How Hot Should Your Wood Burner Be? A Flue Thermometer Guide

For less than £10, a flue thermometer can help you regulate the temperature of your stove - as a rule, a magnetic flue thermometer should read between 110°C and 250°C for a clean and efficient burn.

Getting the most from your fuel means burning cleanly and efficiently. When burning logs in a wood burner, you should regulate the temperature for optimal operation: too hot and you're not only wasting excess heat through the flue, but you could also be damaging the stove and speeding up the wear on components. Too cold and you will be wasting the fuel through smoke (unburnt fuel) and causing creosote deposits in your flue.

Why stove temperature matters

Maintaining the optimal flue-gas temperature will save you money. An overfired or slumbering stove will be less efficient than one running optimally. Running poorly could easily mean burning 20% more fuel for the same amount of heat. With an average household burning between 2 and 3 loose m³ of logs per cold season, that's between £60-100 of fuel costs per year going to waste.

On top of wasted heat, you will end up paying more for stove maintenance:

• Too cold: more regular cleaning & sweeping costs
• Too hot: more regular replacement of consumable parts, e.g. baffle plates, heat bricks etc.

In addition to cost savings, burning at the optimal temperature is safer and less polluting. Creosote buildup increases the likelihood of a flue fire, and a stove running hundreds of degrees hotter than it needs to be will warp baffle plates, crack firebricks and damage door rope. Most manufacturers exclude overfiring damage from their warranty, so the repair bill is yours.

Shutting the air right down to make a load of logs last through the evening feels economical, but it moves the fire into the range where a good share of the fuel leaves as smoke rather than heat. You get a longer burn and less warmth from it.

The three burn zones

This table summarises the symptoms, cost impact and actions for the three burn zones indicated by your thermometer.

Where to fit a magnetic flue thermometer

If your flue thermometer comes with specific placement instructions, you must follow those. As a rule - place the thermometer on the front of the flue between 30-45cm from the top of the stove. Too low and your thermometer will pick up heat conducted from the stove itself. Too high and the gases are starting to cool. Most of the circular spring-based magnetic thermometers are designed to work on single-skin flue pipe, not the insulated twin-walled sections.

If your flue is rear fitted, make sure you place the thermometer on a vertical section to avoid getting an incorrect reading. Avoid seams, joints and anywhere over a damper plate.

Flue gas thermometers are not 100% accurate, but because "optimal" is a broad range this is fine. The thermometer will give enough of an indication for you to avoid running too cold or too hot. Make sure you leave the thermometer in the same place, because the reading depends on your pipe diameter and wall thickness as much as on the fire. It's a good guide to how tonight's fire compares with last night's on your stove, rather than a figure you can compare with someone else's.

How to use the reading to control your fire

In practice, the readings you get from flue thermometers lag behind what's going on in your stove and flue. When you're making any adjustments, make sure you wait a couple of minutes to let things settle before making any more changes.

If your flue is too cool

First, check how long the fire has been going. A cold flue can take ten or fifteen minutes to establish a proper draw, and a low reading in that window is normal rather than a fault.

There are a few possible issues that cause stoves to run too cool. Did the fire start well, with vigorous flames? You need to build from a good base when burning logs to make sure you don't let them smoulder for too long before full ignition. If things never really got started, try adding some more kindling to build the flames up.

Make sure your air intake is fully open - choking the fire at any point can cause the temperature to drop. Often if this is the case you will start to see blackening of the glass.

Are your logs dry? Wet wood doesn't burn as hot as dry wood; heat is used to evaporate the water, and you end up with a cool smoky fire instead of clean combustion. A moisture meter will let you know how dry your logs are. Kiln-dried logs sold under the Ready to Burn scheme must be below 20% moisture, perfect for running an efficient stove.

Does the stove need refuelling? If your fire has died down to embers, the temperature will slowly drop as the coals burn out. This is fine when you have finished with the stove, but if you are carrying on, refuel while the embers are hot enough to ignite the fresh logs quickly.

If your flue is too hot

Full marks for lighting a good fire! However, things now need tempering down a bit.

Has the firebox been overfuelled? Often for a typical 4-5kW stove, one or two 25-30cm logs at a time is enough. If you have a lot of smaller pieces in there, this is like using a load of kindling and will burn fast and hot due to the increased surface area and airflow. If you have put too much in, just let it burn through and remember for next time.

Another option if your fire is hot is to regulate the airflow. On most modern stoves the main air supply for wood is the airwash, which enters above the fire and helps to keep the glass clean. Make sure you are familiar with the controls of your stove and slowly reduce the air intake. Remember the temperature will lag a bit, so a bit of trial and error is likely at first.

Cheat sheet

Too cool

• Check fuel is dry
• Make sure air intake is fully open
• Refuel if required or add smaller kindling pieces to get things going

Too hot

• Avoid overfuelling
• Reduce air intake
• Use larger logs

Understanding the optimal temperature for your stove and flue can help you to burn cost effectively. In addition to buying at the right time of year and buying in bulk, it's one of the main cost savings you can make.