Why an open chimney is like a window you never close

An open fireplace is charming, but in heating terms it is a hole in your living room. Warm air rises, and a chimney is a vertical shaft with a permanent supply of air at the bottom and an opening to the sky at the top. Once the stack effect gets going, your room air is drawn up and out, and replacement air has to come from somewhere — under doors, through floorboards, down the letterbox. You then pay to heat that cold air all over again.

A typical open flue can pull somewhere between 20 and 40 cubic metres of air out of a room every hour, and considerably more on a windy day or when a bathroom fan or cooker hood is running. That is roughly comparable to leaving a small window open all winter. Multiply that across a whole heating season and it becomes a genuinely noticeable slice of your annual bill, quite apart from the cold draughts that make a sofa near the fireplace uncomfortable to sit on.

The good news is that this is one of the cheapest and quickest fixes in the whole retrofit toolbox. There are three broad approaches: temporary plugs, fitted dampers, and safe permanent blocking.

Chimney balloons and draught excluders: the quick win

A chimney balloon is an inflatable cushion that sits inside the flue, just above the fireplace opening. You inflate it with a hand or foot pump until it grips the brickwork, and it simply stops air moving. Most cost between £20 and £40, and fitting takes a few minutes. Sheep's wool versions work on the same principle and some also trap soot and keep birds and insects out.

Measure your flue opening before you buy, as sizes vary from around 300mm square up to 900mm or more. Buy one with a deflation tube you can tuck discreetly out of sight, and check it stays put after the first week as the rubber settles.

Two important cautions:

• Never leave a balloon in place if you intend to light a fire. Remove it first, every single time. Tape a reminder to the mantelpiece if you're forgetful.
• Never fit one in a flue that serves a gas, oil or solid fuel appliance that is still connected. Those flues must remain clear.

If you only ever burn occasionally and can be disciplined about removal, a balloon is the fastest way to feel the difference.

Dampers and register plates for a more permanent fix

If your fireplace is genuinely out of use, a throat damper is a tidier solution than a balloon. This is a metal plate fitted at the base of the flue, opened and closed by a lever, chain or handle. Retrofit versions are available, but the fit needs to be neat — a badly sealed plate lets just as much air through as no plate at all. A chimney sweep or stove fitter will usually do this in a single visit, and many will combine it with a sweep and a smoke test.

At the top end, a cowl or cap on the chimney pot is worth considering, but be realistic about what it does. A cap keeps out rain, birds and downdraught, and a ventilated cap lets the chimney breathe. It does not stop the stack effect pulling air out of the room, so treat it as weather protection rather than an energy measure.

Blocking an unused flue — and why you must not seal it completely

If a chimney breast has been stripped of its fireplace and the flue is redundant, you can close the opening properly. But there's a catch that catches out a lot of homeowners: a chimney that is sealed at both ends with no airflow becomes a cold, damp void. Moisture from the house migrates into the brickwork, condenses against the cold stack, and shows up as damp patches and salts on the plaster.

So the rules are:

• Sweep the flue first if it has ever carried solid fuel, then have it inspected.
• Leave ventilation at both ends. Fit a ventilated cap at the top and a hit-and-miss vent in the blocking board, so the flue can dry out while still preventing draughts into the room.
• Never block a flue serving a live appliance. Combustion gases must always have a clear route to outside. This is a carbon monoxide risk, not a technicality.
• Use a qualified gas engineer for anything connected to a gas appliance.

If you still use the fire, reduce the losses rather than the flue

A wood-burning stove with a closed door and controlled air intake behaves far more like a proper heating appliance than an open grate, and once the fire is out you can shut the air controls to cut the draught. Make sure the register plate around the flue pipe is sealed with fire-rated sealant, and burn dry, seasoned wood — under 20% moisture — so you get heat rather than steam.

Remember that a stove needs a permanent air supply in the room, typically a vent sized to the appliance's output. Sealing a room tight around a solid fuel appliance is not efficiency, it is a hazard. An annual sweep and inspection keeps everything safe and working properly.

Where to start

Work in this order: sweep and inspect, plug or damper any flue you don't use, ventilate the ones you've closed, and only then move on to general draught-proofing. Chimney work pairs well with loft insulation and sealing suspended timber floors, but treat ventilation as part of the design rather than an afterthought. A house that is warm, dry and properly aired is the goal — not one that is simply sealed up.