I have several 10 year old Symmons Temptrol Model A shower units. One has recently stopped all hot water. A second is warm at best. I was told by a friend (not plumber) that I need to replace the mixer valve or the part that stopps scolding. The reason being is the part wears out and defaults to all cold as a safety feature?

What part do I need and can I change it myself?

Symmons valves suck. Buy a Moen. Moens last and are SIMPLE to repair. I hate my Symmons valve that I have had to repair twice, My Moen is older an I have never had to touch it. The Moen is used just as much as the Symmons is and just keeps on working. Symmons valves SUCK.

We do not use Symmons valves either...

Your friend / plumber was correct. The anti-scald device gets stuck. Replace the cartridge and you will be fine. It is the best, fastest and cheapest way of fixing this problem...

Good luck... Milo