Wasp nest in a chimney

A chimney is essentially a tall, dry, sheltered cavity that's rarely used between April and October — exactly when a queen wasp is looking for somewhere quiet to start a nest. Unused flues, capped-off fireplaces, and disused pots are all common sites. The nest itself typically forms partway down the flue, in the void behind a chimney breast, or occasionally right down near the fireback if the fireplace hasn't been used in years.

The two signs that give it away

First is sound: a dull, resonant buzzing carried down the flue into the room, often more noticeable than a loft nest because brick and masonry actually amplify it. Second is debris — small greyish flakes of chewed wood pulp (the material wasps build nests from), plus the occasional dead wasp, collecting in the grate or on the hearth. If you're getting both, you almost certainly have an active nest somewhere in that flue or chimney breast void.

Open flue vs capped chimney vs wood burner

An open, unused flue with a chimney pot is the most common setup we treat — wasps fly straight in and out of the pot, and we can dust the nest from above without disturbing the room below. A chimney that's been capped at the top but still has an open fireplace below is different again: if the cap has failed or was never fitted, wasps get in exactly the same way, but a capped flue with a solid seal usually means the nest, if there is one, is actually in the cavity behind the breast rather than the flue itself.

Wood burners complicate things because most have a stainless liner running inside the original brick flue, leaving a narrow air gap between liner and brickwork. Wasps sometimes nest in that gap rather than in the liner itself, entering through mortar joints higher up the stack. We check which situation we're dealing with before choosing where to treat from, which is why a phone description of what you're seeing and hearing helps us prepare before we arrive — see our signs of a nest guide for what to note down.

What happens after treatment

We treat the entry point with insecticidal dust so returning wasps carry it back into the nest itself, which usually settles activity within a day. We won't advise using the fireplace again until the colony is confirmed dead, and if debris continues to build up in the grate afterwards it's worth a follow-up call — this is covered under our free revisit guarantee. Once it's clear, fitting a cowl to a genuinely unused flue is the single best way to stop it happening again; see prevention advice for other entry points worth checking at the same time. If you've also got activity in the loft above the same chimney breast, our loft nest guide covers that separately.