Water boiler pressure/bleeding questions

Discussion in 'Boiler Forum' started by lordmoosh, Jan 29, 2012.

  1. lordmoosh

    lordmoosh Member

    Messages:
    126
    Location:
    USA
    Hello,

    I have a baseboard heat system with natural gas. We had to cut a radiator out of a kitchen so we could tile the floor. Everything seems to have gone well and the heater is working after refilling it with water. We put a hose on the drain and introduced new water into the system. We turn all thermostats (4 different zones) and turned up one thermostat at a time and left the other 3 off while draining and introducing new water. A lot of air came out and after all was said and done no more air was coming out of any zone. We also opened a bleed cap on one of the pipes in the basement and water was coming out, no air. We did not see any bleed caps in the upstairs baseboard heaters so we weren't sure if we needed to bleed in other places too. The system seems to be working correctly and I set it to 55 degrees so the water supply pipes don't freeze. The boiler temperature gauge was 180 degrees F and pressure was at about 20 PSI at that temperature. I assume if the water was colder the pressure would drop. The boiler is about 4 years old. I am totally new to water heat as my last house had forced hot air heat.

    1) Does 20 psi at 180 degrees F seem correct?
    2) Did we bleed the system properly?
    3) At one point I was draining the system and by accident had the water supply off and the pipes started banging (probably because the heater was on and water was draining and there was air in the pipe because I wasn't supplying water). Is there any possibility of damage? I hit the power switch off when I noticed that. I think the banging occurred for about 5-10 seconds in total before I opened the supply and got it under control.

    Thanks all!
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,344
    Location:
    New England
    1. There should be an expansion tank in the system to account for variations in water volume based on temp - the pressure should be stable. 20psi is on the high side of normal (often around 1 atmosphere - 14-15# - but usually at least 12...there's a pressure relief valve usually set at 30psi that will dump water if the pressure gets that high).
    2. Every system is slightly different. You may need to run more water through it if you don't have heat to all zones to purge out the air. If there's heat everywhere, any small amount of air left should get purged if the air scoop or extraction device is working properly.
    3. Dropping the pressure likely caused the boiler water to flash into steam - mini steam gas explosions. Long term, this will cause problems similar to what happens if the timing in your car is off or you don't have high enough octane and it pings or what would happen if you opened the radiator cap when the coolant was at 230-degrees. If everything works, you're probably okay.
  3. lordmoosh

    lordmoosh Member

    Messages:
    126
    Location:
    USA
    Thanks jadnashua. Do you recommend I drain some of the water to reduce the water pressure to 14-15 PSI at 180 F?
  4. lordmoosh

    lordmoosh Member

    Messages:
    126
    Location:
    USA
    I stopped by the house today and the boiler gauge read 160 degrees and 15 PSI. Next time I am there for a few hours I will keep an eye on the gauge and if it hits 180 degrees/20 PSI I will bleed some extra water out of it until it hits 180 degrees/15-16 PSI.
  5. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,344
    Location:
    New England
    Variations in pressure may indicate two things: your expansion tank has failed or is failing or is saturated with water and there's little room for expansion (a bladder tank is fairly small and has a fitting like you a tire on the bottom - it should normally be full of mostly air; an old style expansion tank is much larger, and those need to be drained periodically and their air valve may be corroded and needing replacement); OR, you may still have some air in the system. Both the air and the water expand when they get heated...air will expand more than the water, and it can raise the pressure more than your expansion tank can handle.
  6. lordmoosh

    lordmoosh Member

    Messages:
    126
    Location:
    USA
    Thanks. I will knock on the expansion tank to see if it is full of water. It looks new, the system is labeled 2007 (just got the house).

    Is the water supply line to the boiler supposed to be left open or closed? I left it closed because I think it was closed when I bought the house. I opened it up only when I had to put water into the system then I closed it.

    There are some little devices on some of the piping around the boiler which have screw type caps on them. One of the devices is called autovent. There maybe 1 or two other smaller ones with caps. I think they are tight right now. I've read that those devices bleed air out of the system over time, is this correct? Should I unscrew them all? How much do I unscrew them, just a little bit so they are not tight?

    Should I aim at 15 PSI as my goal pressure to attain? Thanks.
  7. lordmoosh

    lordmoosh Member

    Messages:
    126
    Location:
    USA
    This morning I checked the boiler and it was 170F at 15 PSI. Looks like it has stabilized at 15PSI. I also unscrewed the auto air bleeder vent caps a few turns. No water came out of them so I left them like that. Hopefully it bleed out any residual air in the system.

    From what I can tell it looks like I should leave the water supply valve closed. Is that right?
  8. Gary in NJ

    Gary in NJ New Member

    Messages:
    90
    Location:
    NJ
    Keep the autofill closed and the bleeder valves open.
  9. lordmoosh

    lordmoosh Member

    Messages:
    126
    Location:
    USA
    What is the purpose of the autovent? Its been open since morning, any side effects? I will close it tonight.
  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,344
    Location:
    New England
    Leave the autovent opened...it is designed to purge small amounts of air that get trapped as it passes, and once enough has accumulated, it automatically opens momentarily to vent it out.
  11. lordmoosh

    lordmoosh Member

    Messages:
    126
    Location:
    USA
    lol. Ok I will unscrew the autovent tomorrow morning. The gauge read at 15 PSI again tonight so it looks like it has stabilized at that value.

    Does anyone know if it is correct to leave the water supply line closed?
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,344
    Location:
    New England
    If you have a leak, the autofill will mask it by keeping the system full of water. It's better to know you have a leak. If the pressure drops, on many boilers, there is a sensor that will prevent it from running. This is similar to the effect you get on your car's radiator with the pressure cap. The pressure keeps the water from boiling. Boiling (steam) can damage boilers designed for hot water, so many have a sensor to monitor the pressure (but not all). Ideally, once the heating system is filled with water, you'd never have to add any ever so leaving the fill valve closed isn't a big deal.

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