Intermittant hot water supply when boiler used earley in morning?

when we use hot water for shower early morning the water
will very mild and temperature will read in boiler appx 35 digree
after 10/15 minutes later water start flow hot normal.
boiler recentley cleaned inside out by usuing special pump..
started to work few days but now still intermittant early morning useage.

has any body experiance this type problem?
is it a sensor or some thing else?