BRITISH GAS is offering a year’s free gas if you buy one of its boilers, potentially saving you up to £800.

But analysts warn that it could prove to be a false economy, as British Gas is one of the most expensive firms from which to buy and install a boiler.

To qualify for the free energy deal you need to be a British Gas customer who uses the firm to supply your gas. The offer doesn’t apply to electricity.

If you are not with British Gas, it will pay you £305.81 in cash after the boiler installation.

British Gas customers can also chose this option if they would prefer money upfront. In all cases, you need to order a boiler from the firm before October 27.

British Gas boiler installations cost upwards of £2,000, but you can get them much cheaper elsewhere.

The cost of installing and buying a boiler on the high street starts at about £1,500 according to comparison firm Uswitch – at least £500 less.

Tim Wolfenden, energy expert at Uswitch, said: “There is quite a large mark-up on the sale and installation of British Gas boilers. You might be able to make equivalent savings simply finding a cheaper deal.”

If you opt for the free energy deal you will have to wait for 12 months before you see any saving, during which time you will still be billed by British Gas.

The amount of money you save will be worked out by measuring your usage for 12 months after the boiler has been installed. A reading will be taken after installation and at the end of the 12-month period, and you will be paid back how much you use based on your tariff. This means you will get more if you are on an expensive deal.

The average British Gas bill is £509 a year for gas, based on a monthly direct-debit tariff, but British Gas will pay up to a maximum of £800. The scheme includes free maintenance for the boiler and your central heating system.

If you leave before the 12 months you will still receive the £305.81 lump sum.

Mark Todd of Energyhelpline, an analyst, said: “Take up the deal if it makes sense, but switch once the free gas is over.” The energy firm Utilita has become the first supplier to tie new customers into 12-month contracts.

The move comes less than a month after the energy regulator Ofgem axed the current rules that allow customers to switch suppliers after 28 days without a penalty.

Utilita customers who decide to leave before the 12 months will forfeit a 7.5% discount that they can carry over to cut the following year’s bill.