[Resolved] Baxi 105e - No Flame on HW - CH fine

[thephatmaster]

Hi all,

***post updated - again - Perhaps it's time for a new thread? - Latest Summary of issues:

• At all times CH is fine - if the CH is running and I turn a tap on, the flame shuts down.

• Day 1 - No ignition on HW side - primary flow microswitch tripped by diaphram fine and pump runs, pump proving switch tripped fine, - but no flame.

- Called company whose sticker is on boiler, they agree to attend to service it on day 3.​

• Day 2 - Wake up to a warning light "HW thermistor fault" - HW does nothing, doesn't even run the pump. I start this thread to find out more about replacing the thermistor.

• Day 3 - AM - Wake up to HW magically working. Thermistor fault light went out over night.

• Day 3 PM - Nice bloke attends and services the (at this stage magically working) boiler. Tells me the water temp is fluctuating a bit, probably because the HEX gunked up - this makes sense. HW still working when nice bloke leaves.

- I go to have a shower about 4 hrs later and same fault as Day 1 reoccurs. No ignition on HW side despite both microswitches being tripped and pump running.​

• Day 4 - No change - still no ignition on HW side - Latest update made (incl this list).

see posts from 5/8/15 below ***

This thread has been started on request as my boiler is now showing a warning light so hopefully this narrows the problem down.

I seen on this forum that the thermistor is a cheap part I may be able to replace myself, and that it 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.

 

[Gasguru]

Does the heating appear to function ok? If it does then the flow thermistor on the LHS pipe running down from the combustion chamber is probably fine. The DHW thermistor is low down just in front of the plate heat exchanger screwed into the flow manifold. You can then compare resistances from each sensor (assuming the pipes are at similar temps). At room temperature say 25 degreees you're looking for about 10K ohms. Of course the thermistor could be intermittent so it's not a reliable test.

 

[thephatmaster]

Cheers Gasguru, I'll try and locate it and look for 10K ish.

I found a sticker on the side of the boiler saying it hadn't been serviced since Jan 2014 so I've called the people whose sticker it was and they're coming Wednesday.

 

[thephatmaster]

The boiler has (again) changed staus overnight and is now magically working.

I had the service company out today anyway (it was last serviced Jan 14) to service it / check the sensors etc. They had the service record for the boiler and it had a recent diverter valve replacement, so it seems the issue is a partially blocked heat exchanger.

The hot water works but has intermittent temp fluctuations, the boiler temp lights also stay on for quite a while after brief use.

I've been advised it's a big job to do a heat excahanger on one of these boilers (due to it's location), and that running some cleaner through it (via the rads) will help?

Anyone have any advice? I've not drained a CH system for years (did one rad replace a few years ago). Filling with cleaner is problematic as I'm a first floor flat and all my pipes go to the loft first, then down to the rads.

Is it worth installing a compression fit "t-piece" and a valve in the loft somewhere to facilitate filling with cleaner?

Draining is easier as I have a rad by the front door (on the ground Floor) which I can drain, via a hose into a drain.

 

[Gasguru]

You previously said the boiler would fail to fire up despite the pump operating and the primary flow microswitch (on top of the diverter) also operating correctly. Blocked/sludged heat exchangers would not stop the boiler from firing up. Find another company...their diagnosis is wrong.

 

[thephatmaster]

That's right gasguru - I did state above that it was working, but by the time I got round to having a shower it was back to square 1 - showing those same symptoms again:

- diverter microswitch activated.by diaohragm;
- pump running; and
- no flame.

Perhaps the HEX needs cleaning as well, as the water temp fluctuates a bit and the survey for the property (I bought it in April) recommended a CH flush / adding inhibitor.

So the questions are:

- Anyone know a decent engineer in the Guildford area; and

- What does make a boiler run the pump etc without firing?

 

[thephatmaster]

I'm reluctant to make a new thread unless mods think it helpful so have provided a chronology in the first post above, which I will keep up to date.

I'm currently on Day 4, the boiler has been serviced but no firm diagnosis has been made of why there is no ignition on the HW side.

Grateful for any thoughts,

Steve

 

[hertsboilers]

People think every gas safe registered operative can repair boilers, it is simply not true.

 

[hertsboilers]

If you can't find a good boiler repair man in your local, you can try Baxi for their fixed price repair, but I don't know if they will cover this boiler which is bit old now.

 

[thephatmaster]

Cheers Hertsboilers - I checked Baxi out and they do cover it (it was installed in 2010) it will be the usual £300 for repair + 1year cover.

My ma's boiler guy is good too (swapped their combi last year - reasonable rate and quick / tidy work) but quite busy, so will see if he can quote also.

To rule out failure of some cheaper components - I'm gonna go home tonight and test: (1) the pump proving switch; and (2) HW thermistor, with a multimeter. If nothing else I can relay my results (for what they are worth) to whoever may be called out.

 

[Agile]

I would never trust a customer's check on anything and would always test everything myself every time. ( Same with other engineers too ).

I would not expect to ever need to charge more than £240 for a fault like that and that's the worst case of needing a new PCB.

Tony

 

[gas112]

One of those that you would need to be in front of to see exactly what its doing re no ignition but is gas at burner 105he is one of easiest combis to fix

 

[hertsboilers]

Agree with gas112, you just need a competent boiler repair man.

From your description, I would point my fingure to the hot water thermistor. Buy a 47k potentiometer from Maplin, and set it to 11kohm, and replace it with existing thermistor connection, to see if your boiler is up and running.

 

[thephatmaster]

Defo - I have a bunch of parts (incl various resistors but no pots) for Arduino projects (one of which is due Saturday, but I'm faffing with this darned boiler), so I'm sure I could cobble something together with 11Kohm value to use for a brief test.

 

[Agile]

Don't even need a pot !

Just a 10.7 K fixed resistor!

Or even a lower value between 4 K and 12 K