Chimney Fans for Fireplaces: When They Help

How a draft inducer works

A chimney fan, also called a draft inducer or exhaust fan, is a motor-driven fan housed in a metal unit that sits on top of the flue in place of a cap. It is wired to a speed control inside the house, usually near the fireplace. When you light a fire you switch it on, and it pulls smoke up the flue instead of waiting for the chimney to warm and draw by itself.

Because the fan does the pulling, the flue runs under steady negative pressure from top to bottom. That makes a fireplace usable on a chimney that has never drawn well, and it lets you turn the speed down once the fire and flue are hot. It is a mechanical answer to a design problem, so it makes sense only when design is the problem.

Fans are also fitted to some heating appliance flues, where they are wired to the burner controls so the appliance cannot fire unless the fan is proven to be running. On a fireplace, you work the switch yourself.

Problems a fan can solve

Fans are used where the cause of poor draft cannot be fixed with masonry work at a reasonable effort. In older neighbourhoods across New York, New Jersey and Philadelphia, the same few situations come up again and again.

In each case the flue itself is sound; it simply cannot produce enough pull for the fire below it.

• A chimney too short for the roofline, where raising it is impractical
• A rowhouse chimney sitting below a taller building or addition next door
• Swirling wind at the top from nearby buildings or tall trees
• A fireplace opening oversized for its flue, where the opening cannot be reduced
• Two fireplaces on different floors sharing one stack, where the lower one struggles

Fix these before considering a fan

Many smoky fireplaces have simpler causes, and a fan on top would only hide them. The flue may be partly blocked by a nest, fallen tile or heavy creosote. The damper may not open all the way. The smoke chamber above the damper may be rough or stepped, slowing the smoke where it should speed up. The wood may be wet.

A tight house is another common cause. Kitchen hoods, dryers and bathroom fans pull air out of the house, and the fireplace becomes the easiest place to pull it back in. If cracking a nearby window stops the smoking, the house needs a supply of makeup air, not a fan on the chimney. Once these are dealt with, many fireplaces draw fine on their own.

Most of these cost less to put right than a fan costs to buy and install, one more reason to rule them out first.

What a fan cannot do

A fan does not make a damaged flue safe. Through a cracked tile or an open joint, heat still reaches the brick and the wood framing around the chimney, whatever the fan is doing. If the fan stops, the damaged flue is back to spilling smoke and gases. Repairs to the flue come first.

Nor does it suit every flue. A fan on a chimney with no liner, or with a liner that does not fit the fireplace, treats the symptom and leaves the cause. And it does not replace routine care: the yearly inspection NFPA 211 sets for every chimney, with sweeping as deposits accumulate, applies fan or no fan.

How the draft is tested first

A fan recommendation should rest on a measured diagnosis, not on one smoky evening. After the flue is checked and swept, the technician watches how the fireplace draws under different conditions, often with a small smoke source or a draft gauge held at the opening, and changes one thing at a time to see what moves the result.

• With the damper fully open, then checked for anything restricting it
• With the kitchen hood, dryer and bath fans running, then switched off
• With a nearby window cracked open, then closed
• Cold at startup, and again once the flue has warmed
• On a calm day and, where possible, with the wind from the usual troublesome side

Choosing and fitting the right fan

Fans used on wood-burning fireplaces have to be made and listed for that heat and that fuel. Some models are made for open fireplaces only, others for stoves or for heating appliance flues, and matching the fan to the appliance below is part of the job. The unit is sized to the flue and mounted on a sound flue top, so a cracked crown or loose top tile is repaired before the fan goes on.

Installation follows the fan maker's instructions, including how the unit is fastened, how it hinges or lifts off for cleaning, and how the power cable runs from the chimney top to the switch. The electrical work should be done by a licensed electrician to local code.

Living with a chimney fan

Power is the obvious limit. If the electricity goes out during a winter storm, the fan stops, and a fireplace that depends on it will smoke. Avoid starting a fire when an outage is likely, and keep working CO alarms on each floor whatever the fireplace setup.

Most of the motor noise stays outside at the chimney top, though some hum can carry down the flue at full speed; the speed control lets you turn it down once the fire is drawing. Soot and creosote collect on the fan as well as in the flue, so the unit is opened or lifted at each sweep and cleaned with it.

Fan, taller chimney or something else

A fan is one of several ways to fix weak draft, and the cause decides which fits. A chimney below a neighbour's new addition might be extended instead. An oversized opening can sometimes be reduced with a raised hearth or a metal smoke guard. Wind trouble may respond to a different style of cap. Glass doors can help a fireplace that smokes only at the end of a fire.

Where the masonry fix would be large, risky or not allowed, as with a party-wall chimney that cannot be raised without the neighbour's agreement, a fan is often the practical choice.

What drives the installation cost

The fan itself is one part. The rest depends on how hard the chimney is to reach, the size and shape of the flue, whether the crown and flue top need repair before the fan can be mounted, and how far the wiring has to run from the roof to a switch inside.

Before recommending a fan, we test the draft and check the flue, the damper and the smoke chamber, then write up both the fan and any repair it needs in a free written estimate.