If you have or plan to install a combustion appliance such as fireplace, a stove or a boiler within the house, specific provisions should be made for supplying combustion air to the appliance, in compliance with the requirements of the Building Regulations TGD Part J (Heat Producing Appliances). You might consider an enclosed appliance such as a stove or fire insert (fire box with glazed door) instead of an open fire as they are much more efficient at producing heat. The other problem with an open fireplace is the large amount of heat loss due to draughts up the chimney. If at all possible, ducting the combustion air supply to the appliance itself or at close proximity will avoid interfering with the efficient general ventilation of the house. This is particularly important if the house is airtight. If you are opting for a MHRV system, it is essential, as a matter of safety and efficiency, that combustion appliances, their supply air duct and their exhaust duct are sealed in an airtight manner from the room. In any case, you should consider installing a carbon monoxide detector in the room where you have combustion appliances.