Moen shower valve issue when water heater drained.

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Robf

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I'm having trouble with my second floor shower. It's a Moen Adler single handle with I'm assuming the posi temp. I'm in the middle of replacing my water heater. Tank is drained, no hot water in the house at all. All of my other fixtures have cold water and good pressure except the upstairs shower. Water comes out but not very strong. Also if i continue to rotate the handle to the hot side I get water that comes out, which shouldn't happen since there's no water being pumped through the hot water lines. I'm suspecting the shower valve is somehow letting cold water past and into the hot water lines. If I don't have the inline hot water shut off closed that leads to the shower, water will back fill through the hot water lines and into the empty water heater. I've yet to contact Moen but plan to later today.
I'm pretty novice with plumbing so any input is appreciated. Thanks!
 

JohnjH2o1

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A balance pressure valve needs both hot & cold water on to operate.
 

Terry

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Yes, rotating the valve to hot mixes it with cold. If you want to take the water heater out of the loop, I suggest picking up a 24" flex connector and installing at the cold supply and hot out for the water heater. Then the emptied water heater can stay empty and using the Posi-Temp will allow showering. Cold only of course without the water heater hooked up.
 

Robf

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So basically the valve needs pressure from both sides or else it will push water into and down whatever pipe doesn't have pressure? In my case being the hot side.
Sounds as simple as me shutting off the cold water leading to the bathroom so no water can circulate back down the hot side. I wouldn't be worried about it but I have to replace the hot shut off since it leaks
 

Robf

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Thanks for the info guys. I'll be installing the new water heater tomorrow and will just shut off the cold water leading to the upstairs bathroom so I can sweat in a new hot water shut off. I thought the cartridge or posi temp valve was bad because it will literally send a constant slow flow of cold water backwards through the hot water lines. Thanks again
 

Jadnashua

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The pressure-balance spool valve might be stuck or worn out, but, it is more to prevent a shutoff or reduction in pressure on the cold side from ending up in all hot (thus, the anti-scald function). Many of them will not allow much of any water through the valve when you do not have both hot and cold supplies turned on.
 

DIYmatt

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Thanks folks- good to know this. Just had a direct experience of this today.

I'm -almost- done the prep for a steam shower and cut off the hot water since I had to move the sink shut-off over to accommodate the steam generator under the sink. I have a Moen positemp shower valve roughed in, backberboard is up, and membrane is underway... Cold stayed on, of course, and water pooled on the floor over the ~30 minutes I worked on the hot line. I panicked at first but after some thought suspected something like this. It was confusing since I don't have a shower arm in yet, just the drop elbow up inside the wall, and the water was coming down the inside of the wall onto where the pan will be. I can see water still in the threads of the drop elbow so I'm confident that's where the water came from. I had all my pipes exposed for a couple weeks, under pressure, so I sure hoped I hadn't missed anything there.

I can't thank you experienced folks enough for sharing here!
 
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