Condensation and other issues that show up in cold weather.

At this time of year when the weather gets cold enough to start affecting your fire or stove you may be surprised or even shocked when things don’t seem to work the way they normally do.

Appliances have Running Temperatures

Stoves have a running temperature and take time to reach this point , you may find they won’t start up so easily and/or are Smokey for the first 10-15 minutes. This is due to the cold air dropping down the flue but once up to running temp the hot air rising displaces the cold air as it pushes its way up and out the chimney. You may need to try heating the metal baffle plate with a blow torch before attempting to light your fire or if it is not metal , you could try the Scandinavian method of building your fire upside down i.e. paper on top> kindling>wood/coal.

Tar or Black Liquid

Today a customer called worried about black liquid tar coming down into their stove. You too may have found a sticky , black liquid in your wood-burning stove and you might have spotted it running down the inside walls of your appliance or settled somewhere in the firebox. The consistency reminds you of tar because that is exactly what it is.

If you have seen this black liquid, it indicates that you are not operating your stove correctly and in particular , burning unsuitable (damp) fuel.

Usually tar is created as a by-product of burning logs on a wood burner when the moisture level of the wood is too high. Since energy is being wasted evaporating water, the stove doesn’t get up to high enough temperature, which leads to cooler gases going up your chimney. Since they are cooler, they are prone to condense when they touch the cold metal of the flue liner.

The condensed gases will either solidify on the inside or the flue or start to drip back down into the stove in the form of the sticky liquid. Whenever the tar does get the chance to solidify it will turn into creosote. A build-up of highly flammable creosote in your stove system significantly increases the risk of chimney fires. This is especially true if you burn soft wood such as pine , the resin being the main culprit.

How to stop the black liquid appearing in your wood burner

There are a few ways to stop the black liquid appearing in your wood-burning stove. The first is to ensure that the only logs you burn are ones that have been correctly seasoned and have low moisture content (<15-20%), this is easily achieved by using a moisture meter.

Ensuring there is a strong draw up your chimney, operating your stove at full capacity and burning it with the air intake fully open for 10-15 minutes once or twice a day will blast off the vapor and burn any sediment that settles on the flue. You can also try using some table salt on the burning wood to try to help absorb any water that is created. Finally , not slumbering your stove over night and also ensuring your room is well ventilated (especially if you keep damp logs in the room too) will minimise the chance of flue gases condensing in your chimney.

It is also important to get your chimney swept regularly as this will remove any residual creosote that has built up in your chimney and prevent it seeping back down your chimney and into the stove. Regular sweeping will also protect your liner from corrosion and eventual destruction.

Wood Burning Properties

A quick guide to wood burning properties

There are a many types of wood to choose from, all of which have their own burning qualities and properties. Although there are references to burning green wood here ; to get the most efficient and effective ‘burn’ in your wood burning stove, only very dry (at least <20% moisture) wood should be used. The following is a small guide only and is by no means comprehensive.

The table below shows the types of woods and their burning properties.

AlderProduces poor heat output and it does not last well.Poor
AppleA very good wood that bums slow and steady when dry, it has small flame size, and does not produce sparking or spitting.Good
Ash*Reckoned by many to be one of best woods for burning, it produces a steady flame and good heat output. It can be burnt when green but like all woods, it burns best when dry.Very good*
Beech*Burns very much like ash, but does not burn well when green.Very good*
BirchProduces good heat output but it does burn quickly. It can be burnt unseasoned, however the sap can cause deposits to form in the flue with prolonged use.Good
CedarIs a good burning wood that produces a consistent and long heat output. It burns with a small flame, but does tend to crackle and spit and the sap can cause deposits to form in the flue with prolonged use.Good
CherryIs a slow to burn wood that produces a good heat output. Cherry needs to be seasoned well.Good
ChestnutA poor burning wood that produces a small flame and poor heat output.Poor
Firs (Douglas etc)A poor burning wood that produces a small flame and poor heat output and the sap can cause deposits to form in the flue with prolonged use.Poor
ElmIs a wood that can follow several burn patterns because of high moisture content, it needs to be dried for two years for best results. Elm is slow to get going and it may be necessary to use a better burning wood to start it off. Splitting of logs should be done early.Medium
EucalyptusIs a fast burning wood. The sap can cause deposits to form in the flue and can increase the risk of a chimney fire if burned unseasoned.Poor
Hawthorn*Is a good traditional firewood that has a slow burn with good heat output.Very good*
HazelIs a good but fast burning wood and produces best results when allowed to season.Good
HollyIs a fast burning wood that produces good flame but poor heat output. Holly will burn green, but best dried for a minimum of a year.Poor
HornbeamA good burning wood that burns similar to beech, slow burn with a good heat output.Good
Horse ChestnutA good wood for burning in wood stoves but not for open fires as it does tend to spit a lot.  It does however produce a good flame and heat output.Good (For stoves only)
LaburnumA very smokey wood with a poor burn.Poor do NOT use
LarchProduces a reasonable heat output, but it needs to be well seasoned. The sap can cause deposits to form in the flue with prolonged use.Medium
LaurelBurns with a good flame but only reasonable heat output. It needs to be well seasoned.Medium
LilacIts smaller branches are good to use as kindling, the wood itself burns well with a good flame.Good
MapleIs a good burning wood that produces good flame and heat output.Good
OakBecause of its density, oak produces a small flame and very slow burn, it is best when seasoned for a minimum of two years as it is a wood that requires time to season well.Good
PearBurns well with good heat output, however it does need to be well seasoned.Good
Pine(Including Leylandii) Burns with a good flame but the resin sap can cause deposits to form in the flue and can increase the risk of a chimney fire.MUST be well seasoned.Good (with caution)
PlumA good burning wood that produces good heat output.Good
PoplarA very smokey wood with a poor burn.Very poor
Rowan*Is a good burning wood that has a slow burn with good heat output.Very good*
Robinia (Acacia)Is a good burning wood that has a slow burn with good heat output. It does produce an acrid and dense smoke but this is of course not a problem in a stove.Good  (For Stoves only)
SpruceProduces a poor heat output and it does not last well.Poor
SycamoreProduces a good flame, but with only moderate heat output. Should only be used well-seasoned.Medium
Sweet ChestnutThe wood burns ok when well-seasoned but it does tend to spit a lot. This is of course not a problem in a stove.Medium (For Stoves only)
Thorn*Is one of the best woods for burning. It produces a steady flame and very good heat output and produces very little smoke.Very good*
WillowA poor fire wood that does not burn well even when seasoned.Poor
Yew*A good burning wood as it has a slow burn and produces a very good heat output.Very good*

In addition there are also the compressed reclaimed ‘eco’ type of logs and briquettes which do tend to burn well and for a decent length of time because they are dense and very dry. You should try to choose a product that does not break apart too easily.
In a nut shell you should always use hard woods over soft woods.

Moisture Meters

The single worst predisposing cause of chimney fires today is wet wood. The recommended level of moisture is less than 20% to avoid chimney fires. You can read more about moisture meters in our site here

This guide was originally taken and adapted from

Moisture Meters

Moisture Meters

Readily available online from large retailers such Ebay and Amazon for around £20 , a firewood moisture meter is perhaps the best investment (next to a stove thermometer) and THE MOST IMPORTANT TOOL for helping you to get the most out of your stove.

Although there are many professional firewood suppliers out there who know their business and when they say that their firewood is well seasoned (sufficiently dry i.e. moisture content = 20% or less) it is what they say it is.
Unfortunately , there are also other firewood suppliers claiming their wood is seasoned , when in fact it is far from it. Your moisture meter will help to confirm that their wood is well seasoned and let you gauge exactly how wet it still is.
You may well find that some of the more professional firewood suppliers are booked solid , so it might be a good idea to get your orders in with them in advance of each season.

Problems burning Wet Wood (>20% moisture)

Without the use of a moisture meter the first you may realise that their wood is not dry enough , will be when you notice your stove isn’t giving out much if any heat at all and that the window is getting tarred up.
It doesn’t just stop there , burning unseasoned wood will tar up your chimney (increasing the risk of chimney fires) as well as reducing the life of your stove , liner and/or the whole chimney itself.

Burning unseasoned wood is inefficient , results in high levels of particulates , which is bad for your health , often makes NASTY problems for your chimney sweep and the environment as a whole !
(resulting in a government backed scheme ‘Burnright’ by woodsure is being promoted now in 2020/21).

In terms of how much heat you get out of your wood stove , the moisture content of your firewood is probably the single most Important + Controllable thing to consider. The moisture in the wood has to be boiled off before you get anywhere near the expected kilowatt per hour (Kw/h which is based on dry wood) rating which is why on so many levels not least financially , using a moisture meter is so important.

Buying Wood

Firstly , it makes sense to check with the supplier what type of firewood you are getting , although in a nut shell you should always use hard woods over soft woods. You can see the Wood Burning Properties of many types wood in our quick guide here. It may be well seasoned , or the supplier may only give you an assessment of the dryness of the wood anywhere from freshly processed to partly dry. When you have a firewood moisture meter then you can tell exactly how wet the firewood logs are , although of course there will always be other tell-tale signs i.e. no cracks in the end grain and a ‘wet’ smell.

Next time you get a firewood delivery from an unknown supplier take out your firewood moisture meter and before your supplier unloads , grab a couple of logs at random , split them in half with your axe and take a couple of moisture readings. If the firewood is well seasoned then it should have a moisture content of 20% or lower , a few logs at around 25% isn’t too bad as long as most of the load is under that.

                                     testing firewood moisture      
Picture 1: Moisture meter 

To take a moisture reading , all you need to do is push the two pins into your chosen piece of firewood. As you can see above in picture 1 , this piece of firewood is at 17% moisture which is ideal.

Anything under 20% is fine but nowadays you can get kiln dried wood which is around 11% and costs a bit more than normally seasoned wood.

If the firewood is a lot wetter than 20% , then it’s just a load of wet chopped logs that will one day eventually be seasoned firewood after you have dried & seasoned them yourself !!
If after using your moisture meter the wood turns out to be wet and the supplier is still claiming that it is well seasoned , you should either pay less for the wood (pays for your meter cost !!) and store it up until it is dry , or simply send it back.

Likely Stories

Of course it is possible that some firewood suppliers are not aware of the issues , or simply do not know how to properly season wood for wood burning stoves.
They often come armed with a multitude of freshly prepared stories to get you to accept their unseasoned wood and might say things like , “freshly felled wood is OK to burn straight away” (this is incorrect , will DAMAGE your equipment !! and cost you time , energy & Money ).
Another common story given to customers is , “because it has been a wet winter , it has been hard to season the firewood logs properly”.

In our own experience , customers have reported some kiln dried log loads ranging from the expected 11% all the way into +40%.

However , seasoning firewood properly and then delivering the properly seasoned wood , is basically their job description and so you should hold fast against accepting these fairy tales.  

Seasoning your own wood

You can also use a firewood moisture meter to see how well/quick your firewood is seasoning , which stacking techniques work best etc.
Crisscross stacking with air gaps works faster and more efficiently than pyramid piling. When you are processing and seasoning a lot of firewood , it really makes sense to do it in the most effective way possible.

Firstly , to properly season firewood the wood must be cut into short log lengths , typically 25 cm and then split down the middle to increase the surface area and aid the drying process. Log diameter should be typically 5-15 cm and a range of sizes is perfect. The split logs need to be stacked under cover , with max airflow around the stack.

How long the logs need to season will depend on the species e.g.  Ash might require 12-18 months whereas Oak will probably need closer to 3-4 years to get really dry and down to below 25% moisture.

Taking a meter reading

To take an exact moisture reading you need to measure the moisture on the inside of your piece of firewood: the firewood will be drier on the outside where the wind and sun have had their effects. So split a piece of firewood down the middle , push the two pins on the end of the moisture meter into one of the freshly split faces of the wood (not the end grain and not the outside faces of the wood) , ideally near where the middle of the piece was before you split it and press the On button. The moisture reading will appear on the screen as a percentage.

Kiln Dried Wood

Kiln drying is a process of force drying which can be done within anything from 60 hours to a week , depending on the type of drying process.  Drying wood this way virtually sucks the moisture out of the log , right from the centre. One thing you will always find with kiln dried logs , unlike really well seasoned logs , is that there will be more of a gradient in moisture content across the log. Kiln dried logs usually show 10-15% on the outside and 20-25% on the inside , with the overall average below 20%. In contrast , one would expect to see more consistent moisture content throughout the log when really well seasoned.

In summary unless you have tested that your seasoned logs are properly dry using a moisture meter correctly , then kiln dried logs are probably a more expensive but convenient alternative.

Be aware that there are many new kiln dried log suppliers, so try and make sure they carry the Woodsure and/or HETAS quality assurance certification for guaranteed quality.