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Thread: Pressure valve flooding house

  1. #1
    DIY Senior Member Melissa2007B's Avatar
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    Default Pressure valve flooding house

    I just woke up to find the pressure valve on our water heater shooting out cold water ( not hot - it may have flooded out the natural gas burner at the bottom ) and flooding the house. ( someone removed the vertical pipe from the bottom of it, if it had one, so it was flooding outside of the bottom drain pan ) It may have been doing this for 90 minutes since I got up before to use the bathroom - the last time I was up and it wasn't doing it. I shut the water off and called the water company and they say it's not overpressure from them. ( though the pressure here sometimes goes to 90 PSI normally )

    We do have a problem when the whole house humidifier runs sometimes. It can take a trickle of hot water from the water heater and cause the temp to go up quite a bit, but it hasn't caused the pressure valve to go off like this.

    What's the likely cause of this? Do I need a plumber or should I just try and replace the valve myself?
    Melissa
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    Retired Defense Industry Engineer jadnashua's Avatar
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    That valve is designed to open if the pressure exceeds 150psi, OR the temperature exceeds (I think) 212-degrees. If it was dumping cold water, it certainly isn't from excessive temperature.

    If it opened periodically for some reason, it may have some mineral deposits on it now that have prevented it from closing, but something opened it up in the first place.

    You can buy a water pressure gauge at a big box store or plumbing supply. I'd pick one up for about $10 and actually check the pressure. Get one with a second, tattle-tale hand to show peak pressure and leave it on for at least a 24-hour period.

    You indicated that the local pressure might get to 90psi, and codes generally require a PRV and an expansion tank to limit it to 80psi. Also, keep in mind that if your static pressure is 90psi, in the early morning, late night situation when few people are using water, it could peak a lot higher while they refill water towers, and because the demand is low.

    If the pressure and temp are below the safety limits, the valve may be defective and should be replaced. This will take some plumbing skills, as the thing needs to be unscrewed from the tank and the drain line from it will need to be removed and a new one soldered into it because that would normally prevent you from unscrewing it with that long pipe on the end.

    If the thing wasn't installed properly and the discharge wetted the gas control valve, then the whole WH should be replaced...a wetted gas valve may never work right again and is a safety hazard.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3
    DIY Senior Member Melissa2007B's Avatar
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    We're in Denver - water boils at around 202 degrees here, but interesting. I had the temp setting all the way up, and the humidifier running pushes it even higher, so I just turned it down and will see now.
    Melissa
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    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by Melissa2007B View Post
    I just woke up to find the pressure valve on our water heater shooting out cold water ( not hot - it may have flooded out the natural gas burner at the bottom ) and flooding the house. ( someone removed the vertical pipe from the bottom of it, if it had one, so it was flooding outside of the bottom drain pan ) It may have been doing this for 90 minutes since I got up before to use the bathroom - the last time I was up and it wasn't doing it. I shut the water off and called the water company and they say it's not overpressure from them. ( though the pressure here sometimes goes to 90 PSI normally )
    Assuming that you in some way verified that the pressure was no longer over 150 psi, the T&P relief valve should have closed. Since it didn't, replace it. Shouldn't be a difficult install other than putting a new drain line on it. (Note, do not reduce the drain line size. Relief devices should never have a smaller outlet pipe size than the size of their discharge.)

    Jim is right, get a peak pressure guage. There is a very high probability that if you do not have a thermal expansion tank, the water heater relief valve is going to hit 150 psi and leak a trickle after shower/bath cycles of the water heater. And without a PRV on the water supply line any impulse to the line could pop a relief valve open as well.

    Before I replaced the toilets in our house the water heater relief valve had no issues. Turns out the old toilets were acting as relief valves trickling a little pressure off at about 120-125 psig when the water heater cycled. When I put in the final new toilet, this source of relief was gone and I started getting puddles at the T&P drain. I confirmed the problem with a peak pressure gauge. So I had a thermal expansion tank installed.

    This might not have been an issue many times in the past, but with utilities now preventing back flow at the meter, and with various appliances having better shut offs, there are fewer sources of relief for thermal expansion.

    We do have a problem when the whole house humidifier runs sometimes. It can take a trickle of hot water from the water heater and cause the temp to go up quite a bit, but it hasn't caused the pressure valve to go off like this.
    I'm not following this. A trickle of hot water to the humidifier shouldn't be causing the water heater temperature to vary greatly. The thermostat on the water heater should kick in when the temp falls a few degrees (I'm not sure how much offset it allows before it is triggered.) If the temperature is spiking from this then I suspect there is some other issue with the water heater. You said in a follow up post that you are running the temp at maximum, why? Is there something about the usage/size of the water heater that is requiring this?

    If you are doing deep cycles with the water heater temp set to max, then the thermal expansion issue will be a bigger one than for someone running a lower water heater temp.

    What's the likely cause of this? Do I need a plumber or should I just try and replace the valve myself?
    See the above and Jim's comments. I suspect you are going to want a plumber because there will be other important work to do to eliminate the root cause. The key is to locate a plumber that is a good troubleshooter and can work out the sequence of events that caused the problem. I wouldn't want to just slap a band aid on this...

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    Nuclear Engineer nukeman's Avatar
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    It is also possible that it was a high temperature situation, even if the water was cold when you checked it. The thermostat could be bad (possibly indicated by the higher temperatures seen when humidifier is running). When the relief valve opens, it doesn't take long to remove all of the hot water left in the tank (a few minutes) and the flow is much more than the burner could continue to heat, even if the burner kept going. As such, you would have cold water pouring out after the initial few minutes.

    The relief valve should have closed after the pressure/temperature reduced, but these valves can stick as they aren't used often and can see scale buildup causing them to stick.

    Basically, you need to figure out the cause (higher pressure/temperature or a defective valve that opened too early) and then we can figure out what needs to be done to fix it.

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    DIY Senior Member Melissa2007B's Avatar
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    What happens with the humidifier is that it draws a trickle of hot water from the top if the heater, apparently drawing just enough cold water in the bottom to trigger the gas heat to come on. Then there's still enough hot water left to make it super hot. We've had to be careful not to get scalded when the humidifier has been on awhile. This is a modular house though, so nothing fancy like overflow tanks and all that.
    Melissa
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    Quote Originally Posted by Melissa2007B View Post
    What happens with the humidifier is that it draws a trickle of hot water from the top if the heater, apparently drawing just enough cold water in the bottom to trigger the gas heat to come on. Then there's still enough hot water left to make it super hot. We've had to be careful not to get scalded when the humidifier has been on awhile. This is a modular house though, so nothing fancy like overflow tanks and all that.
    The proper term for it is "stacking"

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    DIY Senior Member Melissa2007B's Avatar
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    Um, ok. Looks like I just had the thermostat up too high - it was on max temp, and the humidifier ran and popped the pressure valve. What a mess. I also gotta figure how to open that drain pan beneath the heater.

    I turned the temp down to medium and it's fine now.
    Melissa
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    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by nukeman View Post
    It is also possible that it was a high temperature situation, even if the water was cold when you checked it. The thermostat could be bad (possibly indicated by the higher temperatures seen when humidifier is running). When the relief valve opens, it doesn't take long to remove all of the hot water left in the tank (a few minutes) and the flow is much more than the burner could continue to heat, even if the burner kept going. As such, you would have cold water pouring out after the initial few minutes.
    Yes, that was a path I was considering as well.

    Quote Originally Posted by Melissa2007B View Post
    What happens with the humidifier is that it draws a trickle of hot water from the top if the heater, apparently drawing just enough cold water in the bottom to trigger the gas heat to come on. Then there's still enough hot water left to make it super hot. We've had to be careful not to get scalded when the humidifier has been on awhile.
    If that's the case then the first thing I would try is turning down the thermostat at least 10 degrees or so.

    My humidifier uses so little water that I can't imagine it having this sort of effect. Is yours perhaps putting more water through than it is supposed to? Is it cycling the humidifier feed off when the furnace heat is off?

    I will note that my humidier can't cause this sort of problem as it is hooked up to the cold leg to the water heater rather than the hot. The heat source for humidification on mine is the hot air from the furnace. The run of uninsulated narrow tubing and low flow would likely dissipate most of the heat from the hot water line if it was hooked up the other way. So if the problem is "stacking" because of the humidifier, it could be cured by taking the humidifier water from a cold tap.

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    DIY Senior Member Runs with bison's Avatar
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    Quote Originally Posted by Melissa2007B View Post
    Um, ok. Looks like I just had the thermostat up too high - it was on max temp, and the humidifier ran and popped the pressure valve. What a mess. I also gotta figure how to open that drain pan beneath the heater.

    I turned the temp down to medium and it's fine now.
    You beat me to it... But take a closer look at the humidifier. I don't believe it should be using enough water to cause this problem. And even if it is, moving the takeoff to the cold side should eliminate it as a source. The only way this wouldn't work is if it requires a hot feed to humidify.

    There aren't enough btu's in hot water to vaporize much water. Plus unless the line is well insulated, short, and/or the flow is substantial, the water will be close to room temp before it gets to the humidifier. The bulk of the humidification (vaporization btu's) energy comes from the warm air from the furnace. A negative though is that cooler water has a thicker film/is less wetting to surfaces so it will impede mass transfer somewhat until it warms.

    EDIT: Scratch that about changing the source from hot to cold. This humidifier uses the hot water supply as the heat source, rather than warm air from a furnace.
    Last edited by Runs with bison; 01-17-2012 at 07:43 PM. Reason: Update for humidifier type

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    Stacking occurs when the end users make small draws of hot water in short increments. There is a lag time between the thermostat turning on and off. This lag is when the over heating is done.

    Water heater manufactures shortened their dip tubes to help prevent stacking but it doesn't eliminate the possibility of it.

    Most people including most plumbers have never heard of a water heater "stacking". Some find on electric heaters the high limit reset tripped but have no idea why.

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    DIY Senior Member Melissa2007B's Avatar
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    This is an Aprilaire 360 Humidifier and it uses water mixed through a regulator, to come out no more than 140 degrees. Good unit. Saves hauling water to tabletop units every day and the cleaning mess.
    Melissa
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    Lowering the thermostat temp may or may not help you. By lowering the temp it may just take longer for the same "stacking" effect to happen. If you have a closed system make sure your thermal expansion protection is operating.

    I suspect you have a thermal expansion problem or simply just a bad T&P valve. I doubt the valve opened because of 212 degree temps but thats not very common at all compared to thermal expansion and simply a failed T&P valve.

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    DIY Senior Member Melissa2007B's Avatar
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    It's interesting that water boils here at 202 degrees, so I wonder if a 212 degree valve would even work?
    Melissa
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    Quote Originally Posted by Melissa2007B View Post
    It's interesting that water boils here at 202 degrees, so I wonder if a 212 degree valve would even work?
    Maybe the water on your stove boils at 202 but boiling is not the issue inside the water heater. The water heater is subject to your water supply pressure. We are not talking atmospheric pressure.

    If you have a closed potable water supply and use a thermal expansion tank as means of control,remember cold incoming water temps allow for more thermal expansion. Combined with stacking to a really high temp if you did have a thermal expansion tank its volume may not be of sufficient size.

    You did the right thing by turning the thermostat down,thats a start It will not prevent the water heater from stacking but you may not exceed the pressures and temps for the water heater causing you trouble.
    Last edited by Hackney plumbing; 01-17-2012 at 07:29 PM.

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