Why does my fireplace smell like smoke in summer?

Two things line up. Creosote is hygroscopic, so it takes up moisture from humid air and gives off odour as it does — the more humid, the stronger. And in warm weather a house is often under slight negative pressure, so air flows down the flue instead of up it, carrying that odour into the room rather than out of the chimney. In winter the fire reverses both effects, which is why nobody notices in January.

It removes the source, which is most of the answer, and it rarely eliminates the smell entirely on its own. Some odour is soaked into the masonry itself after years, and a damper that does not seal will still let it through. The reliable combination is a sweep plus a top-sealing damper at the crown, which closes against a gasket rather than metal on metal.

Commercial chimney deodorisers work better than an air freshener because they neutralise rather than mask, and they are genuinely useful in the gap between noticing and getting a sweep booked. What they cannot do is remove creosote or fix airflow, so treat them as a stopgap. If a product is being sold as an alternative to sweeping, it is not.

The smell itself is not — it is odour from deposits, not combustion gases, and nothing is burning. What it does tell you is that there is a meaningful amount of creosote in the flue and that air is flowing down the chimney into your house. The first is a fire risk next winter and the second means you would not get much warning if it were combustion gases instead.

Same mechanism, faster. Rain wets the creosote directly if the flue is open, and it raises the humidity everywhere else. A chimney that smells after every rain usually has no cap, or a cracked crown letting water down the stack. That is worth fixing regardless of the smell, because a wet flue also cracks tiles through freeze-thaw.