Central Heating: No Hot Water: Blocked Coils?

8 Dec 2017
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United Kingdom
Hi all, newbie DIY-er here, I was adding some TRV valves to old radiators in my new/first home, and drained the system. Everything went ok, radiators working well once more.

However, after a day I noticed the hot water tank was cold, and the feed & return pipes also cold. It is a standard Indirect Hot Water Tank, feeding off an oil burner in a gravity system.

I initially thought about an airlock, but after following the pipes I can't see where this could happen. It looks like the pipes go straight down from the F+E tank. I also bleed all the rads. After this I read that you can undo the connections on the side of the tank, from the primary & secondary flow pipes. I did this, but no air came out. I didn't open them too far, fearing a flood. Only as far as the water coming out in a steady stream.

I also read that placing a strong magnet on the secondary flow pipe is an indication of metallic sludge in there. I did this and the magnet did stick. Does this point towards the drain down causing sludge to gather in the coils? And if so, is there a way to do a DIY flush on the coils with the tank in place? Is it simply a matter of getting a hose in there and letting her rip?

Thanks in advance, Dan.
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