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Thread: Vaillant boiler leaks from Pressure relief valve

  1. #1
    DIY Junior Member massangler's Avatar
    Join Date
    Nov 2012
    Newbury MA

    Default Vaillant boiler leaks from Pressure relief valve

    I replaced pressure relief valve, and it still leaks. Boiler cycle hi temp shutoff at 200. As it reaches 200, burner shuts off, but temp and pressure continue to rise afterwards. What settings should I use on honeywell controller? It has high, low and diff. dials

  2. #2
    In the trades Dana's Avatar
    Join Date
    Jan 2009


    In general you get better efficiency out of it if you set the low-temp down to 140F (for oil burners), or 130F (for gas or propane), with the biggest possible differential hysteresis. Very few hydronic heating systems actually need a 200F boiler temp EVER, so dial that back to ~180F first, see if that fixes the temperature overshoot. If the aquastat is mis-sensing the temp or has a delay and the thing is overshooting, you may need to re-set the element in the sensor well. If the pressure is going off the charts you may need to re-charge or replace the expansion tank on the system.

    Whether turning down the water temp will work for you from a heating-the-house point of view depends a bit on how much heat emitter/radiation you have. If this is a boiler worth keeping (less than 15 years old, and in decent shape), replacing/hacking around some of the aquastat controls with a smart economizer, it'll do the hysteresis and high temp adjustments automatically. optimizing the system for the actual heat loads. (eg Intellicon 3250 or Beckett Heat Manager, etc.) These typically let you program the low-limit temp, and takes over from there, squeezing out best efficiency it can muster with the system configuration that it can. It won't fix system design issues or oversized boilers, but double-digit percentage reductions in fuel use are common. And the average operating temp (and standby losses) will be much lower.

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    To open, most pressure relief valves need 30# of pressure in the system. Since most domesic systems run about 1/2 of that, something's wrong. First, what is the 'normal' pressure? Then, check your expansion tank. It's probably shot and needs replacement. The, once you get things running as they had been, you can start to tweak the system to make it run better. If you have an autofill, it could be bad. If you have an indirect WH, there could be a leak and potable water is pressurizing the system. Other than that, can't think of any reason other than a bad relief valve that it would open up. The key is to check your pressures first.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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