The Hearth in the House as a System

Summary of lessons learned

The building envelope

Building envelopes are being constructed more tightly to increase comfort and to reduce energy consumption and exfiltration to structural components. Pressurizing buildings is not considered to be good building science because of the damage that condensed room air can do to building components. There is a wide range in leakage rate among the existing housing stock. The natural leakage rate of tight houses is not sufficient for healthy living, which creates the need for mechanical ventilation systems.

The effects of temperature difference

Natural chimney draft and stack effect in houses are both caused by temperature difference. The greater the temperature difference, the stronger the draft. The taller the chimney, the more draft is produced, subject to heat loss from the outdoor portion of the chimney. Temperature difference is the key factor in successful chimney venting by natural draft.

The colder the outdoor temperature, the stronger is the stack effect in a building. The taller the house, the more stack effect is produced. The neutral pressure plane in a building follows the leaks. Stack effect is not significantly affected by the leakiness of the building envelope. Weatherizing of houses can affect chimney venting by causing the neutral pressure plane to rise.

The cold hearth syndrome

Chimneys must be located within the building envelope to avoid the cold hearth syndrome. The chimney should penetrate the building envelope at or near its highest point. External chases must be constructed to be within the building envelope, all the way to the top of the envelope. Backdrafting is a serious failure of the chimney to do its job, whether or not combustion gases are spilled. External chases must be constructed to be part of the building envelope, all the way to the top of the building envelope. The cold hearth syndrome is extremely difficult to correct, so the potential must be dealt with at the design stage.

The effects of wind

Wind flowing over the top of a chimney can produce a driving pressure, increasing draft. Wind may also create adverse pressures at the top of a chimney

because of its direction of flow or turbulence created as it flows over nearby obstacles. Wind is an unreliable source of draft because it is highly variable and may be driving or adverse to chimney venting. Specialized chimney caps may reduce the effects of adverse winds, but are often used in error as a 'cure all' for troubled venting systems. Wind pressure induces air flow through leaks in the building envelope, causing pressure changes inside, which in turn, changes the position of the neutral pressure plane.

The effects of powered exhausts

The more air is exhausted from a building, the more negative the pressure inside will become. The tighter the building envelope, the more negative the pressure inside will become for a given volume of air exhausted. Chimney vented combustion systems act as exhausts, but, aside from open fireplaces, their flow rates are relatively low. Chimney vented systems are vulnerable to backdrafting and spillage due to house depressurization during start-up and tail-out, but are relatively intolerant of depressurization once draft is established. Exhaust-only house ventilation systems cause serious problems for chimney vented combustion systems because their operation spans the vulnerable start-up and tail-out periods. Balanced ventilation systems are needed in tight houses with chimney vented combustion systems. Leaking ductwork or bad design of a central heating system can cause zone depressurization.

Combustion air supplies

Passive air supplies do not supply combustion air, but only flow air in response to the pressure in the house. Passive air supplies of reasonable size are able to provide only a portion of the air requirements of a combustion appliance, depending on the room pressure.

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.

Venting system design

Insulated chimneys produce stronger and more reliable draft than uninsulated chimneys. Flow resistance in a venting system increases the likelihood of open door spillage. Air leakage into the venting system should be minimized because it lowers draft. Chimney venting systems should exceed 15 ft. (4.6 m) in height. The chimney flue should be matched in size to the requirements of the appliance it serves. Most chimneys currently available to the public do not possess the characteristics needed for successful venting.

Appliance design

Open appliances, such as fireplaces without doors or gas appliances with draft hoods, have little resistance to spillage. Appliances with large door openings relative to their flue size are susceptible to open door spillage. Appliances with firebox exhaust outlets in the shape of a narrow slot are susceptible to open door spillage. Appliances with flue outlets below the top of the loading door are susceptible to open door spillage. Appliances with flue outlets or internal passages that route flue gases downward are susceptible to closed door spillage. Appliances with leaky joints are susceptible to both open and closed door spillage. In many cases, the spillage susceptible characteristics of appliances can be overcome by good venting system design.

Energy momentum

Energy momentum is created by the mass of the appliance and venting system. The more energy momentum is created in a system, the more resistant it will be to spillage during tail-out if the building is depressurized.

The human factor in woodburning

Users of wood burning equipment may either induce or prevent spillage. The most common, but least serious, form of spillage occurs when the loading doors are open. The most problematic user influence is to cause the fire to smolder, a condition that promotes dangerous closed-door spillage. The tolerance of householders to the smell of wood smoke can influence the amount of smoke spilled.

Combustion spillage from open fireplaces

Open fireplaces are extremely vulnerable to combustion spillage. The internal design of traditional masonry fireplaces is not conducive to successful venting. The installation of glass doors is the most effective and least expensive way to increase the spillage resistance of open fireplaces. The ratio of flue area to hearth opening is a factor in smoke roll-out. Fireplaces with more than one face open to the room are particularly prone to spillage. To vent properly in a tight house, an open fireplace would need to be of perfect design in all respects and be combined with a sophisticated house pressure management system.

Back to the Table of Contents