High Pressure in the Boiler

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  #1  
Old 07-05-10, 10:52 PM
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High Pressure in the Boiler

Hello,
Once again I'm looking at my heating system. A little back ground on this system. The system was in need of inspection, last year I had someone come in and service it and get it up to speed. At that time the pressure relief valve was leaking and the pressure in the system was regularly hovering at 30 psi. The guy pressurized the tank, cleaned the burners,readjusted the pressure reducing valve (automatic fill valve), checked CO and took some money and left. This guy never fixed the pressure problem. It was set when he left and slowly crept up to the point where the relief valve leaked.
I spent quite a bit of time replacing the valves on the second floor at the radiators (vane type) so they could be shut off and ended up with one that leaks.
So this year I had a technician come in and address the pressure problem and the leaking valve. He tightened the valve (didn't fix it) replaced the expansion tank and went on his merry way with more of my money.

The system has slowly increased in pressure back up to 30psi where the relief valve starts to let off the pressure. Heres the question, can I safely assume that the pressure reducing valve (automatic fill valve) on the system is no longer functioning? The technician last year "adjusted" the pressure at the pressure reducing valve, but i'm not sure how to set it correctly, or if its working any longer. Additionally, does the small leak (at high pressure) add to the pressure problems in the system?

Here are some pictures of the system.


The Valve.



The Plumbing around the boiler.

 
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  #2  
Old 07-06-10, 05:35 AM
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You can isolate the pressure reducing valve (PRV) with the shut off valve just before the PRV. Drain the pressure back to 12 psi and wait. If the pressure holds than the PRV is defective. If the pressure still goes up you may have a bad PRV and a bad shut-off valve. There is only two causes for pressure to go up number one is expansion and the second is water being introduced into the system.
To address both of them independently,
1. Expansion - when you increase the water temperature you will increase the pressure due to expansion of the water, If the tank air pressure is low or the tank is undersized the pressure will go up excessively. The pressure from a cold start to 180 should only change by a few pounds. When the system water cools the pressure should return to about it's starting pressure.
2. Bad PRV - this valve is to maintain the proper pressure in the system. In theory it should never add water once the system is full and all the air eliminated. If pressure goes up the valve is leaking by and must be replaced.
Upon inspection of the photo's you will always run the pressure a bit higher due to the fact the PRV is piped behind the circulator on the return side of the system. The circulator will drop the pressure on the inlet side of the circulator when it starts and fool the PRV into thinking the pressure is low in the system and the PRV will add some water. This will normally equal out to about 20 psi which will not affect the relief valve. The problem is the tank pressure is at 12 psi and the system pressure is higher than that, maybe 20 psi. The tank air pressure and the system fill pressure (static) should be the same.
You stated the pressure went up again after the tank was replaced. I assume you are not running the boiler. So we can eliminate water expansion and must assume the PRV is leaking by.
What type of radiation do you have in your home?
 
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Old 07-06-10, 08:21 AM
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Thank you for the advice. The radiators are fin type essentially just a tube with fins to radiate off of (not sure of the technical name).

The shutoff valve seems to be alright for the system as the pressure has stabilized at 20 psi without it on. So bad pressure reducing valve is probably the issue. Although adjustment is a possibility I'd rather replace it due to its age and fairly easy removal process. I will order a new one today.

So you suggest that the pressure in the system is going to remain slightly high (20) due to the placement of the PRV in the system?
 
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Old 07-07-10, 05:24 AM
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If the valve is leaking by adjusting it would not help. Adjusting pressure only works when you have a positive shut off internally on the PRV.
A 12 psi PRV (standard setting) will creep up to about 20+/- due to pump starting and stopping as piped now. The proper location for the PRV is between the tank and the system to hold 12 psi. The setting of 12 psi is good for a two story home or a tri-level home. If left where it is currently piped the air charge in the tank should be re-adjusted to about 18 - 20 psi, with no water pressure on the tank so the diaphragm does not overextend each time the boiler runs.
 
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Old 07-07-10, 04:08 PM
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I hope rbeck doesn't mind if I add a few more words to his explanation...

On the inlet side of the pump, aka the SUCTION side, when the pump starts up it can momentarily drop the pressure BELOW the static setting of the valve... in that instant that the valve senses that slightly lower pressure, it may open and spit a mouthful of water into the system. Over time, those mouthfuls of water will tend to increase the static pressure in the system.

There is a point in the system piping where the pump starting will not change the pressure. This is known as the "Point Of No Pressure Change" (PONPC)... (fitting name, eh?) ... and this point happens to be the connection point of the expansion tank...

Modern design principles dictate that the pump be "pumping away" from this 'PONPC'. There are a few good reasons to follow this principle, and if you use Google and put "pumping away" in you will find lots of articles to read about it (and probably some porn sites, but ignore them... or not)... also, Google "Point of no pressure change"...

Now, we aren't suggesting you repipe your whole system. There are literally millions of systems that are piped 'old school' and they work fine. This is just some background to help understand what is being said here.

About your expansion tank: There is an excellent PDF file that will help you gain a better understanding about that here:

http://www.amtrol.com/pdf/MC2680EXTROLBrochurelow.pdf

page 2 has a 'cutaway' diagram and an explanation, and page 3 has an example of a properly piped system 'pumping away' from the PONPC.

Another possible solution to your situation would be to fill the system cold to 12 PSI. Then SHUT OFF the manual water valve, and operate with the valve closed. One major manufacturer in fact recommends this. The DOWNSIDE to this is that you must do due diligence in monitoring the system pressure. If you spring a leak, and the boiler runs out of water, you are in trouble. For this reason, modern systems have a "Low Water Cut Off" (LWCO) installed that will shut the system down in the event of low water.

That's my $2.00 worth... adjusted for inflation.
 
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Old 07-07-10, 05:38 PM
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Originally Posted by rbeck View Post
If the valve is leaking - by adjusting it would not help.
Totally correct. Some auto fill valves may have rebuild kits available, but best to just replace the valve.
 
  #7  
Old 07-08-10, 10:20 PM
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Thank you for the clarification of the pump's action on the auto fill valves function. I certainly won't be relocating it, mostly due to the fact that a system that has been in service for 30 years can't be too bad off. Additionally, I don't want to disturb anything unless I have to.

When I have the system down to replace the valve, I am going to re pressurize the tank to ~20psi so the bladder isn't over-extended. I should have the auto fill valve today so I may get to this during the weekend. (Depending on the weather and number of beers that need to be dealt with in the fridge)
 
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Old 07-09-10, 08:07 PM
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I got the new automatic fill valve today and put it in. I isolated the boiler and tested the fill valve which was correctly set around 13 psi. The picture shows the new valve and the red arrow points to the fact that the system is at ~140 degrees in the picture. Yes I'm heating it up in the middle of july, so I can see if I need to do anything else with it. The pressure is @ ~16 psi but is slowly creeping up, I presume it'll top out around 20psi. I checked the expansion tank and it was pressurized to ~15psi so I didn't add any air.

Thanks again guys.

 
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