Central Heating - Water Only Query

[Rautospoon]

I had a replacement Vaillant ecoTech Plus Open Vent boiler installed in my loft about 9 months together with new pump, 3 way valve, Drayton programmer etc.
Since then the system has worked perfectly other than this one query.
A couple of weeks ago I noticed that occassionally neither the pump nor the boiler "fires up" when the "water only" comes on via the programmer both "timed" or "manual". It may well have happened before but I have not noticed it and I always have loads of hot water (the water only is "on" for 30 minutes in the morning and afternoon) so had no reason to think there was a possible issue.
I flicked on the Hot Water Advance on my programmer earlier this morning and initially nothing happened but it "came on" after about 20 minutes.
Is there a valid reason for this - I have the feeling that this happens when I have a full and hot cylinder ?
Many thanks.

 

[muggles]

Faulty 2 port or 3 port valve

 

[CountryFan]

Is the immersion heater on?

 

[Rautospoon]

Is the immersion heater on?

No.

 

[oldbuffer]

If the cylinder is already up to the desired temperatue, the boiler won't fire.

 

[Rautospoon]

If the cylinder is already up to the desired temperatue, the boiler won't fire.

Many thanks for that, so the HW cylinder thermostat 100% over-rides everything ?

As per my original post, I suspected the HW cylinder might be involved !

 

[Murdochcat]

If the tank is up to temperature, the thermostat won’t call for the boiler to fire.

To test this theory, next time you think it should be heating water and doesn’t fire, then run hot water until it does