Is this condening boiler condensing?

[stem]

Thanks for explaining that, it makes sense when you see the numbers.

I'm not aware that the burner is extinguished, although I'm sure it must be, however, I never hear the fan go off whilst the thermostat is calling for heat. (unless it designed to keep running?)

In practice, because the heating is on all day, the thermostats turn most of the radiators off except the bathroom as the lounge is warmed by any sun and the bedrooms are on a low setting, and the kitchen hardly ever comes on anyway. So basically the boiler is running just a tiny radiator in the bathroom.

I think from memory the thermostat is about as low as it can be and still keep the house warm enough, as I was trying to keep the return temperature as low as possible.

 

[D_Hailsham]

I think from memory the thermostat is about as low as it can be and still keep the house warm enough, as I was trying to keep the return temperature as low as possible.

That's sensible. As you probably realize the return temp must be under 55°C for condensing to occur. If this is happening, there will be very little, if any, "plume" emanating from the flue.