Glow worm 330 f5 fault on HW, f6 fault on central heating


21 Sep 2013
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United Kingdom

I have had plenty of information from reading the other topics and am hoping your generosity will help me out on this one.

This is our 2nd winter in this house and the approximately 10yr old British gas 330 (glow worm) started developing faults. It started with a f5 fault on heating water only. A Plumber was called who decided the pump was OK but there's sludge in the system and possibly sludge in the boiler causing this. So he wanted to charge £350 for power flushing to 'hopefully' sort out the fault. That was a lot of money to spend on a hope so I got a fernox x400 myself, left in the system for a week and drained it. I was hoping to see a lot of dark water but to my surprise water was mostly clear but interspersed with dark water every now and again. This was partly because I didn't switch on the hot water that week and only kept the central heating on.
After that the boiler was switched on but the f5 fault hadn't gone away so I keep resetting the safety button. On central heating the f6 fault keeps appearing and disappearing.
Current status is that we just use the electric shower and boiler is doing a good job of keeping the house warm day and night even through these f6 faults.
I thought this was probably an airlock so have let the air out of radiators couple of times but this doesn't seem to have a big effect on the faults.

Called glow-worm and they want to charge £260 to fix this, local Bosch installer (and I assume boiler engineer, rather than a plumber) wanted to charge £89 call out fee to have a look which is again lotsa money.

So I was hoping someone experienced here might be able to suggest if this due to an airlock. If not what should be done about this?

Kind regards
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If you think that £89 is "a lot" then presumably you will also think that my "£84" is also a lot?

But don't you think that its worth paying for a correct diagnosis rather than spending money power flushing and replacing expensive parts?

x400 takes about 4-6 weeks to work ! X800 does it in just four hours though.

Thanks Tony. I couldn't find x800 at the local shop so just went for x400 coz I just wanted to get it done on my day off. So do you think i ought to try the cleaning again and it's not something else? I assume you know all about the f5 being the overheating fault and f6 being the flow thermistor fault.

I guess £84 is not 'a lot' really given the importance of heating but I would feel guilty if I didn't try my best to sort it out and given the money away out of laziness.

I would have liked to pay for the correct diagnosis but £260 is really a lot of money to spend on a 10yr old boiler and what are my chances of getting a correct diagnosis done at £89. I could get lucky and get a proper professional or I could end up paying a young pretender who doesn't really know what he's talking about. Aanyone can advertise them selves as boiler engineer on Google or yellow pages.
£89 too much money!! That won't buy a reasonable coat and a jumper, both of which you'll probably need this winter. You probably haven't had it serviced either, so the new door seal you NEED will cost you a lot more. The seal needs changing every five years.
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Boiler was serviced and new seal replaced last summer so hope it'll be OK
OK, I probably should have been more concise in my query. Seems like topic of discussion has become 'is £89 a lot of money' rather than 'could f5 f6 faults be down to sludge?'
I have paid £50 to get the supposedly professional diagnosis that these faults 'could' be down to sludge in the system. I am wondering if I should continue to clear the sludge myself (using fernox x800 this time) or should I risk £89 to get the same diagnosis again. I thought that kind and experienced souls here might nudge me to decide one way or another.

Tony/Guessman, what would you recommend: clear the sludge or risk a second professional opinion?


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