Back boiler pumping over problem.

[The Bedwetter]

Some weeks ago a friend’s central heating failed. This is a back boiler which only provides heat to the one bedroom flat, so there is no HW cylinder etc. The boiler was still firing but the pump had seized. Upon checking the contents of the F&E cistern I could see that it was full of dark brown sludge. This was entirely drained, refilled and drained again – the contents mostly black. A radiator which matey said been blocked for a good while was removed and backflushed with a hose – lots of black magnetite evident here. A new Grundfoss UPS15 50 pump was fitted and a bottle of Sentinnel X400 poured into the F&E cistern.

Of course the new pumped worked as expected but what was surprising was that there was, upon switching-on, a 'pumping over' through the vent-pipe into the F&E cistern. This continued until the room-thermostat temperature was reached when it stops. This resumes again when the thermostat calls for more heat.

The recommended four weeks that Sentinnel stipulate X400 removal passed and the system contents were once again removed – this time tea-coloured, but pretty clear. The F&E still was a bit claggy so I cleaned this thoroughly. I also put a lot of mains water though the feed pipe, running a hose from the vent pipe to the WC. It took quite a while to get rid of the brown residue but eventually the contents became clear.

Following this the system was refilled using Sentinnel X100 inhibitor. All seems to be working fine and the radiators get hot but there does remain the pumping over. Any ideas as to what this could be? Could there still be some blockage in the pipework that we have failed to remove?

Thanks in advance for any help.

 

[PlumbBobBob]

go round all the pipework in the airing cupboard testing with a magnet if it sticks at all there will be magnatite in the pipe which will need replacing if blocked

 

[mountainman]

The pump could be the wrong way round from the outset, try running it in the opposite direction and see if that solves the problem.

 

[JCplumb]

I was thinking the same as mountainman.
If it's the same rating/speed pump as the old one then it sounds like it's pushing towards the f/e rather than away from it.

 

[Puddle]

As above but also maybe

Try reducing speed of pump,if system was in such a bad way maybe it was updated from gravity to fully pumped without altering the feed and vent pipe work as required

imho

 

[The Bedwetter]

Thanks very much for your responses, guys.

steveb: I will certainly try this but obviously the bulk of the pipework is below the floor, which is tiled. There could be some major upheaval on the agenda if this section of pipework that is blocked.

mountainman: This will be the first thing that I attempt as it seems the most straightforward. Cheers.

JCplumb: It is the same rating as the previous pump though that was a Wilo and it was set on speed 3 whereas the replacement Grundfoss still pumps over when on speed 1.

Puddle: As said above the new pump is running at its lowest speed. Unsure of the history but mate says that the pumping over has only occurred since the replacement.

Just one question, chaps: If there was some blockage in the pipework would we be likely to be getting heat tot all five rads?

 

[mountainman]

It's not likely to be a blockage if the rads are working.
There is another possible solution but try the other ideas you fancy first
MM

 

[The Bedwetter]

Just to convey a big thanks to all you guys. :hurray: Yes, the pump was fitted upside down. Everything is now flowing in the correct direction and there is no pumping over. Thanks again. :smiley2: