The End of the Open Fire?
As our homes become increasingly airtight, does it make any sense to include an open fire in your new home? Possibly, but there are other, better alternatives, argues Tim Pullen
Could our modern, airtight homes be the death knell of the open fire? Tim Pullen explains why this could be the case, and offers some pleasing alternatives
Many older homes have a fireplace or two and increasing numbers of self-builders are including one in the design for their new home. The crackling heart-warming allure of a log fire, with its dancing flames and familiar scent, is still a strong one — as is the idea of a wellinsulated, thermally efficient home.
Obviously, the point of insulation is to retain the heat in the house. Heat escapes through the fabric of the building, so we insulate. But heat also gets carried away on the air leaving the building, and by cold air entering. Around 30 years ago, the Building Regulations started imposing limits on the amount of air that can leave the building — the permeability or airtightness of the building changed from a minimum to a maximum, and that maximum has been decreasing ever since.
Any form of combustion – gas boiler, oil boiler, woodburning stove or open fire – needs air as well as the fuel being combusted. In most cases, the amount of air needed is relatively small and can be supplied either directly from an external air supply or from the room, without undue impact on overall thermal efficiency. The exception is the open fire, where the volume of air needed has to meet the volume of air extracted by the chimney — and that can be a lot.
The effect of an open fire
In a room 5m x 5m x 2.4m, insulated to Building Regs’ standard, including airtightness, the peak heat demand for that room will be a little over 1.2kW. Add an open fireplace and the airtightness in that room plummets and the peak heat demand rises to 2.9kW. You might think that the fire will make up that shortfall itself — and when it is burning well, it will. An average size fire that is burning well will draw around 5m3 of air per minute. The room holds 60m3 of air, so it can supply the fire for just 12 minutes. Thereafter it is drawing air from the rest of the house, none of which is benefiting from the roaring log fire.
However, the chimney will draw a similar amount of air when the fire is not lit, and the lack of airtightness in that room affects the thermal efficiency of the whole house. The house has to be able to supply that 5m3 per minute or the chimney will stop drawing properly and smoke will enter into the house.
What’s more, it can have a financial impact. It is said that a fireplace has the same effect on the heating demands of the house as leaving a window open. A house with 200m2 floor area and 2.4m ceiling height, with insulation and airtightness to current Building Regs, but with a single open
fireplace, will have a peak heat load of 17.5kW and a space heating energy demand of 17,700kWh per year. Remove the fireplace and those figures drop to 10kW peak and 10,500kWh per year. That’s a difference worth about £450 per year. A good proportion of that loss could be saved with assiduous use of a flue-blocker when the fire is not lit. There are problems with these – such as how to insert it when the fire is still lit and you want to go to bed – but they do work otherwise.
An open fire is attractive and it can be lived with in a modern, well-insulated house and maybe the extra running cost is affordable. However, it is difficult to conceive that this is anything but the nailing together of two incompatible ideas.
Alternatives To The open fire
Woodburning stoves many people who want an open fire say a woodburning stove is ‘just not the same’. This is true in all respects except for the smell, the crackle, the look of dancing flames and, of course, its efficiency. A woodburning stove is likely to be better than 65% efficient (an open fire is 30% at best) and typically (but not always) being made of heavy-duty steel, it is a radiator as well.
You can specify models which draw their air supply directly from the exterior, thus maintaining the home’s overall airtightness. “some stoves may be additionally sealed, or ‘leak sealed’, to vastly reduce leakage essential for air pressure tests for modern, airtight houses,” says John nightingale from stoves Online. Bioethanol fires Bioethanol is an alcohol made by the fermentation of plant sugars. It is, notionally at least, a renewable fuel and is now available to fuel domestic stoves and open fires. These machines are available as both stoves and open fires, generally produce 2kW to 3kW heat and do not need a chimney or flue. The fire or stove might cost £500 to £1,500 but you will save £4,000 to £5,000 on the chimney and fireplace. These give the look of a living fire with the convenience of being able to turn it on and off. They do not need an air supply (other than that in the room) so have no impact on the airtightness or thermal efficiency of the house.
convection boxes If nothing but an open fire will do, think about a convection box, such as the Riva Open convector from stovax; this is a modern steel version of the victorian curved stone fire-backs. These conduct the cool air through the fire ensuring that the maximum heat possible is pushed into the room. more interestingly they have a damper control that can be used to limit air flow to control the heat output, which also serves to reduce draughts when the fire is not lit.
Further Contacts
Bioethanol Fireplace bioethanol-fireplace.co.uk Charnwood charnwood.com
Chesneys chesneys.co.uk Imagine Fires imaginefires.co.uk Stovax stovax.com
Stoves Online stovesonline.co.uk