Between natural gas, propane and butane, the gap that matters on site is not a performance level but four physical quantities that drive the hardware. Natural gas type H as distributed in France carries a gross calorific value of 10.7 to 12.8 kWh/Nm³ for a gross-to-net ratio of 1.11, while commercial propane weighs in at 12.78 kWh/kg net and butane at 12.66 kWh/kg according to France Gaz Liquides. Above all, the Wobbe index moves from the 45.7 to 54.7 MJ/m³ band of the second family group H to the 72.9 to 87.3 MJ/m³ band of the third family, and the normal supply pressure from 20 mbar to 28-30 mbar for butane and 37 mbar for propane: it is that double gap, not the calorific value, that forces an injector change and a fresh set-up. On the storage side, the applicable text is the French order of 23 February 2018, in force since 1 January 2020, which replaced the now repealed order of 2 August 1977.
What physically separates the three gases
The reference table worth keeping to hand
The first reflex is not to compare different units: natural gas is billed in kWh derived from a volume, LPG is delivered by the kilogram. The values below are those of the reference gases and the commercial products, not those of a manufacturer's catalogue.
| Quantity | Natural gas H (G20) | Commercial propane (G31) | Commercial butane (G30) |
|---|---|---|---|
| Net calorific value | 9.6 to 11.5 kWh/Nm³ (gross 10.7 to 12.8, ratio 1.11) | 12.78 kWh/kg (gross 13.8) | 12.66 kWh/kg (gross 13.7) |
| Density as a gas at 15 °C | lighter than air | 1.85 kg/m³ | 2.50 kg/m³ |
| Behaviour on a leak | rises and disperses | pools at low points | pools at low points |
| Normal supply pressure | 20 mbar | 37 mbar | 28 to 30 mbar |
| Wobbe index, EN 437 band | 45.7 to 54.7 MJ/m³ | 72.9 to 87.3 MJ/m³ | 72.9 to 87.3 MJ/m³ |
| Boiling point at atmospheric pressure | not applicable, delivered as a gas | −44 °C | 0 °C |
Boiling point and vapour pressure: why butane does not hold up outdoors
Butane boils at 0 °C and propane at −44 °C. That is the only technical reason a butane cylinder placed outside stops delivering at the first frost: the liquid no longer vaporises fast enough to feed the burner, and the appliance locks out with nothing actually faulty. Propane keeps a usable vapour pressure well below zero, which makes it the only LPG usable in an external tank in a continental climate. The regulatory corollary follows directly: it is the saturated vapour pressure at 20 °C that serves as the sorting criterion in the 2018 order, not the trade name of the gas.
Wobbe index: the quantity that decides interchangeability
The Wobbe index divides the volumetric calorific value by the square root of the gas density relative to air. Its useful property is this: at identical injector area and upstream pressure, two gases with the same Wobbe index deliver the same output. It is therefore the Wobbe index, not the calorific value alone, that governs interchangeability. EN 437 sorts gases into Wobbe bands, and the roughly one-and-a-half ratio between the second family group H and the third family explains on its own why an LPG injector is markedly smaller than a natural gas one.
What a change of gas demands of the burner
Appliance category: read the plate before promising a conversion
An appliance is not convertible because a kit exists: it is convertible because its category says so. The data plate carries a category such as II2E+3+, which states the families and pressure couples allowed. A category I appliance, restricted to a single family, does not convert. On a replacement in an unserved area, that check happens before ordering, not on unpacking.
| Gas | Reference gas | Normal supply pressure | EN 437 family |
|---|---|---|---|
| Natural gas type H | G20 | 20 mbar | 2nd family, group H |
| Natural gas type L | G25 | 25 mbar | 2nd family, group L |
| Commercial butane | G30 | 28 to 30 mbar | 3rd family |
| Commercial propane | G31 | 37 mbar | 3rd family |
Injector, burner pressure and rate: what actually changes
Conversion touches three things: the injectors, whose diameter is reduced when moving to LPG, the pressure set at the gas valve, and the electronic rate parameters on modulating premix appliances. All three come from the manufacturer's kit, referenced for the exact model. An improvised conversion with injectors from another model takes the appliance outside its declaration of conformity and voids the warranty, which shows up immediately after an incident.
Set-up and combustion check at commissioning
Conversion ends at the analyser, not at the screwdriver. Record the supply pressure while running at the gas valve test point, then the CO2 or O2 and the CO in the flue gases, at minimum and maximum output, and file the values. The check method and the target values are set out in our article on combustion analysis. Without that record, nothing proves the appliance is set for the gas it is being fed.
Siting and storage under the order of 23 February 2018
The text in force, and the one that must no longer be cited
The order of 23 February 2018 has applied to gas installations in individual and multi-unit residential buildings, common parts included, since 1 January 2020. It replaced the order of 2 August 1977, now repealed. Continuing to cite the 1977 text on a quote or a site report is a mistake that surfaces at the first dispute. The 2018 text sets performance requirements and refers to recognised standards and technical guides for the means of meeting them.
The 4 bar threshold, which decides where the cylinder goes
Article 12 requires cylinders holding more than 3 kg of commercial propane, or of any gas whose saturated vapour pressure at 20 °C exceeds 4 bar, to be kept outside buildings and sited so that a leak cannot enter the dwelling. The criterion is physical, not commercial: it separates propane from butane without naming either product. The same article bars keeping more than one disconnected cylinder larger than 3 kg in a single room. On an LPG plant room, those rules combine with the choice of generator, covered in our article on the LPG propane boiler in rural areas.
Ventilation, flue gas removal and detection
Since LPG is heavier than air, it accumulates at low points: ducts, pits, crawl spaces, inspection chambers. Locating those points belongs to the site survey just as much as measuring the flue. On the discharge side, reusing an existing flue on a condensing appliance means checking its compatibility and, most often, lining it, as set out in our article on lining a chimney flue.
Choosing the fuel according to what serves the site
A dwelling connected to the distribution network
Where the meter already exists, natural gas stays the simplest option to deliver: no storage, no delivery contract, no siting to negotiate. The work concentrates on the generator, the flue and the controls. The question then is not the fuel but the trade-off between replacing the generator and dealing with the heat losses.
Unserved site: a propane tank
Off network, a propane tank reproduces the logic of a natural gas installation, at the cost of a siting exercise: tanker access, distances to the building and to boundaries, ground bearing capacity, tank rental or consignment contract. Those lead times run into weeks and are started at signature of the quote, not at the start of works.
Butane: top-up and cooking, not continuous heating
Butane is not ruled out on its calorific value, which is all but identical to propane's, but on four operating constraints that make continuous heating unmanageable.
- Vaporisation collapses as soon as the cylinder temperature approaches 0 °C.
- Instantaneous rate limited by the cylinder's exchange area, too low for a boiler.
- Handling and frequent changeovers left to the occupier.
- Ventilation constraints in the room where the cylinder is connected.
Positioning gas within a works plan
What grants remain available
Current schemes target the envelope, the controls and low carbon generators. Installing a new gas boiler falls outside the grant routes in virtually every case, including a like-for-like replacement. The measures that remain fundable around a retained gas installation are insulation, controls and balancing: that is where the pricing effort belongs, not on the generator.
Cut the demand before touching the generator
On a retained gas installation, the order of operations decides the outcome: deal with the heat losses, then airtightness, then the weather compensation curve, circuit balancing and thermostatic valves. A generator replaced on an unreduced demand stays oversized and cycles, which degrades the real efficiency whatever the quality of the boiler.
Moving to a heat pump or a hybrid
The switch is decided on data, not on instinct: calculated heat losses, emitter regime, available electrical supply, room for the outdoor unit. On high temperature radiators or in a cold climate, a hybrid keeps the gas generator as back-up on the peaks, as set out in our article on the gas hybrid boiler. To hold the sizing, Argile computes the heat pump output from the survey records and picks the equipment from up-to-date catalogues.
