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Thread: Dripping thermal expansion relief value

  1. #1
    DIY Junior Member psyq's Avatar
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    Default Dripping thermal expansion relief value

    Correct me if I get my terminology wrong. I believe I have a dripping thermal expansion relief value on my hot water heater. My precious and quite costly BTUs are going down the drain
    The main line has a pressure reducing value. Does it look like I need a thermal expansion tank? Should I pressure test the system?

    Lucky I have a nifty picture to illustrate:
    Name:  heater.jpg
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  2. #2
    DIY Senior Member Reach4's Avatar
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    A pressure gauge would be interesting. http://www.homedepot.com/p/Watts-3-4...IWTG/100175467 http://www.homedepot.com/p/Rain-Bird...-P2A/100575619
    Connect it to the faucet on your laundry tub or washing machine hot tap. I don't know what temperature the gauge is good for, but if you don't run the water, things should stay fairly cool at the gauge. The point is, does your pressure rise a lot when the water heater goes on? What pressure is present on the hot water when your relief valve leaks? You might just need a new relief valve.

    You might need an expansion tank. The gauge will be informative.

  3. #3
    DIY Junior Member psyq's Avatar
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    I don't have anywhere easy to screw a pressure gauge in (no laundry tub and washer would be a PITA to get to). What about to the drain for the tank itself?

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    Forum Admin, Expert Plumber Terry's Avatar
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    The drain on the water heater is a hose thread. There is no problem putting a pressure gage on a water heater.

    In your case, an expansion tank would help.
    Unless you PRV has a bypass, it can allow pressure build up when the water heats.
    For starters, you need to know if the PRV is even working.
    Last edited by Terry; 10-22-2013 at 01:07 PM.

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    DIY Senior Member Reach4's Avatar
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    Good idea. However that will be hotter than those other places. Maybe put a garden hose in the path if you are concerned about overheating the gauge. The downside of that the flexible might be that the hose could serve as bit of an expandable pressure tank, messing up your test. Maybe the packaging on the gauge will indicate a temperature range.

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    Retired Defense Industry Engineer jadnashua's Avatar
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    The gauge will handle being on the tank's drain but flush the thing first, or you will probably clog the gauge with sediment when you open the drain valve. It's you that may have problems with the heat, so would need gloves or pliers to maybe install or remove it.

    You can often pick up an adapter for a sink aerator, take the aerator off, then screw in the adapter then install the pressure gauge to it.

    The cable's in the way, is that a T&P valve on the top of the WH? It should be. It will have a test, release lever on the top and probably a tag.

    A pressure relief valve inline to a WH without an expansion tank is a problem waiting to happen.

    I do not know how common closed supply systems are in Canada, but if you have one, and do not have an expansion tank, you should add one, otherwise, there is no way you'll ever stop that valve from releasing pressure while the WH is running...water expands when it is heated, and if there's no place for it to go (a closed system means it can't push back out into the supply, which is one reason they make them, to protect the water supply to others in case yours became polluted some way), the pressure WILL rise, and the pressure relief valve is just doing what it should be. An expansion tank, properly precharged, prevents that pressure rise. You'll need a pressure gauge to determine the proper precharge on an expansion tank. With one, you could probably get rid of the pressure relief valve, and install the expansion tank in its place.
    Jim DeBruycker
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    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    DIY Senior Member Reach4's Avatar
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    Quote Originally Posted by jadnashua View Post
    With one, you could probably get rid of the pressure relief valve, and install the expansion tank in its place.
    You want to re-think that suggestion! Do not get rid of the relief valve.

    If you install an expansion tank, there is no technical reason that it has to be installed at the water heater. It could be on any water heater line that does not have a valve between it an the water heater. I said "technical reason" because I don't know what the codes say about that.

  8. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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    The WH has a pressure relief valve and it is 1/2 the pressure designed for the tank...if your house pressure is excessive, get a PRV, otherwise, you'll be dripping water all the time, or maybe gushing water all the time, depending on the incoming pressure. Those valves generally, are designed for unusual situations, and once they open, they don't always reseal depending on their age and the water conditions...just trouble waiting to happen.

    Code most places requires the expansion tank to be between the WH and any cutoff valve to it on the cold side.
    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 Junior Member psyq's Avatar
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    Quote Originally Posted by jadnashua View Post
    The cable's in the way, is that a T&P valve on the top of the WH? It should be. It will have a test, release lever on the top and probably a tag
    It is. It works, I tested it right away when I heard the dripping.

  10. #10
    DIY Junior Member psyq's Avatar
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    I'll buy the pressure gauge today and get some more information.

    I'm getting mixed up with the PRV abbreviations. Is that pressure reducing value or pressure relief value? The main city inlet does have a pressure reducing value, although logic would indicate I shouldn't need to mess with it if the cold water pressure is ok. 80psi about right?

    The tank is less then 2 years old but our water does have a high mineral content. I really don't want that dripping relief valve to clog up as a result.

    Thanks for the help everyone. Lots of good info.

  11. #11
    Nuclear Engineer nukeman's Avatar
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    PRV = Pressure Reducing Valve in this case. If you have one, yes it will knock the incoming pressure down to say 80 psi. The problem is that when your WH heats the water, the water expands and the pressure will increase. If you didn't have a PRV (or a check valve), the excess pressure would just push back into the city main (so the pressure wouldn't go up).

    The gauge that you get will probably have a red needle that records the highest pressure measured. Leave the gauge on at least overnight and see what the peak is. Also, the gauge doesn't need to be connected to the hot side. The pressure will be felt everywhere in the house. So, you could connect the gauge on an outdoor hose bib (assuming that the bib connects after the PRV). Otherwise, the drain on the WH will do the job.

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    DIY Junior Member psyq's Avatar
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    Quote Originally Posted by nukeman View Post
    PRV = Pressure Reducing Valve in this case. If you have one, yes it will knock the incoming pressure down to say 80 psi. The problem is that when your WH heats the water, the water expands and the pressure will increase. If you didn't have a PRV (or a check valve), the excess pressure would just push back into the city main (so the pressure wouldn't go up).

    The gauge that you get will probably have a red needle that records the highest pressure measured. Leave the gauge on at least overnight and see what the peak is. Also, the gauge doesn't need to be connected to the hot side. The pressure will be felt everywhere in the house. So, you could connect the gauge on an outdoor hose bib (assuming that the bib connects after the PRV). Otherwise, the drain on the WH will do the job.
    I assume the thermal expansion from the heater will cause the pressure to back up the cold line, but it would peak out at whatever the relief value that's leaking is designed for. Is that accurate enough? Lucky hydraulics are so predictable.

    Yes, the bib is after the PRV, and I have one in my garage. Handy!

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    DIY Senior Member Reach4's Avatar
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    Quote Originally Posted by jadnashua View Post
    Code most places requires the expansion tank to be between the WH and any cutoff valve to it on the cold side.
    That makes sense, except that I had somehow got the impression that a "heat stop nipple" on the cold water line to the water heater could act as a one-way valve. That would be a problem. I guess my impression was wrong. An expansion tank on the cold water supply would certainly be more heat-efficient.

  14. #14
    Nuclear Engineer nukeman's Avatar
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    Quote Originally Posted by psyq View Post
    I assume the thermal expansion from the heater will cause the pressure to back up the cold line, but it would peak out at whatever the relief value that's leaking is designed for. Is that accurate enough? Lucky hydraulics are so predictable.

    Yes, the bib is after the PRV, and I have one in my garage. Handy!
    Correct. Without a check valve or some other physical way to separate the hot and the cold, the pressures everywhere will be equal (with some changes up/down depending on which floor you are measuring from). The pressure travels at the speed of sound (nearly 5000' per second in water), so a pressure change at one point is felt everywhere nearly instantaneously.

    Water is considered incompressible, so as it expands within a fixed volume, the pressure increases rapidly (just like how water lines can rupture during a freeze). The pressure increases until the set point of the relief valve. The valve opens and dribbles some water out and the pressure comes back down.

    As I mentioned, leave it on at least overnight. This is when it usually peaks because you aren't using any water. Also, if you didn't have a PRV, the incoming pressure may also be higher during that time (other people not using water or perhaps the water utility is filling their water towers).

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    In the Trades Jerome2877's Avatar
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    Quote Originally Posted by Reach4 View Post
    You want to re-think that suggestion! Do not get rid of the relief valve.

    If you install an expansion tank, there is no technical reason that it has to be installed at the water heater. It could be on any water heater line that does not have a valve between it an the water heater. I said "technical reason" because I don't know what the codes say about that.
    That is a thermal expansion relief valve and is in place of an expansion tank, so absolutely remove it if you install the tank. It is doing the job it was designed for but it is a waste of water.

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