alpha cb24 combiboiler problem

[Paul Barker]

This whole thread started on a red herring,

As already mentioned it's probably a thermister.

Tony it's a useless flow switch design i changed 10 a week. But never for this fault, it always fails the other way, doesn't fire the boiler for hot water.

The diverter valve in the other hand is 10 times more reliable than the flow switch.

The pcb's are bulet proof.


But though you will need a flow switch one day and you might need one now for other reasons it isn't your fault this time.


Unles that is you have a leak on a hot pipe or a seriously running bath tap combined with a leaching diverter valve. Not unheard of but much less likely as a cause than the thermister.

If you asked for my money on the table I wouldn't put it on the pcb or the flow switch. It would go on the running oldfashioned bath mixer or a thermister.

The simple test even the diy guy can perform is isolate the cold feed under the boiler.

 

[Agile]

A common problem though is the diverter valve sticks and does not retract when the DHW demand is ceased.

You can remove the switch and watch the groove in the shaft with an inspection mirror and see if it moves prompty and returns to exactly the same position when DHW is taken.

Tony

Thats funny! I can see my posts but they must be invisible to the OP !

Odd too that people I would have expected to know better dont have any inkling of how a boiler behaves with a demand for hot water when the diverter is in the rest position ( well nearly! ).

Tony

 

[Paul Barker]

It's not common Tony I had two sticking Alpha CB DV's that's all.The older
DV as found on the Baxis was also pretty old by the time it had hydraulic section problems. Never had a microswitch fail by providing a demand when not one, allways failed to provide demand when there was one.

I was advising the op on the basis of what is common to Alphas.

The microswitch makes contact when the white piston drops into a bevel. The domestic push rod has a bevelled end, the primary pushrod has also a bevelled end. Both pushrods rest in the heating position by spring control. In this position the dhw push rod shaft is incontact with the microswitch piston holding the switch in it's normally open (NO) condition.
The demand for water forces water past the left hand side of the diaphragm which pushes it against the spring pressure rightways, so that the white piston of the microswitch drops into the bevel at the end of this pushrod. The primary push rod by spring action moved right to join it but it is the domestic pushrod which creates the conditions for the piston to drop. The primary pushrod has a bevelled end also so that if it works to design it allows the piston to drop, but if it is stuck in the heating position it makes no difference in respect of a false call for the boiler to fire in respect of DHW because it is the DHW pushrod which must move completely to the right to create the demand. The whole thing is enabled by a battle between two springs the more powerful spring to the right of the diaphragm holds everything in the Heating position. When it is overcome by the flow of water beside the left of the diaphragm it gives in and sqaushes up to the right. This allows the less strong spring at the opposite end of the diverter valve to push rightwards so obstructing the primary flow to the central heating flow pipe and opening up the primary flow to the plate heat exchanger. The fact that the pushrod also moves over does not create any demand for hot water but the bevellled end does allow the demand if it reaches all the way as in new condition it would. Divereter valves which leach do so because the less powerful primary spring cannot push the primary side over to fully obstruct the flow pipe or because debris is preventing a good seal. the more powerful domestic spring in an Alpha in any case is always able to return the complement. The Alpha pushrod doesn't have far to move leftwards to push up the piston (about 1 to 2 mm). I have never experienced this theoretical scenario you name in the field.

it is true that a broken white tip would create a permanent demand, but an intermitent nuisance demand as described by OP is much more likely to be caused by a failing NTC.

So in an Alpha CB you cannot get a false domestic hot water demand created by a stuck primary hydraulic section, which in any case is ultra rare on these boilers.

 

[Agile]

I was not refering to you Paul.

The situation explained by the OP is that the switch works when removed and manually pushed.

He also states the heating comes on when there is no demand.

I can only explain that as caused by a ( false ) demand for DHW when the diverter valve manifold is in the heat position thus causing the rads to heat up.

It seems to me that could only be caused by either a faulty switch or the switch being dropped into a bevel.

A faulty NTC would not cause heating to come on ( unless these have any frost protection which I dont think they do but I could be wrong ).

Tony

 

[Paul Barker]

frost protection is why i suggested ntc it is recent enough design to incorporate it. I can't say definitively but would expect it.

 

[mrgas]

The cb's and indeed the 240/280's have frost protection hence suggesting the thermistor.

 

[stedge]

Yes the heating comes on when hot water only is selected. Was fiddling with it yesterday and found that when I turn the hot water tap on and off the boiler fires up and switches off, but not all the time, sometimes it stays on. i squirted a bit of wd40 down the hole where the flow switch goes (is this wise), and I think it helped a bit. So what can be causing this? How does the diverter valve work?

 

[stedge]

Sorry I didn't realise there was a second page thanks for the explanation of how it all works Paul.
As people have suggested, I think the reason why the heating is coming on is because the hot water isn't switching off, so I may have inadvertently misled you. What I forgot to mention in the previous post was when the hot water failed to swich off, if I jiggled the flow switch about it did switch off. So I think this is where the problem lies. Is the diverter valve repairable or do you change the whole thing?

 

[Paul Barker]

Sorry I didn't realise there was a second page thanks for the explanation of how it all works Paul.
As people have suggested, I think the reason why the heating is coming on is because the hot water isn't switching off, so I may have inadvertently misled you. What I forgot to mention in the previous post was when the hot water failed to swich off, if I jiggled the flow switch about it did switch off. So I think this is where the problem lies. Is the diverter valve repairable or do you change the whole thing?

Oh I was wrong 1:0 to tony

now knock me down with a feather

 

[mrgas]

I would change the whole thing

 

[Agile]

On a commercial basis I would replace the whole thing and they are not cheap either.

I would then know that the diverter valve was new and likely to serve the customer for many more years. The cost would be about the same as I would have to charge for the time I took and that cost is better directed at a new part.

However, as a DIYer if you like fiddly things then you could try to clean it. Possibly soaking it in boiling citric acid or if you are very competent perhaps 6% HCl.

Tony

 

[stedge]

Sorry to ressurect this thread but I've just had a look in the diverter valve with an inspection mirror as sugested. The slide moves back and forth when the hot tap is switch on and off, when the tap is switched on the slide moves fast to the right but when the tap is switched off the slide moves very sluggishly back to the left, as if there's no force behind it, certainly not enough to switch the flowswitch. So could it be the diaphragm, or a knackered return spring, which can be swapped without changing the whole diverter valve. I just want to be sure before I fork out on a diverter valve.
Thanks for any help.
Steve

 

[Agile]

The movement back to the rest position relies on one spring being stronger than another.

The failure is as a result of dirt causing greater friction in the manifold section which performs the diverting action.

Tony