DIY Plumbing Question - Incredibly Complicated Shower Valves
January 11, 2017 7:21 AM   Subscribe

DIY plumbing adventures - help with shower pressure and incredibly complicated valves.

We have a shower with one handle for pressure and one for temperature. Our problem is that when you turn the temperature dial to hot, the pressure falls off dramatically. With the dial set to cold, the water blasts out very strong; hot is very weak. The hot water pressure is strong in other fixtures in the same bathroom, and, when I opened up the valve and pulled out some of the guts (more detail below), the hot pressure was awesome for a couple of days before falling off again.

I suspect the problem is a pressure balance mechanism somewhere in the valve. But the valve is incredibly complicated! Here [pdf] is a schematic of the particular valve - it is a Moen Exact Temp 3301/3321. It doesn't help that the valve is 10 years old and discontinued.

I opened up the valve to see what I could figure out. I pulled out the "check stops" and cleaned off some internal filters that were inside them and played around a little bit with tightening them and loosening them. One of the check stops seems like its internal valves are not working right - I could blow air through one very easily but the other one was obstructed/harder to blow through. I switched the position of the two check stops, thinking I would put the "good" one on the hot side and therefore get more hot pressure. For about 2 days, we had great water pressure after I did this, but then the hot water pressure dropped again.

Here's my question - of all of the parts in the valve, where do you think a problem like this would be coming from? Is it the flow cartridge, which is only $12 and relatively easy to replace? Or is it the thermostatic cartridge, which is like $120? Or the "check stops," which seem to have some kind of pressure valve built into them. It's just not clear to me where the valve does the pressure balance. Maybe it's the wonky check stop I already found, but then why did everything improve for a couple of days and then go back?

I have ordered a new copy of the valve on ebay, so I can replace the check stops fairly easily by cannibalizing them from the new one. But I'm worried that the problem is actually in one of the cartridges - either temperature or flow. Thanks for any help!
posted by Mid to Home & Garden (6 answers total)
 
Moen has a lifetime warranty on their faucets. Call them up and complain, and get them to send you the replacement parts for free. My dad does this all the time (he called me a chump when I bought a replacement cartridge for my shower).
posted by fimbulvetr at 8:30 AM on January 11, 2017 [4 favorites]


Seconding fimbulvetr. Sounds more like something on the thermostatic side.

It could also be working as designed- cutting off full-hot flow so you don't scald yourself. What temperature is hour hot water supply set at? If you turn your water heater down just a little, does the problem decrease? For that matter, when you first try to get hot water - when there's still cooler water in the pipes - does the full-hot setting on the shower work, and then slow to a trickle once it senses a slug of actually-hot water in the hot-water line?
posted by notsnot at 8:49 AM on January 11, 2017


You may even be able to get the replacement parts directly from a hardware store. I took in a broken cartridge to find a replacement and the person in the plumbing department just took mine and gave me a new one.
posted by any portmanteau in a storm at 10:11 AM on January 11, 2017


Thanks, all. The pressure-drop does not seem to be connected to an anti-scald function - it cuts out the pressure even before the water is hot. I think it is some kind of pressure balance valve made to protect against a change in pressure from a toilet flush, etc. I think the pressure balance must be stuck somewhere. But this thing has two different cartridges and "check stops" (whatever those are)!
posted by Mid at 11:49 AM on January 11, 2017


One of the check stops seems like its internal valves are not working right - I could blow air through one very easily but the other one was obstructed/harder to blow through. I switched the position of the two check stops, thinking I would put the "good" one on the hot side and therefore get more hot pressure. For about 2 days, we had great water pressure after I did this, but then the hot water pressure dropped again.

This makes me think some upstream part of your hot water system is deteriorating and sending fragments down the line to clog the check stop on the hot water side -- maybe a rubber part degraded by hot water, maybe rust pieces from a decaying fill tube in your hot water tank, possibly rust from a galvanized pipe, and etc.
posted by jamjam at 9:29 PM on January 11, 2017


So I got a copy of the valve on Ebay and I used it to replace all of the replacable stuff -- the thermostatic cartridge, the check stops, and the flow cartridge. I'm not totally sure which one was the culprit, but the pressure is now great. The thermostatic cartridge had a lot of nasty sediment in its screens - like a grey-black sludge.
posted by Mid at 8:23 AM on January 16, 2017


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