So why smoke comes back into your room when you light your fire? When looking at the problems that assail chimneys and fireplaces, some of the more difficult to diagnose are the transient ones, the ones that come and go.
One in particular that comes to mind at the start of the fire burning season is the fireplace that for no apparent reason smoke comes back into your room and smokes the house out when the fire is lit for the first time in the autumn. This can happen every autumn, or it may be that it happens for the first time to an otherwise impeccably behaved chimney.
OK, so it might be due to a summer visitor building a nest in the chimney, but when you have the chimney swept, and it still persists, what do you do then? What is the cause of this problem?
How to diagnose the issue
Diagnosis is pretty simple. Blockages, as when our ‘Feathered Friends’ decide to set up home in the flue leave a chimney dead. Air neither moves up nor down the chimney, and when that hippy era diagnostic, the joss stick is burned, the smoke just hangs there going nowhere.
Using joss sticks as opposed to smoke pellets
(We use joss sticks because most people enjoy the smell, or at least don’t actively dislike it. Also, they don’t make a lot of smoke, unlike a smoke pellet which creates lots of unpleasant smelling smoke. I have seen several times where someone has simply walked up to the hearth, lit a smoke pellet and proceeded to fill the room with pungent, chemical tasting smoke, to the irritation and discomfort of the homeowner. The sensible routine is this. First place an un-gloved hand in the chimney throat and feel if there is a flow of air. If you are not sure, light a joss stick. Smoke pellets really should only be used when you want to demonstrate to the customer that the flue is working well and taking a good flow of air.)
The autumn problem is easily diagnosed, because the air is flowing down into the room, even when there is no wind and the joss stick will show this nicely to the customer. The air will not be in fits and starts, but will be flowing down in a steady stream. It can be very disconcerting when you first come across it. The day outside may be warm and still, but inside a steady torrent of cold air will be falling down the flue. The clue is in its temperature.
Normally, a chimney functions according to a simple rule of physics. Warm things become less dense (and in liquids and gasses), the warmer, lighter portions move upward. So, even when the fire is not lit, the room is warm and the warm air passes up the chimney warming it and maintaining a steady upward flow.
Why does a cold plug of air occur?
Over the summer, when the chimney is unused, the core of the building and the chimney cools until the air in the chimney is quite cool. Then, the air in the flue gets that bit cooler than the air outside and it starts to fall down the chimney. The cool mass of the chimney cools the incoming air and it continues to fall, until an equilibrium is reached and the air stalls and stops moving.
How to resolve a cold plug of air
The only sensible cure is to warm the chimney. If it is summer and that isn’t practical, then the throat of the chimney should be blocked off to stop the flow. If it is autumn and the fire can’t warm the chimney enough to kick start the upward flow and smoke comes back into your room, then a clean source of warm air like a fan heater or hairdryer needs to be positioned so as to send warm air up the flue until the normal state of affairs is restored and the chimney once again functions normally.