LPG vs Natural Gas: Key Differences
LPG and natural gas aren't interchangeable — different operating pressures, injectors and, crucially, LPG being heavier than air change how you install, convert and make an appliance safe.
LPG (liquefied petroleum gas — usually propane in the UK for heating) and natural gas (mains methane) do the same job but behave very differently. They run at different pressures, carry far more energy per unit volume, and — most importantly for safety — LPG is denser than air, so it sinks and pools where natural gas rises and disperses. An engineer needs the right category and the right data for each.
This guide covers the key differences — operating pressures, relative density and the safety implications, injectors and appliance conversion, and calorific value — for engineers working across both fuels. It's guidance only; LPG is a separate Gas Safe category, and all gas work must be carried out by a Gas Safe registered engineer competent for the fuel and appliance.
Key takeaways
- Operating pressure differs: natural gas runs at about 21 mbar at the meter (roughly 20 mbar at the appliance), propane at about 37 mbar.
- Relative density is the critical safety difference: LPG is denser than air (relative density greater than 1), so it sinks and pools low; natural gas is lighter than air and rises.
- LPG has a much higher calorific value per m3 than natural gas, so it needs smaller injectors and far less volume for the same heat input.
- Appliances are fuel-specific: converting between natural gas and LPG needs the manufacturer's conversion kit (injectors, and a pressure/valve reset), never a bodge.
- LPG installations bring extra requirements — cylinder/tank siting, drainage away from low-level ignition sources, and no appliances in cellars/basements where gas could collect.
Operating pressure
The two fuels run at different working pressures, which is why an appliance set for one won't run correctly on the other. Natural gas is distributed to domestic properties at low pressure — around 21 mbar at the meter outlet, with appliances expecting roughly 20 mbar inlet working pressure. Propane runs higher: a domestic propane installation typically operates at about 37 mbar.
Butane, the other common LPG, is usually supplied at about 28 mbar, though propane is the norm for fixed heating installations in the UK. The higher LPG pressure, combined with a much higher calorific value, is part of why LPG appliances use smaller injectors and different burner settings than their natural-gas equivalents.
Different pressure, different setup
About 37 mbar for propane against about 21 mbar for natural gas at the meter. An appliance carries a fuel-specific data badge and injectors — run it on the wrong fuel or pressure and combustion is wrong from the start.
Relative density — the safety difference that matters most
The most important practical difference is weight. Natural gas (mostly methane) is lighter than air, so a leak rises and disperses upward. LPG (propane and butane) has a relative density greater than 1 — it is heavier than air — so a leak sinks and collects at low level, pooling in floor voids, drains, cellars and basements where it can reach an ignition source unseen.
This drives real installation rules. LPG appliances and cylinders are not permitted in cellars or basements where escaping gas could accumulate; cylinder and tank locations, and any relief discharge, must be sited and drained so that leaked gas disperses safely away from drains, gullies and low-level ignition sources; and LPG-appropriate detection is placed low, not high. Treat an LPG leak as something that lies in wait at floor level, not something that clears itself.
LPG sinks — never in a cellar
Because propane and butane are heavier than air, they pool at low level. LPG appliances and cylinders are not allowed in basements or cellars, and cylinder/tank siting must let any leak disperse away from drains and low ignition sources.
Calorific value and injectors
LPG carries far more energy per cubic metre than natural gas: propane's gross calorific value is around 93 MJ/m3 against roughly 39.3 MJ/m3 for natural gas. For the same heat input, an LPG appliance therefore burns much less gas by volume — which is why LPG burners use noticeably smaller injectors to meter that smaller volume at the higher pressure.
It also changes how you check the appliance. On a metered natural-gas supply you can gas-rate at the meter; on a bulk-tank or cylinder LPG installation there's usually no suitable meter, so you verify the appliance by burner pressure and combustion against the manufacturer's LPG data instead. The gross-to-net conversion factor differs slightly too — about 1.09 for LPG against 1.11 for natural gas.
Converting appliances between fuels
Because the injectors, operating pressure and burner settings all differ, an appliance is fuel-specific and can only be moved between natural gas and LPG using the manufacturer's approved conversion kit. That kit changes the injectors and provides the correct pressure/valve settings for the new fuel, and the appliance must then be re-commissioned to the manufacturer's data for that fuel — new injectors, correct burner pressure, and combustion checked with an analyser.
Running an appliance on the wrong fuel or without proper conversion gives wrong combustion — over- or under-firing, sooting, high CO — and is unsafe. LPG is a separate competence: an engineer needs the LPG category as well as the appliance category to work on it, so being carded for natural-gas boilers doesn't authorise LPG work.
Installation and supply differences
The supply itself differs. Natural gas arrives via the mains and a meter; LPG is stored on site in cylinders or a bulk tank and fed through a regulator that drops the tank/cylinder pressure to the appliance operating pressure. That brings its own requirements around tank/cylinder siting and separation distances, regulator and changeover arrangements, and pipework and testing appropriate to LPG.
Everything else — sizing pipework for the load and permitted pressure drop, tightness testing, ventilation, flueing — still applies, but to the LPG figures and procedures rather than the natural-gas ones. The principles carry across both fuels; the numbers, the density-driven safety rules and the competence required do not, which is why LPG is treated as its own discipline.
Frequently asked questions
What pressure does LPG run at compared with natural gas?
A domestic propane installation typically operates at about 37 mbar, against about 21 mbar at the meter for natural gas (roughly 20 mbar inlet at the appliance). Butane is usually around 28 mbar. The different pressures are one reason appliances are fuel-specific.
Why is LPG more dangerous in a basement?
Because LPG is heavier than air (relative density greater than 1), a leak sinks and pools at low level rather than rising and dispersing. In a cellar or basement it can accumulate to a dangerous concentration near ignition sources, which is why LPG appliances and cylinders aren't permitted there.
Can I run a natural-gas boiler on LPG?
Only if the manufacturer supplies an approved conversion kit and the appliance is converted and re-commissioned to their LPG data — new injectors, correct burner pressure and combustion checked with an analyser. You also need the LPG Gas Safe category. Running an unconverted appliance on the wrong fuel is unsafe.
Why can't I gas-rate an LPG appliance at a meter?
Bulk-tank and cylinder LPG installations usually have no suitable meter to time, so gas rating at the meter isn't possible. Instead you verify the appliance by burner pressure and combustion analysis against the manufacturer's LPG data.
From guidance to action
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How Gas Safe registration works, the ACS route, and keeping your categories current.
