Choose a Fireplace for Beauty and Warmth
(Page 3 of 6)
October/November 2003
By John Gulland
2. Open fireplaces demand more air than modern houses leak. To adjust for this situation, either a window must be opened or some other allowance made for the fireplace to get the air it needs to work. The little 4- or 6-inch-diameter ducts often installed to provide outside air to fireplaces for combustion are far too small to supply even a fraction of what is nec essary. So without that open window, even a fireplace with an outside air duct will be smoky and fussy to operate, as many users can attest. But obviously, no practical heating system in a cold climate could depend on an open window for proper functioning.
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3.Unless conditions are perfect, smoke spills into the room. The absence of a barrier between the fire and the room means that smoke can come out of the fireplace opening into the room, a problem called "spillage." The small currents of air from people moving around in the room and pressure changes at the top of the chimney caused by wind are enough to trigger such spills. Many people are unaware that the pleasant, woodsy smell of fireplace smoke is a health hazard and that even slight smoke spillage should be unacceptable inside a house.
STRATEGIES FOR IMPROVEMENT
Add glass doors (to reduce excess air flow) to all the open fireplaces above and you can increase their net efficiency to some extent. In the case of masonry fireplaces, glass doors allow some of the heat from the fire to soak into the masonry for slow release into the room. In the case of fireplaces with air circulation chambers, glass doors can boost internal temperatures, increasing heat transfer to the circulating air.
The majority of glass fireplace doors, however, are not tight-fitting, so they reduce air flow only by about half. The tempered-glass panels also block most of the heat from getting into the room and they are prone to breakage when exposed to intense or uneven heating, making them unsuitable if you want large, continuous fires to produce heat for your home.
So, while tempered-glass doors can boost efficiency by reducing excess airflow, they also reduce radiant heating efficiency and that is why they don't tend to produce a large net increase in fireplace efficiency.
Some fireplace doors use a ceramic glass like that used in woodstoves. It's a type of glass that permits much of the radiant heat to pass through and has the added benefit of far greater heat resistance than tempered glass. The drawback is that it is significantly more expensive.
And even if a glass door set were gasketed, producing a tighter seal to reduce excess air, and had ceramic panels instead of tempered glass, to allow radiant heat to pass through, the panels still would get dirty. The tighter they fit, the quicker they'd soot up, too. Having to clean the glass after every evening's use would be bad enough, but you'd find a thick haze would form long before your evening had passed, spoiling your view of the fire.
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