I get short amount of hot water in shower.
The sink in the bathroom is always hot.
We have a boiler, no hot water heater.
The sink never runs out of hot water ,the shower lasts about four minutes..
I think it is the mixing valve behind the shower knob, a single shower valve. Could I be right? Badjim61

I think it is the mixing valve behind the shower knob, a single shower valve. Could I be right?
Bang on Jim, What's the brand name of your valve? Does it have Posi-Temp or Anti-Skald? Let me know. Regards, tom