CH not warm enough

[31dn]

Hi,
I'm having problems with my CH system.
Baxi Solo 60 RS boiler, open vented.
All radiators in house are tepid.
Decided to replace pump, drained system, refilled (adding 'cleaner'), bled all radiators etc.
Boiler is firing-up, burner on all the time but flow water from top of boiler is tepid. Pump running ok. Boiler thermostat set to high but feel that it's not firing enough ?
When system does work properly (don't know why!), all rads in house are very warm, flow from boiler to pump also very warm & can hear the boiler working nicely (from outside flue).

Thanks in advance for any suggestions.

 

[MANDATE]

Sorry I can't help you, but I recall examining a faulty 'gas valve' just out of interest a couple years ago.
There were two coils and two solenoids and two valves. It appeared one valve could not open until the first valve had opened to allow pressure to pas from one chamber to another.
It just struck me reading your problem that it might be related if one of these valves is sticking.
It could explain it if it sometimes works and sometimes not. Also if the burner is not on full.
The fault on the one I was looking at was one of the coil windings, it seemed to work ok when cool but failed when hot.