Why a Fireplace Smells Worse in Warm Weather, and How to Stop It Before Summer

The fire has been out for weeks. The room still smells of smoke. On the first warm afternoon of spring it is noticeably worse, and it gets worse again on humid days and after rain. Plenty of Melbourne households assume this means something is wrong with the chimney. Usually nothing is broken. The chimney is doing exactly what a chimney does in warm weather, and it is simply doing it with a winter of deposits inside.

The short version is that the draught has reversed. Through winter, warm air in the flue rose and carried smoke out. Now the outside air is warmer than the air sitting in the chimney, so the flue draws the other way — downward, into the house — and everything it picks up on the way is what you can smell.

Why the draught reverses when it warms up

A chimney works on temperature difference. Warm air is lighter than cold air, so while the air inside the flue is warmer than the air outside, it rises and pulls room air up behind it. That is the draught, and in winter it is strong even when the fire is out, because the house is heated and the flue is warmer than the outside night air.

In spring that balance flips. A masonry chimney is a large mass of brick that stays cool for a long time, particularly the shaded side and the part inside the roof space. On a warm afternoon the air outside is warmer than the air inside that cool flue, so the heavier air in the chimney sinks. It falls into the firebox and out into the room, bringing the smell of the flue walls with it. The warmer the day and the cooler the chimney, the stronger the effect, which is why the first hot days of the season are when people notice it.

Humidity makes it worse for a separate reason. The deposits on the flue walls give off much more odour when they are damp, so a warm, humid day, or the day after rain, produces the strongest smell of all.

What the smell actually is

The smell is creosote and soot. Every fire leaves a layer of it on the flue walls: a mix of unburnt tar, ash and condensed smoke that builds up across a season. In winter you never smell it because the draught is carrying everything up and out. Once the draught reverses, the air passing over those deposits is coming into the room rather than leaving it.

The amount of deposit varies with how the fire was run. Slow, smouldering overnight burns, damp or unseasoned wood, and a fire that was regularly choked down all leave more creosote than hot, well-ventilated fires. A season of low fires is why some houses have a faint smell and others have a room that is unusable on a warm day.

Rain adds to it. An uncapped flue lets water straight down onto the smoke shelf and the deposits above it, and wet creosote smells far more than dry. Water also carries the tar into porous brick in the smoke chamber, where it is harder to remove. A chimney that smells strongly after every shower is telling you about the top of the chimney as well as the inside.

Why the house pulls air down the chimney

The reversal is not only about temperature. A modern house spends most of the day pulling air out of itself — the rangehood, the bathroom exhaust, a laundry fan, a ducted system on fresh-air mode — and every one of those needs replacement air to come in from somewhere. In a well-sealed house the easiest path is often the open flue. So the fireplace becomes the make-up air inlet for the whole house, and the smell arrives whenever the kitchen fan is on.

This is why the smell can be worst in the evening when someone is cooking, rather than in the heat of the day. It also explains why a house with the doors and windows shut smells more than one that is open: with the windows open, the fans have an easier route and the chimney is left alone.

What stops it: clean the flue, then close it off

The smell has two parts — deposits, and a draught carrying them — and the fix deals with both.

• Have the flue swept. Removing the creosote removes the source. A chimney sweep at the end of the burning season, rather than the start of the next, means the deposits are not sitting in the flue through the warm months producing the smell, and it clears the way for an honest look at the flue walls while they are bare.
• Close the damper. Once the fireplace is finished for the year and cold, the damper is the simplest barrier between the flue and the room. Check it actually seals; a warped or rusted damper leaves a gap that defeats the purpose.
• Plug an open flue. Many older Melbourne fireplaces have no damper at all. A chimney balloon or a fitted flue plug closes the opening for the season and is removed before the first fire.
• Fit a cap if there is not one. A cap keeps rain out of the flue, which stops the wet-creosote smell, and keeps birds and debris out at the same time.
• Give the house another air path. Cracking a window while the rangehood runs stops the fans drawing through the chimney. It is a small habit that removes a lot of the evening smell.

When the smell means something more than deposits

Most warm-weather chimney smell is creosote and a reversed draught, and a sweep plus a closed damper deals with it. A few versions are worth a closer look. A smell that is sharp or sweet rather than smoky, or that is present in a chimney that has not been used in years, can be a nest or a dead animal on the smoke shelf, and spring is nesting season. A smell that persists after a proper sweep usually means the tar has soaked into porous brick in the smoke chamber, or that the flue has an open joint letting deposits from a neighbouring flue through — both of which show up in a chimney inspection rather than a clean.

If the fireplace is a gas log fire or shares a chimney breast with a gas appliance, a flue that is drawing downward is worth having checked regardless of the smell, because it can affect how those appliances vent.

Timing it right

The instinct is to book a sweep in April, before the first fire. For a smell problem the better time is now, once the last fire of the season is definitely done and the chimney is cold. Sweeping at the end of the season means the flue sits clean and dry through spring and summer instead of full of deposits that get wetter and smellier as the weather warms. It also means the damper can be closed on a clean flue, which is the combination that makes the smell stop rather than fade.

The chimney will reverse its draught again next spring, because that is the physics of a cold masonry flue on a warm day. What changes is whether there is anything inside it for the draught to bring down.

Frequently asked questions

Is a smoky smell from an unused fireplace dangerous?

The smell itself is a nuisance rather than a hazard, but it is a clear message that the flue is dirty and that the chimney is currently pulling air down into the house. That second part matters if the fireplace is a gas log fire or shares a wall with a gas appliance, because a downdraughting flue can interfere with how those vent. It is worth dealing with rather than living with, and a sweep and inspection is the way to know what is actually up there.

Should I leave the damper open or closed over summer?

Closed, once the fireplace is definitely finished for the season and completely cold. A closed damper is the simplest thing standing between the flue and the room, and it stops most of the summer downdraught smell on its own. Two cautions: never close it on a fire that is out but still warm, and check that it actually seals, because a warped or rusted damper leaves a gap that the smell comes straight through. If the fireplace has no damper at all, a chimney balloon or a fitted flue plug does the same job.

Why does it smell worse after rain?

Because water has reached the creosote. Rain coming down an uncapped flue, or getting in through a cracked crown or open mortar joints, wets the deposits on the flue walls, and wet creosote gives off far more odour than dry. The smell of a chimney after rain is the same smell as a cold wet campfire for the same reason. A chimney cap keeps most of the rain out, and if a flue smells strongly after every shower it is worth having the top of the chimney looked at as well as the inside.

Will cleaning the chimney get rid of the smell completely?

Cleaning removes the deposits that produce the smell, so in most cases yes. What it does not change is the downdraught, which is a physics problem rather than a dirt problem. If the flue is clean and the damper is closed the downdraught has nothing to carry and nowhere to go, and that combination is what makes the smell stop. Where a smell persists after a sweep, the usual reasons are creosote soaked into porous brick in the smoke chamber, a nest or debris the brush did not reach, or a damper that is not sealing.