baxi 105e Hot water problem

[jonesy7]

Can anyone help to stop me going completely madddd.
Have got a Baxi 105e Instant combi boiler. it has the following problem:
When a hot water tap is turned on in the house i get a slug of hot water followed by cold water. On further investigation it seems that on request the boiler ignites, then there is a whoshing noise like a kettle boiling and the burner turns off for about 30 - 60 secs then ignites again but almost immediatly the whooshing noise again and the burner turns off again.
The boiler works fine for the central heating mode and also there is no problem with the pre-heat mode where the water in the internal tank is kept hot. It is only on the hot water request that this is happening. There is plenty of flow through the hot water taps around the house its just hot and cold slugs of water that come through (more cold than hot). I have changed the DHW heat exchanger, and the DHW/heating diverter valve and the DHW pressure valve but the problem still occuring.
I need to solve this one quick as the missus likes baths but we dont get enough hot water for her to have one and she is starting to nagg!

 

[bengasman]

Was the plate hex blocked?

 

[jonesy7]

Thanks for reply. Not as far as i know. the origional heat DHW exchanger was not blocked when i exchanged it! Cud it be trapped air problem in the exchanger? Any suggestions. I cud take it off and reinstall it to see if it works. is there a way of getting air out other than filling through the system inlet valve and pump?

 

[bengasman]

Very unlikely to have air in the boiler. I trust it is a sealed system, and no tanks in the loft?

 

[baxpoti]

Try replacing the NTC stats

 

[jonesy7]

Thanks for replys.
Yes the system is a sealed system, no tank in anywhere in the house.
Sorry but having one of my "thick" days , what are NTC Stats? and what do they do. ta

 

[Agile]

NTC resistors used as temperature sensors!

Tony