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.

This article explains the need for adequate combustion air for fuel burning appliances in buildings, for both safety and for proper equipment operation.

Sketch at page top and accompanying text are reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

The sketch and the article note 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.

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 in this IMAGE) 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.

• 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.

 

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:

• Combustion Air Details for Tight Houses - [PDF] use your browser's back button to return to this page
• Combustion Air Details for Tight Houses - [PDF] part 2
• Combustion Air Details for Tight Houses - [PDF] part 3

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Should I bring fresh air to the return air plenum or to the building utility room?

Hello. I have negative air pressure in my finished basement. I do not see any fresh air intake for combustion. I think I need to add fresh air to help with the negative pressure. Should I connect fresh air intake duct to my return air plenum or should I bring fresh air intake duct into the utility room that has gas furnace, gas hot water heater and gas dryer? On 2020-11-18 by techpainting -

Reply by (mod) -

Fresh air for combustion needs to be made availble to the burner, not to the building air plenum

How big does the combustion air supply hole have to be?

How can I figure how large a combustion air hole to make in my door for a stacked laundry in a closet inside our house? On 2020-06-19 by Jon -

Reply by (mod) -

You need a better one square inch of opening per thousand BTUs. That's before considering the reduction in flow because of a louvered on the vents. Yes you can provide outdoor air or possibly sufficient air from indoors. - same as for Cole

Combustion air for boiler in a closet

My 125000 BTU boiler is in a 4 foot by 5 foot closet with absolutely no ventilation will this greatly reduce the efficiency of the boiler and also and the drawing at the top of the page are the permanent openings considered horizontal vents On 2020-01-02 by Cole Million -

- by (mod) -

You need a better one square inch of opening per thousand BTUs. That's before considering the reduction in flow because of a louvered on the vents. Yes you can provide outdoor air or possibly sufficient air from indoors.

Watch out. Inadequate combustion air means your heating system is dangerous, risking fatal carbon monoxide gas poisoning.

Can I seal off combustion air supplies where I have direct venting equipment?

I have a utility room with a sealed combustion / power direct vent gas-fired Bryant furnace and a natural draft gas-fired 80 gallon water heater.

Combustion air is provided low from the crawl space and high from the attic. The utility room is off the garage and is entered via a door from the garage.

When the water heater is off, lots of warm heat flows from the utility room to the attic in the winter and is a contributor to ice dams. I am considering changing the water heater out for a power direct vent / closed combustion HTP Phoenix unit.

If the utility room has two power direct vent appliances (furnace and water heater) both with direct outdoor combustion air and each with its own exhaust (both with its own concentric inlet/exhaust pipe), can I permanently close and seal off both the floor and ceiling air supplies

? Both appliances would no longer require combustion air from the room. I've talked to my local inspector and he said it would be permissible to close off the upper and lower combustion air sources.

Does the code address this anywhere? On 2019-06-02 by Steve D -

Reply by (mod) -

I don't think you'll find an explicit code Citation for every possible mechanical system equipment arrangement.

Instead the way the code writers and I both would duck and weave on this actually interesting question would be to suggest that if you are meeting the installation requirements set by the manufacturer of the equipment that would be the final technical word.

So I expect you're fine in what you want to do. It should be easy enough to identify the brand and model of each of the pieces of equipment and to review what they tell you about combustion air.

Negative air pressure in my basement - how do I fix it ?

Hello. I have negative air pressure in my finished basement. I do not see any fresh air intake for combustion.

I think I need to add fresh air to help with the negative pressure. Should I connect fresh air intake duct to my return air plenum or should I bring fresh air intake duct into the utility room that has gas furnace, gas hot water heater and gas dryer? On 2020-11-18 by techpainting -

Reply by (mod) -

Fresh air for combustion needs to be made availble to the burner, not to the building air plenum.

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Continue reading at COMBUSTION AIR DEFECTS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see COMBUSTION AIR REQUIREMENT FAQs - questions & answers posted originally at this page

Or see these

Recommended Articles

• BACKDRAFTING HEATING EQUIPMENT - home
• CARBON MONOXIDE - CO
• CARBON MONOXIDE - CO
• CARBON MONOXIDE WARNINGS: HOME HEATERS
• COMBUSTION AIR DEFECTS
• COMBUSTION AIR REQUIREMENTS - home
• COMBUSTION GASES & PARTICLE HAZARDS - dangers of inadequate combustion air
• COMBUSTION PRODUCTS & IAQ - relationship between fuel burning appliances and building indoor air quality
• COMPLETE COMBUSTION, STOICHIOMETRIC

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COMBUSTION AIR for TIGHT BUILDINGS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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