Droning noise when CH switches off?

[UnginSah]

For the last month whenever the CH system switches off either via the the room thermostat or via the CH timer I get a loud droning noise which continues for around 3-5 minutes and then ceases. It seems to be eminating from the loft where my pump and motorised valves are but I haven't really got a clue as to why its just started of late.

Also I do know that my CH system is sludged up so can anybody recommend me the best product for de-sludging it, tried Fernox but that didn't really help.

Thanks in advance.

 

[oilman]

I suspect your pump is struggling against the sludge. Do you hear the noise while the boiler is on? The pump is kept running to ensure the heat is taken away from the boiler. What type of motorized valve is it?

With the amount of sludge being enough for you to notice it, you should powerflush the system, otherwise anything you do otherwise will be wasted effort.

 

[UnginSah]

Any ideas why the noise greatly increase when the system switches off, during normal use the systems seems fine. I understand the concept of pump overrun to cool the boiler down when the system shuts down but am totally baffled by the droning noise when it switches off.

 

[galwag]

As you have mentioned motorized valves I would presume there are two of them. If this is so then you should also have a bypass valve fitted. This is normally a valve fitted on the pipe between pump and m\valves and links flow & return pipework. If this valve is a gate valve or ballofix type valve try opening a little further (1/4 to 1/2 turn ) and see if this cures noise

 

[ChrisR]

from the other site...

You say Motorised valveS. Is there a bypass route for pump overrun when they are both shut?
Which Fernox product did you use?