A balanced flue is *room sealed*, that is the whole point.
Balanced flue has inner & outer tube, IIRC...
- Inner allows air to enter for combustion ONLY
- Outer allows heated combustion products out ONLY
The only draught would be room convective draught caused by air flowing over the heat exchanger.
Class-1 chimney gas fire is conversely very different.
- Some are only 45-55% efficient (some go 72%+)
- Thus 28-55% of their heat goes up the chimney
- Air is drawn through through the house via house vents
- Thus they can cause a howling draught under a door
The most common cause of a howling draught is a full bore chimney pot without rain guard, being sucked by the prevailing wind, combined with the huge thermal mass of the chimney length. That draught can spin a newspaper from front to back without difficulty if placed near a doorway and reduces efficiency yet further - it is how a gas fire can "suck a house cold".
The often forgotten trick with open hearth fires or gas fires when out is that they STILL suck air all night long, that thermal mass & prevailing wind can combine to "suck the house cold". Stick a sausage outside the relevent door when going to bed, you may find the house is significantly warmer.
This is why some people end up with Class-1 flame-effect or gas-fires AND central heating which "fight one another", the central heating is having to preheat the air so as to avoid draughts which vanish up the chimney. A door sausage can save =A350/yr and avoid such problems - BUT there must still be sufficient air for combustion (just some chimney draw like an 9" square extractor on maximum without a fire).
So any "draught" was due to convective effects across the balanced flue fire's heat exchanger, or it was a class-1 flue type fire (using chimney).