it reads around 27. So while not 100% accurate, I think it is pretty close, particularly given its age.

Also, I read you other post on how to test the pressure with another 30 psi gauge on the drain, which was very helpful. I am hesitant to follow that procedure right now, since the drain valve probably hasn't been opened in 30 years and is likely to weep once I move it. I think that's a better project for the warmer months when I don't need heat right away.
 
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Old 02-10-13, 08:40 PM
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The old tank leaked air. I could drain it (with the valve to the expansion tank closed) and let it refill (about half with water), and in a day or two it would be completely filled with water again.

My choice of the 30 size tank was based on two things. First was the recommendation of the owner of the local plumbing supply shop (who is usually very helpful). Second was the watts website calculator:

Non-Potable Water Expansion Tank Sizing - Support - Water Safety & Flow Control -Watts


supply and air pre charge 12 psi
30 psi relief valve
initial temperature 90
max temp 155
I estimated there are 100 gallons in the system, but I really have no idea the capacity. Is there a convention for the number of gallons based on btu output? (the boiler is 131250 btu).



Perhaps the temp settings I put in are wrong, or my estimate of the system capacity is off.

Any additional input would be appreciated. I was starting to think that maybe the expansion tank was undersized, but I was hesitant to spend more $$ until I confirmed that to be the case.

Would the ETX60 or ETX90 be more appropriate?


Thanks again for your help.
 
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Old 02-10-13, 09:59 PM
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From the calculator I say etx 60 at a min.


Max temp 180f

Min temp IMO 140f

But I would be inclined to put a 90 in..... Borderline if you drop the temp to 130......
 
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Old 02-10-13, 10:15 PM
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Thanks for the advice.

The aquastat is set to start the pump at 140.

From what I observed, the pipes at the aquastat are about 90 degrees just from the pilot, and the temp never went above 155 even when the pump was running constantly.

That's how I arrived at my input for the calculator. Sounds like I was off.
 
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Old 02-10-13, 10:21 PM
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The boiler should make 180f hi..
 
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Old 02-11-13, 06:46 AM
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When sizing the expansion tank you need to allow for the 'extremes'... that would be a cold boiler, which I would take as 70, and the high limit setting of 180...

With the max pressure, you don't want to spec the actual relief valve setting, you want at least 10% less, so not more than 27 PSI... and I would use 25 PSI for 'headroom'.

There's no reason not to continue using the 30, you can tee off and install a second tank if you want. No sense in trashing a brand new tank. I would think that a 30 and a 60 would be more than enough...

A rule of thumb or 'sanity check' that I use is:

Water volume expands a bit less than 4% from 70 to 180 degrees.

If you have 100 gallons in the system (which may be a good guess...) when the water is heated, you would need a tank with an ADMITTANCE of at least 4 gallons.
 
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Old 02-11-13, 03:54 PM
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Thanks again. I picked up an additional expansion tank this morning. I may not have time until the weekend to sweat the new pipes and install everything, but I'll keep you updated when I get to it.
 
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Old 02-16-13, 10:49 PM
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Put in the additional tank this morning. While I had everything apart I also installed a bleeder valve in the system by the expansion tanks so that I can check the air pressure periodically without draining the whole system.

Ran the system from 64-75 degrees without blowing the pressure relief (went up to around 21 psi instead of 30, so everything seems good).

Page two of the following link may be helpful to anyone else trying to size an expansion tank:

http://media.wattswater.com/ES-ETX-ETSX.pdf


Thank you to everyone for all of your help.
 
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Old 02-17-13, 07:59 AM
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I can check the air pressure periodically
Reminder... this is something that should be done at LEAST every two years, if not annually at the start of the heating season. Now that you've got the valves installed, it's real easy!

Did you add a 60 ?
 
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Old 02-17-13, 12:42 PM
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I ended up using a second 30. Looking at the specs, I realized that the 60 is not twice the capacity of the 30 - it is only slightly larger. So with the two 30s installed I have the equivalent of somewhere between a 60 and a 90.

My other rationale - although not necessary good logic, was that the second 30 fit better above my boiler, and it was cheaper.
 
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Old 02-17-13, 12:47 PM
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Symmetry is always nice too!
 
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