Low hot water pressure from combi boiler

I have started having a problem with my combi boiler that when the hot tap is running the boiler fires for about 20 seconds and then stops. Although the cold water pressure is good, pressure from the hot tap is very low. Could it be that the pressure is high enough to trigger the burner but low enough that the water overheats and the burner is stopped by the protection thermostat?

I was wondering any reasons why the hot water pressure would be so low?

The central heating side works insofar as the burner runs and the system gets hot. The pump seems to get pretty warm too - I don't know if this is normal - there's no air in the radiators and no noise when the CH is running so don't think that there's air in the system. My feeling is that this isn't related to the hot water problem - but I could be wrong?!

You'll have noticed that I'm not a professional plumber but would welcome any pointers that you might have if there's some relatively straight forward things that I can look at or help diagnose the problem so that I'm able to discuss it with the professional who comes to fix it if that's what's required.

 

Thanks for your reply - sorry that I didn't see it earlier, I didn't get emailed the update. Grrr!

Mains pressure seems OK. The filling loop is right before the water in to the combi (as you'd expect). I took this off and squirted it into the kitchen sink and pressure seemed good.

Kitchen sink is very close to the boiler. Pressure from cold tap there is fine, pressure from hot is very low. Pipe run is approximately the same length save for the hot running through the boiler of course.

 

Thanks. It's a Ravenheat RSF 82e. I can't see a filter or flow restrictor as separate items marked on the schematic.