Heating a Cold Garage or Outbuilding Gym: Carbon Monoxide, Fuel-Burning Heaters and Electric Heater Fire Safety

A garage, shed or garden-room gym is often the coldest room a household uses, and a portable heater is the obvious fix. Heaters and fuel-burning appliances carry two separate risks: carbon monoxide from anything that burns fuel, and fire from heaters of every kind. This article summarises guidance from the UK Health Security Agency (UKHSA), the NHS and the London Fire Brigade. The UKHSA figures quoted relate to England and Wales, the NHS page is written for England, and the London Fire Brigade advice is general fire-safety guidance that is not specific to London.

Why carbon monoxide matters in a small, closed gym

UKHSA describes carbon monoxide (CO) as a colourless, odourless gas that cannot be seen, smelled or tasted but can kill. Its November 2025 blog says around 20 people die each year in England and Wales from unintentional CO poisoning, with many more hospitalised, and that in England 68% of CO deaths occur in autumn and winter. UKHSA links the winter peak to more heating being used with less ventilation, and says smaller rooms containing CO sources are particularly at risk because the gas concentrates more quickly in confined spaces. A shut garage or shed is exactly that kind of space in cold weather.

Where carbon monoxide comes from

The NHS says CO can be made by fires and appliances that burn gas, wood, oil or coal. Its list of appliances that can produce it includes gas boilers, gas or paraffin heaters, wood, gas and coal fires and portable generators. Both the NHS and UKHSA add that engines are a source too: leaving a vehicle or lawn mower engine running in an enclosed space such as a garage can cause a build-up, and UKHSA advises keeping a window or door open if a vehicle, generator or engine is started in a garage or shed. A boiler or gas appliance fitted in the same garage as the gym counts as a CO source in its own right.

Electric heaters do not burn fuel. The London Fire Brigade ties the CO risk to solid fuel, gas and paraffin heaters, while electric heaters carry a fire risk instead.

Alarms, servicing and warning signs

UKHSA calls fitting CO alarms the single most important action. It recommends alarms that meet BS EN 50291-1 in every room with a fuel-burning appliance, and regular testing with batteries replaced as needed. The NHS gives the same advice and suggests one alarm for each room containing a gas, oil, coal or wood appliance. The London Fire Brigade says it is important to always fit a CO alarm in rooms with solid fuel, gas or paraffin heaters, and notes that CO can make people drowsy and less able to escape.

UKHSA advises that all gas appliances be serviced every year by a Gas Safe registered engineer, and the NHS says boilers, heaters and cookers should be serviced regularly by a qualified and registered engineer. UKHSA lists three warning signs on gas appliances:

• floppy yellow or orange flames instead of crisp blue ones;
• soot marks around the appliance;
• increased condensation inside windows.

The London Fire Brigade adds that anyone installing, repairing or servicing appliances should be registered with the Gas Safe Register for gas, HETAS for solid fuel and OFTEC for oil, and that people should never do this work themselves.

Symptoms that call for action

UKHSA lists headache, dizziness, feeling or being sick, weakness, confusion, chest and muscle pain and shortness of breath. The NHS adds tiredness, flushing and loss of vision. Both describe a pattern as the important sign: symptoms that come and go, worsen in one room or building and ease outdoors. If CO poisoning is suspected, UKHSA says to stop using appliances that might be producing it, open windows and doors, go outside immediately, get medical advice as soon as possible and stay out of the building until professional advice has been received. The NHS says to call NHS 111 for urgent advice, and to call 999 or go to A&E if someone is finding it hard to breathe, has suddenly become confused, has lost consciousness, or has weakness or chest or muscle pain. For a suspected gas leak, both give the free National Gas Helpline number, 0800 111 999, open 24 hours a day.

Portable heaters and fire risk

The London Fire Brigade says fires involving heaters seem to have a particularly high fatality rate, and that fires typically start when bedding, blankets or clothes are placed too close to them. Its advice for all heaters is to:

• keep heaters well away from bedding, blankets, curtains and furniture, and never use them for drying clothes;
• sit at least one metre from a heater;
• where appropriate, secure heaters against a wall so they cannot fall over, or fit wall-mounted heaters;
• turn a heater off and let it cool before moving it;
• keep heaters in good working order and check that they are not on a recall list.

For electric heaters specifically, the Brigade says over 160 fires are caused by electric heaters every year, that a third of electrical fires ending in a fatality involved heaters, and that an old electric heater should be tested by a qualified electrician or replaced. Towels, mats and clothing in a gym are fabric in the same sense as bedding, so the clearance advice applies to them.

For gas heaters, the Brigade says cylinders should be changed in the open air, or with all rooms ventilated if they must be changed indoors, and that spare cylinders should be stored upright and outside where possible and never in basements, under stairs or in cupboards containing electric meters or equipment.

The bottom line

Heating a cold gym safely depends on matching precautions to the heater. Anything that burns fuel, including a boiler in the same garage, needs a BS EN 50291-1 alarm, ventilation and regular servicing by a registered engineer, and engines should not be run in a closed space. Electric heaters need clearance, stable positioning and replacement or testing when old. Symptoms that ease when leaving the room are a reason to get out and seek medical advice.

Sources

• UK Health Security Agency, “How to protect your home and family from the invisible danger of carbon monoxide” (18 November 2025)
• NHS, “Carbon monoxide poisoning”
• London Fire Brigade, “Portable heaters, gas fires and open fires”