Alpha CB24 hot water

Hi,

We have an intermittent problem with an Alpha CB24 combi boiler in that sometimes the boiler doesn't produce hot water.

We've had a plumber round a couple of times, when it's exhibited the fault, but Sod's law, by the time he comes, it starts working!

I've had a quick look on the forum and I found at least 2 posts where the DHW on this model of boiler was not working and the solution didn't seem conclusive, but was in the same general area.

When the DHW fails, you turn the hot tap on and the light B doesn't turn on and the boiler doesn't switch on/fire. If the Boiler is turned to continuous CH and the room thermostat tweaked to ensure the CH goes on, then eventually the DHW will come through.

From looking at the fault finding manual, case L page 32 (http://www.alpha-innovation.co.uk/c...s/CB24 28 Installation and Servicing Inst.pdf),
it seems to indicate four possible faulty parts (and their approximate costs from a quick google)-
PCB. ~£186
DHW switch ~£35
Diaphragm. ~£23
Diverter valve ~£156

The boiler is about 8 years old.

I have a multimeter so could probably do some measurement to eliminate PCB, but I'd not be happy swapping out any components and would prefer a gas-safe plumber to do it.

Any advice on which is most likely or suggested next steps?

If it helps, pic of inside.
A lot of gunk around the brass compression fittings - not sure if this is normal?

9/10 times intermittant hw is the hw flow switch (the horseshoe shaped one).

Scaled up joints is normal on alpha's, they leak for fun.

more than likely the DHW diaphragm, a few engineers will want to change the complete diverter valve cant really blame them to be honest but the cheaper alternative is just change the diaphragm

Thanks for the feedback.

petit_pablo said:

9/10 times intermittant hw is the hw flow switch (the horseshoe shaped one).

Click to expand...

Is it possible to test this with a multimeter? The cable connected to the switch is easy access with a little push fit type electrical connector. Is it possible to disconnect this and monitor the voltage across the switch before and after turning on the Hot water?

ianmcd said:

more than likely the DHW diaphragm, a few engineers will want to change the complete diverter valve cant really blame them to be honest but the cheaper alternative is just change the diaphragm

Click to expand...

If it is the diaphragm, is this easy to get at? In the maintenance manual there's an exploded view of the whole diverter (fig 27) but not obvious where the diaphragm is.
I guess it may be swings and roundabouts - a more expensive part and less labour time or vice versa.
There is probably less risk for the future also, if the whole diverter was swapped.

if you decide to replace the DHW diaphragm then you need to remove the diverter valve anyway but once removed is not a major job to replace the diaphragm, it can be tricky lining the new diaphragm up though, as for testing the switch with a multi-meter, the switch works in reverse of how you would expect, i.e. it will make contact when removed as opposed to breaking contact when removed,I worked for Alpha for 3 years and never replaced one of these switches once yet they are the most ordered part for your boiler, I will leave the rest up to your own deductions

The DHW switches are hopeless on this boiler model....one of the most unreliable switches ever fitted to a boiler.

Unclip it from the harness and the diverter...it just pulls up and off. Check the resistance...should be an ohm or less. Press the little plunger in and out at least a dozen times. When they fail you get a fluctuating or high resistance.

If the switch is ok then clip it back, turn the tap on and off several times and check the switch is operated using the multimeter again.

Genuine switches are a rip off price but a copy part is here...

http://www.euroboilerparts.co.uk/fs432-alpha-cb24-cb28-microswitch-3013211-280-p.asp

Diverters are a PITA on these boilers...everything can start leaking and you'll need all the seals. You can replace the diaphragm in situ by pulling out the pump.