Chimney Draft Problems; Excessive Updraft

Q: I am told that I have the problem of too much draft. I recently had a new woodstove installed with a stainless steel flue. When I fill the stove for a long burn, I have trouble reducing the temperature – it shoots to over 800 and completely shutting down the air supply doesn’t cool it until much of the wood is burnt. What are your opinions and possible solutions?

A: First, we should note that non-catalytic approved woodstoves like your stove reburn the exhaust gases in a secondary burn chamber at the top of the firebox at approximately 1200 degrees, so stovetop temperatures of 800 degrees are to be expected from a freshly loaded fire.

Second, it is a fact that today’s approved woodstoves aren’t as adjustable as many of the pre-airtights. In years past, we’ve had experience with woodstoves that had air intake controls which could be closed all the way, resulting in a smoldering fire, excessive creosote formation in the chimney, and a thick plume of black smoke pollution pouring out of the chimney.

Approved woodstoves are designed so the fire can’t be smoldered: even when the air control is closed as far as it will go, the fire still gets enough air to burn efficiently.

That said, some approved woodstoves are less responsive than others, and your complaint is one we hear frequently from woodstove owners with the same stove that you have:- lack of controllability, resulting in hot, short duration burns and the need for frequent refueling.

We believe the problem most likely arises from this design’s need for an air control that remains particularly wide open even when turned down as far as it will go, to keep the fire burning briskly enough so the stove will meet emissions requirements.

When you’ve got a chimney flue with an extra-strong updraft attached to a stove with an air control that doesn’t turn down far enough to compensate, it can be very hard to control your rate of burn.

If your stove is raging out of control, and you’re unwilling to trade it in on a more controllable model, the only remaining avenue of relief is to attempt to reduce your chimney updraft.

There are two techniques we know of to reduce chimney updraft, and both involve some attendant risk. Barometric dampers, often used in conjunction with oil-burning furnaces, are installed in the stovepipe and have an adjustable, weighted flapper that is drawn inward by the updraft, allowing room air to enter the pipe to reduce negative pressure at the stove in much the same way as the thumb slide on a vacuum cleaner hose reduces suction power below.

The problem with barometric dampers is, the reduced updraft might adversely affect the secondary burn, reducing efficiency and increasing emissions. Further, the intrusion of room-temperature air into the flue cools the flue gases, causing increased creosote formation. Finally, if the increased formation of creosote leads to a chimney fire, the resulting extreme updraft will pull the barometric damper WIDE open, and could allow the chimney fire to rage out of control.

The other technique is the manual stovepipe damper. A manual damper is a metal disc inside the stovepipe, attached to a handle on the outside. Manual dampers reduce air flow through the stove by mechanically blocking the flue.

By turning the handle, the disc may be oriented parallel with the flow of exhaust (no resistance), perpendicular to the flow (maximum resistance), or at any angle in between. Manual dampers also cause problems: reducing the airflow through the stove can adversely affect the secondary burn immediately, and will almost certainly do so later on, as the flue cools and the updraft is reduced. The price paid is reduced efficiency, increased emissions and excessive creosote formation in the flue.

If you attempt to reduce your chimney updraft with either type of damper, you do so at your own risk. None of the woodstove manufacturers we know of recommend the use of stovepipe dampers except in stoves designed for their use (Pot Belly stoves and the old visor fires for example) and some specifically forbid their use.