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Thread: Pressure Relief Valve-Furnace

  1. #1
    Carpentry/Handyman Service wrdtlpf's Avatar
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    Default Pressure Relief Valve-Furnace

    Hi all- Hope you guys can help. I just installed a new pressure relief valve on my furnace. Old one was constantly dripping. I replaced a Watts 374A(550,000 BTU) with a Watts 335M1(510,000). Guy at supply house said it would be fine for the small Slant-Fin furnace I have. Both are factory set to 30 lbs as indicated on the tags. The autofill valve fills very slowly so I moved the valve lever and locked it straight up while I went around and bleed radiators. I came back to basement and water was everywhere because relief valve wasn't holding. Was it just because I had fill valve wide open and exceeded the 30 lbs or is there something you have to do when you install a new relief valve? Thanks guys, Bill-

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    Moderator & Master Plumber hj's Avatar
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    YOU did it by not allowing the automatic filler to control the amount of water going into the system. The relief valve did EXACTLY what is was supposed to, and a "furnace" does NOT have a relief valve.

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    Carpentry/Handyman Service wrdtlpf's Avatar
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    Not a plumber, carpenter. Kinda re-acquainted myself with things and figured it was because I had autofill wide open and as HJ reminded me that was the case. I closed autofill and all's ok now. The box says Watts "Relief Valve" on it so that's what I called it and have heard it called but maybe that's not technically correct when talking about a furnace? Gotta look into the Extrol tank now because that can cause the 'pressure relief valve' to leak if not set correctly I believe?

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    In the Trades Jerome2877's Avatar
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    Obviously its a boiler not a furnace and a boiler has a relief valve. The system pressure should be set around 15 psi and the relief goes off at 30 psi. You may want to replace the Pressure reducing valve as well depending on its age, it could be causing the issue as well. Also yes the expantion tank could be a problem as well.
    Last edited by Jerome2877; 08-21-2010 at 07:47 PM.

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    Carpentry/Handyman Service wrdtlpf's Avatar
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    Geez, yes, a boiler. Furnace hot air. I use furnace as a generic term I guess. Felt like I was getting yelled at, just looking for some help and advice, thanks Jerome2877.
    Last edited by wrdtlpf; 08-21-2010 at 10:01 PM.

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    Plumber jimbo's Avatar
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    The house where I grew up had a boiler ( still does, 65 years later, but not the original!). Single pipe steam heat. My dad always referred to it as "the furnace".

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    Moderator & Master Plumber hj's Avatar
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    As Abraham Lincoln once said, "If I call a dog's tail a leg, how many legs does it have?" You can call something anything you want to but that does not change what it is.

  8. #8
    Carpentry/Handyman Service wrdtlpf's Avatar
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    The vent above the purger/expansion tank was also leaking, I replaced it; does that mean the tank is waterlogged and the bladder is bad? I haven't isolated tank yet but then tank pressure is 27lbs. How do I correctly check the tanks pressure? Pressure gauge on boiler is reading 31-32lbs and it hasn't called for heat/hot water for quiet sometime now and new relief valve isn't leaking but will when boiler is running. I have installation/operating manual but can't find what pressure is recommended for this boiler. Forum says DIY welcome so that's why I'm here. Thanks guys-

  9. #9
    Master Plumber-Gas Fitter shacko's Avatar
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    Quote Originally Posted by wrdtlpf View Post
    The vent above the purger/expansion tank was also leaking, I replaced it; does that mean the tank is waterlogged and the bladder is bad? I haven't isolated tank yet but then tank pressure is 27lbs. How do I correctly check the tanks pressure? Pressure gauge on boiler is reading 31-32lbs and it hasn't called for heat/hot water for quiet sometime now and new relief valve isn't leaking but will when boiler is running. I have installation/operating manual but can't find what pressure is recommended for this boiler. Forum says DIY welcome so that's why I'm here. Thanks guys-
    If the gauge on the boiler is showing 31-32lbs you are at the limit of the relief valve, every time it comes on it's going to leak, you have to get the pressure down to 12-15lbs; your auto fill may be bad. Are you sure you have an expansion tank with a bladder?

  10. #10
    Carpentry/Handyman Service wrdtlpf's Avatar
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    Yes, an Extrol tank model 30, precharge of 12lbs. It's reading 27lbs but I think I need to isolate it from the system to take a proper reading, is that right? Autofill was taking forever to fill system after I drained it down to swap out air vent and relief valve, as well as, install 2 new outside sipgots. Is that an indication that is bad, a very slow fill time? Is water leaking form air vent a sign that the Extrol tank is bad? Thanks shacko

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    Plumber jimbo's Avatar
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    Quote Originally Posted by hj View Post
    As Abraham Lincoln once said, "If I call a dog's tail a leg, how many legs does it have?" You can call something anything you want to but that does not change what it is.
    A furnace is whatever you want it to be;
    fur·nace

       /ˈfɜrnɪs/ Show Spelled [fur-nis] Show IPA noun, verb, -naced, -nac·ing.
    –noun 1. a structure or apparatus in which heat may be generated, as for heating houses, smelting ores, or producing steam.

  12. #12
    In the Trades Jerome2877's Avatar
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    Is the pressure constantly at 27 lbs? You may have overcharged the system when filling it. Try bringing the pressure down to about 15 psi then watch to see if it goes back up right away. If thats the case I would suspect the auto fill valve/ PRV.

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