Worcester Bosch 35CDi II RSF boiler making banging noises

[StuB]

We have a Worcester Bosch 35CDi II RSF boiler that we've had for about 10 years and it's developed a problem.

When we turn a hot water tap on at a low rate it ignites quickly and starts working as expected and then after about 10 seconds it starts making banging noises and the pressure gauge jumps up and down and the flame goes out.

Turning the tap off and then back on again starts the process all over.

If I turn the tap on full then the water is heated as you'd expect and there is no banging.

First off does any one have any suggestions as to the fault? My thoughts are that its getting too hot and the pressure is increasing too much and shutting down.

Secondly is anything like the Homeserve repair service any good? It's £240 for a repair and 12 months of cover which doesn't sound too bad if it solves the problem.

Alternatively is it worth looking at just replacing it if it's 10 years old and put any repair costs towards a new one?

 

[mogget]

I can't comment on your gas boiler as I'm not gas registered, but homeserve have a terrible reputation!

I think worcester themselves have a fixed price repair scheme which is often recommended, might be worth giving them a call.

Others will be able to comment but I don't imagine there's many on the forum at 5am...

 

[StuB]

Thanks for the info on Homeserve, they appealed as they can come quickly.

 

[StuB]

With the pump spindle cover removed am I correct in thinking that spindle should always spin when there is a demand for HW or CH?

It's not always spinning and when it doesn't is when the pressure goes mad and the boiler cuts out.

 

[mogget]

If your boiler has a plate heat exchanger (iirc the old cdi rsf series does) then yes it should always spin.

This is getting into rgi territory now and unless another member here can help you, I would advise that you call a gas engineer to repair your boiler.

 

[StuB]

If I put a multimeter on the power wires to the pump as soon as there is a demand for CH the wires show 240V and then as you say the pump should run but sometimes it doesn't.

When it's not working with 240V to it if I put a screwdriver to the pump and my ear I can hear a high pitch noise and then if I bang the pump with my hand it often bursts into life.

Once it's spinning the boiler works fine so my guess is the pump is shagged.

I'm trying to get someone to come and take a look and I'm waiting for a couple of calls back but in the meantime I thought I'd try and work out the cause of the problem.

 

[mogget]

Sounds like you're right about the pump, it should spin with 240v on it. It may be a diyable repair if you can change it without doing gas work, but I am am not sure on the model of boiler you have. It may just need a new capacitor if the problem only occurs on pump start. If it stops during a demand the capacitor is unlikely.

 

[StuB]

The pump is a Bosch 59506607 such as this.

http://www.directheatingsupplies.co.uk/worcester-87161431080-pump-g-fos-59506607-87161431080

I've only seen the pump not start, once it's running it stays on from what I can see. There is a big capacitor in the plastic box attached to the pump. Does somewhere like Maplins sell the capacitor, I can't easily see a part number on it?

 

[StuB]

Actually the Capacitor comes out easily it's a MD MKP 2/400 II

Any idea where I can get one of those from?

 

[mogget]

I tried to post a link but my tablet computer is playing up. There's some used but working Grundfos selectric ups15-50s on eBay for around £20-25. You could buy one and rob it for parts, I think the motor head itself would probably fit your pump, but I'm not 100% sure.

That said the link you posted seems a fair price for what is available to diy. Heatingspares247 have it a shade cheaper.

 

[StuB]

I tried to post a link but my tablet computer is playing up. There's some used but working Grundfos selectric ups15-50s on eBay for around £20-25. You could buy one and rob it for parts, I think the motor head itself would probably fit your pump, but I'm not 100% sure.

That said the link you posted seems a fair price for what is available to diy. Heatingspares247 have it a shade cheaper.

Thanks for that, I don't have a problem with the new price and swapping shouldn't be that difficult although there doesn't seem to be much movement in various pipes without removing other items.

Can anyone else confirm the likelyhood of the capacitor being at fault as that would be easier to swap?

 

[mogget]

I can't help any further with the capacitor except to say you could check with your multimeter if it has a capacitance setting.

If you bought the new pump you would only have to undo the 4 Allen bolts and swap the motor head, after draining down.

 

[StuB]

Thanks, that's a very good point. I hadn't thought about just swapping the motor.

 

[StuB]

I can't find an exact match however Maplins have a 2uF 460V capacitor meant for motors for £3.50 so it may be worth a try.

 

[mogget]

As long as it will physically fit then that's got to be worth a go. Peak-peak voltage of the mains is about 330v I think.