Potterton Titanium 40 (Baxi Platinum) CH shut down/overheat ?

[HP74Shaun]

Hi..

Heating Engineer here, ready to give up !!

We have been looking into an over heat issue with a Potterton Titanium Combi 40kw, Customer thinks about 5 years old.

They have had 3 engineers out previously, all walked away.

Boiler was firing on both CH and DHW and then cutting out after 8-10secs indication likely pump issue/circulating problem/blockage etc.

System was power flushed 1 year ago to date
Mag Filter installed
Previous engineers replaced Diverter Valve Cartridge & Motorised Diverter Head, Clean out Plate HEX.
Still fault on both.

Customer lives in Hard Water Area

We attended :

Changed the Plate HEX and this cured DHW, this now works perfectly and boiler cycles correctly up and down within range depending on secondary water flow.
CH still overheating reaching 85-90 then shutting down.
Changed both thermistors even though within range,
Stripped out flow and return pipework to main HEX incase any lodged blockage in sensor pocket. Stripped full diverter valve brass body and filters.
Stripped down circulator and cleaned all ( MagDrive Grundfos Pump )
Completely cut out and separated rad circuit from boiler and tested the boiler CH through 50m coil of Speedfit !!
Replaced PCB as a final resort because it looked like the pots were not reading correctly on CH.
We have also ran cold water through the heat exchanger using filling loop through full CH circuit and out through Mag filter return and flow rates are good.

Boiler still overheats and shuts down after 8-10 secs flow pipe heats up far too quickly !

After all that ! .. my question would be would you change the Main HEX or the Pump, we can physically hear and feel the pump spinning and this would also be required for DHW which is working perfectly so be it to only circulate through boiler and plate, therefor i would lean more towards the main HEX, but my issue is would it not overheat and fault quicker in DHW mode cycling to full power with a scaled up main HEX and not on CH first because boiler is relying on secondary latent heat only to cool the primary water ?? Or is the pump faulty and can only manage to circulate water through HEX and plate and not a larger circuit ? HEX and Circulator are £600 each ! Even considering sourcing a recon Main HEX to test !

Apologies for long winded rant if i wasnt already bald id of pulled my hair out by now !!

Any opinions gladly welcomed.. !

 

[ShaunCorbs]

Have you cleaned the filter in the return iso ?

As hot water is ok now it’s not a boiler issue to say would look at the system

 

[HP74Shaun]

\Yeh stripped out return iso and pump first thing then as mentioned cut into flow and return and tried to run CH on 50m of Speedfit and still cut out.
One more thing we did notice we went back today and when asking for either a DHW or CH flow temp range on both it was giving us 10 degrees more than demand.. e.g set pot to 55 degrees for DHW and it would deliver 65 degrees but then cycle correctly and hold it at that once we had changed the plate HEX. quite happily.

The same with the CH temp setting, we ask for a 70 degree flow rate it would give 80 if asking for a 45 degree flow it would give 55 degrees then cut out in all test even at the lower temperatures but once cut out on CH it would cool right down to 25-30 then re-fire taking far too long to re-sequence and fire up again ? We could also physically feel the flow rate pipe cooling once it overheated confirming the pump was pulling in cooler return water ?