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Thread: Too much pressure

  1. #1
    DIY Junior Member Dave in Mass's Avatar
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    Default Too much pressure

    My boiler is a Dunkirk, about 5 years old, with Taco PC700-2 control using outdoor reset. The house, near Boston, has cast iron radiators and cast iron baseboard, and is comfortable. The boiler also has a zone for indirect water heater. The problem is that in very cold weather, recently as low as zero, with a boiler design temp setting of 160 and an outdoor design minimum set at 15 degrees, the boiler heats up to just above 170, pressure rises to 35psi or slightly more, and it will discharge water thru the pressure relief valve every heat cycle. It continues to do this, but less, now that outdoor temp is about 40. My uninformed suspicion is that the outdoor reset is correctly allowing the boiler to exceed 160 to compensate for low outdoor temp. To avoid triggering the pressure relief valve should I change the setting for outdoor design temp, perhaps to zero or 5 degrees? I hate to do it for the few days needed in most years.
    Is it reasonable to introduce a switch that would turn off the outdoor reset thermistor and defeat the reset feature?
    Could the pressure causing water discharge be eliminated with a larger Extrol tank? It is size 30 now, and because of space concerns it can't be exchanged for a larger one. I would have to tap a second tank into the piping elsewhere. Would it cause any problem doing that? All wisdom and insight gratefully received

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    DIY Senior Member Reach4's Avatar
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    I think the Extrol tank has a schrader valve for control of a precharge pressure. Have you removed the pressure *totally* and then measured the precharge pressure? I don't know what the pressure should be for you, but I am thinking it should be set to whatever pressure would cause water to be added to the system. I think the initial precharge is 12 psi, but you can change it with a tire pump to fit your system.

    If water comes out, the expansion tank needs to be replaced.

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
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    Your expansion tank is probably shot. Two quick checks: rap on it, and if it feels heavy and gives you a thud rather than a bell sound, it's probably bad; if it releases water if you check the pressure, it is bad. But, also keep in mind that when reading the pressure on an OPERATING ET, it will always read the system's pressure. Now, what that should be depends, but 12-15psi is pretty typical for most residential systems. As already said, to check the static, air precharge on the tank, you have to remove all water pressure first, otherwise, it will just read the water pressure.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4
    DIY Junior Member Dave in Mass's Avatar
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    Thanks to those that read my problem. You were on the right track. It turned out that the schrader valve on the expansion tank was not holding pressure. Recharged it to 15 psi and most of the problem is gone. However, the tank appears to be undersized and I may add a supplemental tank after the heating season. Luckily there is a capped nipple available to connect to.

  5. #5
    DIY Senior Member Reach4's Avatar
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    When you turned the water back on, after increasing the precharge to 15 but before you turned on the heat, what pressure did the water climb to? If that number was 15 PSI, then I think 15 was optimum for the precharge.

  6. #6

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    I agree with you reach4

    Quote Originally Posted by Reach4 View Post
    I think the Extrol tank has a schrader valve for control of a precharge pressure. Have you removed the pressure *totally* and then measured the precharge pressure? I don't know what the pressure should be for you, but I am thinking it should be set to whatever pressure would cause water to be added to the system. I think the initial precharge is 12 psi, but you can change it with a tire pump to fit your system.

    If water comes out, the expansion tank needs to be replaced.

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