Just replaced water heater and all faucets are running hot except for the shower, what could be the problem?

The shower is running only a little warmer than luke warm, and the pressure seems low on hot. This is turning the faucet all the way to the left on hot. It used to be burning hot before the problem started, and it also goes cold way faster too.

Wow this is strange, are you sure you connected to lines to the heater correct? Is this tank or tankless? I hope an expert will be along shortly to help iron this out. Good luck

Hi Eve...

It could be as simple as the temperature has been set to a different temperature than the old water heater was set at and you'll just need to reset the temperature to the new water heater, maybe? Here, most shower valves have temperature limiting devices that will automatically keep the temperature lower than the rest of the faucets to begin with, so a small temperature drop could seem much more dramatic at the shower. Check the temperature setting at the water heater, but it should not exceed 125F maximum.

If the temperature setting is appropriate, then the only other thing that makes sense to me is that the mixing cartridge at the shower valve has become plugged with sediment and will need to be cleared to fix the shower valve.

Here, shut the water off to the shower and disassemble the valve. Clean the cartridge and the insides of the valve and then, using a towel to cover the shower valve (cartridge out) pulse the hot and cold water on to flush the hot/cold water lines of any sediment in the water lines. Reasssemble and test your work.

If that doesn't fix this then you will most likely need to replace the mixing cartridge entirely, but try as above first, OK?

Let us know how you make out...

Mark