It’s fire lighting season again, and to many that ignites a hunger for that cozy feeling you can only get from a wood burning stove. Stoves have become hugely popular over the last decade and because they are so efficient compared to an open fire will continue to be a very popular way of enjoying the crackling warmth of a wood fire. Wood burners have become a bit of a status symbol, highly desirable and a design statement.
Like this lovely shot of a Broseley Evolution 5 SE Deluxe demonstrates, a wood burning stove complements the popular ceiling to floor glass look. Although it creates a great view of the outdoors, can can make the temperature feel a bit outdoorsy too, and that’s where the stove comes in. They add a bit of localised warmth in the room without having to heat the whole house as well. They will of course heat a bit more than the room they are in, but after they have warmed their immediate area first.
So yea they are stylish, handy and economical,however its not all plain sailing. If you live in the city you may already be aware of the Clean Air Act. This was brought in to reduce pollution from domestic fires, a major source of domestic heating back at the time. Most of the houses in Belfast at the time had back boilers, many still do. The legislation restricted the use of fuels that were not smokeless. Coal and wood were instantly off the menu. The only remaining choice was the more expensive and less interesting to look at smokless fuels. With less of a flame and more of a glow the solid fuel choice was kind of bland and lacking the charm of the wood-fueled fires out of town. Stove manufacturers saw this and set to work, because the legislation wasn’t against the fuels but rather the production of smoke, which is un-burnt fuel. They found that if they can make a stove that can burn wood efficiently and crucially without producing smoke, it was game on, and so they did. Smoke Exempt stoves or DEFRA stoves as they are sometimes marketed, came about. The options for those folks who love where they live but couldn’t have a wood burner suddenly looked better.
How does a Smoke Exempt stove work? Well in short it burns more completely. When you set fuel alight it gasifies, or basically turns to gas, which is flammable.
From the second it is made in the fire bed the hot gas sets off for the chimney pot, the stove needs to burn all the flammable gas before it leaves the stove to prevent smoke being visible from the flue. So modern stoves usually have a few tricks up their sleeve. When the air enters the stove is is diverted to three main areas, the primary feed from under the grate, the secondary feed which comes down the back of the glass, sometimes called airwash, this keeps the glass clear. Finally there is the third or tertiary feed this is introduced at the top of the stove where the gases are hot but low on oxygen. The secondary and tertiary feeds help complete the burn and release the energy in the fuel as heat and reduce the amount of un-burnt fuel (smoke) being wasted before the gas leaves the stove. This is great news for homeowners, high efficiency and good for the environment.
Great, but what has this got to do with chimneys? Well, a stove and a chimney are a partnership and one will not work well unless the other is suited to its partner. So its horses for courses. Stoves come in all shapes and sizes some have small flue outlets others have large ones. Houses on the other hand have fairly standardised flue sizes, most being around the 8 inch diameter which is fine for an open fire.
How does a flue work? It provides a path for hot gases to leave the grate and exit the building. The flue does not suck the gases out, the gases leave on their own accord, because they are hot, and hot air rises. So then what has a flue to do to help the gases get out? They need to keep the gases hot so they still want to rise. They need to provide a path in the same direction that the hot gases want to go, up! Lastly they need to be the correct diameter, large enough so that the flow is not restricted and small enough that the rate of flow is not too slow so the gases linger too long in the chimney. Lingering is a problem, the longer the hot gas spends in the chimney the more heat it looses to the surrounding chimney, and the less it wants to rise. The problem you have is then poor draw. Most noticeable when the flue gases are cooler. When lighting the fire and when refueling the stove are too good examples of this. Open fires, have large flues, they are also open so the gas in their flue gets to rise unimpeded because it draws air directly from the room to allow the hot gases to keep their momentum and exit the chimney. Stoves are described as closed appliances. This means all the gas in the flue comes through the stove. This means that there is not all that air entering the flue directly from the room like there is in those 8 inch diameter open fire flues. You can see now that the diameter of the open fire flue is not correct for the stove with its more metered flow. So what size does it need to be?
Should it be the same as the outlet on my stove? Not necessarily, it cannot be smaller than the outlet on your stove. This would restrict the flow. Building regulations document L for us here in NI or J for the rest of the UK say that multifuel stoves under 21killowatts should be fitted to a minimum diameter of 6 inches or 150mm flue, unless the outlet on the appliance is larger than 6 inch. If it is then the minimum is the diameter of the outlet on the stove. What if my stove has a 4 inch flue? Well if it is a multifuel stove that can burn coal then it needs to be connected to a 6 inch minimum diameter flue. Why so big? Multifuel stoves can potentially burn and produce smoke which layers up in the flue system, and even with an annual clean can still pose a blockage risk so that is why 6 inch is the standard. Why do suppliers sell 5 inch flue? 5inch flue is perfectly fine for gas and oil fired appliances because they burn cleaner anyway. With solid fuel, the exception to the 6 inch rule are the Smoke Control stoves. They have been tested to produce less smoke these are permitted to be fitted to a 5inch or 125mm flue, provided the outlet on the stove is 5inch or less. On the grounds that less soot will gather inside the flue system between cleans.
So you want to install a woodburner? if you live in a smoke control area you will need a smoke control stove. You will almost certainly need to adapt your openfire flue to suit this, often by installing a liner, and yes if your smoke control stove has a 5 inch or smaller flue you can fit a 5 inch liner. Great, I’ve found a super cheap liner online… Not always the best idea unless you know what you are looking at. Not all liners are made equal, they can vary in thickness, grade of material and how they are constructed. Essentially they are made from a ribbon of stainless steel which is folded and crimped together in a spiral to make the tubing shape you recognise as a liner. The liners have an outer ribbed skin and an inner smooth skin. The outer skin gives the liner its strength and flexibility the inner skin gives it corrosion protection and wear resistance. A cheaply made liner will use a thinner ribbon and not fold the outer skin as deep making it not as strong and prone to collapsing and unwinding during fitting or cleaning.
Then there are grades of material. 316 and 904 stainless steel. Simply put they are blends of stainless with different percentages of other metals like Chromium in their mix. 904 has more chrome in it than 316. This makes it more resistant to acid corrosion than the 316 and that is why it should be the only choice for a multifuel stove that can burn sulfurous fuels like coal. The sulphur in the coal ends up in the soot and when it gets wet makes sulphuric acid, which, you can imagine is fairly nasty to the steel. So that’s why we specify 904 for multifuel. Yes it is dearer than 316, but not by much and it is because of the chrome, nothing else. The 316 is fine for woodburners, they have the same flue acidity. Installing the liner often costs more than the cost of the materials so it is always better to get the best quality of liner that you can afford. Liner failure means it will have to be removed and a new one installed. If you are lucky you will discover it when cleaning, if not then you could be poisoning the house with a silent killer, carbon monoxide. So for a few pounds more it really is better to safe than sorry. There are so many outlets selling liners and it can be really difficult to tell them apart without knowing how they are made and what they are made from, and that is why we only fit liners that we are familiar with and we are satisfied with their quality and suitability for the job. We cannot fit a liner sourced elsewhere as we have no guarantee that it is what it says it is.
Liners, like other flues will have a designation number that looks something like this T600-N-D3-Vml40010-G(25) All the parts of this code refer to various tests and standards that the liner has met in official testing and a supplier should be able to supply this on request.The T600 means it has passed the test for an operational flue temp of 600°C, the G on the end means it has passed a test to withstand a simulated chimney fire test for 30 minutes. The other parts in the middle refer to material corrosion resistance, thickness and if it can be used in positive or negative pressure applications.
So to sum up, if you want a woodburning stove and you are in the city and have an open fire. You will need to adapt your flue by lining it. You can fit a Smoke Control stove to allow you to burn wood, a multifuel smoke control one will allow you to use smokeless fuels too, but you can only fit a pure wood burner to a 5 inch flue (provided the outlet on the stove is no bigger than 5 inch). Even smokeless fuel will need a 6 inch flue. Going for the wood only option? then 316 grade is fine otherwise we would recommend the 904. We have been repairing and maintaining flues and chimneys for over 25 years now and our reputation is key extremely important to us and to some of our large clients like the National Trust, and this is why we insist on only using liner from trusted sources. We work closely with our suppliers and because of our repeat business with them, we can negotiate preferential prices on top grade materials which we pass on to our customers.
Give us a call if you are thinking of installing a stove or you want to service your existing stove. Happy stove season everyone.