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Boiler Temperature/Pressure Too High?

HTM Member Posts: 29
Thanks to several helpful posters on this site I was able to purge air from one of the zones in our hot water system that wasn't producing heat late last year. If memory serves me, Dan the Heater Man offered great detail. Belated thank you. Honestly, I held off for awhile, being a little gun shy that I'd get it right. As it turned out, no need to hesitate. Heat restored!
System is an old Weil-McLain P-5663E-W series 3 boiler, Beckett AF burner, Honeywell L7224U electronic aquastat, five zones (zone valves), Superstor indirect hot water.
A month ago I repeated the exercise on a different zone. Another good result. However, a few days ago I checked on a loud clacking noise coming from the valves/large diameter pipes (manifold) above the boiler. Temperature/pressure readings were much greater than I'm used to seeing - about 225 degF/22 psi. Essentially, water in the boiler was boiling. I thought these were supposed to be in the area of max 180degF/15 psi. Also noticed the expansion tank was empty, which had never been the case before. Flipped the boiler switch to "off".
My best guess is that the boiler drew off water from the expansion tank to try to make up for too much water drained while purging air. Estimate approximately 7 gallons removed
initial purge and 10 gallons the second. The next morning (after a cold night), used the "fast fill" feature to add water to cold system for 8-9 minutes. I don't know how much volume that might have been. Is there a valve that automatically adds water to the system when needed?
After re-start and then running a few hours, boiler temperature and pressure were still quite high: 220/225degF and 21-22psi. Since then, I've lowered thermostat readings in the house to reduce calls for heat and boiler running time. This has managed to hold temperature and pressure no higher than 210degF and 18 psi, but pioneer living isn't comfortable. These readings still seem higher than they should be. A plumber acquaintance of mine who does heating came by but doesn't have any good answers or solutions.
The boiler sounds fine when it does operate, but I'm reluctant to run it normally until I can learn more: for example - appropriate operating temperature and pressure levels;
whether the aquastat is working properly re. limits that can be set/maintained; should the
expansion tank be about 1/2 full of water (it's empty, now); system possibly still short of water.
Or, something else I should look into.
Thanks in advance for reviewing and your guidance.


    EBEBRATT-Ed Member Posts: 14,534
    As far as the expansion tank goes if it's a steel compression tank (no air bladder) it should be about 1/2 full of water. If you have a bladder expansion tank it should have 12 psi on the air side of the tank when it is removed from or isolated from the system with 0 water pressure against the bladder.

    The boiler should have 12 -15 psi when cold and around 20 when hot.

    Operating temp is set by the boiler hi limit aquastat which should be set at 190. No need to have the boiler hotter than that.

    To test the high limit take the cover off the aquastat and turn the thermostat up to start the boiler. Then turn the high limit down. The burner should shut off and the circulator may still be running. Set the high limit to 190

    I am guessing you purged or vented the zones without adding water to the system. There should be a pressure reducing valve that adds water automatically ...maybe it.s not working.

    The procedure is to fill the boiler maintain 15 psi while purging and venting all the zones.

    Then it should operate ok.

    If the temp rises over the high limit setting you probably need a new control.

    If the pressure rises much over 20 or so your expansion tank is the problem most likely or it could be your PRV feeding water and leaking by or it could be the superstore coil leaking

    In any event,
    1. test the high limit
    2. purge and vent and start with 15 psi in the boiler

    Then report back with any problems

    Pictures will help
  • HTM
    HTM Member Posts: 29
    Thank you, Ed.
    During the two purging exercises I used the fast fill feature to bump up the pressure and improve draining. I did not fast fill after purging. I assumed the valve feeding water automatically compensated for what was drained.
    Raised thermostats a bit over the weekend to monitor boiler temp and pressure. Approximately 205-210degF/16-17psi. Room temps 2-3 deg below our normal. Both are Improvements over a few days ago.
    I will try testing the high limit tonight.
    Much appreciate your assistance.
  • HTM
    HTM Member Posts: 29
    Ed - Removed aquastat cover. Red digital readings of temp started @ 148F. Boiler gauge showed 180F and 13psi. Boiler had been off awhile.
    It began running and after a bit, red temp. was 162F. Boiler gauge 190F and 14 psi.
    Last aquastat reading 189F. Boiler gauge 225 and 17 psi. A little later, the boiler shut off
    on its own.
    I don't know how to go about resetting high limit. Aside from that for the moment, is the temp. reading in the aquastat supposed to match boiler gauge temp?https://photos.app.goo.gl/txHvGFLk1K5bNHbw9
  • WMno57
    WMno57 Member Posts: 1,024
    Your photos show a plain steel air over water compression tank. There are three rust stains on the side of the tank. Are these pinholes? Was this tank abandoned years ago? Was the expansion space in your system partially filled radiators, which you recently purged?

    You will get more help if you post your photos on this site.
    I DIY.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,736
    edited December 2021
    HTM said:

    ...is the temp. reading in the aquastat supposed to match boiler gauge temp?https://photos.app.goo.gl/txHvGFLk1K5bNHbw9

    YES, Close but may not be exact.

    It appears that your boiler temperature gauge and the boiler Aquastat L8224 are taking temperature readings from different places in the boiler. But if the water is moving thru the system, the temperature should not be 30° different. Perhaps the temperature gauge is incorrect. They are inexpensive gauges. Do you have a digital thermometer? The kind with the probe that is wired to the digital meter, or a meat thermometer? If so, Record the temperature then remove the aquastat probe from the boiler and place the thermometer in the opening and wait for the thermometer to settle on a temperature. Record that reading. Then place the probe from the Aquastat back in the boiler and wait for that temperature to settle. if the temperature is close on all three readings then you can be sure the aquastat probe is working just fine.

    Are the thermometer / pressure gauge and the Aquastat Relay L7224U in the original location on the your boiler? they should be HERE

    If they have been moved at some point over the years, then a picture of the current location can help in solving your problem.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,736
    edited December 2021
    I just looked at your pictures and the Aquastat and temperature gauge are in the correct places. You may have some build up on the Well Adaptor inside the boiler that is making the transfer of temperature move slowly to the Aquastat. That Temp-Alt-Press. gauge is not original. It may have been changed some time ago.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • HTM
    HTM Member Posts: 29
    Sorry the photos weren't transmitted well.
    No pinholes or ever a sign of leakage in the many years we've been in the house. Tank installed prior to
    our arrival. I will treat the surface rust before it gets too far along.
    Earlier this year the tank seemed to be about half full when tapped on the side. Can't say when it went empty as I didn't examine it before or just after the two purgings. Boiler use after initial purging was very late in the heating season, then indirect hot water spring, summer and into late fall.
    The air/water removed during purging came from an isolated zone, so I would answer that the expansion space was partially filled pipe, baseboard and radiators (purging #1), partially filled pipe and baseboard
    (purging #2).
    Thank you.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 6,736
    edited December 2021
    I looked at your pictures. You do have an auto feed valve. It is baby blue

    The manual valve must be left open for the auto feed to work. If the manual service valve is closed on the feed line then no water will get in to maintain pressure.

    Some purest believe you should only use the auto feed to fill the boiler and remove air from the closed system. After completed, then close the manual service fill valve. I was like most technicians, leave the manual valve open and allow the auto fill to maintain pressure all year long.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • HTM
    HTM Member Posts: 29
    Thank you, Ed, and to all who have weighed in to help. Hope you are enjoying retirement.

    1. Feed line valve is and has been open. That was confirmed hearing water moving in the line during fast
    fill last week. The fast fill volume added was guesswork, though. Assume valve's automatic fill feature
    would supply any additional water needed. That presumes it is working properly.

    2. Late this morning, outside temperature a little higher than last two days. Boiler has not been on for
    a few hours. Aquastat showed 136F, gauge 159F/14psi. Seems to be a great enough difference that
    one or the other could be off. Believe it's worthwhile to try an inexpensive test with a replacement
    temp/pressure gauge.

    3. Any theories on how the expansion tank emptied? Should it always have some water in it?

  • HTM
    HTM Member Posts: 29
    The boiler must have come on and I didn't hear it.
    At 12:40 pm: Aquastat 174F. Boiler gauge 205F/14psi.
  • HTM
    HTM Member Posts: 29

    My plumber acquaintance came by yesterday. He believes actual boiler pressure (per gauge) is understated. Also that the system needed more water. Recommended operating boiler at gauge pressure 20 psi.

    PRV fast fill opened to add water, with gauge pressure settling at around 21psi. Started at 14 psi. Fast fill thumb screw retained in this position several hours. Boiler cycled on/off a couple of times, but didn't go higher than 22 psi.

    I went against his recommendation to keep fast fill open going forward. The instruction on the PRV's ancient paper tag advises back off tension on thumb screw when fill or purge is completed. Although boiler gauge showed a consistent 21-22 psi over many hours, I was uneasy going overnight with fast fill open, pressure jumping and possibly waking up to a flooded floor.

    No issues overnight. Boiler is maintaining 21-22 psi. My not adding water after the two purgings caused a major shortage in the system. My mistake
    not appreciating this is a closed system. I will continue to monitor readouts but how to determine whether/when there is enough water in the system and whether automatic fill is working properly?

    Much appreciate comments and assistance.