Central Heating expansion constantly running

[Dunston]

I have a standard open vented central heating system with a fairly new <4 years Potterton Profile 50. When it was installed, the plumber told me that these do trip the thermal overloads every now and then. I thought nothing of this and true to word I kept resetting it. In the end it got so bad that BG replaced the overload switch and sensor. This seemed to fix it for a while but I was never able to have the temperature controller over 2 otherwise it tripped again.

I have sinced noticed that there is a constant stream of water coming out of the expansion pipe in to the header tank. It's not gaining water as it never overflows. My suspicion is that the original plumber got the flow and return wrong on the boiler, or BG replaced the pump the wrong way around (or both).

To prove this I assume that if I power the system up from cold and hold both pipes above the boiler I will be able to tell which way the water is flowing by which pipe get hot first?

How can I tell whether the pump is going the right way? we have a 3 way valve, should this be before or after the pump? Also, I feel that the radiators get too hot. Could it just be the thermostat on the boiler?

 

[Mike Jackson]

If the flow and return are reversed at the boiler it could cause the overheat stat to trip as the boiler stat would then be on the return allowing the boiler to get a lot hotter. I doubt this would cause the pumping over though.

There are several likely causes for this. If it has only started doing this recently I would suspect that BG upped the pump speed to try and stop the overheating problem. This could possibly be cured by turning the pump down again but the best solution would be to raise the open vent.

The other main cause of pumping over is incorrectly positioned cold feed and open vent connections. They should be between the pump and the boiler.

Whatever the cause of the pumping over you need to get it sorted as this will cause corrosion to the system and considerably shorten it's lifespan.

Mike

 

[mcmoby69]

another possibility is sludge in the system. this will restrict the bore of the pipework and slow it down. this could also cause overheating and the water being pumped round will flow the easiest way, in your case out of the expansion and back down the cold feed. creating another circuit!

 

[Dunston]

The overflow pipe is about 14-18" from the top of the loop to the surface of the water in the header tank. I can't tell how long it's been like this but just noticed it as I was changing the cold water tank over the weekend. As you can imagine it has a header tank full of hot water all of the time.

The heating system is also affecting our thermostatic shower, the hot water gets too hot for the thermostat so it restricts flow to the shower. If the water in the HW cylinder is cooler we get a more powerful shower. I've tried turning the stat on the cylinder right down, which improved things but not a lot.

If I remember correctly, the flow connection is on the left as you look at the boiler from the front?

I drained the system about 2 years and flushed it through the best I could with a hose connected to the mains water and then refilled adding MB1. The water was dirty but not excessively so. We have no cold spots in the system so I can't see that it's blocked anywhere. It certainly doesn't rattle, in fact it's very quiet.

Cheers Malcolm

Sorry, one last thing. Because there were insufficient cores in the cable from the airing cupboard where the pump is to the boiler, the plumber could not setup a pump overrum. I thought Potterton did a overrun relay but I can't find it anywhere. Any ideas?

Malcolm