Black gunge and underperforming pump

[mjburrows]

Moved into 4 bed detached Victorian house last year. Wasn't particularly warm over winter and have decided to investigate. Found boiler is short cycling, 2 minutes on 2 minutes off. Boiler switches off when outlet temp reaches about 75 C, and inlet temp is 55 C. Cleaned out system with Sentinel x800, but that has had no effect.

Found bypass valve was always open so took that off and found it jammed with black gunge. Put in a new one which stayed closed until I foolishly turned down pressure, now that one will not close!. Replaced it temporarily with stop ends.

Knowing boiler output and delta temp over output, calculated flow rate through primary and hot water tank circuit at 16 litres / min. Have put pressure gauge where bypass was, after pump and before motorised valves. Static pressure is 1.7m. This increases to 2.4m (0.7m above static) when pump is running and hot water tank circuit motorised valve is open. Gauge reads 3.2m (1.5m above static) when pump is running and valves are closed. Pump is BG Grundfos multihead G that I cant find curves for. Pump setting 3. Have curves for selectric and super selectric which I understand are similar and give a maximum head of 5 to 6 metres, and a head of 4.5 to 5 metres at a flowrate of 16 litres / min

So it looks as if the pump isn't producing the required flowrate and head. The piping immediately before and after the pump strongly attracts a magnet, and water drained from the pump is dirty. Could the dirt be affecting the pump? If I replace the piping could I clean the pump? I see there are cheap refurbished BG Grundfos multihead Gs available - are they any good? I'd love to hear your thoughts on how to proceed. Thanks

 

[picasso]

What's the boiler make and model ?

 

[DP]

Cleaned out system with Sentinel x100, but that has had no effect.

With X100? Is this a joke?

 

[Mottie]

With X100? Is this a joke?

That’s just inhibitor, isn't it?

 

[SpecialK]

You may be able to dismantle and clean the impeller on the pump. Possibly full of gunk?

 

[mjburrows]

Not a joke. It's a mistake. I should have written x800

 

[mjburrows]

That’s just inhibitor, isn't it?

my mistake, I should have written x800

 

[mjburrows]

With X100? Is this a joke?

Just a typo. Should have written x800.

 

[mjburrows]

What's the boiler make and model ?

Potterton suprima 70

 

[Elkato531]

If the system has been flushed with the pump in place, the impellor is likely to be blocked with debris from the rest of the system. If parts of the system are strongly attracting a magnet, then those parts are probably partially or almost fully blocked with magnetite build up. If the system is open vented, check the area where the feed and vent pipes connect into the system. Flushing won't remove these deposits and the pipes will need cutting out cleaning/replacing. And, if x800 has been used and not completely flushed out, it will destroy the pump so flush thoroughly before fitting new pump.

 

[Madrab]

I think you need to stop worrying too much about facts and figures and pressure etc though I am intrigued to know how you managed to get pressure figures to one decimal point, standard pressure gauges aren't that accurate.

If the system is filthy and you can get a magnet to stick to certain spots then that is a certain indication that there are restrictions to flow and the pump can't circulate properly and the boiler is exceeding it's high set limit, hence the cycling.

If it's go to the point where you can attach a magnet to sections then cleaner will not be able to shift that amount of build up, it may get better but you'd be there for ever and a day to clear it all out. Time to start checking and replacing fittings/pipe section, strip the pump out as that may have become clogged up after the clean, that and the boiler's HEX/internals may also be restricted now but that's a whole different ball game.

 

[fixitflav]

The piping immediately before and after the pump strongly attracts a magnet, and water drained from the pump is dirty.

Silly question, but is the pipework copper? In a Victorian house it might be steel, so you'd expect a magnet to stick!

 

[mjburrows]

Silly question, but is the pipework copper? In a Victorian house it might be steel, so you'd expect a magnet to stick!

Yes, it's copper

 

[fixitflav]

Yes, it's copper

OK

 

[mjburrows]

You may be able to dismantle and clean the impeller on the pump. Possibly full of gunk?

Good suggestion. The impeller was nearly full of baked in crap. I spent 20 mins poking it out with a bit of stiff wire and now produces a 5m head rather than the 1.5m before. Thanks!