Shower valves run hot only in the last few degrees of rotation

[mlefebvre]

In my house, I have 6 shower/tub valves, all about 15 years old. Each valve, including the kitchen faucet, only gets hot water in the last few (3-5) degrees of handle rotation before full hot. Once hot water is running, moving the handle only a few degrees off full hot gets cold water. Any ideas anybody?

My theory is there must be a big pressure differential between hot and cold, so that it takes calling for lots of hot water to overcome the cold. However, hot and cold water flow seems about the same.

Thanks,

Mark

[cacher_chick]

And you have a tankless water heater?

[mlefebvre]

No, conventional gas, set to about 120 or 130.