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Thread: "Superstore" 40 gal hot water tank discharging through overflow - why?

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  1. #1
    DIY Member McG's Avatar
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    Default "Superstore" 40 gal hot water tank discharging through overflow - why?

    We were away for a few days and shut off the oil burner/boiler (Weil-McClean Gold) while we were gone. We turned the boiler on when we got home. A day after returning home I heard water running in the cellar - the overflow from the Superstore was discharging into the one gallon pale on the floor and overflowing. The cellar floor was soaked. I flipped the spring loaded pressure release valve at the top of the discharge pipe and the discharge opened up full force. Releasing the valve did not return the flow to a strong trickle instead it was a stronger flow. Finally I turned off the cold water feeding the tank and shut off the boiler. What's going on and how do I correct the problem?

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    Retired Defense Industry Engineer jadnashua's Avatar
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    I'm assuming that the place where it was leaking was the T&P valve?

    That is designed to release under any one of two conditions: excessive pressure, or excessive temperature. Weeping a little is likely if the expansion tank is shot (or there's none installed). But, a strong flow is not normal. What was the temperature of the water being discharged?

    If I had to guess, I'd guess that your expansion tank is shot and the T&P valve has some mineral deposits on it, and once it tripped, it wouldn't reseat. So, I'd check the expansion tank, and then replace the T&P valve unless the boiler was running away, and the tank was excessively hot.

    It's possible, if you have excessively high water pressure, that your PRV is shot. You should also check the incoming water pressure to rule that out.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Member McG's Avatar
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    Thanks. The temperature of the discharge is not excessively high. Yes, I believe it is the Temperature and Pressure Relief valve. And yes, I have a very old expansion tank, shaped like a half barrel and attached to the ceiling between two floor joists. The fella who installed the boiler said the old expansion tanks were better than the new ones and never need replacing because there's no bladder to fail. Anyway, he left it. Two years ago, a year after the installation, I called him and he drained my expansion tank after tapping on it and saying it will full. I assume an expansion tank must be capable of filling and emptying itself. Would you recommend I drain the expansion tank as a plan A to resolving the problem?

    Your reply synopsizes what I found described on this web page http://www.factsfacts.com/MyHomeRepa...sureRelief.htm

    2 hours later --
    I drained the expansion tank, collecting 20 gallons of water. I turned off the feed before opening the drain. I had to blow into the hose to release air locks, kind of like blowing into a straw immersed in a bottle of soda. After gurgling in the tank, water flowed again slowing to a trickle. But this expansion tank does not interface with the hot water storage tank. The cold water in and the hot water out do not pass connect with the expansion tank at any point.

    I opened the cold water-in line to the storage tank and immediately water gushed from the relief valve pipe. Flipping the valve up and down had little effect. Does this mean I have to replace the valve? It is only three years old, same as the boiler and tank.
    Last edited by McG; 10-25-2010 at 06:32 PM.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    A home might have a couple of expansion tanks, one if it has a boiler, and a second one for the WH. The old style tanks eventually fill up with water, so they require periodic maintenance. The newer bladder styles work until the bladder dies, then you replace it. I'd rather have it work like a switch until it fails than require periodic maintenance.

    If things are cold and the pressure isn't exceedingly high, then yes, the T&P valve probably needs to be replaced.
    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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    DIY Member McG's Avatar
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    House PSI is 79. Bought Watts Det-5 expansion tank. Max precharge is 80psi (79 is awfully close to 80). Bought new T&P valve for Superstore. Ready to install? I learned there is no airscoop on the boiler. Maybe we don't need one with the old fashioned expansion tank. Don't know. There is no pressure reducing valve, or backflow preventer on the cold water inlet to the WH. It may be obvious to those who know - but which line of the WH does the expansion tank go on, the cold water inlet or the hot water outlet?
    Last edited by McG; 10-31-2010 at 03:26 PM.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    Potable water expansion tanks are designed for COLD water use only. One designed for a boiler, obviously, needs to work on the hot side, but is NOT rated for potable use.

    An expansion tank is ONLY required when there is a closed system. That is almost always the case on a boiler, and is true on the potable water system if there is a check valve or a PRV in the system. So, if the expansion tank is for the boiler, then it should be rated for hot water use, and sized for the volume of hot water inside of the boiler and heating runs. If it is for the WH, then it should be sized for the volume of the tank and the normal temperature rise.

    A boiler operating pressure is normally only in the 12-20# range (could be higher, depending on system design and building height).

    PRV will normally be on a WH and another one on the boiler. They have different ratings for pressure and temperature, normally. Make sure to match the type and ratings when replacing.

    There are normally things that can corrode in a boiler...air (oxygen) in the water makes it happen. If there's none, it generally stops. So, a means to extract that air is generally a good thing, but on a good system, there may not be much. You WILL have some when you first fill the system, and any time you need to add water, it will likely contain some air as well. A pump might create a vacuum, and suck some in, if there is a leak, or the act of heating and cooling the water will generate a change in pressure that can do it.
    Last edited by jadnashua; 11-01-2010 at 12:07 PM.
    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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