These work by having a slight slope to the back of the fire which causes a small build up of heat, just above the burner, while some of this heat allows a to enter and heat up the room, the vast majority is pulled up the flue. This is all to do with really sciency physics as the pressure builds up from the expanded air (Heated air) causing it to rise in the flue, due to the pressure differential from the outside to inside the warm room, this creates suction up the flue and continues to draw (suck) more gas and a lot of heat from the room. Most of these fires will be a maximum of upto 40% efficient, and the only reason certain models provide a output of upto 4.5kW is that they inject more gas in to burn. Most will stick to 6.9kW input as a vent in the room is required above this figure, but should you find a High Output Open fronted fire, you will need to place in an 100cm3 air vent as a precautionary measure to safety. This means when the fire is off, you will have an opening in the wall one side and a long open chimney to allow a very nice draft.
e.g, Standard Inset Gas fire with a 30% efficiency – 4.52p per/kWh (Swalec 26th Jan 2010 – lowest possible rating available) and a 3.5kW output means it will be inputting 11.6kW into the product meaning this will cost 52p per hour while you are only actually receiving 15.8p of heat! That’s wastage of over 36 pence per hour. Now this may not sound a lot but it does add up significantly throughout the year average household using their fire for 4 hours a day (4 x 36p = 144p/per day wastage), 6.2 days per week (144p x 6.2 = £8.93 per day wastage), 20 weeks of the year (£8.93 x 20weeks = £178.56 per year on average) This by no means is the worst it can be, this is just average for a 30% efficiency there are lower ratings still available on the market, some with NO HEAT OUPUT giving a 0% efficacy!! Decorative Gas Fires
These are a small substrain of Standard Gas fires, but these items provide so little output that the manufacturers feel it is not a selling feature so do not pay Gastech to measure the efficiency. These fires usually have the heat output of a light bulb and an efficiency of around 1-2%! Usually large open fronted hole in the wall fires. But the main use for them is in gas baskets and cast iron trays. These items do not have the space available to create a higher efficiency. They have large fixed openings which usually require class one flues, they then use a small gas burner, all these added together means one big waste of cold hard cash. Watch out for them!