in combi boiler, hot water become too hot and too cold?

when running the shower, look at led display - usually between 40ish to 50ish, when the burner goes out quickly isolate the water supply to boiler - black lever below boiler - then look at led display, if in the 90's then I would suspect a blocked domestic heat exchanger.

Big A
July 2009

the 2nd thermistor is under the outer casing on the right hand side screwed into the hot water output pipe

paul
June 2009

maybe your 2nd thermistor - i tested mine as follows
01, remover outer case ( 3 screws underneath )
02, turn off power switch
03, turn off water supply ( Black tap bottom right)
04, open hot water tap ( to drain off water )
05, remover black wire to thermistor and black sleeve ( carefully )
06, unscrew thermistor with spanner ( catch bit of water with cloth )
07, now you will need a blank plug the same size thread as the thermistor to block the hole up
08, screw the the blank plug in its place
09, connect the thermistor back on its wire
10, close the hot tap
11, turn on water supply to the boiler, check for leaks
12, switch power back on
13, test boiler by turning on the hot tap

hope this helps.

paul
June 2009