no hot water upstairs (hot faucets run cold)

[flusckwy]

I live in MEL /AUS and i have a DUX tankless hot water system (on-demand gas system). Since two days ago, our upstairs bathroom and downstairs master beddroom do not have hot water--water pressure is good from hot taps but only run cold water (i mean cold, not warm at all!!) But downstairs kitchen and laudry room have great supply of hot water..i believe the water circulate in my house as from Laudry (the closest to hot water unit outside), to Kitchen, to upstairs and finally to the downstairs master bedroom (this is why master br has no hot water either..)

We have always been very cautious when taking shower. we turn the hot water on and run for a while then need to be very careful in turning cold on--very slowly bit by bit to adjust the temp. Otherwise the water becomes totally cold (very BAD/SENSITIVE TEMPER)...But normally if we restart the hot water will come back... but this time two days NO HOT WATER AT ALL.. it happend last year and now again.. no one can take a warm shower now.. i checked the unit outside it works fine... can anyone please help? thanks

 

[picasso]

I cannot find any instructions for your water heater so at a guess I would say the diaphragm is worn, thats if your tankless heater has a diaphragm, if you can find a link to the instructions that would be helpful.