I assume you have a boiler, not a furnance. I also assume that you have a tankless coil, whereby the boiler makes the hot water for both heat and domestic hot water.
I had the same system in my home until just recently when I had a new boiler and separate hot water heater installed.
Does the hot water seem short at the same time the house heat is running? I know in my old system I would run out of hot water very fast whenever the house heat was running, as the base boards where drawing off most of the hot water. If you didn't have a tankless coil before, you will find that your hot water will not last long. There are only a few gallons of hot water inside the boiler, and as soon as you turn on the hot water those few gallons disappear fast.
Anyway, after I installed my new boiler and separate hot water heater I am very pleased regarding the steady, consistent hot water.
So, it this applies and you can't stand the changing hot water temperature as you shower, then a separate hot water heater would do the trick, but not without spending some money.