It used to be called "downdraft", and the quick fix was to "Add another 3 ft. length of flue, chop down the tree, or add an "H" top".

Today it's called "negative pressure" and it's what causes a smoking fireplace - and affects the quality of the air in the living space.

In reality true "downdraft" is extremely uncommon. 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, 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 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 affect on the fire that the homeowner and installer should be wary of - it's also the affect 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 manufacturers 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 are 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 making an "H" top.

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 mid point by ".33d"

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