We have an old furnice (15-20 years old) that I think heats the hot water on demand as we have no hot water tank (apart from the overflow). We use oil as a fuel and have hot water radiators in one zone and baseboards in a second zone. The problem is that when we go to take a shower it starts very hot and after a minute we have to keep turning up the heat because it gets colder until we can't turn it up any more. Then it's luke warm at best. After 8-10 minutes it starts getting hot again. Any idea what's going on and what I can do to fix it. I'd like to hear that I don't need a new furnace right now and there's a fix I could do on my own. Let me know if you need any more details to better analyze the issue.

Also my wife seems to think that it is only happening in the winter when we have the heat on, but I think she just has a short term memory.

Thank you in advance for your help

(I actually put this in the plumbing section first, but the sticky in that forum indicated this may be a better place for the question)

This is a heating question so you are in the right area. I think you have a combi-boiler. It makes hot water for heat and domestic hot water. I'll leave the advice to others later today. I don't work on combis.