Baxi 105e no HW - OLD THREAD - See new one

[thephatmaster]

Evening all,

I've reluctantly posted this as a google search of your forum has suggested (several times) that if I have no HW, then the most probable answer is that the change over valve diaphragm or microswitch is broken.

I'm a reasonably competent DIYer and have done the usual stuff (fixing minor plumbing leaks, oven element replacement, washer belts and washer motor bearing and brushes etc), so, I have hope that this will be easy(ish) to fix with hand tools and a bit of patience.

After reading many threads on here - I've had a look and can't see an issue with either the diaphragm or the microswitches. I wonder if the fault lies somewhere else, or (most likely) I'm missing something - grateful for any help.

Apologies for the long post, I've tried to give as good an explaination of the fault as I can.

Symptoms are:

• Sudden DHW failure, no previous symptoms I am aware of;
  
• CH works fine;
  
• The boiler does not light after I open a HW tap;
• Pressure is 2 bar;
  
• Boiler reset seveal times using the dial, switching off the mains power, and one followed by the other;
  
• Green LED with "tap" symbol lights up when a tap is open; and
  
• No fault lights.

Tests I have done so far:

(A) Change over valve diaphragm rod visual inspection:

1. Checked the rod extends when a HW tap is opened (it does);
2. Checked that the rod extends far enough to operate the microswitch (it does); and
   
3. Checked for leaks around the gland nut (no leak).

(B) Test of microswitches:

I have seem a diagram of this part in this thread:
//www.diynot.com/diy/threads/hello-baxi-105e-dripping-water.71801/

• The switch I'm referring to as the "front" microswitch is the one pictured in that diagram.
• The switch I'm referring to as the "rear" microswitch is mounted above the orange part in that diagram, and actuated by a rod protruding upwards when a HW tap is running.

Tests:​

1. Front microswitch operated by hand - no water running. This results in a pumping sound but no ignition of the boiler. A rod rises up and operates the "rear" microswitch.
2. Rear microswitch only operated by hand - no water running. Nothing happens.
   
3. Front and rear microswitches operated by rod (e.g HW tap open). Same result as above, i.e. pumping sound, but no ignition.

I'm confused as to why manual operation of the front microswitch doesn't prompt the boiler to light, and wonder whether the rear switch is faulty (I don't know what it does)?

So my questions are:

(i) Why is my DHW not working (obviously!)?

(ii) What does the "rear" switch do? Is it safe to connect the terminals (via two spade connectors soldered to a short piece of wire) to test it?

(iii) Am I barking up the wrong tree and the answer is still the diaphragm, for some reason I don't understand.

Thank you for your patience getting to the end of this mammoth post. Any advice you have would be greatly appreciated.

Steve

 

[thephatmaster]

Hi there,

I've woken up this morning to a flashing HW sensor error light. Now the boiler simply does nothing when the hot tap is opened.

I've seen on this forum that this can be checked by switching the sensor (or the wiring) with the CH sensor as they are both the same. I can see what I think is the HW sensor in a pipe on the bottom left of the boiler - no idea where the CH one is though.

Are there any other checks I can do, I have a multimeter I can test the sensor with, but don't know the values I'm looking for.

Kind regards,

Steve

 

[picasso]

A blocked main and/or plate heat exchanger can cause this, it causes enough restriction to stop the "rear" microswitch from making (its the pump proving switch) as you said you can test by using a short piece of wire when the tap is running to see if the boiler fires up, failing that it could be a thermistor, potentiometer, loose or broken cable or pcb.

 

[picasso]

The thermistor resistance should be approx 11k @room temp.

 

[thephatmaster]

Hi Picasso - apologies for the confusion - in relation to your first post the pump proving switch was being operated fine by the rod in the pump.

I have taken your advice and started a new thread - perhaps the mods can close this one? - so we can deal with the potential thermistor issue.

The new thread is here:
//www.diynot.com/diy/threads/baxi-105e-no-hw-fault-light-showing-hw-sensor.439536/

 

[bernardgreen]

Falsing sensors on a gas fired boiler can create dangerous situations. Most of the sensors are SAFETY devices intended to STOP the boiling firing up in un-safe conditions.

Falsing a sensor to make the gas valve open for just a couple of seconds could allow enough gas into the combustion chamber to cause an explosive ignition.

DO NOT USE TRIAL AND ERROR, GET A GAS SAFE REGISTERED PERSON TO REPAIR YOUR BOILER

 

[thephatmaster]

Thanks Bernard,

There is a company's sticker on the boiler - looks like they've ben servicing it for a few years now.

I'll give them a bell.

 

[picasso]

Falsing sensors on a gas fired boiler can create dangerous situations. Most of the sensors are SAFETY devices intended to STOP the boiling firing up in un-safe conditions.

Falsing a sensor to make the gas valve open for just a couple of seconds could allow enough gas into the combustion chamber to cause an explosive ignition.

DO NOT USE TRIAL AND ERROR, GET A GAS SAFE REGISTERED PERSON TO REPAIR YOUR BOILER

What are you on about ? falsing sensors ? its a hot water thermistor, the op wants to put a multimeter on it to see if its out of spec,good grief.

 

[bernardgreen]

A blocked main and/or plate heat exchanger can cause this, it causes enough restriction to stop the "rear" microswitch from making (its the pump proving switch) as you said you can test by using a short piece of wire when the tap is running to see if the boiler fires up, failing that it could be a thermistor, potentiometer, loose or broken cable or pcb.

This what I mean by falsing a sensor. A short piece of wire to short out the sensor. Yes the sensor may have failed. But also the sensor may be open circuit because the condition it is monitoring is un-safe.

 

[picasso]

Its a pump proving switch, it does nothing other than prove the pumps running,(funnily enough) as you think its dangerous enough to have highlighted bits of your post in red, perhaps you could tell how bypassing this switch for a few seconds would cause an explosion ?

 

[bernardgreen]

Its a pump proving switch, it does nothing other than prove the pumps running,(funnily enough) as you think its dangerous enough to have highlighted bits of your post in red, perhaps you could tell how bypassing this switch for a few seconds would cause an explosion ?

I said by passing A switch and did NOT specify a specific switch. I am sure you know the effect of prolonged by passing the air pressure switch when the fan is not running properly.

 

[picasso]

 

[Steelmasons]

Lol...