Which gas fire

It depends on the size of the room mainly...you wouldn't want too much heat in a small room and vice versa...as a guide 3kw would be suitable for an average sized room with no other heat source

 

Decorative fuel effect gas fires are not very efficient. Typically about 40%. So on a 10KW input fire only 4KW max heats the room and 6KW goes straight up the chimney.

Another consideration is that no additional ventilation is required normally up to 7KW input. Bigger than this and an air brick to the outside will be needed. Manufacturers instructions will detail what ventilation, if any, is required.

 

The heat output kW (1000 Watts) rating of a gas fire is the same as for an electric fire (e.g. a gas fire with a heat output of 3 kW is equivalent to a three bar electric fire).

Decorative Fuel Effect (DFE) fires are, as Pugley says, not very efficient. I've encountered some that were as low as 20% efficient. A DFE fire sits in the bottom of an existing fireplace, and looks like a grate with artificial coals on top. Normally, this type of fire requires purpose provided ventilation with a free area of at least 100 cm² (even if its heat input is less than 7 kW - but check the manufacturer's instructions).

Inset Live Fuel Effect (ILFE) fires have efficiencies from about 45% to 67%. This type of fire is built into a metal case, with a bezel that seals against the fireplace/surround and the more efficient ones have a hood at the top that is part of a convector. An ILFE fire does not normally require any additional ventilation unless, as Pugley said, its heat input is greater than 7 kW.

The luminous flames of fuel effect fires means that complete combustion of the gas is not achieved. A by-product of this incomplete combustion is carbon monoxide, which is highly poisonous. In recent years, flueless ILFE fires have become available. A catalytic converter is used to convert the carbon monoxide into carbon dioxide, but the heat input of these fires is usually limited to only 3.5 kW. Without a flue, they're 100% efficient, but would require permanent ventilation in any room small enough to be heated by just a 3.5 kW gas fire. The ventilation is not only required to provide fresh air for combustion, but also to remove the products of the combustion process - water vapour and carbon dioxide.