Changed radiator now F&E Tank is overflowing

[JPOxford]

Hi there,

I had to remove and drain a towel radiator a few weeks ago as both valves were leaking and needed replacing. I bunged the system rather than draining it because it's a big house and the refill process takes hours. The water that drained from the radiator contained some black sludge particles but it wasn't too bad.

The valve replacement went smoothly, the radiator refilled and everything seemed fine...

Except that since this operation, the overflow pipe from the F&E tank has been pouring out water when the heating comes on. It gushes for a short time then stops. There is no continuous drip.

I've done some research and it suggests that a blockage somewhere may cause a release of pressure into the F&E tank. The whole system was drained twice last year and this didn't happen so it feels unlikely that a blockage could have occurred in such a short time, plus the water in the rad wasn't too bad. I've bled the radiators so I'm assuming it's not an airlock.

Can anyone suggest the likely culprit here? Any advice appreciated.

 

[Johntheo5]

Gas or oil boiler/
Internal or external circ pump?
If external, post make/model/mode/setting or/and a post of any LEDs when running.

 

[JPOxford]

Thanks for the response!

Oil boiler, Worcester Bosch, 4 years old, regularly serviced. Last serviced end of January. Internal circulation pump located in the airing cupboard, also changed 4 years ago.

No adjustments made to anything else, other than changing the valves and draining/refilling the rad.

 

[Johntheo5]

And the pump details ?

 

[Agile]

When cold what is the depth of water in the header tank?

Only needs to be about 100 mm.

But when hot expansion increases that to about 200mm.

 

[JPOxford]

And the pump details ?

Grundfos UPS3 15-50/65. Three LEDs, when firing the I is green, II and III are amber.

 

[JPOxford]

When cold what is the depth of water in the header tank?

Only needs to be about 100 mm.

But when hot expansion increases that to about 200mm.

My job for tomorrow is to go into the loft and investigate the depth. I wondered if I could have somehow knocked the ballcock when putting in the bung but there's no constant drip, just a spurt when the heating comes on. I will check and lower the level if it's too high.

I've just put the heating on to see how long it takes for the water to come out of the overflow. 30 mins and nothing yet.

 

[JPOxford]

The heating was on for one hour before the water came out. There are 14 rads and it's a fairly big place so that feels to me like it would be down to the water level being too high and the expansion reaching a critical point and driving the level above the overflow.

But just guessing until I go up and look in the tank.