Hi, The problem is the divertor valve (probably the actuater). The boiler is trying to heat the water in the heat exchanger to enable instant hot water from a tap.
Usually takes a few seconds.
In your case the valve hasn't diveted so the water is flowing through the rads.
Engineer should check actuator motor firstly.
Simple fix, shouldn't cost much.
Hope this helps!
allgasinstaller., September 2016
