Had the same problem, although my heater was a bit newer. It turned out that limescale was the issue. Guys from Hot Water Nurse came and solved the problem. Apparently, water here is full of carbonates.
What's wrong with my gas hot water heater? It is about 16 years old, and the water has been getting cooler lately. I now have it turned up to hot, but only get warm water from it. I can't afford a new one.
If you can’t improve the energy efficiency of the house, and have a fixed radiator grid, then get a gas boiler every time. A heat pump will at best increase your heating costs by a factor of 1.5, and at worst not be able to heat your house properly. Listed buildings, where the windows cannot be replaced with energy-efficient windows, are particularly strongly in this category.
If you live in a listed building with poor energy efficiency, get out of there in the next few years and into an energy effiicent building. You’ll have real problems with heating if forced (at some more distant future time) to edinburgh boiler installation without ability to insulate properly or rebuild the structure of the building with larger radiator grids or underfloor heating.
With your boiler being 16 years old, it could be that it is very inefficient, however, as long as it still starts with no problems and the flames are always on in the boiler; it could be that you need some maintenance.
There might be air in your radiators. The way to tell is if the top half of your radiators are cold even though the bottom half is hot. This means there is air in the system and the radiators need to be bled - which means letting the air out. This might be having some impact on the water temperature overall.
There might be limescale in the pipes I think this is the most likely cause (as my mum had this issue last year), it could also be sludge as this builds up over time. However, I think it's likely that there is a blockage in the pipes preventing the water heated from the boiler getting around. You can buy product to flush out the system. I would recommend getting a plumber though!
The pump might be broken The boiler may well be doing it's job, but if the water pump isn't working, the hot water wont move from where the boiler is heating it up.
The best way to locate the problem is to feel where the pipes are warmest up to. Is there a point where it starts to cool? Find that and you will hopefully find the issue.