Gravity fed central heating, rads cold at top

[PartyFops]

But when I bleed them nothing happens, no hissing of air or water. Has the pump gone?

[PigletJohn]

You have a small tank in the loft. Is there any water in it?

[PartyFops]

Just been up and checked, water is stone cold and heating is on if that makes a difference?

[PartyFops]

Sorry meant to add yes water is in there and ball cock working fine.

[PigletJohn]

Then I am sorry to say you have a sludge or sediment blockage in the feed and expansion pipe. This is the 15mm pipe that comes down from the small loft tank and probably joins the larger circulating pipes beside the cylinder.

If you hold a magnet against the Tee where the pipes connect, you will probably find it is attracted to the copper pipes because they are full of black iron oxide from the steel radiators. The blocked section has to be cut out and replaced. If you are fond of plumbing you can do it yourself (empty the loft tank first) or a plumber can do it. You do not have to pay boiler engineer rates. It is a very common fault with old open vented systems. Sadly it is now too late to try a DIY chemical clean because the chemicals cannot flow down the blocked pipe.

If you pay someone, ask what they would charge to install a magnetic system filter at the same time, and show you how to empty it. It is no harder than emptying a vacuum cleaner.

[PartyFops]

Thank you Piglet John, you are amazing!!

Can I just add, just incase it makes any difference, the system was drained down only 2 weeks ago and 4 out of the 9 rads were replaced. I gave texted my heating engineer who is due out this week anyway to plumb in my new LPG hob.

[PigletJohn]

if you look in your feed and expansion tank, you might find there is mud in the bottom, as well as drowned spiders, mice and birds. Sometimes scraps of insulation fibre too.

Unless you bale it out and sponge it clean, this accumulated sludge washes down into the pipes when refilling after a drain, and contributes to blockages.