Baxi Instant 105e CH Fault

[chillman]

I am new to this forum, but wondered if any one can help with the following problem.
My Baxi (instant 105e) is now outside it's guarantee and this is the first problem encountered.
The CH is only working intermitantly and the timer clock switch is not always functioning.
Hot Water is working fine, boiler fires up and the "pre-heat" thermostat activates the boiler. So that side of things is OK.
Last night the boiler would not fire up when switched to "continuous" and did not activate when switch moved from Off to Timed to Cont.
The Hot Water LED was permanently on, as though someone was calling for hot water, with the fan running, but when CH Thermostat was rotated the boiler would not fire up.
Decided to drain CH system at boiler, using release valve until pressure down to zero in case of air lock. I also rotated pump with screwdriver in case this was jammed. Once done I re-filled to 1bar pressure and restarted boiler.
CH started straight away and I used timer clock to switch on & off a few times, all seemed OK.
Got up this morning to cold house and no CH again!!
Timer clock is the mechanical push pin type and is rotating and keeping time, it just doesn't seem to want to switch on the CH.

Can anyone help me?
Is is just a new timer clock I need?
Or is it something even more expensive, please help it's gonna be very cold this weekend!!

Thanks.

 

[gazthepottertonengineer]

sounds like your hot water pressure diferential is sticking either that or the microswitch which sits on top of it

pressure diff is part nos 248728

 

[weargas]

Just an addendum to gaz's suggestion. I've had stacks of these where its been the pins in the primary water section behind the dhw section that have gotten coated in shyt and don't return fully(we're talking about 1mm here). The two halves are held together by a clip. It's possible to remove the clip and ease the dhw section (complete with microswitch) forward slightly, just enough so there's no communication between the pins on the primary section and the dhw section. You can then (or have someone else) turn on and off a hot tap. If the dhw demand light goes out (or starts flashing in dhw preheat mode) when you switch off the tap then the primary side is the problem. If the dhw demand light stays on then the dhw section is the problem.