Home Ownership Fireplace inserts are implicated as hazards far more frequently than wood stoves or fireplaces. Inserts usually cause problems when they're installed incorrectly, operated carelessly and maintained poorly. Design flaws in inserts can scuttle the best intentioned wood-heat husbandry. The worst offenders are models with no direct connection to the chimney flue. They vent smoke and gases directly into the space around them. If your chimney has an excellent updraft, the smoke goes straight up and out. But if it doesn't, the smoke sometimes takes the line of least resistance — through your living room.

In an apparent attempt to ensure good updraft, standards suggest that the flue opening on a fireplace insert be at least 20 by 20 cm and not too large. Too-large flues are especially hazardous in combination with airtight inserts that aren't positively attached to the flue.

Without a big flue and its massive updraft, fireplaces would smoke. But airtight inserts and wood stoves don't need a large flue. However, their air-starved burn sometimes produces less complete combustion and more creosote. When the hot, volatile smoke hits a large flue, it cools and deposits as creosote. Regular cleaning will reduce the build-up of creosote, but there is always the chance that a hot fire will melt any that remains. If your insert is not connected to the flue with a stovepipe and flue coverplate, the creosote will drip down onto the top of the insert. The next thing you know, you have two fires — one inside your insert and one on top.




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December 4, 2008