Radiant floor heat boiler overpressuring and blowing steam

Discussion in 'Boiler Forum' started by rypkr937, Oct 1, 2018.

  1. rypkr937

    rypkr937 New Member

    Joined:
    Jul 5, 2018
    Location:
    Western PA
    I have a radiant floor heat system that runs off of a gas boiler.

    I purged all of the air from the floor loop and closed the shutoffs assuring the floor loop is tight. I then hooked a drain hose to the boiler drain valve and flushed the boiler side of the system assuring it was water tight.

    I left the boiler at 20psi with no air in the system, opened the shutoffs to the loop and turned it on. After it started heating up it blew the pressure relief valve and a mix of air and steam blasted out until it tripped the lower water cutoff and killed the system.

    My initial thoughts were I had too much water PSI in the boiler so I repeated the above process, but this time turned the boiler back on with only 5psi in the boiler. I also left the garden hose connected to boiler drain valve and when the pressure began to rise I throttled it open letting some water out. This worked as it heated up for a few degrees and then suddenly a sharp rise in pressure and it blew the valve open again.

    Why is my boiler getting overpressure? I'm guessing the second time that I may have let out too much water and created an air pocket that quickly heated and created a dangerous pressure situation? Yikes if so.

    But the first time I fired it up what could have been the issue?

    Thanks for any help.
     
    Last edited: Oct 1, 2018
  2. Reach4

    Reach4 Well-Known Member

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    Sep 25, 2013
    Location:
    IL
    Was this a brand new system, or are you bringing an existing system back up? If existing, you probably have a thermal expansion tank. That may have failed. It should be pretty much empty of water when the water pressure is normal, before you turn on the heat. Knock on it to see if it sounds hollow, or has water in it.
     
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  4. rypkr937

    rypkr937 New Member

    Joined:
    Jul 5, 2018
    Location:
    Western PA
    Existing system that I turned back on. I replaced the pressure tank last year because it failed. I briefly tapped the Schrader valve to assure it wasn't water logged and got no water. It's oriented with the Schrader on the bottom so if it had failed I would expect to get water.
     
  5. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
    If the Schrader valve is on the bottom, you could test for a leak that way. If it is on top, knock on the tank as if you were knocking on a door.

    What air precharge pressure is your expansion tank? That should be a the recharge pressure or a little more.
     
  6. Dana

    Dana In the trades

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    Jan 14, 2009
    Location:
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    Why 20 psi that's as very high pressure for a residential system.

    The highest pressure it needs to be at idle is about half that to prevent "kettling" on the boiler's heat exchanger plates. Beyond that it's all about elevation.

    Measure the vertical height between the highest point on the system and the pressure gauge on the boiler in feet, then multiply x 0.433, add 3 psi. eg:

    Say it's a 2 story house,with 10' separation between floors, with baseboard radiators with plumbing 6" off the floor in the top floor, and the gauge on the boiler is 6.5' below the subfloor of the first floor. That means the top floor baseboards are 0.5' + 10' (first floor to second floor difference) + 6.5' = 16' above the pressure gauge. 16' x 0.433 psi/foot= 7psi. Add 3 psi to cover any dynamic pressure difference related to pumping, and you're at 10 psi. If it's a 3 story that adds 10', and another 4.33 psi, call it 15 psi.

    It would have to be a 4 story or higher house to call for 20 psi, measured at a basement boiler room.

    If the expansion tank is not properly precharged to a few psi above your calculated system psi it can hit it's expansion limits and have that rapid spike in pressure as it heats up, hitting the maximum expansion range of the tank before reaching full temp. If the expansion tank is undersized for the system volume it would have a similar symptom.

    Just because the bladder hasn't failed, dripping water out of the schrader valve doesn't mean the expansion tank is properly charged. The valve can leak air and lose charge over time and need to be re-charged to the proper level (after replacing the schrader valve with a fresh one.)
     
  7. rypkr937

    rypkr937 New Member

    Joined:
    Jul 5, 2018
    Location:
    Western PA
    I didn't have a reason to fill to 20psi other than I knew that the relief valve lifted somewhere around 30psi so I thought that was an appropriate spot. It's not baseboard but radiant floor pex that snakes around quite extensively, I would have thought more pressure would be better but correct me if I'm wrong (2 story home).

    The second time I filled it, it was to less than 5psi and it still blew.

    I don't think the expansion tank is undersized, it's the same size that's been I the system for 15 years - it was however replaced 11 months ago because it did fail then.

    Once I do your calculation to my second floor I should drain the boiler side so there is no pressure in the tank and set it to the same pressure that I determine the boiler to operate at, correct?

    I should note one other symptom is that the air scoop and hyvent valve is about a foot in line from the expansion tank and as the system started circulating and heating up it was letting out quite a bit of air. More than I have ever noted in the past. I just replaced it a few days ago because it started leaking when I was purging the loops in my system.
     
  8. Dana

    Dana In the trades

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    If it's the same size tank as the one it replaced the problem is almost certainly that it's lost (or never had) the air charged correctly.

    If there is an isolating valve that can allow you to remove the tank without draining the system that would be the surest way to guarantee that you're stating with zero psi on the wet side of the bladder.

    Set the system to at least 12 psi, even if that isn't necessary for 0.433x vertical distance + 3psi. Pre charge the tank to 14-15psi, and use soapy water to verify the the valve isn't blowing bubbles. If it is leaking a bit of air, tighten the schrader valve- see if you can't get it to stop, or replace the valve.

    Any time you open up the system air gets in. Any time you let the system drop to 5 psi it can even draw in air at the top of the system that's under negative pressure. So it's fine that the vent is doing it's job and auto-purging the air.
     
  9. plumber69

    plumber69 In the Trades

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    Jun 20, 2014
    Location:
    Prince Rupert, British Columbia
    If your blowing water and steam... sounds like a valve may be stuck closed or the pump isn't working right.
     
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  10. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Occupation:
    Retired Systems engineer for defense industry.
    Location:
    New England
    If water isn't circulating properly past the heat exchanger in the boiler, it will flash to steam. Steam is like 540x more volume than the liquid, rapidly spiking the pressure until released by the safety valve. Are all of the valves properly opening or opened, if manual?

    If there's an overtemp sensor, it may not respond fast enough to prevent the problem.
     
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  11. Dana

    Dana In the trades

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    Jan 14, 2009
    Location:
    01609
    That's a good possibility!

    A co-symptom of low flow would be an unusually fast temperature rise, even if it's not hitting the high-limit before it blows.
     
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