Wood Furnace Chimneys: Dos and Don’ts

The fire is undoubtedly the most important part of a wood-burning furnace. It’s what provides the heat that warms the home. But, to paraphrase an old adage, “Where there’s fire, there’s smoke.” All of that smoke needs to be vented safely — and that’s where a chimney comes in.

Chimneys seem simple. After all, they’re just a pipe that carries smoke out of the home. But between temperature variables, pressure differences, length requirements, metal types, elbows, and more, there’s a lot more to chimneys than meets the eye.

Because of all these complex factors, it can be hard to know how to build a safe chimney for a wood heating appliance. But we want to help you unravel the mysteries.

At HY-C, our Fire Chief furnace division has been manufacturing forced-air wood-burning furnaces for over 40 years. From the chimney connection point all the way to the chimney cap, we know what it takes to build a proper furnace chimney.

In this guide, we’re going to cover the dos and don’ts of building a chimney for your wood-burning furnace. We’ll outline important considerations for:

1. The chimney connection from the furnace
2. The chimney itself
3. The chimney relative to the roof

By the time you’re finished here, you’ll have an informed understanding of wood furnace chimneys to ensure you can design your system for safety and efficiency.

The Chimney Connection from the Furnace

The furnace and its chimney are two separate systems that work together to vent smoke out of your house. Bridging the gap between those two systems is the chimney connection, a length of pipe connected to a port on the back of the furnace (which usually has a 6-inch diameter).

This pipe must follow several rules and standards. First and foremost, it should never be connected to a chimney that’s servicing another appliance. Aside from that, there are several other rules it must follow, and we’ll cover them one by one here.

Chimney Connection Pipe: Metal Types

When selecting the type of metal for your connector pipe, you have two options:

1. 24-gauge single-wall black stove pipe
2. Stainless steel pipe

It’s important to note that you should NEVER use galvanized steel pipe for your furnace chimney connection. This is because galvanized steel is coated with zinc. When the furnace causes this zinc to heat up, it burns away, turning into a gas.

This zinc gas can potentially spread throughout your home, causing what’s called metal fume fever. Breathing these fumes can be very dangerous, causing a myriad of symptoms from a fever and chills to nausea, vomiting, and fatigue.

Chimney Connection Pipe: Clearances to Combustibles

All wood heating appliances have what are called clearances to combustibles — distances from which parts of the furnace must be kept from flammable materials. These differ from furnace to furnace and even from certain sides and parts of the same furnace.

As an example, on our Fire Chief FC1000E furnace, the connector pipe’s clearances to combustibles are 18 inches. This means that you should keep combustible material (like paper, wood fuel, wooden structure supports, drywall, etc.) at least 18 inches from the pipe at all times.

Chimney Connection Pipe: Rise and Run

The length of the connection pipe itself and the angle at which it’s connected to the chimney are important installation factors. Getting them right ensures both efficient operation of your furnace and safety from smoke and heat.

As far as the length of the pipe (i.e., its “run”), it must not exceed 5 feet. If the connector pipe is longer than this, it can cause flue gasses to move too slowly, resulting in smoke rollback into the home. A long connector pipe can also cause flue gasses to cool too quickly, resulting in creosote.

As for the angle of the pipe (i.e., its “rise”), the pipe must maintain 2 inches of rise for every 1 foot of run. Imagine your connector pipe ends up being 3 feet long. This means that the end of the pipe that connects to the chimney should sit 6 inches higher than the end that connects to the furnace.

Maintaining a proper rise ensures a good draft. If the pipe were straight and had no rise, the gasses from the fire would quickly stagnate in the pipe and roll back into the home. Rise helps facilitate a natural draft, ensuring all flue gasses flow forward into the chimney at an acceptable rate.

Chimney Connection Pipe: Elbows

When connecting a furnace to a chimney, the orientation of the furnace and the layout of your home may necessitate the use of elbows to allow everything to fit properly. If you do need to install elbows, though, it’s important never to install more than 2.

Elbows mean that smoke and flue gasses need to change direction. If they change direction too often (i.e., if you use too many elbows), the gasses will slow, potentially creating a backdraft. Best practice indicates that two elbows — but no more — are fine to use.

Also, if you do need to use elbows to connect your furnace to your chimney, using 2 45-degree elbows is preferable to using 1 90-degree elbow.

The Wood Furnace Chimney

Now that we’ve covered the connection pipe from the furnace to the chimney, let’s talk about the chimney itself. When construction a chimney, there are three options from which to choose:

1. A clay-lined masonry chimney
2. A masonry chimney with a 6-inch stainless steel liner
3. Class A chimney pipe (rated to 2,100 °F)

Clay-lined masonry flues are common in older homes. Provided that they’re cleaned, inspected, and okayed by a chimney professional, they are suitable to use as a chimney for a wood-burning furnace.

As a wood furnace manufacturer, though, we’d recommend having a masonry chimney lined with a 6-inch stainless steel liner for use with a furnace. Lined chimneys are better suited to furnaces. They’re less likely to experience damage or develop creosote.

If you don’t have a preexisting masonry chimney in your home, you can always opt for Class A chimney pipe. This type of chimney pipe is manufactured specifically with wood heating appliances in mind, so it’s always a good bet.

Even with these three options available, there are a couple of important things to keep in mind regarding a furnace chimney, namely flue diameter and flue draft.

Wood Furnace Chimney Diameter

Whichever type of chimney you connect your furnace to, it’s vital that its diameter be no narrower than 6 inches and no wider than 12 inches.

If the chimney pipe is too narrow, it will inhibit a proper draft. This can lead to backdrafts and smoke spillage back into the furnace (and out into the home). A chimney with a poor draft will also develop creosote more quickly, increasing the likelihood of a chimney fire.

On the other hand, if the chimney pipe is too wide, it can cause overdrafts. This phenomenon can potentially supply too much oxygen to the fire, causing it to burn very hot and potentially warp or damage the furnace.

For these reasons, it’s important to use a properly sized chimney flue. Also, to prevent exacerbating any of the problems listed above, the diameter of the chimney stack should stay the same throughout the length of the chimney.

Wood Furnace Chimney Draft

By now, it should be clear that a furnace chimney should maintain a good draft. That means that air, smoke, and flue gasses need to move through the chimney at an acceptable rate. Specifically, though, the flue draft should not exceed 0.08 water column inches (WCI) of static pressure.

Static pressure is essentially a measurement of air resistance in a system. Static pressure is measured in water column inches. For reference, 1 water column inch is the amount of pressure required to raise a column of water by 1 inch. The higher the measurement, the more static pressure (i.e., airflow resistance) there is in the chimney.

For this reason, it’s important to keep the static pressure in the chimney as low as possible. You can do so by following all of the recommendations listed in this guide. Also, you can ascertain the static pressure measurement (in water column inches) using a draft gauge.

Wood Furnace Chimney: Through the Roof

Finally, when building a wood-burning furnace’s chimney through your home’s roof, there are some vital specifications that need to be followed to ensure a proper draft.

For starters, the chimney must extend at least three feet higher than the highest point of the roof through which it passes. This not only helps to improve the efficiency of the chimney’s draft, but it also prevents downdrafts (i.e., when wind pushes smoke and other gasses back down the chimney).

The chimney must also be at least 2 feet higher than any part of the building within a horizontal distance of 10 feet. Maintaining this distance also helps to improve draft and prevent downdrafts. It assists in preventing blockages caused by ice, snow, and debris, too.

Lastly, the furnace chimney’s cap should be properly sized to fit the chimney flue. It should be installed in such a way that keeps out rain and nuisance wildlife while also maintaining proper airflow and preventing downdrafts.

How Do You Maintain Your Wood Furnace’s Chimney?

There they are: the dos and don’ts of installing a chimney for your wood-burning furnace. After the chimney is built, though, how do you maintain it to ensure it stays properly operational and creosote-free?

One option, of course, is hiring a professional chimney sweep. Cleaning and maintaining chimneys is their job. They’ll be able to point out any problems and make any repairs that need to be made.

On the other hand, you can also use a DIY chimney cleaning kit. These drill-powered kits allow you to do the job of a chimney sweep yourself, cleaning and maintaining your own chimney on your own schedule.

We actually manufacture one of these kits; it’s called the SootEater. The guide below has more information on how it works and how to use it to help keep your brand-new furnace chimney clean, safe, and operational.