Boiler Pressure too high!

[Anonymous]

I have a boiler for home heat (hot water/baseboard) and the pressure relief valve is leaking because the pressure within the boiler is too high (the needle is pegged off the scale).

What could be causing this? I haven’t verified that the pump is faulty, but it appears to be working. The system seems to be adequately bled and the vent seems to be working.

I have little plumbing experience, but have lots of mechanical and electrical experience.

Please help!
Dan

[SylvanLMPParticipant]

HI Dan, Lucky you asked the right heating expert in hydronics (ME)

OK the probable causes of excessive pressure in a hot water heating system are the following

1- The expansion tank is flooded

2-The expansion bladder gave up either the ghost Or the bladder lost some of its charge

3- The automatic feeder NORMALLY set 12-15 psi is malfunctioning allowing street pressure in to the system

4- Someone opened by pass OR the fast fill feature of the automatic feeder

5- If you have a tankless coil this coil sprung a leak and is allowing FULL main pressure into this system

6- This boiler has excessive air in the system that is allowing this air to compress and thus build up pressure (air can be compressed and water cant)

7- Your Aqua stat controller (high limit) is not working and allowing excessive pressure/temperature relationship.

Most lightly it is the expansion tank OR tankless coil is applicable

[fourth yearParticipant]

The pump is a water moving device, it does not change to total pressure of the system. If the pressure is normal when the system is cool and high when it is hot, then you do not have enough air in the system, notably in the compression tank as opposed to a comment that you have too much air. Air, being compressible, is used as a moderating medium in heating systems to keep the pressure from making large changes as the water is heated. Your pressure reducing feed valve may have lost its adjustment, in which case you can back the adjusting screw out a little, or it may need to be replaced.

[SylvanLMPParticipant]

Hey Dan read this before the helper gets your home blown away.

I think the helper needs to start from 1st year again as it sure sounds like his instructors have FAILED in his training.

Why You Should Never “Dead-Head” A Circulator, by Dan Holohan – PlumbViews >> MasterPlumbers.com – The Largest Plumbing Reso

[Harold KestenholzParticipant]

fourth year, MOST of your statements are correct:

wrote on 21 October 2000 at 02:40 PM:
The pump is a water moving device, it does not change t[he]o total pressure of the system. … Air, being compressible, is used as a moderating medium in heating systems to keep the pressure from making large changes as the water is heated. Your pressure reducing feed valve may have lost its adjustment, in which case you can back the adjusting screw out a little, or it may need to be replaced.

The following statement was incomplete and misleading:
If the pressure is normal when the system is cool and high when it is hot, then you do not have enough air in the system, notably in the compression tank as opposed to a comment that you have too much air.

The intended idea was correct; but the wording made the statement incorrect. A properly sized and filled expansion tank will permit a low (usually the 12 psig fill valve setting) when the system is cool, and a higher pressure (less than 25 psig to prevent the relief valve from popping or weeping) when the system is at its hottest.

Adding more air to the expansion tank would raise the pressure and cause the relief valve to open, so letting air out might help by lowering the pressure.

Adding more air by adding another or larger expansion tank might solve a problem related to the improper sizing of an expansion tank.

Sylvan’s reference to Dan Holohan’s pump deadheading proposes the problem of raising the temperature and pressure of water at the pump by heating water trapped between closed isolating valves. This is an isolated situation where the pump does raise the local operating pressure within the system.

The small difference in wording is important when writing technical articles or refining code language.

[fourth yearParticipant]

Adding air to the compression tank requires injecting it either with a pump or draining the water out and allowing air to replace it. Either of these should be done with the system at zero pressure otherwise the tanks will not start with the correct amount of air. Nothing was said about trying to do it with the system at operating pressure, since that would involve changing the volume in the system and doing that with either air or water would increase the pressure.

[Harold KestenholzParticipant]

Correct again, fourth year. This added detail is a real help to the novice. Writing what was unsaid brings the unknown to the attention of the reader. You will be a good instructor to your apprentices when you follow this practice.

It is usually the part that is left unsaid that causes confusion. But, you will notice that Sylvan leaves almost nothing unsaid and there is still confusion. Information-giving is an indefinite practice, it only becomes education when the student can accept the information as useful.

[SylvanLMPParticipant]

Hi Harold,

please re read this

“I have a boiler for home heat [hot water/baseboard) and the pressure relief valve is leaking because the pressure within the boiler is too high (the needle is pegged off the scale).”

Since it says HOT WATER wouldn’t you also look for a tankless coil that may have sprung a leak?

Then after you shut down the domestic C/W valve leading to this coil wouldn’t you then Drain down this system to within the proper operating pressure of this system by taking the height of this building and figuring . 434 times the height of the highest radiator above the boiler or let it just balance itself out like a normal cold water start up?

This is how I set my attitude gauge on “normal” reading PLUS an extra 4 PSI for a positive displacement (BUT why confuse the helpers trying to learn the very, very basics)

Watch the gauge and if no change then open the domestic valve again as this would show a defect right away correct?

Now if the gauge doesn’t move isolate the expansion tank drain same Or if a bladder type remove from system check the charge and then re install back into service correct so far?

Now drain that the obvious is taken care of check the automatic feeder.

Then you
of course you can check the “flow control valve” is indeed opening when the circulator comes on.

I was trying to give Dan a step by step way to trouble shoot this kind of system as Dan is a mechanic in his own right.

Harold between us didn’t you ever see a low pressure hot water boiler operating in excess of 75 PSI relief valve setting?

I have inspected and serviced low pressure hot water boilers operating just under 160 PSI @ 250 degrees F.

Checking the obvious I thought was the way to go. Step by step, and Never assuming

[Dan SzwarcParticipant]

OK, here is some more information. First, I thank you all for such quick replies.

The expansion tank appears to be disconnected from the system. The valve to it is closed. It has always (7 years) been this way.

I opened a drain valve on the expansion tank and some really nasty, dirty water came out. I did not completely drain it, but I might flush it tonight. I have no idea if there is an expansion bladder in the tank. Is there any reason why the expansion tank would be locked out of the system (other than it leaking and the previous owner being too cheap/lazy to repair it)?

I shut off the feed to the automatic (12PSI) feeder and that did not eliminate the leakage, therefore the reducer valve is NOT faulty.

What is a tankless coil? I do not have any idea what references to a “tankless coil” mean. Like I said, I’m not a plumber, but an electrical engineer which means I think I know everything, but I don’t.

The Aqua-stat was replaced last year when high voltage fried it. The system worked fine all winter last year. It could be faulty again, but what exactly does it do?

I have checked a few (not all) bleeders and got no air. I will rebleed the entire system to verify as little air as possible is in it. Is there a minimum amount of air required in the system?

A reminder that this is a HOME boiler system. Not steam. It is in a ranch so there are floors above ground level.

Thanks for your help in advance!

Dan
[Edited by Dan Szwarc on 23 October 2000]

[Harold KestenholzParticipant]

If you do not want to suffer the possible consequences of this service procedure, ask a qualified professional to do this for you.

A hot water heating system is a sealed system. Water expands about 4% by volume when heated from about 50 to 240F, so an expansion tank accepts this ‘extra’ water. If there is no acceptable place for the water to go, the water will go out of the safety relief valve.

In the years around 1910 to 1930, the combination of an automatic feed and relief valve was used to add and release water as the wood or coal stoves cycled only 3 times per day and the amount of water lost was negligeable. Todays boilers cycle 3 to 6 times per hour. This brings in too much corrosive oxygen.

Expansion tanks permit the system to be a truly sealed system. The automatic feed, per manufacturer instructions, should remain shut until service is required to prevent fresh water makeup due to leaks. If the automatic feed water valve had been closed, the need for repair would have been more obvious and immediate because you would have bubble noises in the system and no heat from the upper radiation.

The total internal volume of the expansion tank must be about 4 times the expanded water volume made when heating the water from 50 to 240F. You can find how much water will be produced by letting your system cool down to room temperature (with the automatic water feed valve open) then close the feed valve and run the burners until the boiler shuts down on the high limit control. Just turn the thermostat all the way up.

Watch the boiler temperature to make sure the boiler temperature does not rise above this point. Shut the power switch down if it does rise too high. Cool the system and replace the Aquastat, then start the process again. If you do not want to suffer the possible consequences of this service procedure, ask a qualified professional to do this for you.

Make sure you have a bucket under the relief valve pipe to catch the extra water. As you have the mathematics to calculate the internal volume of an expansion tank, you will size it properly. It is OK if the new expansion tank is a little larger than required. The manufacturer has tables that indicate the acceptance volume of their tanks. If you would like more professional software, Amtrol has a web site with calculation tools.

The Aquastat is the electric control used to sense the water temperature within the boiler. The minimum function of the aquastat is to limit the highest temperature of the boiler. Residential boilers are required to remain below 250F. the Aquastat will shut off the power to the burner at 240F. The word Aquastat is the property of Honeywell. Other companies use a more generic name like water temperature limit control.

A tankless coil is a long coil(s) bundle that fits within the boiler to permit cold domestic water to enter the coil and leave (heated) to pass on to the taps. this requires a more complex limit control to set the boiler at a minimum and maximum domestic water temperature as well as limit the boiler maximum temperature.

There should be no free air in the system. When water enters the system from the street or well water there is about 4% of the water that is air by volume. By heating the water, the bubbles become larger and rise to gather at high spots in the system. This water should be bled from the system periodically by opening bleed valves when bubble noises become noticeable in the pipes. Automatic vents should not be left open, as oxygen can enter through openings by the law of partial pressures. Water inside a hydronic (hot water) heating system will retain about 2% entrained air that can not be removed. The air will become nitrogen as oxygen rusts ferrous metals within the system. After this, the water within the system is valuable as deoxygenated treated water. Be sure that the fresh water added is of good quality without excessive hardness or corrosive properties.

To see the proper piping, pictures of hot water systems and tankless coil instruction, visit http://www.hydronic.net

[Harold KestenholzParticipant]

As an additional precaution, it is time to replace the old relief valve with the manufacturer recommended model and pressure. They are inexpensive and a new, unobstructed safety relief valve is a worthwhile investment.

[SylvanLMPParticipant]

Excellent posting Harold as usual EXCEPT the

“A hot water heating system is a sealed system. Water expands about 4% by volume when heated from about 50 to 240F, so an expansion tank accepts this ‘extra’ water.”

Harold ever go into the older buildings where there is no Expansion tank (sealed system)

Only an open tank in the attic with an over flow to the basement or out doors

This of course doesn’t sound like the case I would think the existing tank was disconnected for some strange reason possibly a bad Air Trol tank fitting or the owner had no idea what it was for.

As for the tankless coil I happen to like using a Holby mixing valve exclusively to maintain domestic hot water under a manageable temperature to the fixtures other then the dish washer.
I like 160 deg wash water in residential a dedicated line.

[Dan SzwarcParticipant]

I thank you all very much for all the help you have offered.

I believe what I have learned is that I need to call the experts on this one. I could probably replace the relief valve and maybe a few other things, but I will let the experts determine why the expansion tank was locked out of the system and what else there might be wrong.

I certainly have learned a lot, but not enough to be confident that I won’t make things worse. Maybe if it was March instead of October. We need heat too much this time of year.

Thanks again!
Dan

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