Central heating expansion tank problem

[Ronanh]

I Realise this topic has been discussed many times but I couldn't find a solution to my particular issue!
Basically the hot water in the c/h system is overflowing into the expansion tank and then circulating back through the system. Tho I've just discovered that this doesn't happen if I turn the flow pump down to 1.
There are about 8 rads upstairs but I only have one of them turned on. I'm afraid if I turn on any more rads I will need to increase the pump speed and consequently will cause the water to start overflowing again!
I had been doing some work on the boiler before this started which necessitated draining down the system so I'm not sure why this problem should have started now?
Any thoughts and suggestions most welcome.

Ronan.

 

[HERTS P&D]

I had been doing some work on the boiler before this started Ronan.

What were YOU doing to the boiler?

Andy

 

[Ronanh]

I had to take the boiler out altogether in order to weld a new inlet (the old one had completely rusted!). This was done successfully and the boiler reinstated. All was working fine apart from the above problem.

 

[mogget]

I had to take the boiler out altogether in order to weld a new inlet.

Oo err this doesn't sound good to me, what fuel does your boiler run on - I hope it's not gas!

mogget

 

[Ronanh]

No! Oil fired. I took the damn thing out to my workshop to do the welding. It was either that or fork out for a new boiler.

 

[mogget]

Fair enough - if your system is pumping over, it's usually caused by 1)a blockage (or possibly airlock) between the vent and cold feed or 2) improper cold feed and vent positioning. I can never remember the proper positions but if you do a google image search you'll get plenty of hits. You could try the magnet test - put a magnet on the pipe around cold feed and vent area - if it sticks the pipe is full of magnetite sludge (assuming your pipework is not ferrous).

Quite often, the pipework around the cold feed and vent needs cutting out and replacing.

I would hazard a guess that a google search for 'central heating pumping over' will probably find you the info you need.

I should add that I am not a professional, just an experienced DIY'er.

mogget

 

[Ronanh]

Thanks Mogget. I will have another look at the setup tomorrow and see if your suggestions work.

 

[halfway]

Turn on the rest of the rads without changing the pump settings and see what happens.