Hot water temperature problem Baxi 105 HE

[Luke78]

Hello again all:

Firstly: Personal background, as I know people are likely to ask:

1) I AM GSR
2) Most of my work so far since qualification has been on pipework, fires and cookers with the odd boiler service.
3) At the advice of other members I have made arrangements to go on the Baxi fault-finding course, but couldn't get a date until September...
4) I have done a couple of model specific courses with Worcester Bosch so have some sundry grounding in the innards of boilers.

Hope that explains the level I am coming from!

Having said all of the above, I have so far found forum members to be extremely helpful and am very grateful of any and all help and information.

I have done a search but couldn't find anything directly related to my query, however am happy to be directed to any known pre-existing threads.

I have been requested to service a Baxi 105HE, (haven't visited the property yet) and am trying to get as prepared as possible; manual gives good clear instructions, with pictures and everything, so that in itself shouldn't present any difficulty, but...

Query 1: Manual does not appear to specify any combustion figures in service schedule... Guessing this is due to age of appliance and requirements at the time...
Do I just ensure the appliance is within the 0.008 ratio?
There are no instructions given for adjusting CO2 at gas valve or throttle etc as I have found and experienced on WB boilers in any case!

Query 2:
Customer reports DHW from taps only
gets hot if run at "nearly full pressure", takes 30 seconds - 1 minute to get hot and sometimes goes cold again before heating up properly...

Without benefit of in depth fault finding training I have studied manual and reasoned as follows:

As hot water is eventually provided problem lies either with DHW heat exchanger being furred up, or with sensors that call for DHW?

According to manual, sequence is that flow from hot taps operates the DHW flow switch which then prompts the diverter valve to change to the DHW supply position...

This I understand...

What I am struggling with is why it should go cold again before then going hot. I vaguely recall this scenario being described on a WB training session and think it may relate to a DHW flow sensor defect, but have referred to my notes (now 12 months old plus!) and can't find any direct reference.

Possible theory is that filter in DHW flow switch is clogged? Perhaps causing it to initially detect flow and then after initial flow decreases (owing to valve being partially blocked?) it no longer senses demand and therefore rest of system thinks demand has ended...
Or there is a fault with DHW flow switch itself?

Would like to help customer if I can and unfortunately won't have training for another few weeks yet!

Would be grateful if anyone could point me in the right direction, or even just tell my reasoning is flawed!

What should I be checking?

Any help and replies much appreciated.

 

[ShaunCorbs]

Be the diaphragm/ diverter / rubber seal

 

[Deleted member 32524]

Sounds like you are on the right lines. As shaun suggest check for heat going down flow. And that flow switch, bridge it out and see if that helps.

 

[Luke78]

Hi Shaun

Thanks for your reply.

I am guessing you have reached this conclusion based on experience! Would be interested to know how I could deduce it from the symptoms, of course.

I would hazard a guess that the failure of the diaphragm means that water passing through the diverter valve doesn't all get sent to the DHW heat exchanger and instead some is still sent to the CH circuit...? Hence the initial heat followed by the much more gradual re-heating.

Happy to be told I've totally misunderstood, of course...

Will certainly look at the 3 way valve assembly.

Thanks again

 

[ShaunCorbs]

As it’s a 105 there’s a big seal in the front left housing

When a hot water tap is opened this seal pushes a pin out

Which makes contact with a micro switch witch fires the boiler and heats the water

It could be this is warn and not fully pressing the micro switch

 

[Deleted member 32524]

Ah know it now. Works off differential pressure. Good shout. No flow switch. Diaphragm common on these.

 

[Luke78]

Thanks all.

Really grateful for your help and will report back once I have seen the boiler.

Incidentally, as nobody mentioned it, am I correct in assuming that the only parameter for CPA is that the boiler must be below 0.008 CO/CO2 ratio?

 

[Luke78]

Hi all

Visiting customer today and will report back.

Having re-read, I assume if I detatch the microswitch from the front of the valve and close it manually, the boiler should fire...

Also, service instructions advise cleaning of water inlet filters, so guessing that is worth a punt before dismantling the diverter valve?

 

[ShaunCorbs]

Correct

Open hot tap

If boiler doesn’t fire manually click and hold the front micro switch should fire

 

[townfanjon]

As above , don't disconnect anything , check the rod connects with the switch

 

[Luke78]

Thanks guys.

At boiler now, all controls working fine.

Open tap and push rod pops out to close microswitch.

DHW Temperature dial on front was nearly set to zero which may have had something to do with it... Have turned it up a bit to about 2/3 and now seems fine... Seeing a 25 deg rise from ambient temp.

Inlet gauze filter was spotless.

However... seems to be evidence of a past leak around DHW Temp sensor area (photo attached). Guess this can't be healthy!?!

Am attempting to clean up and see if scale etc returns in a week or two.

Any thoughts/opinions welcome.

Many thanks for help so far.

 

[ShaunCorbs]

Yes needs sorting if you can get it out

But tbh case looks a bit past it I would advise time for a change soon

 

[Luke78]

Just an addendum to say I have done my beést to clean up with a mini wire brush and managed to get some paint over the rusted area below the valve. Idea being that if there is still any leakage it will be easier to discern new from existing detritus...

I have suggested to customer that a repair may be possible to delay replacement temporarily, but boiler is probably towards end of it's life.

Once again, really grateful for everyone's help and input; this forum is an excellent community and credit to all concerned.