What causes a downdraft?
Unfortunately the fire has no control over the air movement within it when it is cold, it has to come from external pressures, from either outside the home pushing down the flue system or possibly from inside the home with other extraction devices causing a negative pressure that is then pulling in down the flue, it may also be a combination of both of these effects at work simultaneously.
If it happens again, try and take note of whether the house is all closed up and if any external ventilation or extraction is running that could be creating a negative pressure situation, and then also take a note of the wind direction and strength outside. By monitoring these you will then be able to discover what is causing your downdraft issues.
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