How to Use Log Burner Vents: Wood And Multi-Fuel Stove Air Controls Explained

The proper use of air vent controls is one of the most advanced skills a log burner or multi-fuel stove owner can master.

The basics are deceptively simple. More air means a hotter, faster burn. Less air means less heat but a longer heat output. In most situations, you’ll want the vents fully open when starting the fire and will want to lower them when the fire is started.

Easy, right? Wrong.

In practice, even experienced owners of wood and coal heating appliances often struggle with understanding how to use them to get the most efficient burn out of their kiln-dried firewood or wood briquettes.

The situation is made more difficult by the fact that every stove is different:

• Most stoves sold before 2022 are not Ecodesign compliant. Turning the vents all the way down will kill off the flames. Additionally, they may burn very inefficiently on lower settings, so you will need to open the vents a bit more than you think to use a long-burning fuel like 8-Hour Night Briquettes.

• Most new stoves purchased in the UK after 2022 are Ecodesign compliant. What does that mean? Better overall efficiency. The stove will work better on lower airflow settings, so you can take full advantage of your Fire Logs and Night Briquettes.

• Defra-approved stoves of a clearSkies Level 3 or above are a different beast entirely. Buying a stove like this is like driving a Rolls Royce. You don’t have to think about anything. It all just works. These stoves are super-efficient on all air vent settings. And even if you turn the vent all the way down, you’ll still get a good burn from 8-hour Night Briquettes, thanks to an integrated stopper that’s tailored to your stove.

To make things easier for new and experienced log burner owners alike, we’ve created this article. In close collaboration with wood stove expert Sune Nightingale (director of Firepower and Stoves Online), we’ve gathered all the information you’ll ever need on wood burner stove air vents.

So you’ve ever wondered whether your wood burner vents should be open or closed, how to adjust them for the best (and most efficient) heat output, or why your log burner keeps smoking, this ultimate expert guide is 100% for you. We’ll cover everything you need to know about wood burner vent controls. From how they work to why there are several of them to how to know if they’re functioning properly to optimising your fire for maximum efficiency.

Let’s dig in!

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Understanding Your Stove’s Air Vents

Most wood-burning and multi-fuel stoves on the UK market today will have three types of air vents. Understanding the way all three of them function is key to getting the most heat out of your wood (or coal) fire.

1. Primary Air Vent (First/Bottom Vent)

This vent is found at the bottom of the stove, usually sitting somewhere near the ash pan. Its primary function is to feed air under the fire, which makes it essential when lighting a multi-fuel stove when using coal. If you’re burning wood, this isn’t as important as wood typically burns from the top, not the bottom. Once your wood fuel is burning properly, it should be set to a minimal position. 

Importance for wood burning: Minimal

2. Secondary Air Vent (Second/Top/Airwash Vent)

This vent is typically located above the firebox or near the stove glass. It helps burn gases released by wood, making the combustion more efficient by giving you more heat and less smoke. It is this air vent that helps keep your stove glass clean, which is why it is often called the stove’s “airwash system”. This is the most important vent for wood burning as it is what controls how hot (and fast) your wood fuel will be burning.

Importance for wood burning: Extremely important

3. Tertiary Air (Pre-Set Vents)

Tertiary vents are those sometimes tiny holes you find at the back of the firebox. These provide a little bit of extra oxygen to help reburn smoke and gasses released from the combustion process. This allows to increase the efficiency of the burn and decrease energy waste. These are typically non-adjustable and start working automatically when the stove is hot enough for air circulation to occur naturally.

This being said it’s wise to check these air vents for blockage periodically and ensure that the air access is unobstructed. This will help your stove be as efficient as possible.

Importance for wood burning: Increase efficiency.

What Air Vents Are Most Important For Wood Burning?

The secondary air vent (aka top or airwash vent) is by far the most important for wood burning. Open it at the start of the combustion process to give your fire full access to air, then lower it to a desired setting to control the heat output of the fire. Tertiary (pre-set) vents are also important for increasing the efficiency of a stove, but these are unadjustable, so just keep them blockage-free. Primary air vents (aka bottom vents) are exceptionally important for burning coal in log burners but aren’t as important for burning wood.

How to Use Your Stove Vents (Step-by-Step Expert Guide)

Now that you understand what each vent in your stove does, it’s time to go through how to use them in a real-world wood-burning scenario to maximise efficiency.

Stove Vent Settings to Use When Lighting a Fire

When lighting a fire, you main goal is to give the flames as much fire as possible. In order to do this, you need both the primary (bottom) and secondary (top) air vents to be fully open. If the fire is still struggling to catch, you can even open your log burner door a little to help it get even more air.

Stove Vent Settings For a Freshly Started Fire

Air flow control settings for stoves that are actively burning differ depending on the type of fuel you burn. If you’re burning firewood or wood briquettes, you will want to close the primary (bottom) air vent and control the fire using the secondary (top) air vent. If you’re using coal or smokeless fuel, you will want to keep the primary (bottom) vent open. If you encounter overfiring issues when burning coal, adjust the burning intensity through the primary vent.

Stove Vent Settings For Maintaining a Steady Burn

Once your wood, coal, or smokeless fuel is burning nicely, the airflow controls should be used to control the fire. If your wood fire is burning too fast, you can slow it down by slightly closing the secondary (top) air vent. If it isn’t burning strong enough, you will want to open it more.

Using Stove Vents to Slow Down the Burn for a Longer Burn

There are situations when overall burn time is more important than momentary heat output. Such situations include warmer winter days, summer fires, and when using a long-burning wood heating fuel like 8-Hour Night Briquettes. Regardless of the situation, you will want to wait for your fuel to start burning nicely, then reduce the airflow controls. 

The degree to which this needs to be done varies from stove to stove. Defra-exempt stoves (clearSkies Level 3 and above) are “very good” at being turned down, so you can set them very close to the minimum setting. If you own a less efficient stove, try lowering it to a medium-low setting first and then adjust as you see fit. If you see smoke at any point, it means you’ve turned your stove down too much and need to open up the airflow.

Air Vent Controls When Adding New Logs to the Fire

If you’re about to add new oak logs or sawdust briquettes to an already burning fire, you can make the process effortless using air vent controls. First, open the secondary (top) vent fully for about a minute to bring extra oxygen to the fire (you may see the flames becoming brighter). Then, add your new fuel to the fire and wait for it to catch fire. Once it’s burning nicely, adjust the vents back to normal.

Common Air Vent Problems and How to Fix Them

Why does my log burner smoke when I open the door?

If your stove starts smoking when you open the glass door, then you likely have a problem with poor airflow or a cold chimney. Try opening the vents fully for about a minute before opening the door. If it solves the issue, then the problem is poor airflow, and it most likely can be solved by opening the vents more. Another cause could be that you haven't pre-heated your stove and chimney before adding your main wood fuel (the top-down fire-starting method solves this issue). If neither of these addresses your problem, you may want to check for blockage in your flue.

Why is my stove glass turning black?

Black stains on stove glass are typically indicative of a fire that isn’t burning hot enough, the use of wet fuel, or airflow controls that are set too low. This can be fixed via a combination of these tips: a) only burn quality Ready to Burn-certified kiln-dried firewood and wood briquettes, b) open the secondary vent more, c) running the stove extra hot (air vents fully open) to burn off the soot.

My fire keeps going out – what am I doing wrong?

If your wood fire keeps going out, then you’re either burning poor fuel (e.g. wet wood), you’re closing your air vents too soon (or too much), or your stove has ventilation problems (e.g. flue blockage). If you’re burning seasoned firewood, split a log in half and then use a wood moisture meter to check if it's dry enough (see our full article on wood moisture meters). Secondly, try to keep your secondary (top) vents open more. Thirdly, try waiting a bit longer before closing your primary (bottom) vents after you light a fire.

My chimney is producing a lot of smoke – what should I do?

In most cases, excessive smoke production is caused by poor combustion. This can be caused by overly low air flow settings on your secondary air vent (the top one). If this isn’t the problem, check the wood you’re burning with a wood moisture meter to ensure its of lower moisture level than 20%. Lastly, check if your chimney/flue is clean and that there’s no blockage.

UK Regulations You Need to Know

Smoke Control Areas & DEFRA-Approved Stoves

If you live in a smoke control area, then you can only install a DEFRA-approved wood-burning stove. This is the law. There are no ways around this. 

EcoDesign 2022 Regulations

Since 2022, all new stoves sold in the UK must be EcoDesign compliant. These stoves burn fuel more efficiently and produce less pollution. Stoves manufactured before 2022 may still be installed if they are not EcoDesign compliant, but there are a few of these left on the market, so this will only concern you if you’re purchasing a second-hand stove.

Defra’s “Ready to Burn” Scheme

Ready to Burn is Defra’s wood fuel quality assurance scheme. It ensures that the wood you burn is of a moisture level of 20% or lower. A separate Ready to Burn scheme exists for smokeless fuel. In order to ensure that the fuel you burn is efficient and won’t smoke, only use Ready to Burn certified.

Closing Thoughts on Log Burner Vents

If you’ve reached this point, then congratulations! You now know more about air vent controls than about 95% of all users. Put these tips into action the next time you start a fire, and bookmark the page for future reference!