hot water scalding hot in cylinder!!

[ih]

my system
boiler potterton profile 100E Boiler stat 1-6 (6 being max) i have it on 4
Indirect cylinder
cylinder stat Honeywell L641A1039
Valve 3 port Honeywell V4073A1039
honeywell programmer ST6400c
This morning found HW very hot at the taps investigated and found that cylinder stat was not clicking further also switched HW demand and the stat would not cut off the boiler so i think the cylinder stat is knackered.
2.I still think the water is very hot with boiler stat @4 am i right that at 4 setting the water temp should be around 73degree? unfortunately donot have thermo to check water temp my question Is it possible for the 3 port valve to heat water when only ch is demanded ? If so how can i check this?
thanking in advance for all help/advise

 

[MarkCharnock]

If you feel the pipe going from the three port valve to the cylinder and its getting hot then the three port vave is staying in the open position which means it is broken

 

[twgas]

I assume you mean the cyl stat didn't click when you turned it down - if the water was as hot as you claim it wouldn't because the "start" point was lower than the actual temp ie. it had already clicked. probably nothing wrong with it.
the boiler stat on 4 would give very hot water if the three port is stuck as mark suggested in his post - I would tend to agree with him.

 

[ih]

thank you guys
Mick you are spot on about temperature on the cylinder stat iam can now confirm that the cylinder stat is ok.
2. the 3 port has auto marked one side and man on the other side i have always found it to be on auto! ( can i unstuck it or it needs replacement) As per mark i checked the pipe going from 3 port to cylinder this is hot when CH is on demand and luke warm/cold when hw is on demand1 your conclusion guys. thanks