Combi Boiler Hot Water Intermittent

[Chris_J]

Hello all,

The hot water on my Icon 23T has developed a fault overnight. This morning I turned on the shower as usual to find it was cold, so I went downstairs to investigate the fault further.

Boiler readings were fine and pressure was normal. I tested the boiler by turning the hot water tap on and off to see if the boiler fired correctly. The diaphragm is moving out fully and depressing the microswitch. However, there were two problems:

1. Most of the time, you would hear the microswitch click and the fan come on. Normally, ignition kicks in after 1-2 seconds. Is was taking about 5-10 seconds, but still firing.

2. Some of the time, the switch would click, but nothing would happen (hence the cold shower.)

Initially, I thought it may have been a faulty microswitch, but that doesn't explain why it would be delaying the ignition sequence, since the switch has done it's job by that point. Just to add, pushing the overide button on the thermostat receiver produced no faults with the ignition.

This was just a quick test this morning, but I managed to get hot water after a couple of tries. Any ideas where I should start looking properly?

Thanks in advance!

 

[Gasguru]

So you are saying the heating works everytime then?

Have you tested the contact resistance on the DHW microswitch?

 

[Chris_J]

Yes, CH seems to work everytime (as far as I can tell anyway - problem only started this morning)

I haven't tested contact resistance of microswitch. What values should I be looking for?

I'll have a propper look in the morning.

Cheers.

 

[Gasguru]

Less than an ohm.

 

[Chris_J]

Thanks mate, will get the multimeter out tomorrow.

 

[Chris_J]

Ok, finally got around to testing and the switch which checks out okay.

Boiler didn't make the fault again all weekend, but decided to this evening. Normally when I turn the tap on, there are actually two clicks one after the other. The first is the microswitch being depressed, then a relay energises straight after and fires up the fan and the ignition sequence etc.

When it plays up, I just hear the microswitch clicking and nothing else (as if boiler power is switched off). Doesn't fire up for about 5-6 tries, then after leaving the tap running a bit longer, suddenly decides to fire up.

Could this still be a faulty microswitch or something a little deeper?

 

[Chris_J]

Ok, bit of an update. I've noticed now that the fault only develops when the hot water hasn't been used for a while (i.e not working when I return home from work.)

To get it to work, I have to turn tap on and off until the fan kicks in, then I know it's going to fire up. After that, it's all fine until the next day.

When the fault is happening, the central heating will fire every time (by using manual override on stat receiver.)

Would this then rule out the possibility of being a stuck fan/faulty relay etc, since it doesn't at all effect the central heating side of things?

I removed the switch again today and cleaned the contacts. Resistance across the terminals is about 0.05 ohms when closed.

Any help anyone??

 

[Agile]

Temporarily stick a bit of thin cardboard over microswitch operating arm to see if its being fully depressed by the pin at all times.

Tony

 

[Chris_J]

Thanks for the reply.

I've checked the operation of the plunger when it's faulting, and it comes out as far as completely depressing the switch to the point where the face of the plunger is flush with the body of the microswitch - i.e as far as it could go without pushing the switch out of its housing.

I could try the cardboard trick, but it might damage it...

The diaphragm was replaced about 2 months ago, so the whole setup has been disturbed.

Could this be a faulty connection somewhere? Is it just worth replacing the switch, or am I falling into the trap of replacing random parts!

Thanks.

 

[Agile]

Operating the DHW demand switch will first bring on the pump!

Does the pump come on in the fault condition?

Tony

 

[Chris_J]

I didn't realise the pump comes on when there is a hot water demand?

I'll have a check tonight.

Cheers.