Ventilation for Fires

It' another "Ask Andy" for the Cambridge Carbon Footprint newsletter
Question:
Our builders are insisting that we install a ventilator in the living room, as we have an open fire. We don’t want more draughts. What should we do?
Answer:
Your builders are correct. Requirements for ventilation are set out in the Approved Document J “Combustion appliances and fuel storage systems” of the Building Regulations and applies to alterations. It says you need a "permanent open air vent" and that the vent should be at least half the area of the 'throat' of the fireplace chimney. That would be an air brick in the wall or a grille in the floor. It’s a lot of ventilation, but would be justified if you have gone to a lot of effort to seal your windows, doors and floor. I have done this myself and found the room filling with smoke when the fire was lit, as the chimney would not draw properly from the sealed room!
Open fires can look lovely and become the central attraction for the home. But they have two problems. When the fire is not lit, the flue will continue to suck valuable heat from your home, and cause draughts in the room. This is easily solved with a ‘chimney balloon’ or an old pillow in a plastic bag. At the same time, you might place some temporary obstruction over the fixed vent.
When the fire is lit, the amount of air drawn up the chimney is huge. A modern fire place with a properly deigned ‘throat’ at the base of the chimney will control this, but it will still draw much more air than is needed to ensure efficient burning of your wood. This will also cause a draught from the fixed vent, so the best place for it is in the floor near the fire.
If you replace the open fire with a stove, you remove the need for a ventilator and you still have a wonderful feature for your home. A stove with less than 5kw output does not require fixed ventilation, and when the fire is not lit you can close the vents and the room is well sealed. Some stoves have efficient “air washers” on the window, so you can see the logs burning. On ours, we leave the door open if we are wanting to enjoy the fire, but actually closing the door makes it easier to control and uses less wood.