The boiler only comes on intermittently for hot water, but when the heating is on there is no problem. If hot water is wanted when the heating is not on, we turn the heating on then open a tap and turn the heating off and the hot water stays on. The thermostats work on both heating and hot water.
Ariston Euro Combi SX 20 MFFICE
Type C with sealed combustion chamber
GC No 4711602

Ken Ball, September 2006

you need to check the diaphram in the divertor valve. if the chrome pin doesnt come out enough then the boiler won't fire the diaphram cost £20 at the most. quite easy to change

baz, October 2006

I had i similar problem with my Sime Super 90 Plus it suddenly stopped giving hot water unless the heating was on after countless searches both on this site and others and including the instalation manual fault finder it appeared to be the Divertor Valve after speaking with the mantainence man at work he suggested looking at the divertor valve (big brass type thing with 5 ish pipes connected) there is piston at the top that connects with a switch to change from heating to hot water it's either in a lever type connection or inside a plastic type setup with wires connected as mine is but it will depend on your make so i had a look and discovered that it wasn't reaching the switch so i just increased the thickness of the tip on the piston and it worked the reason yours may be intermittent may be a similar reason hope this helps

matthew johnston, September 2006