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.

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 a wee while. This is due to the cold air dropping down the flue , 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 top/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 byproduct of burning logs on a woodburner 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.

How to stop the black liquid appearing in your woodburner

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 use 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.

Rodtech training course : Chimney Sweeping and Advanced fault finding

I have been talking about going on a training course for a few years now and i’ve finally gone down and got my City & Guilds accredited , “Chimney Sweeping and Advanced fault finding” certificate via Rodtech at their Heats Centre training facilities in Hingham , Norfolkshire.

I wasn’t sure how much i would know it being English rules & regulations but i was glad to find i had already self taught myself the majority of the written course information via the internet, so it wasn’t too much of a struggle. It was however, voluminous and the class room day was a long one, for all concerned the trainer too bless her cotton socks. I thought she was very good and confident so was surprised to hear her admit to a wee bit nervousness.

The second day i found invaluable , i found lots of holes in my own knowledge so it was great to fill them in. The trainers being a family of dedicated & active sweeps were a great bunch and really decent people. Picking their brains was in itself invaluable, as was being allowed to play with clean stoves with no fear of breaking them…exponential learning curve hahaha. I met sweeps from England and Estonia, which i think is the first time i have set eyes on other ‘real’ chimney sweeps.

The third and final day , was the day of reckoning and exam conditions.. yes i did get a wee bit nervous on the practical and i don’t know why ? The exam , i think i dropped only 2 or 3 questions … the best part by far though was the advanced fault finding. I dropped only one flaw in it but wow it was a learning process too… i am by nature quite analytical but still managed to learn a lot. Somethings it is true you must learn by doing and having competent brains to pick was just fantastic.

All in all a very worthwhile time spent and i will most certainly be back to do their oftec stove installation course as soon as i can.

Reasons for maintenance of stoves and risks of neglect.

Any kind of appliance that does something, especially those with moving parts, will need to be maintained regularly. When it comes to maintenance people can normally be divided into three kinds:
Those that know and appreciate the situation and they will maintain their devices/appliances
Those that do know but think they can ‘risk it’.
Those who just simply are not aware. [Installers may be to blame here?]
Once you own a wood burning stove you become responsible to ensure it works safely, burns cleanly and hopefully lasts as long as possible. Unfortunately, many stove owners are unaware of the basic requirements needed to ensure it all keeps working.

Wood burning stove maintenance

Stove maintenance can be broken down into three distinct sections. Much of the daily maintenance can be handled by the owner but some of the more complex maintenance will require professional help.

  1. Regular owner maintenance checks and stove servicing.

Ensure the stove is cold with no burning embers before undertaking any of the tasks below which are mostly straight forward, once you get used to them.

  • Clean out the ash on a regular basis. (This may seem obvious but it’s often overlooked and the old adage of a little ash in the fire bed for burning wood is often abused.)
  • Regularly check all controls for smooth movement.
  • Burn only seasoned or kiln dried wood that is the correct size for the firebox. Burning wood that is low quality or high in water content will damage your appliance and liner, may cause a chimney fire and is a nasty air pollutant.
  • Check firebricks for cracks monthly.
  • On a monthly basis, the baffle plate should be removed and the flue way checked for soot build-up and potential blockage. If you are not confident with removing and replacing the baffle, get your chimney sweep to show you how or do it for you.
  • Clean the glass. If you cannot see the flames then you do not know how well your fire is burning. Remember moderate flaming combustion is what you need.
  • The last part of owner maintenance is planning regular expert maintenance from your local experienced chimney sweep.

2. Chimney sweeping

Regular sweeping of the chimney is imperative to the correct and safe functioning of appliances. Chimney sweeping is a profession that requires a diversified knowledge skill set, the correct equipment and experience.

It is widely accepted that all chimneys should be swept annually as a minimum. Some chimneys require sweeping twice per year and rarely some even more frequently, depending on use and a variety of other factors. A professional chimney sweep will also provide important advice and confirm with you the required sweeping frequency for your usage, which will be recorded in his notes also.

3. Solid fuel servicing

Solid fuel servicing is often an unknown variable when people purchase stoves and similar appliances. Whilst it is true that many stoves often last a very long time, it is also true that their internal components often do not.

The internal stones and grates of an appliance should be designed to survive the rigors of use however they do decay, wear and occasionally break. Repeated opening and closing of doors degrades rope seals which will eventually require replacing. Controls and hinges can become loose or seize with time. Castings can even crack.

Door Glass can be broken but more often becomes scratched or milky with use and replacing it is not as straightforward as replacing the glass of a house window. The glass used in stoves is often a clear pyro-ceramic which is held in place with heat proof fixings and sealed with a specially sized fiber glass rope. It is specially treated and pre-cut with beveled or smooth edges to prevent early failure

Many owners find it most beneficial to find a professional sweep who also undertakes servicing as this can save time, money and give peace of mind. Experience is very important when servicing is involved as there are so many appliances, that only time and practice can teach.

Potential risks of maintenance neglect

There are many possible risks to the stove user who avoids or neglects the necessary maintenance and sweeping intervals:

Carbon monoxide poisoning even death
Chimney fire
House fires
Invalidated warranties
Invalidated home insures
Shortened appliance lifespan
Greatly increased running costs
Excessively polluting our air quality
Angry neighbours

These risks are very real and significant.

If you are unsure of what you must do yourself then call your local chimney sweep and ask advice. Although the advice in this post is good general advice please note it is not specific to your particular appliance.
You must take professional advise specific to your particular installation [ I.E. Make sure your installer educates you !!!], you must also read and follow the guidelines set out by the appliance manufacturer in the instruction manual. You should also be aware of your home insurance company policy regulations and or small print.

Buckled Baffle Plates

It always comes as a shock to new wood burner or multi fuel stove owners that their baffle plate (also known as a throat or deflector plate) is classed as a stove ‘consumable’ and is therefore NOT covered under their stove warranty in the same way that most of the other components are.
This is because baffle plates are positioned at the top of the fire chamber specifically to deflect flames and heat back into the stove instead of letting them go straight through the flue system. This is partly why stoves are much more efficient at delivering heat to your room than an open fire.
However, it’s also why, even on top quality stoves, baffle plates tend to burn out.

Baffle plates are positioned at the hottest possible part of the stove and take the full brunt of the very high temperatures day-in and day-out. So whether your baffle plate is made of traditional cast iron, heavy steel plate, stainless steel or vermiculite board, you’ll eventually have to replace it.
How often this needs to happen depends on the type of fuel you burn, the size of the fuel load and how you burn your fuel.

Some stove owners can burn through their stove baffle plate in a matter of months and conversely some owners can make theirs last for years. A baffle plate on a boiler stove, for example, will not tend to last as long as a baffle plate on a non-boiler stove, simply because boiler stoves generally need to be burned ‘harder’ over longer periods to maximise the heat to the hot water, especially when there are lots of central heating radiators connected to the system. As a matter of interest, smokeless mineral fuels (eg Homefire Ovals) do not have the same flame height as an equivalent wood log load and can therefore be easier on a boiler stove baffle plate (but not necessarily on your CO2 emissions).

If you regularly need to replace your stove baffle plate then you should review how you operate your stove by referring to your owners manual which should contain advice on recommended fuel types and fuel loads, as well as instructions on how to effectively operate your stove.
For sure, quickly burning out a baffle plate means that you are probably not only wasting money on replacement baffle plates but also wasting money on fuel. As a chimney sweep i need to be able to remove the baffle plate so i can use the power sweeping equipment and clean effectively. If it is buckled and cannot be removed then i am in the unfortunate situation of having to charge a call out fee and you need to call your installer (if you can still get hold of him).

Damaged baffle plates are increasingly more common these days, (which is one of the reasons for writing this article) with speculation on the quality of metal materials these days and/or installers failing to ensure the customer is sufficiently educated in the use and maintenance requirements of their appliance.
It is often the case that you are over-firing your stove and producing excess heat which is wasted through the flue system. An easy to use magnetic flue pipe thermostat will help you to monitor your flue gas temperature and can clearly indicate when you are over-firing your stove.

In my opinion a flue pipe thermostat is one of the best, ‘value for money’ stove accessories you could buy. It helps you to maximise the efficiency of your stove and also alerts you when your stove is potentially dangerous by being over-fired.

As it is very easy to abuse a stove and burn out a baffle plate within a single heating season by simply ignoring the manufacturer’s recommended fuels and operating instructions, it’s also easy to see why most manufacturers now exclude baffle plates, fire grates, fire fences and glass from their normal warranty terms.

Warped , corroded and finally broken Baffle plate

The picture above shows a burned out cast iron baffle plate which is about three years old. The owner used mainly softwood (it was free) which required a substantially bigger fuel load than the equivalent hardwood load needed to generate the same heat.
***This meant that the fuel and flames were too close to the baffle plate and the extremely high temperatures which were created caused the baffle plate to warp and eventually fail (there is NO middle left).***
The stove had a rear fitted flue and it wasn’t until the owners noticed the flue pipe was glowing red that they realised they had any kind of problem.

Cool fuel rules hit the right hot spot

There are many fuel types available to the solid fuel user: coal, peat, wood logs, compressed wood products, smokeless coals. With so much negative information in the press these days … which is the right fuel for you to burn?

Firstly, you must look at your appliance and follow the manufacturer guidelines as burning the wrong fuel type can be dangerous and at the very least invalidate your warranty. Good sweeps are generally a hive of knowledge.

If you care about the environment, and you should, then you would want to focus more on renewable fuels such as wood logs.

Selecting and purchasing wood logs

Ash and beech are some of the best woods logs to burn however most logs burn perfectly well (see our burning wood post) if they are dry enough and have been adequately seasoned.

Wood should be purchased from a reputable supplier who should have to comply to regulations and have a vested interest in planting more trees to replace what has been felled.

Wood logs must have less than 20% water content when they are burned. This can ascertained several ways, (see our post on moisture meters) but the easiest and probably most reliable at a price is to purchase logs from woodsure who will have done all checks and tested the moisture content in random batches. You will also be able to complain if the wood you purchased is too high in water content.

If you choose to purchase unseasoned logs then you must season them (typically for 18 months or more) and after seasoning you must test the moisture content with an electrical conductivity moisture tester prior to using them. (again for more on this see our post on moisture meters) Users burning wet wood are one of the biggest problems and challenges our industry faces but it is one of the easiest fixes too.

Why mixing fuels is a problem

Wood and coal burn quite differently. Wood burns through a process of gasification where the volatile oils within become heated, turn to a gas, mix with oxygen above the log and then eventually combust as a mixture of gases.

Coal on the other hand is primarily a carbon, to burn efficiently oxygen must enter and mix from below the fuel. Combustion takes place inside and not above the coal itself.

There are also distinct differences between wood and coal burning types of appliances, such as a grate below or just flat metal base.

Some issues associated with the incorrect use of fuels

  1. Wood on top of coal

If you try to burn wood on a bed of coal you would need to open the top air intake, thus drawing air above the coal and not through it from below. Therefore, the coal would burn poorly and produce high levels of carbon monoxide and carbon dioxide, which acts as a fire suppressant/extinguisher and the effect is compounded by the inadequate heat from the coal.
Wood burns by first emitting volatile hydrocarbons which then become gaseous.
Normally these hot gasses mix with oxygen and burn with the yellow flame we associate with wood. However, in the reduced oxygen environment above a coal bed the volatile gas emitted by the wood log will instead travel into the chimney where it then deposits as a flammable creosote.
Another issue is when combustion temperatures and oxygen levels are too low, then the later hotter stages of gaseous combustion will not effectively take place. During these later phases, carbon soot particles should be burned.
If these particles are not burned they will then exit the chimney as fine dust pollution harming our air quality with all the associated risks.

2. Coal on top of wood

So we understand that coal requires oxygen from below to burn effectively, think bellows in a foundry. If we burn wood below a coal bed we will have a situation where our coal is bathed in carbon dioxide leaving an incomplete burn. The second issue here is that wood produces lots of ash which in turn blocks the ash grate, further lowering the amount of available oxygen to the coal.
If the coal used is smokeless, such as anthracite then high levels of sulphurous compounds will also be emitted. These compounds are highly corrosive and will eat through stainless steel liners and most pot coverings in next to no time. The one exception to the wood underneath coal scenario, is when kindling a fire with wood. Here it is very likely that the air intakes or even the door will be open in order to provide enough oxygen for both fuels for a limited time.


The term slumbering describes the process of the user loading the already burning appliance and then closing down the air supply. This is in an attempt to make the fire burn for long periods, typically overnight.

The big problem here is the fact that fuel requires oxygen for combustion. When oxygen is limited in a combustion reaction, there are several results that occur as are shown below.

  • Much of the volatile content within the fuel is emitted due to the reduced heat but it does not combust. It rises up the flue coating the chimney with volatile fuel, greatly increasing the risk of chimney fire and the requirement for sweeping.
  • Carbon requires lots of oxygen and heat for the particles to combust. When the oxygen supply is limited, dangerous sized carbon particles are released. Many of these particles make it to the outside air as fine dust pollutants , polluting the environment and causing health problems such as respiratory and lung diseases.
  • Heat or the lack of. In order for a fuel to release its stored heat, it must react with oxygen, it being the catalyst for a combustion reaction.
  • Cost. Slumbering will shorten the life of stainless steel liners, increase the need for servicing, sweeping and maintenance. It will reduce the heat value that you should get from the fuel, making it less cost effective too.

In conclusion

Ideally you would only burn dry wood logs in an environment that has adequate air/O2 for moderate flaming combustion to take place. This is achieved by adjusting the air supply so that flames are not sucking up the chimney but not limited too much. Smoke should not be visible in the firebox, only in the fire.
Also by adding only as much fuel as is needed to fill the fire box with flames, because burning too much or too little fuel at a given time is another mistake to be avoided.

Your chimney sweep should give tips on fuels and how to achieve adequate flaming combustion, fuel storage, types, sweeping frequencies and many other important factors surrounding good use practices.