Alpha CB28 hot water / diverter valve problem

DoctorV · 26 Jun 2006

I have a CB28X. I have to open the hot water tap almost fully before the boiler will fire. If the flow rate is not very high the boiler will not fire. I have changed the DHW flow switch and no change.

I guess the problem is with the diverter valve, so my questions:

Can I get a diagphram kit for the CB28 diverter valve?
If I can get a diagphram kit is this likely to solve the problem and will I need any additional seals?
Should I just change the whole diverter valve (££££!)?

Thanks,

Lee

ChrisR · 26 Jun 2006

Can I get a diagphram kit for the CB28 diverter valve?

Yep

If I can get a diagphram kit is this likely to solve the problem and will I need any additional seals?

Likely, yes. It comes with what you need - costs enough too.

Should I just change the whole diverter valve (££££!)?

Only if you send me the old one so I can sort it out and sell it as a new one

...

There's always a chance that the inner parts of the diverter valve manifold are a problem, but swapping the diaphragm is statistically a good bet.

DoctorV · 27 Jun 2006

Thanks Chris,

Do you know where I can get a CB28 diaphragm kit from? (Alpha won't sell me any parts directly). Is it the same as the CB24? (I've seen a kit for a CB24)

ChrisR · 27 Jun 2006

Same animal.
http://www.partscenter.co.uk/hrpc/default.html

Product: 449921 ALPHA 3.013389 DIAPHRAGM KIT

DoctorV · 30 Jun 2006

Cheers Chris,

I have the part, going to have a go at fitting it next weekend, too much on this weekend (England cricket and football).

I have a handfull of washers in the box, I'll fit these where they match up with the old ones. It also comes with a tube of sealant. I've never fitted one of these. I'm assuming the diaphragm casing splits in two and the diaphragm is pinched by the casing all around, my guess is the sealant helps seal these faces - both sides. I'll let you know how I get on.

If you read this before next weekend any tips on where the sealant goes?

Cheers again,

Lee.

DoctorV · 30 Jun 2006

PS after you confirmed the part number I got it from Gas appliance spares in Preston www.gas-spares.co.uk.

They charged 22.96 inc VAT and delivery. They will give a further 20% discount if you are trade.

Parts Center wanted £27.11.

Gass Appliance Spares might be worth checking if you need bits in the future. Only downside is they print your name, address, credit card number and expiry date on the invoice. Not good if the parts go missing in the post!

ChrisR · 30 Jun 2006

Thanks for the tip but I/we wouldn't dream of spending 20 quid on a diaphragm!

If I remember correctly - that is NOT sealant it's silicone GREASE.
DO NOT put it on the diaphragm or it WILL leak. The shaft seals might need a little of the grease.

pete267 · 1 Aug 2007

Gents,
Sorry to reserect this old topic but my parent have intermittant hot water. They have an Alpha CB28 that intermittantly fires up when the hot tap is turned on. The boiler is 6 yrs old and they have had this problem before, the plumber (now moved abroad) fixed the problem by changing the whole housing that contains the diaphragm, total charge of £165. I'm assuming its the diaphragm again as they can get the boiler to fire if they turn the tap full on.

Any ideas + idea on where to obtain a diaphragm & seal kit if required.

Thank You
Pete

Mikast · 7 Jun 2011

Thanks to all the posts on here about this fault, my CH had to be on to get DHW through but eventually that gave up the ghost too!!
Got a diaphragm kit and replaced, all ok. Only issue was that I couldn't remove the diverter unit due to corrosion, so gave up following the instructions and just removed the pump and split the unit in place.
Will inform if any other issues follow.

Thanks to all.

DoctorV · 6 Aug 2011

Mine has just started showing signs of failing again! Need to get a high flow rate for boiler to fire up.

It seems these diaphragms last about 5 years. I've ordered another.

Boiler now 10 years old and so far:

-Diverter Diaphragm number 1 (£23)
-Expansion vessel (had to fit external one since flue exited rear and that prevents removal of old one! - About £20 - screwfix)
-DWX heat exchanger (split and mains pressure slowly kept overpressuring radiator circuit!, I think it was about £85)
-Diverter Diaphragm number 2 (another £23)

I guess this is normal with modern boilers. I once had a very old glow-worm unit that went on for 20+ years with no failures. Just don't make things like they used to..........

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Alpha CB28 hot water / diverter valve problem

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