Alpha CB28 Intermittent/cycling Hot Water

Joined
18 Sep 2007
Messages
7
Reaction score
0
Location
Kent
Country
United Kingdom
I have an Alpha CB28 about 4 years old.

Heating works fine but when drawing hot water the burner cycles on and

off.

The off time for the burner is always about 10.5 seconds

The on time varies when tested but is always the same for that test

i.e. 22 secs on 10.5 off 22 secs on 10.5 secs off 22 secs on........

or 41 secs on 10.5 secs off 41 secs on 10.5 off 41 secs on.....

or 37 secs on 10.5 secs off 37 secs on 10.5 secs off 37 secs on......

It does this when only Hot water is required or if the heating is already on

As I said earlier Heating on its own is fine.

I am really baffled

Any help will be greatly appreciated
 
Sponsored Links
I think your boiler has an ignition circuit which sits on the gas valve.

If so then I have seen a faulty one giving that kind of action.

But before changing it I would need to be sure all the rest of the boiler is working properly.

Tony
 
Sponsored Links
The rythmic nature implies pcb problem but be cautious becaue these pcb's are bullet proof.

What if the rythmic behaviour is not as clockwork like as you think? In any case it's odd how the on time varies.

Looks more likely to be blocked dhw heat ex but that usually goes right to pump or thermister failure lockout.

Microswitch does drop out intermittently when it is starting to fail, then totallly fails.

It would be easy for an alpha service agent to fix.
 
Thanks for your help Guys

The references to a thermistor interests me are we talking temperature

sensors here?

If so which one?

As far as I know the DHW H/X is as clean as a whistle.

Cheers
 
I doubt if it's thermisters. There are two, both can be affected as pcb monitors both during hot water performance and the primary one has the hardest life. I have yet to come across a white one that has failed. Black ones pretty good too. Grey ones good, any colour is crap and should be replaced.

most likely cause other than sludge is microswitch dropping out, will eventually fail completely. You cannot prove it with a multimeter as it passes multimeter continuity test but fails in use. If it ever drops out at all, replace it.

I have never had a bad pcb for that model. Never had a bad pump, never had a bad white thermister. Had plenty of bad microswitches and sludged dhw heat ex's. Every 4 years diaphragm is under suspition, can go sooner if it's had a hard paper round, but I probably change 10 microswitches to one diaphragm, maybe 1 in 4 times it's sludge.
 
Agile said:
I think your boiler has an ignition circuit which sits on the gas valve.

If so then I have seen a faulty one giving that kind of action.

But before changing it I would need to be sure all the rest of the boiler is working properly.

Tony

Surely that would affect HTG aswell. Plate heat-ex SLUDGED not scaled would be first inspection.

What makes you think its clean?
 
This is quite rare and the only one I have encountered I did not have time to investigate.

I am guessing it may be a weak flame failure detection and the 10.5 sec is a time constant to retry?

It could also be flame failure due to lift off which is not happening on a lower CH setting.

I only mentioned that I had seen one ignition PCB doing that, not that I think its the most likely fault which statistically would be a blocked plate HE.

A simple thermal survey will identify a blocked plate.

Tony
 
Acually Tony you might have it with flame lift.

There were a few vertical flue terminal problems in this generation, flame lift and really bad co alarming on analyser. feel outer flue for hot spots to check seals, but probably in the terminal.

Combustion makes popping noises as it relights itself.

most likely to show itself on dhw as max flame.
 
Thanks again

I will remove and check the DHW HX and the diverter valve assy.

As these are the only things I can do

Already proved the switch by shorting.

Regards
 
You mean you've unplugged the switch from the leed and shorted out the leed with the water running, and the behaviour is the same?

You can eliminate diaphragm from suspicion by listening for the tiny click of the flow switch, if you get the click it has worked.

So all you have to do is clean your plate heat exchanger, the black side.
 
Hi Paul

Yes the behaviour is exactly the same with the switch shorted

Thanks for the info I will give it a try when my Wife okays the lack of hot

water for a day or half a day (may take some time)

Regards
 
OK

Finally got HX off

There is Black Gunk inside

Question is ..... What do I clean it out with ?

Regards
 
OK

Got lots of black bits out of HX using a combination of Sludge Remover,91% solution of Sulphuric Acid,Water pressure and tapping with a small hammer,

NOT all at the same time!(BE CAREFUL WITH ACIDS!)

Have reassembled system refilled using the rest of the sludge remover
and I am hopeful not to say quietly confident!

Watch this space

Regards
 

DIYnot Local

Staff member

If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.

Select the supplier or trade you require, enter your location to begin your search.


Are you a trade or supplier? You can create your listing free at DIYnot Local

 
Sponsored Links
Back
Top