Adjustable pressure relief valve
I have a hot water coil on my oil burner. There is a adjustable pressure relive vale that is leaking. It is located where the cold water pipe enters the coil. I would like to replace it but I am not sure what the pressure should be. Any help would be appreciated.
Comments
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If it is on the domestic hot water side it can be replaced with a standard fixed t&p valve. Are you sure it is a relief valve and not something else? Can you show us a picture of it?
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The PRV may not be at fault. If your water service includes a backflow preventer, you need a small expansion tank on the domestic hot water piping to make room for the additional volume of water as it is heated. Most municipal water meters now have a BFP.
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Bburd1 -
This is the valve.
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The chances are that you have anywhere from 50 to 80 PSI city water pressure. That is why the original installer selected the adjustable relief valve. I would select 100 PSI to 120 PSI for a cold side on the tankless coil DHW heater. Do you know what your city water pressure is under normal conditions? That would be a determining factor. As @bburd said about your water meter having a backflow preventer, if that is something that was recently added when the new meter was installed, your system may not be able to handle the expanding hot water in your potable water piping. That would cause the water in the piping to expand to a point where it might cause a pipe to burst or a relief valve to operate (release water). A therm-extrol for DHW service may be able to solve the problem.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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No backflow preventer. And the installer added a vertical length of copper pipe to the system to act as a expansion tank. This is the type of coil I am talking about.
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Hi, Does your valve look something like this? It might have been used so there is no temperature sensitive part. I'd measure the static water pressure and then set the valve about 20 psi higher. It would be good to use a pressure gauge that has the red "lazy hand" so you know what the highest pressure has been over, say a 24 hour period. If you have a pressure reducer or back-flow preventer, you may get pressure spikes when the water heater fires. In no case do you want the system exposed to pressure over 80 psi.
Yours, Larry
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oh, because in a steam boiler it could get hot enough for the temp part of a regular t & p valve to operate
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Not a T & P just pressure only. Usually a 15-175 lb valve.
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Most residential water heater T & P relief valves are set to open at 120 to 150 PSI. I would think that 20 PSI above normal pressure is a bit low. since most systems operate around 50 PSI, I believe the rule of three (a common engineering concept) applies here. Don't exceed three times the operating pressure, as the component with the lowest tolerance to pressure is likely to be able to handle three times the design pressure of the system. So a 120 PSI setting would be acceptable for the relief valve in your situation. It's high enough to not weep at the possible 80 PSI and less that 3 times the normal 50 PSI of the system's design pressure. But I'm not the expert on this. I wonder what @hot_rod or @Jamie Hall might add to this conversation as their backgrounds are from the manufacturer and AHJ inspection point of view.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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150 psi is the most common T&P rating. I think water heater tanks are tested to 300 psi, operating pressure 1/2 of that at 150. So I think that is where the 150 psi comes from.
Possibly there is a label on the tankless coil with a working pressure rating? That would tell you the correct pressure relief setting.
I see Moen shows 175 psi max. on their faucets. I thought it was 150 psi, however.
Plumbing codes seem to indicate a maximum of 80 psi. A pressure reducing valve shall be installed if pressure exceeds 80 psi.
A thermal expansion device would be a wise addition on any hot water generator.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
1 -
That's a Watts 530C. Set it to 125 or 150. The 100XL T&P (non adjustable) comes either 125 or 150 psi.
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If the water supply pressure is below 80psi, as per code, and you have a thermal expansion device, I would think 100 psi-125 psi would be plenty for the relief.
I don't know if these mini exp tanks, designed for tankless water heaters, ever get used on tankless coils in boilers? If the coil holds 2 gallons or less it should be adequate.
If not a small tank type like the ST-5.
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream
2 -
You can get fixed pressure only relief valves set for several points between 100 and 150 psi from someplace like supplyhouse.com too. I would rather see a fixed relief value so that someone that doesn't know what they are doing can't adjust it.
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yeah, no need to have an adjustable relief valve?
Bob "hot rod" Rohr
trainer for Caleffi NA
Living the hydronic dream1 -
Fixed preesure relief is probably much cheaper.
We have all seen tankless coils installed without a PRv and they have been fine for years. The coil only holds a small amount of water so there is not much expansion which is taken up by the H & C cold water piping and as long as there are no check valves or pressure reducing valve it can back up into the street.
That being said it still should have a PRV
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