How propane and butane differ from natural gas: density, liquid storage, boiling points, odorant, energy and air, and why appliances must be converted.
LPG (liquefied petroleum gas) is propane, butane or a blend of the two. Natural gas is mainly methane. Fuel properties and appliance requirements differ, so changing the supply requires an approved conversion for the actual appliance.
Where a leak goes
- At comparable conditions, propane has a relative density of about 1.5. Its still-air tendency is downwards, making drains, gullies, cellars and other low spaces particular accumulation risks.
- Natural gas has a relative density of about 0.6 and tends to rise in still air.
Airflow, mixing and release conditions affect both fuels. Where a leak is found does not tell you which gas it is, or everywhere it may go. Keep any required enclosure dispersal routes clear, assess appliance combustion-air requirements separately and follow the actual detector model’s positioning instructions.
Stored as a liquid, used as a vapour
LPG liquefies under modest pressure at normal temperatures, shrinking to a small fraction of its gas volume, which makes cylinders practical. Liquid sits at the bottom with vapour above it. The appliance draws vapour from the top and more liquid boils to replace it, taking heat from the cylinder walls, so a hard-worked cylinder gets cold and can frost.
- Propane boils at about −42 °C at atmospheric pressure and is suited to outdoor storage. Available vaporisation still has to meet the winter load; the boiling point is not a capacity guarantee.
- Commercial butane’s boiling point is around −2 °C. It is less suited to cold outdoor supply and is commonly used with suitable indoor portable heaters under their storage and ventilation conditions.
- Escaping liquid propane chills sharply as it vaporises and can cause cold burns.
Odorant
The added odorant helps make an escape noticeable. A smell is a warning to act, but no smell does not mean the installation is sound. People may fail to notice an odour, and smell does not measure gas concentration. Neither odorant nor an alarm replaces proper testing or the emergency response.
Energy and air
Comparing vapour volumes at the same temperature and pressure, LPG supplies more energy per cubic metre than natural gas. It therefore takes a smaller volume for the same heat input; useful heat output also depends on appliance efficiency. For equal volumes at the same temperature and pressure, propane also needs more air for complete combustion. That does not mean an appliance of the same heat input needs its ventilation multiplied by the fuel-volume ratio.
Converting an appliance
A natural-gas appliance cannot simply be connected to propane. A proper conversion needs:
- confirmation that the exact model is approved for the intended fuel
- the manufacturer’s conversion procedure, replacing injectors or other parts where specified and applying the required settings
- a suitable regulated supply, with pressure and capacity matching the model
- complete recommissioning, including the specified tightness, pressure, heat-input, combustion and labelling checks
Many cooker conversions require different injectors. Some adaptive boilers instead have an approved fuel configuration that does not require replacement parts. Follow the actual procedure: a parts swap, pressure adjustment or menu selection alone does not replace the remaining checks. Flow through an injector depends on its opening, the pressure difference and gas density; heat input then depends on that flow and the calorific value. A wrong injector gives a mismatched flame that can cause poor combustion and carbon monoxide. A normal-looking flame does not prove the conversion is correct.
If LPG escapes
Get people clear and prevent ignition: no electrical switches or phones where gas may be present. Contact the LPG supplier's emergency service from a safe place, and isolate or ventilate only if it is safe. Call 999 for a fire.