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Creeping pressure on boiler

jjjj Member Posts: 9
Hi fellow contractors! I've been at his p&h business over 30 yrs. Got a Vaillant 80% eff. 25yr. old boiler driving me crazy. New #30 amtrol exp. tank and new pressure reducing valve still get pressure creeping up to relief valve leakage. Boiler sounds a bit limed up when it gets to temp. about 170-180. Exp. tank hooked to bottom of air scoop and has clear passage thru. I even put an additional #30 tank on return side off boiler drain just to try SOMETHING. Worked for 2 wks. got a call again relief leaking. House is only 1600sqft. ranch. Also has 40 gal indirect fired water heater. What am I missing here? Is it the liming up? Even with 2 tanks? I appreciate anyones input.


  • Sounds like

    your indirect water heater has a small hole in the heat exchanger.  The high pressure on the domestic side is bleeding through to the low pressure hydronic side, lifting the relief valve.

    It's time to sell them a new indirect.
    Often wrong, never in doubt.
  • kevkev Member Posts: 94
    had same

    Problem on a new install. It would take about a week to climb to 30 lbs. Pin hole in the indirect coil.  I was certain somebody was messing with that boiler feed.
  • billtwocasebilltwocase Member Posts: 2,385
    ditto on the indirect coil

    but also if the feed valve is left on, try keeping it closed, also make sure the expansion tank is not isolated by zone valves, IFC pumps, or flow checks.
  • jjjj Member Posts: 9
    Creeping pressure

    Thanks fellas! I've never encountered this problem in all the years. Can't believe I didn't think of that. Great site to get other expert opinions. I appreciate the feedback so promptly.
  • Don't feel bad....

    The first time it happened to me was after 32 years in the business and I replaced all the components you did- fill valve, larger expansion tank. It was so frustrating to get those calls from the home owner that the relief valve was leaking again. And I remember doubting the new cause theory because practically, you can't test for a compromised heat exchanger with such a slow leak; at least, I didn't think so.

    And what happens if you replace the indirect, hand the owner's an invoice for $2,000 - $3,000 and it happens again? In my case, it didn't and I was greatly relieved.
    Often wrong, never in doubt.
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