Originally Posted By: Tom Brown
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Hi all.

Central heating system has been a little noisy of late. In the airing cupboard upstairs are my immersion tank, my central heating pump and what looks like a 3-way diverter valve which is a 3-way brass casting with a squarish metal box on the side with a lever. Thursday night soon after the system switched on there was a loud knocking from the airing cupboard, I couldn't place the sound to the pump or the diverter valve. The pump was still running. After this three of the radiators downstairs did not heat - the remaining three did. All the radiators upstairs heated.

I called British Gas. The engineer changed the pump; pointed out to me some sediment around the pump and half a bowl of quite blackish water. He recommended a powerflush of the system and quoted ?549.00. I fell over! All the radiators heated but after about half-an-hour after he left the knocking occurred again.

Talking to a neighbour, he suggest that it is not unlikely to have a build up of sediment around the pump and a system 13 years old shouldn't need a powerflush. I drained down the system. The water was a little dark but not too bad. I clean out the expansion tank; let some clean water run through; added Fernox to the system as it filled. Bled the rads downstairs then upstairs before switching on the system. Switched on the system and bled the rads again. The system is running quietly and smoothly but the three rads downstairs wont get hot. Clean water is coming out of all the rads on bleeding.

I still can't believe the system needs a powerflush but do not know what else to try.

How does the divert valve work, what is the lever for and could it be this? The new pump has a switch, which I think are speed setting, the engineer set it on level 2. It also has a central nut. What is this for?

Any suggestions greatly appreciated.