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

Open topped partitions for combustion air (C) Daniel Friedman

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:

See COMBUSTION AIR REQUIREMENTS 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

 




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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-11-22 - by (mod) -

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

On 2020-11-18 by techpainting

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 2020-06-21 - 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

On 2020-06-19 by Jon

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 2020-01-02 - 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.

On 2020-01-02 by Cole Million

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 2019-06-03 - by (mod) -

I don't think you'll find an explicit code Citation for every 19 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.

On 2019-06-02 by Steve D

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 2020-11-22 - by (mod) -

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

On 2020-11-18 by techpainting

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

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 2020-06-21 - 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

On 2020-06-19 by Jon

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 2020-01-02 - 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.

On 2020-01-02 by Cole Million

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 2019-06-03 - by (mod) -

I don't think you'll find an explicit code Citation for every 19 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.

On 2019-06-02 by Steve D

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?


...

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