boiler cold

[john3565]

grant combi 90 v 3 boiler fault
is when boiler on for hot water only the boiler will not keep to temp.the boiler fires when divertor valve operates switch and water then heats up ok.the rotating temp switch on front seems to work.will fill up bath no problem.in morning the boiler is stone cold,but ok after heating is on.
overheating light not on any ideas as allready replaced divertor valve and wexchanger

 

[john3565]

just add to.
turned on heating and boiler fires.
lower thermostat on front and boiler keeps firing
flow and return pipes red hot
boiler still firing
lovely hot rads but boiler firing all time.
what type of thermostat do i require as two wires intop off boiler at thermostat location