Hot water coming through cold tap at 16 degrees All aspects of plumbing questions and answers, help, tips and information

[pipefitter]

fitted new heating system boiler and heat exchanger

new pipe work to about 12 sinks some fitted with blending valves

first thing in the morning when i run cold tap
hot water at 16 degrees comes through for about 5 mins

pipework is lagged
pipework is not crossed

any ideas