In the past, fireplaces used to be an integral part of everyday home design, but not so (much) these days. It took many years of research to get fireplaces and flues to work efficiently after being beset with countless problems. It was realised that for a fire to burn effectively a constant air supply would be required to enter the room. A draught-proofed room starved the fire of oxygen preventing it from burning brightly and causing downdraughts and smoke filled rooms. It was also realised that the size of the fireplace aperture governed the rate at which the fire would draw air to the flue and being too small would cause it to draw at an excessive rate.
Eventually, most of these problems were ironed out, and the rising warm air in the flue would be replaced by oxygen entering the room at moderate rate. Air supply was then further adjusted via a damper, which increased or decreased the intake into the fireplace providing control over the burning rate of the fire as well as providing a constant air circulation in the process.
The absents of fireplaces and flues in the modern home goes hand in hand with poor air circulation, contributing to the condensation problem. In a home that has no fireplaces, the provisions provided for ventilation should be in use, or alternative methods introduced so as to provide it.
Homes with fireplaces, (which are blocked) can have the added problem of the air that is trapped within the flue, condensing, causing damp patches to appear on the chimney breast. A chimney stack that is not in use should be vented together with an adjustable air vent in the fireplace (or where it used to be). This will allow air to circulate within the flue, preventing condensation, as well as providing the much needed air changes within the room that it serves.
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