Alpha 240P DHW Troubles

[TinUK]

Hi all, I have an Ocean Alpha 240P combi boiler. I had my diaphagm replaced a year ago to solve a hot water problem, found I needed it from these forums so as a result I gave plumber exact details for repair, which was nice.
However, I have a new problem... again with the DHW.
When the hot tap is turned on and the CH is not currently on the boiler registers the need for hot water but does not fire up, the fan kicks in though. BUT when the CH is running and I request hot water, the boiler cuts off. HOWEVER it seems that if the heating has been on for a while the hot water does not turn off the boiler and I can get piping hot water.
I can see the diaphagm is doing its job.

Hope you can shed some light on this.

Thanks,
Martin.

 

[Rob]

hmmm..sounds like the micro switch could be dropping out.

 

[TinUK]

Thanks for the reply,

I think the microswitch is ok because the boiler registers the hot water demand by lighting the lamp and whizzing the fan etc, just seems only thing missing is the flames in the sealed part.
Cant quite put my finger on the circumstances which make it work, seems to be when the central heating has been on for 15mins or more, otherwise turning the hot tap on while heating is on turns off the flames in the sealed part of the boiler, only after approx 15mins or more of heating will the boiler stay burning.

Bizzare...

/martin.

 

[ChrisR]

Check the primary flow switch. Its pin might move a little when you turn HW on, enough to unswitch it - or not switch it on.
Tip - use the CW inlet valve to turn HW on and off!

 

[Agile]

I dont fully remember this model but expect it has an NTC sensor to measure the DHW temperature.

If so this should be checked to ensure the boiler does not think that the DHW is already hot enough.

Tony

 

[TinUK]

Thanks Agile for the suggestion, that sounds like the most probable cause, there is a plumber coming tomorrow, I will post what he finds for future reference.

Thakns for your inputs folks..

Martin.

 

[Paul Barker]

As Tony says, but also bear in mind that the resistance to flow in the primary is greater when diverted round DHW heat ex than when diverted round radiators. Therefore a weak pump proving mechanism may work in CH mode but not in DHW mode, and can be tricked by running heating first then running a tap, as you have found.

I would start by changing the thermisters anyway, and I always change both as the primary thermister is also sensed by the pcb during dhw operation.

 

[ChrisR]

pump proving mechanism may work in CH mode but not in DHW mode

There's an echo in here!

Sensors should be about 10k at room temp. Sometimes worth taking the primary one out to clean the sludge off it. You can also try swapping them - they're the same. So if any change in behaviour, change them both.

 

[TinUK]

Well the engineer came and looked at the boiler, went off to get a part and returned with a DHW temperature sensor!! I now have hot water.

You were right. Nice one guys.

Martin.