How an LPG boiler works
An LPG boiler works in the same way as a mains gas boiler: it burns gas to heat water for your radiators and taps. The difference is where the gas comes from. Propane is stored as a liquid under pressure in a bulk tank or in cylinders outside, turns back into a gas as it leaves the tank, and passes through a regulator that drops it to the pressure the boiler needs. Liquid Gas UK, the LPG trade association, gives about 37 mbar as the typical domestic supply pressure for propane, compared with about 20 mbar for mains natural gas.
Inside, the boiler is almost identical to its natural gas version, with the gas valve set for LPG and a data plate or conversion label saying which gas it uses. Modern LPG boilers are condensing, with a condensate pipe to a drain.
LPG combi boilers
An LPG combi boiler heats water on demand with no hot water cylinder, and it is common in smaller off-grid homes. Larger homes may have an LPG system or regular boiler with a cylinder. The gas supply is the same either way.
What should the pressure gauge read?
The gauge on the front of an LPG boiler shows the water pressure in the heating system, not the gas pressure, and most manufacturers ask for about 1 to 1.5 bar when the system is cold. It will rise a little when the heating is running. Check the exact figure in your model's user manual.
If the needle drops below about 1 bar, you can usually top it up yourself with the filling loop, exactly as on a mains gas boiler. Our guide to boiler pressure explains the red and green zones and what a normal reading looks like. You cannot see or adjust the gas pressure; only a Gas Safe registered engineer can test it, using a gauge on the boiler's gas valve.
Common LPG boiler faults and what they look like
Most LPG boiler faults are the same as on any gas boiler, but a few come from the gas supply, so rule those out first.
| What you notice | Likely cause | What you can check |
|---|---|---|
| Boiler tries to light, then locks out with an ignition code | Tank or cylinder empty, supply valve closed, regulator fault | Tank gauge, cylinder indicator, whether an LPG hob lights |
| Lockout soon after a cylinder change | Valve not fully open on the new cylinder, air in the pipe | That the cylinder valve is open and the changeover lever is set correctly |
| Low pressure code, needle below 1 bar | Water pressure has dropped | Top up with the filling loop |
| Lockout in freezing weather, gurgling from the boiler | Frozen condensate pipe | The outside section of the plastic condensate pipe |
| Yellow or orange flame, soot, pilot or burner problems | Poor combustion | Nothing: stop using it and call an engineer |
| Smell of gas near the boiler, tank or cylinders | Leak | Follow the gas smell steps below |
Fault codes you may see
An empty tank or a closed valve shows up as an ignition failure, because the boiler cannot light with no gas. The code depends on the make: Vaillant F28, Worcester Bosch EA 227, Ideal L2 and Baxi E133 are common examples. The same codes appear on mains gas boilers for other reasons, such as a faulty ignition electrode or gas valve, so on an LPG boiler check the supply first and the boiler second.
Running out of gas
A bulk tank running dry is one of the most common reasons an LPG boiler stops. Tanks have a gauge, and many suppliers offer a remote level monitor or automatic deliveries. With cylinders, an automatic changeover valve swaps to the second cylinder when the first empties, and many show an indicator when the cylinder in use is empty. If a bulk tank has run completely dry, contact your supplier before you try to relight anything.
How to reset an LPG boiler safely
You reset an LPG boiler with its reset button or setting, like a mains gas model, but check the gas supply first. Resetting with no gas available only adds more lockouts.
- Check you cannot smell gas. If you can, do not reset. Follow the steps in the warning below.
- Check the supply. Look at the tank gauge or cylinder indicator, and make sure any valves you use are open. If you have an LPG hob, see if it lights.
- Check the water pressure. If it is below about 1 bar, top it up first.
- In freezing weather, check the condensate pipe where it runs outside.
- Press reset once, following your user manual, and give the boiler a few minutes to try to light.
- If it locks out again, stop. Note the code and call an engineer.
Our guide on how to reset a boiler shows where the reset button is on the main brands and why repeated resets can leave the boiler locked out for longer.
Write the fault code down before you reset. The display usually clears after a reset, and the code is the most useful thing you can give the engineer.
Servicing and upkeep
An LPG boiler should be serviced once a year by a Gas Safe registered engineer qualified for LPG, which is also Gas Safe Register's general advice for any gas appliance. The service is much the same as for a mains gas boiler, but the engineer checks the working gas pressure and combustion against the LPG figures in the manufacturer's manual. If you rent your home, your landlord must arrange a gas safety check on LPG appliances every 12 months, just as for mains gas.
Between services:
- Keep the tank or cylinders clear of anything that can burn, with a clear route for deliveries.
- Look after the regulator. On cylinders, Liquid Gas UK advises replacing a regulator that shows signs of wear, leakage or damage, and replacing old regulators every 10 years. If you rent a bulk tank, check your agreement for who maintains the regulator and the pipe to the house.
- Keep air vents and the flue clear. Never block a vent fitted for the boiler.
- Watch the pressure gauge. A boiler that needs frequent topping up has a leak or fault.
LPG safety: gas smells and carbon monoxide
LPG is heavier than air, so a leak does not rise and disperse like mains gas. It sinks and can collect in drains, gullies and cellars, which is why the advice for an LPG leak is slightly different.
If you smell gas, turn off the supply at the tank or the valves on top of the cylinders, and at the emergency control valve if you have one, if it is safe to do so. Do not use light switches, electrical devices or naked flames, and do not smoke. Open doors and windows, particularly at low level, and get everyone outside. Call your LPG supplier's emergency number, which is usually on the tank, a cylinder label or your bill. Keep people away from cellars, drains and low areas.
Our guide to what to do if you smell gas near a boiler covers the steps in more detail.
An LPG boiler can produce carbon monoxide if it is burning poorly or the flue is damaged, just like a mains gas boiler. Gas Safe Register lists the warning signs: a floppy yellow flame instead of a crisp blue one, black or brown marks on or around the appliance, a pilot light that keeps going out and more condensation than usual on the windows. The NHS lists headaches, dizziness, feeling sick, tiredness, confusion and shortness of breath among the symptoms of carbon monoxide poisoning. Fit an audible carbon monoxide alarm and test it regularly.
Who can work on an LPG boiler?
Only a Gas Safe registered engineer whose registration covers LPG can service or repair an LPG boiler. Gas Safe ID cards list the types of gas work each engineer is qualified for on the back, so check that the card shows LPG as well as boilers, or search the engineer's number on the Gas Safe Register. Oil engineers registered with OFTEC do not cover LPG unless they also hold Gas Safe registration for it.
The same applies to converting a boiler between natural gas and LPG, which is only possible with the manufacturer's kit or procedure.
Running and replacement costs
LPG is usually more expensive to run than heating oil. The Energy Saving Trust describes LPG as the most expensive of mains gas, oil and LPG, although it produces less carbon dioxide than oil. LPG is not covered by the Ofgem price cap, so the price depends on your supplier and contract.
If your boiler does need replacing, Checkatrade gives a typical total of about £2,500 to £3,500 for an LPG boiler supplied and fitted. Prices vary by region, boiler and how much pipework or flue work is needed. For an older boiler with a costly fault, our guide to whether to repair or replace a boiler helps you decide.
When to call an engineer
Call a Gas Safe registered engineer with LPG on their card if:
- the boiler locks out again after one reset and you have confirmed there is gas in the tank or cylinder;
- it keeps losing water pressure or is leaking;
- you see a yellow flame, soot or scorch marks, or your carbon monoxide alarm sounds;
- a fault code points to a part such as the fan, gas valve, pump or circuit board.
Never remove the boiler case or work on the regulator or tank fittings yourself.