Mixing valve repair question

My house is about 25 years old - probably the original shower plumbing - in the five years since I've been in the house, the temperature adjustment knob for the shower has always had to be turned almost all the way to hot to get the shower nice and hot. The adjustment knob is a single one-piece knob - the kind that ya pull out to control the water, and turn left for hot and right for cold. It's getting to where need to turn it all the way to hot to get a decent temperature shower - I imagine that means that the mixing valve is on its way out.
Before I go replacing things that I don't understand, or call a plumber to look at it, when a mixing valve goes out, does it tend to fail as I've described, or might it be a problem with the hot water supply line?
Thanks!
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Whatever . . wrote:

What temperature do you measure at your kitchen sink or other spigot with straight hot water?
nate
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The temperature at the kitchen sink on full hot is a little bit hotter (but not much) than the shower turned all the way to hot - I have the gas hot water heater turned down a bit. Also, the shower is a shower stall - no tub, no other faucet besides the shower head.
Thanks!
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If the water going to the valve is not hot enough, messing around with the valve is not going to solve anything. First thing is to check the temp at other faucets-- with a thermometer-- and compare them to the shower temp. Also, when the shower is on full hot, does the water flow seem to be the same as that on full cold? I would at least set the w/h to the "normal" range on the dial, and see what happern. Good luck Larry
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Eau contraire, mon ami. The mixing body combines the flow of water from two separate pipes. If either port is restricted, the flow is diminished. I thought I had to replace a mixing body once, and when I got it apart, it had "crud" in it, and cleaning it solved the problem. But yes, checking temperatures can identify the problem if it is not in the mixing chamber.
Steve
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On Jan 30, 7:30pm, "Whatever . ."

Turn it up. -----
- gpsman
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wrote:

Turn it up. -----
- gpsman
But not over 120.
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Mixing valves aren't rocket surgery, but they are a little picky. Particularly getting them out after being there that long. Usually manufacturers sell a tool for that. I've had good luck fixing them, even a German model I thought I'd have a booger of a time getting parts for. Many times, it's just hard water scale accumulation that has it partially plugged, but you have to disassemble to get it out. Pay close attention to how it comes apart, even if it means taking pictures or making a drawing. BTDT.
Steve
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Is this something recent? Mixing valve or anti scald valve? Some anti scald devices have a way of setting them so the water will be as hot as you safely set when turned on all the way to hot. Behind the handle there may be a stop to adjust.
Keep in mind, if it was OK all summer and how is not hot enough, it may be because the cold water is the coldest it has been in a couple of years here so you need a different ratio when blending. In my shower, in winter I just crank it all the way, but in summer, I back it of just a tiny bit to get the temperature I want.
Go to the manufacturers web site and see if they have instructions on how to clean or replace parts. They have not changed much internally so you may be able to do a simple fix with readily available parts, or make a simple adjustment.
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On Jan 30, 6:13pm, "Whatever . ."

Another thing to consider (and more of an FYI) is occasionally there is an extra valve installed after the mixing valves (after the mixing point), in a different location. Like the shower pause valve to save water. When there is demand for hot water in a different location, the cold water will back-feed (in most valves) into the hot water supply.
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