System Boiler problem Aaaaarrrrhhhh!!!

[1norab]

Hi, I have an Ideal mini S28 System boiler. the problem I'm having is the CH works fine but if i press for DHW, the boiler cuts out. If I call for DHW only, the boiler fan/pump starts and the motorised valve starts. But just as you think the boiler is going to fire up - Nothing. When I put the boiler into system mode the lamps (x3) tell me its a Faulty Primary Circuit fault (No water or absence of flow).
Now, I'm no expert (If I was, I wouldn't be typing this I suppose), but please correct me if I'm wrong, the thermistor and the Primary circuit flow switch cannot be at fault if the CH is working. I was going to change them but then this occured to me, so I didn't. Could be the PCB or the Main switch membrane ? but in case it 's not, what else could it be?
Any suggestions would be appreciated greatly.

 

[gas4you]

Is this really a Mini S28? Or do you mean a combi C28?

The S28 would not be heating the HW within its self but would be relying on the external programmer, cyl stat and motorised valve to tell it to fire for DHW.

The S28 wouldn't know whether it was being asked to heat HW or CH, so the fault would point to the motorised valve.

If it is a C28, then thats a different matter.

 

[Agile]

I think that you are confused here. If you had a system boiler then you would also have a big hot water cylinder!

If the boiler has a black diverter valve motor then its a C28 combi boiler. The following assumes its really a combi.

If it works OK on CH but not on DHW and the fault IS caused by the flow switch is not operating then thats usually because the plate heat exchanger is blocked.

In the last case however the flow switch usually stays energised when it goes into DHW mode from CH mode.

Contrary to what Dave has said, on the system boiler there IS a fairly rare condition which causes the flow switch to fail to activate on HW even when it does on CH !

Tony

 

[1norab]

Thanks for your replies... The boiler is an S28. I have never had a system boiler before now, just moved into this house and there it was! I didn't realise the DHW wasn't heated by the boiler but now you mention it, it makes perfect sense. as I do have a cylinder right next to it (and an expansion vessel above).
The motorised valve on the DHW side appears to be working as all the springs/sync move accordingly. So I'm guessing its not that??? I have been informed (rightly or wrongly), it could be an air block?

 

[Agile]

IF you are competent, then you can remove the boiler system flow switch and set the system to HW and then operate the flow switch with a plastic rod within four seconds of creating the demand and see if the boiler fires up.

While the flow switch is off push the pin back to see how much resistance there is. Then repeat that with only heating on. Then with CH + HW.

Tell me the result and then I will give you further advice.

However, if the flow switch is found to indeed be the problem then check what happens in each of the three senarios:-

HW only

CH only

CH + HW

Tony

 

[1norab]

Tony, The boiler does fire up when I remove the flow switch and set the system to HW as you suggested, with the switch pressed.
There is no resistance on the pin in any of the senarios..HW only/CH only/CH+HW.

Gary.

 

[Agile]

If what you say is really correct then you have a blockage in the primary HE of the boiler

OR

much more likely dirt in the eye of the pump!

OR

very unlikely a problem with the flow switch diaphragm

OR

rare, but possible, the bypass screw on the left of the flow switch diaphragm is loose and needs to be screwed fully in.

Tony

 

[1norab]

Tony, Thanks for your advice. I shall now check to see if this sorts it. But forgive me if I sound like an idiot... would these potential faults also prevent the CH from firing up

I have just rechecked the test using the plastic rod pressing into the flow switch. It took the boiler 4 attempts to fire up for HW (i didn't release the pressure on the switch at all). But it did fire up!

What I then noticed was that the pin protruded approx. 11mm. when CH only was pressed.
If I pressed for HW only it came out approx. 8mm.
If I pressed for CH and then pressed for HW to come on 30 seconds later, the pin would retract to 8mm again!!?

The pin retracting 3mm is disconnecting the switch? Question remains, what is causing the pin to retract?