My indirect-fired water heater (SuperStor Ultra) was occasionally releasing hot water from its relief valve, likely affected by a pressure relief valve near the water meter. To fix it, a plumber came and added an expansion tank (Amtrol Therm-X-Span) on the hot output of the anti-scald valve (Legend T-41) which is connected to the water heater. Now sometimes, when I turn on the cold tap of a faucet, hot water briefly comes out! Also, if I touch the "cold" water input pipe to the anti-scald valve, it feels as hot as the hot water input pipe.

Why am I now sometimes getting hot water from my cold tap? Did the plumber make a mistake in one of the connections? Is it okay that the expansion tank is on the hot water side of the water heater? Is it okay that the expansion tank is after the anti-scald valve?

How do I prevent hot water from coming out of the cold water pipe? Do I need a check valve to be installed on the cold water pipe?


full resolution: [i.imgur.com]


full resolution: [i.imgur.com]

I've never run into this situation ? possibly the spring in the mixer is affecting the ability of the X tank ??
expansion 101...
you have a tank full of cold water..
you heat that water thereby increasing it's volume..
the added volume expands into the cold water pipe and goes harmlessly out to the street main. (no problem)
but when you add a check valve to your water main in the form of a pressure reducing valve this can not happen.
so now the increased volume of water creates additional pressure in the water heater
if this pressure rises above the setting on the relief valve, the relief valve opens and the excess pressure is relieved.
solution.. install an expansion tank on the cold side of the tank. this tank acts like the water main on your street and absorbs the increase in volume which keeps the pressure stable..
so. reading and thinking about what i have just written would lead one to believe the tank is in the wrong location ??
BTW, why do you have a mixing valve at the water heater ??

The tank is "suppose" to be on the cold water side, although it is rated up to 200 degrees. Installing it after the mixing valve was not a good idea either. I think switching the tank to the cold side would solve the problem.

I agree. The expansion tank will work and absorb the excess water pressure whether it is on the cold inlet side or the hot outlet side. But by putting it on the cold side you avoid this type of problem since only cold water will ever enter and leave the expansion tank. The expansion tank does not need to be installed really close to the water heater, although near it is usually a handy place.



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Packy, possibly the tank is undersized and to compensate, the temperature has been increased. Just a guess.

He's probably heating the tank with 180 degree boiler water , in case of a faulty control the mixing valve will provide a safety as far as scalding .

If that mixing valve does not incorporate check valves, and the plumber didn't install them, that's a problem.

i looked up that model of mixing valve. it has no check valves.
i thought the problem just started since the x-tank was installed ??

I'll speculate that when a faucet is opened the expansion tank becomes the source of pressure for a very brief period of time. That said, it points to a failed mixing valve. The reason for check valves is that it is possible for the valve to fail completely, and scald someone. I'd replace the mixing valve, either with one that has integral check valves, or make sure they are used, if installing a valve without them.

The mixing valve was installed by my previous plumber (now retired) in 2010 when he replaced an old indirect water heater (9 years past its warranty date) that busted open a week after we bought the house. That plumber also installed the pressure relief valve near the water meter. He set the new indirect hot water heater to 140F, saying it would prevent legionella bacteria. He then installed the anti-scald mixing valve so that the hot water going out to the fixtures was reduced to a safer 120F.

We never experienced hot water coming out of the cold tap in the past 7 years since the mixing valve was installed. But ever since the new plumber installed the thermal expansion tank 3 weeks ago, we've noticed hot water briefly coming out of the cold tap at least 3 times.

easiest attempt at finding the problem might be to shut the water, unscrew the tank, put a brass nipple and a threaded shut off in the pipe and re-install the x-tank. then the x-tank can be isolated from the house by closing the valve. if the problem persists then it is not the problem..if the problem goes away, it is the problem and needs to relocated to a cold pipe near the heater.
installing a valve will take 1.5 minutes..



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The x-tank is on the hot/out of the heater. Are you saying that moving it to the cold/in side of the heater could possibly solve a problem?

Yes.