There are a few reasons why this could be happening.
Short Answers:
- Negative pressure in the room – this can be caused by a household electric extraction fan or severe pressure difference in a windstorm. Open a window to equalise the pressure.
- Severe down draft due to surrounding structures, hills, trees or roof layout.
- Most commonly, this is an indication your flue is blocked. Clear the obstruction and investigate the cause. Check the moisture of your wood and make sure you are burning good, dry wood. The flue pipe can block very quickly if you are burning wet or gummy wood. Make sure you are using a reliable chimney sweep as the Pyro is different from other wood fires.
Download down draft troubleshooting info HERE.
More Detail Explanations:
All of our fires are hand built to exacting standards so every fire leaves us in an identical way, there are usually only ever 3 or 4 variables which can affect the performance of the fire and these are the installation, the house, the fuel and the operator. Any combination of these or each on their own can have dramatic effects on how the fire will or will not perform.
INSTALLATION & FLUE.
The New Zealand Standard for flue installation requires a minimum of 4.6 meters from the top of the hearth to the top of the flue system, this is usually sufficient to allow adequate draw on the fire in most installations however, occasionally this is not enough and so we at Pyro Fires supply 4.75 meters of flue in our standard kits to allow a little bit extra for draw.
Every now and then this may still not be enough to give good draw and an extra length of flue may be needed to provide the fire with sufficient draw for good performance, unfortunately there is no requirement in NZ for the installers to test the flue for draw before commissioning the fire and so this can often be overlooked.
A draw test can be done a couple of ways, the simplest is to just attach a piece of newspaper above the door opening on your fire and whilst it’s not going watch to see which way the paper moves most often, if it is being drawn into the fire then this is good drafting, if it is continually being blown out towards the room then this will indicate poor draw.
More specific testing can be done with a flue draught meter but unfortunately most installers fail to use or even own one of these in NZ.
Quite often operators may see a drop in the performance of their fire following the flue being swept, this is an indication that the flue has not actually been completely cleared of all the build up and so a quick check of the flue system and using a smartphone or a camera is a worthwhile process. Remove the flue from the spigot of the fire and take a picture up the flue pipe to ensure you can see a clear ring of daylight (you can use a mirror for this too instead if preferred).
Then using the same equipment also check down into the top chamber of the fire through the hole and see if there is a pyramid type build up in there which could be restricting air flow before putting the flue back in. If the fire has a wetback installed there could be a significant build-up of soot and creosote on the element itself which can also cause some restriction in air flow.
If there is little or no draw on the fire then the fire can only ever operate at a reduced temperature, this means the unburnt hydrocarbons and other volatile organic compounds will not be combusted inside the fire chamber, this results in lost heat and the reduced temps create a condensed sticky residue in the flue pipes called creosote, if the method of operation doesn’t change then this becomes a symptom of its own cause and more build up will occur due to the ever reducing amount of draw from the flue system on the fire until it reaches a stage where the flue provides no draw to the fire and the smoke then finds any other means out of the fire chamber possible. This can happen over the course of a year or just a few weeks or even days in some cases.
HOUSE.
If the fire has been installed into a relatively new build home, which are being constructed to be far more air tight than the older style housing then this can have an significant impact on the free air available over time to the fire, especially when other forms of mechanical extraction from within the home are used such as range hoods, clothes dryers, wet room extraction etc. Even basic heat transfer systems that move air around within the home can have a similar effect if the room that the fire is in becomes enclosed, ie. the air is being drawn out of the room the fire is in with the doors closed and deposited into rooms elsewhere in the house can create a negative air pressure situation in that room.
In severe cases the flue pipe can actually end up being used as the means of ventilation into the room space causing the flue gases to reverse their normal flow and be drawn into the house. This is not a fault of the fire but a flaw in the inadequate ventilation planning of the house construction to accommodate a solid fuel combustion appliance.
A further point of note on the house topic is the location of the house in relation to its surroundings and the termination of the flue system which can create a situation which is often referred to as downdraft.
Below is some more detailed information from the New Zealand Home Heating Associations website further explaining downdraft in more detail;
In days gone by a smoking fireplace was most likely to be caused by wind pressure on the house or surrounding trees or nearby buildings. The solution is to terminate the flue outside the negative pressure bubble, or in less severe cases simply to open a window on the wind affected side to equalise pressures.
While these conditions still affect good fireplace operation today there are other considerations to be taken into account.
But let’s be quite clear that external pressure factors account for only about 25% of performance problems – the other 75% can usually be attributed to poor installation (including properly sealing inbuilt heaters to the fireplace facing) and flue installation & termination problems. It is however important to understand the problems negative and positive air pressures within the home can pose.
Houses today have become more “energy efficient” – meaning better insulated, and more airtight. Aluminium windows provide an almost perfect seal against drafts. And fans are everywhere! Power flues, clothes driers, extractor fans, dehumidifiers, air conditioners. ceiling fans, microwaves, and computers all rely on fan assistance.
No longer is pressure inside the home stable – it’s changing all the time to a greater or lesser extent. Even something as simple as recessed lights cause a draft – particularly when they are in the high part of the home.
Then there’s the problem of “stratification” of heat within the dwelling. As we know, warm air tends to rise to the higher point of the house and if there are any leaks in this upper region, the warm air will leak to the outside (if the outside pressure is lower than that inside). In order to try to maintain the internal pressure, air will be drawn in through gaps, cracks or other fissures in the lower section of the house structure. However, if the lower section – usually that in which living takes place – is sealed to prevent uncomfortable drafts, while the upper section is left unsealed. the structure will still endeavour to equalise pressures. And what easier way than to pull the extra air from the flue system! The effect of negative pressure again.
And in the pressure war it’s those appliances which rely on natural pull, created by normal differential pressure, which miss out. Unfortunately these appliances cannot overcome the relatively high pressures created by a fan.
To understand what causes chimney draft let’s go back to basics. When a fuel is burnt, the products of combustion are hot, and expanded. As a result, the molecules are distributed more widely and therefore these gases are less dense and lighter than the surrounding atmosphere, and rise. The hotter their temperature, the lighter the flue gas, and the greater the tendency to rise. Now poke these light hot rising gases through a vertical tube, and you generate a continuous pull – or pressure. But the pressure in the flue is very low, and most modern wood heaters use a lot of the pressure just to overcome the restrictions caused by the design of the air entrainment and heat exchanger system. Obviously maintaining a constant warm flue temperature helps – hence the need to insulate some flues. Decreasing the diameter hence increasing velocity, or increasing length of the flue can helps too (remember the “add another 3 foot of flue” trick!)
But remember the pressures we are talking about are minimal, (much less than the pressure that your lungs create when breathing!) and a change in pressure around the appliance can result, at best, in a restriction of airflow to the heater, poor combustion, and low flue temperatures, and creosote formation. At worst, the negative pressure caused by electric extraction fans, if it’s powerful enough, will completely starve the heater of combustion air and “seek” further air supply to meet its needs as well. More often than not, it will come from the path of least resistance and easy availability – from the fireplace or heater! Result? Smoke from the fireplace. In turn the fire burns poorly, and if it is starved of enough combustion air, will eventually go out.
But it’s not just the effect on the fire that the homeowner and installer should be wary of – it’s also the effect on the quality and health of the air within the house. For a healthy environment the U.S. ASHRAE code sets a standard requiring sufficient ventilation in a home to provide an air exchange rate of .35. This provides for proper control of pollutants. The more tightly a home is constructed the harder it is to achieve this air exchange objective. In reality, in the dead of winter, people simply won’t leave a window open to allow for enough excess air to overcome this problem.
So what to do?
Determining and testing for negative pressure zones within a dwelling is a fairly sophisticated and time consuming job – yet one which is not outside the ability of the average installer, providing they understand the affect positive and negative pressures have within the home.
An article by Paul Stegmeir published in the June 1994 issue of Hearth & Home “Testing and diagnosing Negative Pressure Problems” outlines the equipment and procedures required for such a test.
But there are a few basic points to check before resorting to such measures.
Firstly, make sure that the chimney system can operate at its optimum. Ensure that the flue from a wood heating appliance is installed in accordance with the manufacturer’s instructions – and ensure that it terminates outside the pressure bubble which is formed over the house. A rule of thumb is that on a house with a good roof pitch, the flue should terminate about 600 mm above the ridge. A dwelling with a flat roof poses the worst pressure scenario. In this case a flue should terminate about 2 metres above the roof. Remember too that every bend which is put into the flue system reduces the potential flue pull, and the flue should be extended to take this into account. Make sure that the flue is well insulated, and the joints sealed.
Check for airtightness of the home – what is the outside cladding? – what type of window & door joinery is used? Are the walls and ceilings insulated? Are all possible air outlets in the upper sector of the house sealed to prevent escape?
What other influences may affect air pressure within the home – unsealed fireplaces – extractor fans – clothes driers – central vacuum cleaning systems. How often and when are they used? Make sure that the home owner is aware of the correlation between fan operation and the possibility of negative pressure within the dwelling. It maybe that simply opening a window during fan operation may solve the problem – or a permanent air supply may be required. A simple smoke test may help to ascertain airflows around the heating appliance when extraction and other fans are operating.
Are there any buildings or trees adjacent to the dwelling or flue termination point which could affect chimney operation?
If there is any reason to suspect that there could be any factor which could contribute to a negative pressure situation, you should consider installing a permanent air vent to allow fresh air into the home and to equalise inside and outside pressures.
If the problem still exists when these potential causes have been checked, then it may be necessary to resort to the methods recommended by Stegmeir.
Remember that negative pressure which results in egress of combustion products from a heating appliance is a serious problem. It is symptom of poor internal air quality – an unhealthy living environment, and it is quite probable that such a situation may also be causing the build-up of other unseen hazardous pollutants into the dwelling. Adequate air change is vital to ensure good appliance operation and the expulsion of these pollutants so that risks to health are minimised.
What appears to be a very simple chimney cowl is in fact very carefully designed and to operate properly, it must be made to close specifications.
The “H” top is the only cowl which may reduce the effects of true downdraft. But remember that true downdraft is rare, and, providing that the manufacturers chimney specifications are followed, in most cases negative pressure is caused by other factors. Check these possibilities before going to the extent of fitting an “H” top cowl.
The design of the “H” top is based on strict proportions. It comprises a tee shaped tube fitted with vertical tubes at the end of each horizontal.
If the diameter of the flue is “d”, then the vertical end tubes must be “2d”. The internal measurement between the verticals is “3d”. The vertical must penetrate the verticals at the midpoint by “.33d”
WOOD FUEL.
A number of things should be considered when using wood for heating. An understanding of the various types of wood fuel that are available including its advantages and limitations, and it is essential to know how to light and maintain a good fire.
Wood fuel ranges from soft woods like pine, to hardwoods like manuka. But whatever wood is chosen, the key to a successful fire is to ensure the fuel is as ‘Dry’ and as ‘Seasoned’, as possible.
Green wood can hold up to its own weight in moisture and sap and it takes time to get rid of this. While surface water does not really matter because that will evaporate quickly, it is important to reduce the sap levels within the cell structure of the wood itself. Softwoods will season quite quickly, in about 6 to 12 months, but it can take from 18 months to 2 years for hardwoods such as manuka to dry to an acceptable level once split and prepared correctly.
All wood fuel needs to be processed, a fallen tree will go into a survival mode and attempt to retain as much of its moisture content as possible when felled, depending on the species it can successfully retain this internal moisture for many years if left fallen on the ground. The only way to kick start the drying process is to open the timber up by breaking its circumference and exposing the heartwood to the elements.
Gathering and stacking wood in the open air over the summer period is advantageous because the warmth of the sun and good air circulation will automatically evaporate some of the sap. When the wood gets wet from seasonal rain, the rain water replaces sap and because water is more quickly evaporated, the fuel dries faster.
However simply because a piece of wood is dry on the outside, it doesn’t mean that it is dry enough to burn. Conversely, even if the outside is wet, if it is seasoned properly, it will often burn beautifully. The drier the wood, the cleaner the burn, the less likely is creosote formation and unburnt smoke being exhausted from the flue.
Most woods make suitable fuel, pine is common and its high resin content and loose cellular structure means it burns faster than some others, so be prepared to make more trips to the wood shed. Macarocarpa and gum are also excellent fuels although marcarocarpa tends to spit and spark more than a lot of other fuels and in some appliances this may cause servicing problems because of fly ash.
It is suggest avoiding native timber for fuel, unless it becomes available through demolition or natural attrition. Some native species though are considered a nuisance timber in some areas of New Zealand, and could be used for fuel. These make good fuel – provided it is dry – but remember, drying hardwoods will take a long time.
If you knock two pieces of seemingly dry wood together if it “rings” rather than “thuds” it is likely to be dry, regular use of a moisture meter will ensure you know just how dry your wood fuel is. If you place a piece of timber on a good fire base if three sides are burning within 15 minutes, the fuel can be considered to be “dry.”
Split larger logs, so that the largest surface area of the internal wood is exposed to the atmosphere, stack the wood loosely, on bearers, with the ends facing a prevailing wind, cover with a plastic sheet on a light frame to create a warmhouse effect, with the sides open to the prevailing breeze so it can flow freely through your stack.
Do not use it until it is fully seasoned, do not stack rotten wood – it has very little useful heat in it and leave the bark on split wood – it helps to provide natural protection from rain once the fuel is seasoned out.
The process of wood combustion is quite simple and microscopic examination of wood shows the channels which carry the liquid nutrients up and down the tree; consequently the properties of wood are very different along the grain than across it. Heat moves along the grain about fifteen times faster than across it, therefore, solid wood across the grain does not conduct heat and is an effective insulator meaning it does not readily burn, it is the gases and the carbon which burn and to do this requires high temperatures to start the process of making the most of these. When a fire is lit, even by rubbing two sticks together, the gasification process starts and it is the combustion of these gases with air that produce heat which we see as flames and smoke. When heat cannot penetrate wood easily, i.e. across the grain, the volatiles given off are not rich enough nor hot enough to burn efficiently. Efficiency is clearly not a primary consideration in the more panoramic designed appliances.
To get the heat out of wood the fuel must pass through several stages, firstly, free water, that which is not chemically bound with the wood, is driven off – even wood at 20% moisture content still has to get rid of 2 litres of water for every 10 kilos of wood. In the second stage the wood breaks down into the volatile gases, liquids and charcoal. Finally the charcoal is also gasified, burning with a very short flame close to the char surface that appears to glow. In Pyro Fires all stages proceed simultaneously.
Burning of the volatile gases delivers over 60% of the heat stored in the original log but few heaters can recover the major portion of this heat as the volatiles must be over 600°C and mixed with hot oxygen to burn them. Now these are difficult conditions to meet and here’s why; if the main air supply comes from under or around the burning logs the glowing char consumes all of the oxygen, it takes only 5cms of glowing char to consume all the available oxygen. At that point incomplete combustion continues as characterised by increased carbon monoxide and tars which mostly go up the chimney where the unburned volatiles deposit on the flue walls as a highly flammable, gummy substance known as creosote. It is wrong to introduce cold secondary air above the fuel as it cools the gases below their ignition temperature and now they won’t burn at all. The requirement is to introduce a highly pre-heated but variable volume of air for the different stages of combustion; this is done very efficiently by the secondary air tubes inside the Pyro Fire.
All fires consume large volumes of air in order to extract the oxygen required to burn their fuel, 1Kg of wood needs 3.7m3 of air to burn completely although this is only a theoretical minimum for “stoichiometric combustion”. Such ideal combustion does not exist in real life as only some of the oxygen in that amount of air can be used and therefore “cool fires” need some 200% to 300% excess air to get the oxygen they need, therefore some 7 to 10m3 of air per Kg of wood pass through the firebox cooling the core temperature inside it and cooling air below 600°C kills the reaction needed to burn the volatiles. In most fires the air needs of the fire make it work against itself making it inefficient and polluting, the excess air it uses only goes up the chimney/flue with all that gas, tar and particulates. A Pyro Fire only uses superheated air in its secondary burn cycle ensuring there is no cooling of the firebox and no excess air consumed.
OPERATOR.
The Pyro Fire is a high performance unit and so a good understanding of the various principals of wood burning and the combustion process will ensure that your fire will perform to the best of its proven abilities.
If installed the wetback will reduce the amount of radiant space heating available from the top plate by around 10% however this is more than returned with an 18% increase in efficiency through the wetback element creating a total efficiency of 85% through the appliance.
Regarding the overnight burn ability of the Pyro Fire this is 100% dependant on the quality and size of fuel you put in it, you will need to have a good ember bed established, then add 2 or 3 dry hardwood logs measuring approximately the full depth of the firebox long by 100-120mm thick into the fire box, allow the flames to establish a strong burning presence on the front ends of the logs and then ensure that the Turboslide is fully closed meaning that the air flow into the fire is controlled by the fire itself. The further back in the fire chamber you have the fire the longer it will burn for, this should mean you have some hot embers left in the back of the fire chamber in the morning ready to be brought forward to establish another fire. With a Pyro Classic you should be able to achieve a 12+ hour burn time with a good wood fuel as explained above based on the fire consuming between 1-1.5kg of fuel and hour.
As a point of caution you should never insert a fresh log which is too large or placed in the fire too late in the burn cycle to ensure a flaming combustion, doing this will cook the wood fuel on the remaining embers releasing unburnt volatile gases into the combustion chamber which will eventually reach a concentration point of ignition, this can result in a sizable explosion inside the fire chamber and as well as making you jump may cause some damage to the unit.