Shower water is luke warm on some days


Two of the four showers in our house have the same problem. Some days the water is hot and fine, but some days the water is just luke warm and not comfortable for showering.
When the shower water is luke warm, nearby sink continues to provide hot water.
The shower components are Delta make and the controller is a plastic knob. http://www.deltafaucet.com/bath/details/1348-WS.html
We have two heaters on the attic and we believe both of these showers are fed by the same heater. We don't know for sure.
The plumber that looked at the problem quickly diagnosed it to be a problem with the cartridge. He did so without doing much analysis. I read up on the cartridge and I am not fully convinced that replacing cartridge will resolve the issue - but I am new to all this. Since we recently had an expensive plumbing repair in the house, I really don't want to spend time and money for experimentation.
I am only looking to see if you all agree with our plumber's diagnosis. Thank you very much for your time and help.
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Sounds like the mixing valve in the faucet needs replacing. The mixing valve controls the flow of water to prevent scalding.
You will need new seats and springs at the same time as they should not be reused. Total cost of part less than $20 most places. Off the truck prices may trouble that.
Colbyt
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On Tue, 18 Aug 2009 08:29:24 -0400, "Colbyt"

I seriously doubt that is the problem because the problem exists on two faucets. It would be a hell of a coincidence if that was the problem.
Gordon Shumway
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On Tue, 18 Aug 2009 04:36:30 -0700 (PDT), Sudarshan Karkada

I don't know what the problem may be but it certainly isn't the cartridge. The problem exists between each faucet and the water heater. Otherwise you would only have the problem at one shower. My advise would be to contact a competent plumber.
Gordon Shumway
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On Tue, 18 Aug 2009 04:36:30 -0700 (PDT), Sudarshan Karkada

One of the two water heaters may be at fault.
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Phisherman wrote:

It's too bad you don't feel like doing some troubleshooting, Sudarshan. It would be simple enough to turn off the water going to one of the heaters temporarily so that you could find out which heater supplied which showers.
It's a 15 minute test that would give you some really useful information.
--Winston
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