YAPH wrote:
Not possible. Where are you going to recover the heat from? The cold air
is coming in to replace air being burned in the appliance and going up the
flue as Carbon Dioxide and Water (as steam). There isn't a corresponding
flow of warm air out of the room, as there is with mechanical heat
recovery ventilation units.

Ah, good point.

The sensible energy-efficient approach is to replace the inefficient and
potentially-unsafe open-flued boiler with a high-efficiency room-sealed
one.

This isn't practical in this situation, much though it's obviously the best
option.

The main thing is to reduce draughts while not interfering with combustion.
Does such an arrangement draw in cold air by convection? So would, for
example, a louvred path be OK because the combustion causes
lower pressure in the room that would suck in fresh air? Or would the
boiler happily churn out CO before it causes a significant pressure
difference?

What sort of airflow is required for safe combustion in the average gas
boiler?

Approx 5cm^2 per kW of heat input to the appliance, less 7kW of the
appliance's rating (a certain amount of ventilation is assumed to be
available "adventitiously", i.e. from natural draughts). So for a 14kW
appliance you need 7 * 5 = 35cm^2.

I'm a bit puzzled by the units. That means a grille 7cm x 5cm, presumably?
But what's the amount of air input you need to the boiler (ie in cm^3 per
second)?

Theo