Hello, I'm a keen DIYer but a plumbing novice.

I have a conventional vented CH system with header and storage tanks and gas boiler. Radiators are piped in 10mm "microbore". The whole installation is 20 years old.

Although the HW works fine, the heating has never been efficient and over the last 18 months the radiators have struggled to heat up. The problem is getting worse. When I try to bleed the rads, it's obvious there's v. little pressure.

I've replaced the circulating pump and the 3 port motorised valve but to no avail.
Although it appears to be heating OK, can it be my boiler that needs replacing?
I don't know much about boilers, but assume that internally there's some kind of matrix of thin pipes much like a car radiator... Can these fur up or corrode over the years causing a blockage?

I hope somebody (any plumbers out there?) can advise. I don't want to be sold a new boiler if it's something else that's wrong...

Thanks in advance

You could have a build up inside the boiler vessel itself. Stop replacing parts unless you know it is the problem. Might be better to call a pro on this one.

What's the water pressure? Could the core be getting plugged with soot causing a poor burning of fuel? Could one or more orifices be fouled with a spiders nest or something?