Alpha CB28

cozycats · 1 Mar 2013

Apologies if the answer is already on here, I did search, but anyway...
Alpha CB28, heating side is fine, but on the dhw side
turn hot tap on, boiler fires up for about 40-50 secs (hot water starts coming through tap) then cuts out for about 10 secs before reigniting after which everything is fine.
If I restrict the flow down to 4-5 l/min it doesn't cut out.
during the period it cuts out the red(A)led is constant, and green(B) led flashes quickly. According to the manual this indicates pump failure, which makes no sense regarding the above??. The fault was intermittent but has now become persistent which seems to indicate mechanical wear.
The first thing I did was to replace the dhw flow switch, which was well worn but it made no difference.
I would really appreciate advice as the last time it went wrong it cost me £300 just to get the fault diagnosed, (diverter valve, completely different symptoms)
Many thanks to all of you who freely help and advise!!!

ianmcd · 1 Mar 2013

your boiler is getting too hot when HW running could be your plate heat exchanger is scaled up if you are in a hard water area, it does seem you have a flow restriction of some sort within the boiler.

cozycats · 1 Mar 2013

Thanks for the quick reply, but sorry should have also stated have a 'proper' water softener

wouldn't ever be without one, they are fantastic! agree though that water temp seems a likely cause somehow, I had wondered about the 'seasonality' adjustment thing, but it doesn't really say anything about that in the manual

Also wouldn't the boiler 'cycle'
rather than behave after one cut-out?

ianmcd · 1 Mar 2013

The seasonality valve just alters the flow rate of the cold water coming in to your boiler depending on the incoming water temperature so as long as you have a decent flow rate at your HW taps then nothing wrong with the seasonality valve

cozycats · 1 Mar 2013

OK thanks, there doesn't seem to be any problem with the flow rate once the boiler kicks in again, but will watch that, needless to say the boiler is behaving perfectly tonight

ianmcd · 1 Mar 2013

If the problem is intermittent the most common problem with your boiler is the thermistors, your boiler has two one for the system temperature and one for the DHW flow temperature, both are the same and are not expensive I would have them replaced as a pair if it was me and this should solve your problem , you will need them replaced at some time anyway and I always replace both at the same time

cozycats · 20 Mar 2013

Thanks Ian, well I've replaced the thermistor, and no change at all. I've run the hot tap at certainly no more than 2l/min and once over it's initial cutout and restart it runs continuously. Doesn't this rule out the diverter valve? could something be sticking causing an overheat problem until water flowing through sorts it? Any other ideas please....
Many thanks
Andrew

gas_man_gaz · 20 Mar 2013

sounds like the plate heat exchanger isn't transferring the heat sufficiently, causing the primary circuit to overheat and the boiler to cycle.
dirt lining the ch side of the plate heat exchanger would create the same symptoms as scale

cozycats · 20 Mar 2013

Thanks for that.. how do I sort it out though? dismantle and wash it all out (as if it were scaled up) or is there another way? quite happy to tackle plumbing side of things but will not touch the gas side. what is the 'cyclone' thing doing? Not really any info in the manual about it. I guess it is supposed to filter out crap from the system, but no info on how to clean it?
Again... many thanks

gas_man_gaz · 20 Mar 2013

Personally I would replace the plate heat exchanger, it is possible to clean but you would have to use quite aggressive chemicals to get good results.
Whilst replacing i would judge wheather it is scale or dirt which is causing the fault, if its dirt fit a filter on the return to the boiler and top up the inhibitor levels with fernox. if the sys water is really poor consider getting the sys flushed.

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Alpha CB28

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