Anti-scald valve sticking

[Rory]

My system was plumbed up just about a year ago and includes an anti-scald valve for the DHW as it exits my passive storage tank. A week ago, I went to bed with the tank temp up to 170 for the first time in a while, and was woken the next morning by my wife claiming there was no hot water for her shower, which was a bit of a shocker. I discovered that the ASV was only letting lukewarm water through despite plenty of hot water in the tank and in the DHW line leading to the valve. I took it apart, flushed it, and things were all good again. This morning the scenario repeated itself, including the fact that I had a really good charge of heat last night. My initial thoughts last week were that some crud had entered the valve and I flushed it out, but now I'm not so sure. Is it possible to develop an air lock, or is it likely that the additional excursion caused by hotter than normal temps is leading to a stuck valve? More importantly, what can I do other than replacing the valve, as it's practically a brand new piece in terms of its expected life?

 

[Fred61]

It's 31 years since I installed one of these so you can place any value you want on what I have to say. I seem to recall that the instructions stated (Yea I read the instructions--That's when I could see) that the cold water inlet pipe needed to go below the valve by some amount and then come up into the valve connection. Now, I don't know if this requirement is for mixing accuracy, pressure, crud abatement or what, but I did install it as prescribed. What did get my attention was all the installations I saw, in subsequent years, whether professional or DIY weren't installed the way I did mine. Either I was the only one that didn't know that it didn't make any difference or most other installers never read the instructions.

 

[Rory]

Perhaps anti-scald valve was the wrong term to use. A bit of googling seems to indicate that ASV's are typically installed at a tub or shower to eliminate a rush of hot water when someone opens a cold faucet elsewhere in the system. If I recall correctly, my plumber called it a mixing valve, it's purpose is to prevent my dhw from exceeding a set value as my tank temperature fluctuates. I didn't call it that in order to avoid confusion with the blue wax valves that are typically used like a Termovar to prevent cold water from entering parts of the system. At any rate, it has an internal spring similar to an automotive thermostat or a Termovar. As far as the geometry, the cold and hot enter at opposite ends along a horizontal run, while the mixed water exits upward. The cold is actually fed from a tee in the dhw supply line as it is going down into the tank. I think if it sticks again I may try pulsing the water without taking the valve apart, as I'm not seeing any evidence of crud when I take it apart.

 

[Gooserider]

Could also be that the valve has just failed...

Another possible thing is that the valve isn't responding as precisely as it ought to... What kind of valve is it? I know I remember NoFossil on his personal site mentioned that he had done some pretty extensive research on these DHW mix valves, and found there was only one model that spec'd to fairly tight tolerances on maintaining a set temperature - might be worth checking.

Another odd thought is to wonder if you are getting heat transfer through the valve from the hot side to the cold side, and if so, what that does to the way it works if both sides are hot... Considering that it is supposed to be serving a safety function, I wonder if it would shut off the output flow if it couldn't mix down to it's set value?

Gooserider