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I am currently redoing the shower/tub surround, and would like to know what valve I currently have. It is a Moen, and uses the 1225 cartridge (crystal pull out handle, pull for volume twist for temperature). On the side, it looks like "85811". The house was built in 2000. High-end materials were not used.

I ask about the current valve, because we are having a terrible time with controlling temperature. Even turning on the sink (hot or cold) will affect the shower temp, and things like dishwasher and clothes washer make the shower useless with all the temperature changes. But from what I understand, this valve is some sort of mixing valve that should be handling temperature fluctuations like this. If this valve does this, then would a fancy thermostatic valve accomplish anything? Or is there some other cause.

We have 2 shower/tub combos, and both behave the exact same way.Right side

Left

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  • I replaced the valve with a Delta Multichoice valve. Although not a thermostatic one, the change in temp quality has been amazing. All issues I had before are gone -- toilet flushes, washing machines, and even faucets can be used without issue while showering. – izzmit Jan 1 at 19:09
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This is neither a pressure-balanced or scald-protecting valve. It is very simple Moen mixing valve that mixes cold and hot and adjusts flow according to your manual input with the knob.

There are various tricks to help reduce the effects caused by other users; the simplest is to install a flow-restricting shower head.

Changing the valve for a pressure-balanced unit will help but only to a point. If you really want to solve the problem you need to take a hard look at your water delivery scheme and investigate options like up/down-sizing parts, adding a hot water recirculating strategy, etc. Maybe consult an expert (licensed and bonded reputable plumber sourced through personal recommendations).

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  • Pressure-balanced units use a type of... piston/spool/element that moves when pressure drops significantly from either the hot or cold. When the element moves it simultaneously restricts input from one side and opens input from the other side. its an fairly effective way to solve your problem when pressure fluctuation is the issue. Another temp-control feature you will see is a high-limit stop on the hot water side, preventing one from opening the hot input beyond a user-controlled setpoint. The thermostatic control units are the most expensive option (I don't know if they are the "best"). – Jimmy Fix-it Nov 24 '20 at 23:58
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What you need is a thermostatic mixing valve. Good ones aren't cheap but they will do the trick. I was having the same problem in my house a few years back and when I remodeled the bath I put in a Danze and it works great. There are a number of good ones on the market but read the reviews. You get what you pay for.

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