Potterton puma 80e, DHW not hot enough

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When I first open a hot tap, the boiler kicks in and starts running as normal for about 45 seconds and the hot water comes out very hot, then the burner stops while the tap is running hot, then the boiler tries to kick in again (quick little usual sound just before the burner goes on) but DOES NOT start! the water becomes just warm then cold then the burner kicks in again.
Then the same cycle again...
Please note:
1-The CH is working fine. All the radiators get hot enough all the time.
2-The DHW flow from the tap is now (somehow) lower than normal/usual

Looking for some diagnostic please. Any help would be much appreciated in this cold weather.
Thanks
 
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Pop the front outer case off and check the Red light on the circuit board remains lit whilst the tap is open.
If it goes out it may need the DHW microswitch adjusting or the DHW circular brass flowswitch assembly is suspect.
 
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The Red light on the circuit board does indeed go off while the tap is still open.
Will now look into the microswitch/flowswitch. Thanks Gasguru for the suggestion.
 
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Put in a new microswitch. Now the red light on the circuit board stays on and the burner restarts then stays on ALL the time while the tap in open and even when I close the tap!!!
Also the burner started by itself when NO tap was touched (all closed)!!!
Please help.
 
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Sounds like you either haven't adjusted the microswitch right (it's screwed in too far) or the pin from the brass flow switch assembly is sticking.
If you isolate the cold inlet you can unscrew the flowswitch hex part...dissemble the spring and pin, re-grease and should be ok.
Otherwise replace the DHW flowswitch.
 
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1-You mean unscrew this part (red Arrow)?
2-What sort of grease would do the job?
Thanks again.
flowswitch.jpg
 
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Yes, silicon grease on the pin. Long time since I've worked on a Puma but I think you'll find when you unscrew the hex lump there's a smaller gland part to unscrew to access the pin.
Those boilers are 20 years plus now...and they were a total dog the day they left the factory. Any surviving will have rewarded Baxi many £££££ in parts revenue.
In the long run it would have been cheaper for the owners to have ripped it out the day after install and get something decent.
 
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Greased the pin but didn't work. Ended up replacing the flowswitch and that fixed it.
Thank you Gasguru for your support. Really appreciate it.

>>In the long run it would have been cheaper for the owners to have ripped it out the day after install and get something decent.

Any decent model you could recommend?
Thanks
 
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