The combustion air supply

A few years ago, building scientists and hearth specialists were convinced that bringing combustion air from outdoors to the appliance was the answer to venting failure problems. Now we know better.

Ideally, a hearth operating on natural draft takes its combustion air from a neutral pressure environment so that the air supply is steady and consistent. Perfection would be an air supply provided at neutral pressure and with no resistance to flow. Fireplaces and stoves are designed to consume combustion air in response to the amount of chimney draft produced by the waste heat of their own flue gases. A stove or fireplace will be fussy to start and likely to spill when it tries to get its combustion air from a seriously depressurized building. The extent of building depressurization is deducted from potential chimney draft, like this:

theoretical draft - depressurization = net draft

Building depressurization competes directly with chimney draft. If the chimney wins the competition, the flow direction is up (successful venting); if the fan causing the depressurization wins, the flow is down (spillage or backdrafting).

Several years ago, when large exhaust systems—like downdraft kitchen range exhausts—began to cause spillage from fireplaces in reasonably tight houses, a consensus briefly formed around the idea of bringing combustion air from outdoors. The theory was that the flow of air from outdoors would be free of influence from building pressure, allowing the combustion appliance to get the air it needed. In the late 1980s such certainty existed among technologists and regulatory authorities on the issue of outdoor combustion air that it was made mandatory in some building codes. Unfortunately, the decision to enforce mandatory outdoor air requirements was made before research was done to investigate how these air supplies actually work.

The idea that spillage from combustion systems can be reduced or eliminated by providing a supply of outdoor air to the appliance is not supported by research results. Laboratory and field reports have revealed that providing outdoor air is not a simple or effective cure for spillage, and that some designs could create a fire hazard. Two forms of outdoor air supply have been used: passive make-up type air supplies and direct-to-combustion chamber air supplies.

Passive make-up air supplies

A passive make-up type air supply is one that is not connected directly to the appliance combustion chamber. Since it is connected only to the house environment and not to the appliance, it flows air into a house only when the pressure inside is lower than the pressure outdoors at the duct weatherhood.

Table 4 shows that the standard 4 inch diameter air supply connected to the air circulation chamber of a factory-built fireplace would be able to flow only about 10 CFM at -5 Pa room pressure, the allowable depressurization for such fireplaces. Since this type of fireplace consumes between 40 and 60 CFM of air while operating, a 4 inch air supply is far too small. Enough air for a combustion appliance could only flow through the duct if the house were severely depressurized, but that level would cause the appliance to backdraft.

A passive make-up air supply of reasonable size would only be able to moderately reduce the effects of powerful exhausts on room depressurization, it would not neutralize them. At normal house pressures and in reasonable diameters, this type of air supply is insufficient to provide the air requirements of most fireplaces.

For example, to provide all the air requirements for a factory-built fireplace with bi-fold doors at -5 Pa room pressure would require a flex duct with a diameter in the range of 8 to 9 inches (see Table 4). Such a large air supply duct would be difficult to install properly and is probably impractical.

Table 4 Air flows through passive supplies of various diameters at -5Pa room pressure

Air Flow		Duct Diameter
L/s	CFM	in inches
5	(10)	4
10	(21)	5
15	(31)	6
20	(42)	6
25	(53)	7
30	(64)	7
35	(74)	7
40	(85)	8
50	(106)	8
60	(127)	9
70	(148)	9
80	(170)	10
90	(191)	10
100	(212)	10
120	(254)	12
140	(297)	12

Note: this table assumes an air supply consisting of a screened weatherhood, three elbows and a total length of 10 feet in rigid duct. If flex pipe is used, add one inch of duct diameter. Wind effects around the house would alter the flows significantly.

More importantly, it is misleading to think of the hole in the wall approach as supplying combustion air. In fact, passive air supplies provide air only in response to pressure differences. In cold weather, if a passive make-up air supply is located below the neutral pressure plane of the house (and there is no wind effect and no exhaust systems are operating), air will flow into the house. If, on the other hand, the passive inlet is located above the house neutral pressure plane, air will flow out. It is useful to keep in mind a key physical principle: AIR FLOWS TO ZONES OF LOWER PRESSURE through any available opening.

Air will flow OUT through a passive inlet located above the neutral pressure plane and IN through a 'hole in the wall' located below the NPP.

Wind effects around the house also affect the direction and volume of flow through a passive inlet. If the weatherhood of a passive inlet is on the windward side of a building, wind pressure is likely to force air into the building; if the weatherhood is on the downwind side, the negative pressure zone created by the wind is likely to draw air out of the house, possibly depressurizing it.

Another drawback of passive supplies is that they are often plugged by householders because in cold weather they can lead to uncomfortable cold air pooling at floor level.

The real problem with the passive make-up air strategy is that it does not reliably supply combustion air, nor does it reliably reduce combustion spillage. Under favorable conditions it may tip the balance of driving and adverse pressures in favor of successful venting. This is why some hearth specialists have reported performance improvements after the installation of a passive supply. However, it is also possible for a passive supply to cause spillage if air is drawn out of the house into a low pressure zone caused by wind effects. A remedial strategy that only works sometimes, and that may make the problem worse, is not a good strategy. A passive make-up air supply is really nothing more than another uncontrolled leak in the house envelope. A leaky house envelope is no guarantee of successful venting.

Direct combustion air supplies

Research has shown that properly sized air supplies routed from outside directly to a fireplace or stove combustion chamber can supply the total combustion air requirements after the system has reached operating temperature, provided the firebox is sealed tightly from the room with gasketed glass doors.

However, two key findings from the research serve as cautions against the widespread use of direct combustion air supplies.

1. Smoke leakage can occur, even when the appliance has tightly sealed doors. If a powerful exhaust ventilator depressurizes the room to a level greater than the draft produced in the chimney, combustion gases will leak from any available opening, such as gaps in gaskets and the joints between factory-built chimney sections. Because air flows to zones of lower pressure, a tightly sealed combustion/venting system will spill a smaller volume of combustion gas into the room than a leakier system, but it will still leak unless it is perfectly sealed. Perfect sealing would be difficult to achieve at the time of construction or installation and is unlikely to be permanent.

When outdoor air is supplied directly to the combustion chamber, exhaust gases will spill into the room if the room pressure is lower than the draft pressure created in the chimney.

2. Direct air supplies can reverse flow direction if the weatherhood is exposed to a negative pressure in excess of chimney draft. Hot exhaust gas passing through a combustion air duct constitutes a potential fire hazard. The pressure effects of wind force around buildings can be far more powerful than the pressures produced by chimney draft. Chimney draft ranges from zero to about 50 Pa in normal residential installations, whereas high wind effects can produce pressures around houses up to 100 Pa.

When a strong wind creates a negative pressure zone around a combustion air supply inlet, it may overcome chimney draft and cause a reversal of gas flow.

Evidence of wind-induced reversals in combustion air ducts is becoming more common now that so many systems have been installed. When diagnosing venting problems in systems with direct outdoor combustion air ducts, look for soot or staining inside the duct. If there is any evidence of reversal, disconnect the duct and plug the hole in the house envelope.

It has been suggested that a direct combustion air supply to a woodburning appliance would eliminate its air consumption impact on other chimney vented combustion equipment in the building. However, when their doors are closed, most woodburning appliances exhaust comparatively little air from the dwelling (see Table 2), so the risk of reversal of a ducted combustion air supply usually outweighs any advantage gained by bringing air from outdoors.

An open fireplace, in contrast, can exhaust such a large volume of air that it could affect the operation of, for example, a conventional gas-fired furnace or water heater. But direct combustion air supplies cannot effectively be connected to a fireplace without doors because insufficient pressure difference is created to drive the flow. Other strategies are required to deal with the impact of open fireplaces on the operation of spillage-susceptible chimney vented equipment in the building. (See Spillage from open fireplaces.)

There may be circumstances in which a direct outdoor air supply is considered necessary. If it is decided to supply combustion air directly to a firebox, it should be done with full awareness that spillage is still likely if the room becomes seriously depressurized and, for safety reasons, steps should be taken to control temperatures on combustibles adjacent to the air supply duct in case wind effects lead to a flow reversal.

Despite the fact that it is enshrined in some building codes and its adherents are often vocally forceful, there is no scientific evidence to suggest that outdoor air supplies, either direct to the combustion chamber or indirect supplies to the living space, are reliable and effective remedial measures for combustion spillage from the appliance for which the supply is intended.

The house as a combustion air supply chamber

A hearth vented by natural draft needs a reliable and unrestricted supply of combustion air. Since passive outdoor air supplies in reasonable sizes are ineffective and since direct combustion air supplies are unreliable and potentially dangerous, other options must be considered. The most obvious alternative to outdoor air is to take combustion air from inside the building.

The advantage of taking combustion air directly from the room in which the appliannce is installed is that the building envelope moderates the effect of wind on the air supply by damping out wind-induced pressure fluctuations. The pressure inside the house will still be affected by wind to some extent, but the flow resistance offered by the envelope tends to remove the peaks and valleys of high and low pressure caused by wind gusts.

The main disadvantage of taking air from inside the house is that the pressure environment can be adversely affected by powered exhausts. However, depressurization caused by powered exhaust flows is predictable and manageable, unlike the more random and unpredictable effects of wind on outdoor air supplies. The worst-case indoor air pressure environment can be measured using the house pressure test described later, and can be controlled either by limiting exhaust flows or by installing a powered make-up air system.

In general, therefore, hearths that are vented by natural chimney draft should draw the air for combustion from the room in which they are located. Where necessary the indoor air pressure should be controlled to minimize depressurization.

Summary

passive air supplies do not supply combustion air, but only flow air in response to the pressure in the house compared to outside
passive air supplies of reasonable size are able to provide only a portion of the air requirements of most hearths at house pressures normally encountered
directly-ducted combustion air supplies may supply all the air requirements, but spillage will still occur if the room is depressurized to a level of pressure greater than that produced in the chimney
directly-ducted combustion air supplies can reverse flow direction when wind effects create a zone of negative pressure at the outdoor weatherhood
air flows to zones of lower pressure
appliances that are vented by natural chimney draft should draw the air required for combustion from the room in which they are located

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