Pressure relief valve question

[BillS22]

My hot water tank pressure relief valve weeps a little water ever heat cycle. Not a lot, say a tablespoon or two. Should I replace it?
My temperature setting on the controller is on B. The adjustment range has four settings:
A B C and Very Hot.

Thanks
Bill

 

[Reach4]

Often this means that you need a thermal expansion tank, or if you have one, replace the failed unit. Get a pressure gauge and monitor what happens to the water pressure after you use much hot water and then do not use any water while the water heater heats.

 

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[Gary Swart]

I think Reach 4's answer is correct. Your T/P is working properly. You apparently have a pressure regulator valve that is preventing the expansion caused by the heated water from being absorbed by the city main. This trips the T/P when the pressure reaches the limit and then when the pressure is back to normal, the T/P closes. This is corrected by adding a thermal expansion tank in the cold water supply line ahead of the water heater.

 

[Jadnashua]

You don't necessarily need a PRV to get a closed system (which is what is the main cause of a T&P valve weeping during firing)...your utility may have a check valve on your supply, which will cause the system to be closed as does a PRV.

If the T&P valve does shut off, it's probably good, but repeated opening can cause some mineral deposits and those can prevent it from closing eventually.

 

[Dj2]

I would replace the T&P valve first.

 

[Jadnashua]

I would replace the T&P valve first.

The first thing you should do is to check the pressure to verify the valve is working properly or not! No sense in replacing parts until you've determined why it is opening...it's highly likely that it's just doing what it is supposed to do. The T&P valve can open for one of two reasons: pressure exceeds 150psi (easy to do during heating in a closed system without an expansion tank), or the temperature exceeds about 205-degrees F (both of those can vary slightly). It would likely be very obvious if your water was too hot, but you may not notice the excessive pressure. Note, that pressure will drop almost instantly once you open a valve since the pipes don't balloon much (the hoses can flex lots more than the copper), but once that excess spurt has relieved the tension on the supply lines, your pressure returns to 'normal', whatever that is. A cheap pressure gauge with a second tattle tale hand will show you the peak and current pressure. Install one and leave it there for a day or so, and you'll know whether you need to add or replace parts.

 

[hj]

quote; I would replace the T&P valve first.

PRobably a good idea if it is fairly old, but will probably NOT cure this problem. They either need a leaky faucet or an expansion tank.

 

[Mikey]

I've got the same problem in a US Craftmasters 66-gal "Energy Smart" WH. Incoming pressure is typically 80psi, incoming temperature around 55-60°F, output temp 130°. Watts' calculator says I need a crazy big expansion tank. I plan on installing a PRV in the supply to get things down to 60psi, but my real question right now is what size T&P valve to get. Apparently they have different stem lengths to get through the insulation on these super-insulated tanks, but I have no idea how much there is in my tank. I'd like to have the correct size on hand when I take the old one out. US Craftmasters has been unresponsive.

 

[Jadnashua]

The size of the ET is determined by the combination of the size of your WH and the incoming and ultimate outlet temperature (the temperature rise or Delta T). It's easy to calculate how much water expands with temperature change, then multiply that by the actual volume of water being heated. You don't need to know those values...most all of the sellers of the ETs have calculators that do it for you.

The ET needs to be between the inlet of the WH and any shutoff valve to it. That gives you some latitude on where to place it. IOW, it is rarely is attached directly to the inlet of the WH.

 

[Mikey]

Well, there's the problem. If the Watts test gauge is correct, I'm seeing 150psi (at least) at the water heater now and then. T&P valve is working as designed, apparently, but a new T&P relief valve and an expansion tank are on order.

 

[Reach4]

I've got the same problem in a US Craftmasters 66-gal "Energy Smart" WH. Incoming pressure is typically 80psi, incoming temperature around 55-60°F, output temp 130°. Watts' calculator says I need a crazy big expansion tank. I plan on installing a PRV in the supply to get things down to 60psi, but my real question right now is what size T&P valve to get. Apparently they have different stem lengths to get through the insulation on these super-insulated tanks, but I have no idea how much there is in my tank. I'd like to have the correct size on hand when I take the old one out. US Craftmasters has been unresponsive.

When I try http://www.watts.com/pages/support/sizing_DET.asp with 80 degrees in (reading their note next to the entry) shows 1.69 gallons needed. Their 2.1 gallon tank would be big enough. Even going to 55 degrees, still makes it. However a bigger tank will not hurt and may help.

Set the precharge to 80 with the Watts (the max per their spec) if your water gets to 80 PSI or more coming in. With a Therm-X-Trol, you could set it higher if necessary, because those allow a higher precharge.

I would not install the T+P now, but test now and then. If it does not shut off after operating the lever, change the T+P. I think that is what the test lever is mainly for -- to see if it turns off the water on its own.

 

[Mikey]

I have been turning the T&P off now and then as soon as I noticed the wet floor drain. The pressure gauge confirmed the T&P had good reason to open; wish it noted the time as well. I finally figured out Watts' sizing app and got the same results, so it looks like the plt-12 will do the job. It'll go in at the same time I replace the old gate valve with a ball. Therm-X-Trol specifies mounting inlet-side-up only, which is a minor problem for me, so I'll probably go with the Watts.

Having trouble finding a Watts SL-100XL T&P to replace the one in there now, so there will be a slight delay in replacing that anyway.

 

[Reach4]

Therm-X-Trol specifies mounting inlet-side-up only, which is a minor problem for me

What I see says "Mount vertically only."

Having trouble finding a Watts SL-100XL T&P to replace the one in there now, so there will be a slight delay in replacing that anyway.

If you have not ordered it already, I would not do so. You can find those locally pretty easily, and odds are the one you have will last longer than the WH.

 

[Mikey]

Amtrol's installation manual excerpt:

I interpreted "downward position only" as meaning inlet-up.

 

[Reach4]

Amtrol's installation manual excerpt:

Yep. There it is. I was looking at a different document I guess.

 

[Jadnashua]

I think that they want the water pressure to be pushing up on the bladder, rather than having water pressing down on it via both pressure and gravity. It probably won't make a huge difference, but why push your luck!

 

[Mikey]

I've been trying to come up with a reasonable reason, and all I can come up with is that Amtrol wants the water on top of air for some reason. I guess you could more easily check for a leak in the membrane. Or, in an earthquake, the tank may be less susceptible to breaking off the line. Watts explicitly allows "up" or "down", or sideways, for that matter, if its weight is supported.

When I plug in my numbers, I get an acceptance volume of 0.97 gal, but a total volume of 2.65 gallons, so the PLT-12 is the conservative choice.

 

[Reach4]

Or, in an earthquake, the tank may be less susceptible to breaking off the line.

I expect that is more of the thinking. I think they are wanting to avoid lateral stresses. I think they are being overcautious, making their product seem inferior as a side effect.

 

[Mikey]

Now, there's an idea... thanks. I like the valve to isolate the tank, wouldn't be a bad idea to placard it to remain ON for normal operation.

PS: I'm sure local codes will require earthquake protection.

 

[Reach4]

There would certainly not be a need for a full port valve. I think 1/8 inch tubing would be enough to carry the flow, but since they use 3/4 inch MIP on the tanks, that would require more adapters.