Combustion air details for tight houses  (C) Daniel Friedman Combustion Air Details for Heating Appliances

  • COMBUSTION AIR for TIGHT BUILDINGS - CONTENTS: Combustion air: how to provide adequate combustion air for combustion appliances in newer tighter buildings and in older homes that have been made more airtight to save on energy costs. How to provide adequate combustion air without wasting building energy through air or heat leaks. Adequate combustion air is essential for building safety (avoiding potentially fatal carbon monoxide poisoning) and essential for proper operating of fuel-burning appliances
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This article discusses methods for providing adequate, safe combustion air for fuel-burning appliances in tight buildings - how to provide outside combustion air for heating appliances. Sketch at page top and accompanying text are reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

This article explains the need for adequate combustion air for fuel burning appliances in buildings, for both safety and for proper equipment operation. Figure 1 (at page top - click to enlarge) notes that by confining the gas furnace in a separate room, adequate air for draft and combustion can be supplied [from outdoors] without adding infiltration to [and cooling] the house.

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Details for Providing Adequate Combustion Air Without Losing Building Heat

The author notes that

... standard formulas used to compute whether the indoor air supply was sufficient assumed a minimum rate of air infiltration of 0.5 air changes per hour.

In newer, tighter homes and in older retrofitted houses, air for combustion is not as readily accessible. If an appliance does not get enough air for complete combustion, its efficiency drops [increasing heating costs], soot can build up [potentially leading to fatal carbon monoxide poisoning in some cases], condensed water can collect in and corrode the flue [dangerous flue gas leaks].

Moreover, in a tight house, an exhaust fan - for example in the kitchen, greenhouse, or clothes dryer [or a whole house fan or bathroom exhaust fans] - may create a negative air pressure strong enough to draw toxic gases back into the house.

For both safety and energy reasons, then, more designers are deliberately supplying outside air to combustion appliances.

Simple Homeowner Tests for Adequate Combustion Air & Adequate Chimney Draft

An easy test of adequate draft in a gas appliance is to hold a just-blown-out match near the vent hood and see if the smoke is drawn up the flue. This chimney draft test should be performed under worst conditions: in warm weather (the chimney stack pressure will be lower in warm weather), with the house closed up (shut windows and doors, especially the windows and doors feeding the utility room where the appliance is located), and running all of the building's exhaust fans at once.

See BACKDRAFTING HEATING EQUIPMENT for a detailed description of the test procedure to check for adequate combustion air and adequate draft.

Two Methods for Supplying Combustion Air for Heating Appliances in Tight buildings

The National Center for Appropriate Technology (NCAT) [and other sources such as the Uniform Mechanical Code and the National Fuel Gas Code] suggest looking for carbon build-up around the burner and looking for flue corrosion. Two approaches are given for supplying outdoor combustion air to fuel-burning (oil, gas, wood, coal) heating equipment:

  • The heating appliances are fully isolated from the living space with partitions. Figure 1 (page top and shown in more detail here) provides two vents to the outside, one within a foot of the ceiling and one within a foot of the floor. If vertical ducts are used to bring combustion air to the appliance each vent should be sized at one square inch of free vent area per 4000 BTUH of the appliance input rating.

    The high and low vents allow heat to dissipate from the enclosure and they allow combusting gases to escape should there be any backdrafts. The double vent arrangement permit a freer flow of combustion air to the heating appliance such as a furnace or gas fired water heater.

    In cold climates steps may be necessary to protect plumbing in the outside-air-cooled utility area from freezing, and the specific details of any combustion air system should be checked against local building codes and importantly, against the installation instructions from the equipment's manufacturer.

    The figures given above are for gas fired appliances. At oil burner school we were taught that for oil burning appliances we wanted to see one square inch of free vent area per 1000 BTUH of oil fired appliance input rating. Remember this is "free area" so vents that are louvered and screens need to be larger to allow for the effects of that obstruction - DJF.

Open topped partitions for combustion air (C) Daniel Friedman

  • Confining partitions are left open at the top [saving construction costs] (image at left). The open-topped partition has a duct delivering fresh air to the bottom of the enclosure. The logic is that the basin created by the partitions will trap [the incoming] cold air and minimize its mixing with the conditioned [heated] house air. NCAT provides a booklet with these details.

Direct-Vented Combustion Appliances

Some heating appliances, furnaces, boilers, and water heaters, are designed to isolate the combustion process from the living space entirely, avoiding the need for complex combustion air and venting schemes.

Direct-vented combustion appliances are designed and tested to burn fuel and draw combustion air properly even when high winds hinder draft. Typically such systems include two sets of piping or ducts between the appliance and outdoors, one bringing combustion air in directly to the appliance burner, and a second venting combustion air outside. The two vents might appear on some systems as a single larger diameter double-walled pipe containing actually two vents, the smaller located inside the larger.

Combustion Air for Air Tight Woodstoves

Typical airtight woodstoves require only 10-25 cfm of combustion air - much less than an open fireplace (50 to 150 cfm or more) or to older non-airtight woodstove. But in tighter homes it may be necessary to provide combustion air or a draft inducer fan even for these appliances.

Just as modern energy codes provide a vent to supply outside air to open fireplaces, outside combustion air can be supplied to an airtight woodstove through a floor vent or a wall register that is ducted in turn to outdoors - a method that adds to cold air infiltration into the building.

A more promising approach described in the original article Combustion Air Details for Tight Houses(page 3) [PDF] is to supply combustion air through fixed ducts right to the appliance air inlet.

See COMBUSTION AIR for additional details about the requirement for combustion air. See COMBUSTION GASES & PARTICLE HAZARDS for an explanation of the dangers of inadequate combustion air.

See COMBUSTION PRODUCTS & IAQ for the relationship between fuel burning appliances and building indoor air quality. More about carbon monoxide - CO - is at CARBON MONOXIDE - CO and at CARBON MONOXIDE WARNING.

Also see the safety warnings at BACKDRAFTING HEATING EQUIPMENT.

Here we include solar energy, solar heating, solar hot water, and related building energy efficiency improvement articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Original article in PDF form:


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