Heating appliance backdrafting:
This article discusses the causes and cures of back drafting and flue gas spillage hazards in tight houses.
We explain the difference between flue gas spillage and back-drafting. While the trend is toward power-vented appliances, most furnaces, boilers, and water heaters still use atmospheric or “natural” venting. Atmospheric venting relies on the natural buoyancy of warm air in the flue or chimney to carry exhaust gases from the home.
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The strength of the draft depends on the temperature difference between the flue gases and outside air, the height of the chimney, and the indoor air pressure.
In newer, tighter houses, significant negative pressures can be generated by kitchen and bath exhaust fans, gas dryers, and unbalanced air flows in the home’s air distribution system.
Unbalanced pressures can also be caused by leaks in return ductwork, by the use of building cavities as ducts, or by the simple closing of bedroom doors in homes with a central return register. Leaky return duct- work in a basement may be enough to backdraft a water heater or furnace.
See COMBUSTION AIR for TIGHT BUILDINGS for details on how to provide adequate combustion air in tight buildings.
Spillage vs. backdrafting. If a naturally vented appliance lies in an area of the house with strong enough depressurization, the flue gases will spill into the home. When the flow reversal lasts for 30 seconds to a minute, it is called spillage; longer sustained spills are called backdrafting, a far more serious condition.
If the gas-log fireplace shown at left is not properly vented, spillage of combustion gases including possibly carbon monoxide into the building interior may be continuous. In tight quarters there is also the risk of oxygen depletion.
Watch out: Both of these are potentially fatal hazards.
While the manufacturer of the gas log shown in our photograph advertised that the burner includes a safety device that would shut off the fire in the event of dangerous oxygen depletion, we found that we were unable to operate this appliance without setting off the carbon monoxide detector alarm in the room, and the alarm continued to sound even when the detector was placed in an adjoining hallway.
Continuing from from Best Practices Guide to Residential Construction:
Once backdrafting begins and the flue gets cold, it may be sustained for a long time. Research has shown that negative pressure of as little as 5 Pascals (Pa) creates a risk of backdrafting with naturally vented boilers and furnaces. Numerous studies have documented the prevalence of high negative pressures and frequent spillage in new homes built to current codes but not intentionally built airtight.
If the heating equipment is well adjusted and has adequate combustion air, the flue gases will contain primarily water vapor and carbon dioxide, along with nitrous and sulfur oxides, and particulates.
If the burner is malfunctioning for any reason, it may put out large quantities of carbon monoxide and turn a backdrafting situation deadly. Fireplaces and poorly sealed woodstoves are most likely to reverse flow late at night when the fire is smoldering, producing a weak draft and high levels of CO.
There are three key elements to preventing backdrafting:
Watch out: as we discuss also
at AIR FILTER LOCATION, a common backdrafting problem found by heating service technicians and by experienced home inspectors is the addition of an improperly-located return air or "makup air" opening in a warm air heating system return duct or in the return duct at an air handler that works for both air conditioning and heating air supply.
When the supply of cooled or heated air to occupied building spaces is inadequate and the cause is suspected to be inadequate return-air in the system duct design, an amateur retrofit "repair" for this condition is to simply cut additional return air openings in the return duct.
But if those openings are cut too close to a heating appliance, especially gas fired heating appliances, the in-draft created by the return air opening cut close to the gas burner can interfere with adequate combustion air supply to the burner, causing dangerous or even fatal carbon monoxide production, and backdrafting into the occupied space.
See COMBUSTION AIR for TIGHT BUILDINGS for additional details. - Ed.
A chimney or flue that is too large, too small, or blocked by a bird’s nest or loose brick will not draw properly and will be prone to spillage problems. Uninsulated chimneys on outside walls are also prone to poor draft and to condensation problems that can deteriorate flue materials. These problems should be fixed first before addressing problems inside the house.
at CHIMNEY INSPECTION DIAGNOSIS REPAIR we discuss chimney inspection and diagnosis including unsafe venting and fire hazards.
Heating systems with fan-powered exhaust systems can withstand higher negative pressures than natural-draft appliances. Some types of fan-powered systems are much better than others, however. In order of effectiveness, the choices are:
By comparison, an atmospherically vented furnace can backdraft with as little as 5 Pa of negative pressure, and a gas water heater will have spillage at 2 or 3 Pa. Fireplaces can start having problems at about 3 Pa. Canadian codes limit negative pressures in homes with atmospherically vented equipment to 5 Pa. U.S. codes do not currently address the issue.
To keep indoor depressurization to a minimum, do not oversize bathroom and kitchen fans, and avoid the use of downdraft and island fans, which can draw 600 cfm or more. If large fans must be used, they should be interlocked with a supply fan to provide makeup air.
Or see VENTILATION, BALANCED
Canada’s 1995 National Building Code requires that in homes with fuel- burning appliances vented through a chimney, any exhaust fan with a net capacity greater than 160 cfm must have fan- supplied makeup air.
The makeup air fan should be sized to reduce the net exhaust rate to no more than 160 cfm and can be delivered to an adjacent room or through the forced-air distribution system. For example, a 300 cfm exhaust fan should have at least 140 cfm (300 minus 160) of makeup air.
How much an exhaust fan will depressurize a house depends on the tightness of the house. A 1993 study of several newly built energy-efficient homes in Minnesota found that exhaust airflows of 300 to 550 cfm depressurized the homes to 5 Pa, the level at which natural-draft appliances start having spillage problems.
Other studies indicate that a 600-cfm exhaust fan can produce negative pressures from 3 to over 20 Pa, depending on house tightness. Without an adequate source of makeup air, a fan this size (or a combination of exhaust fans running at the same time) will pull air from the path of least resistance—often a nearby chimney or flue. Unless makeup air is provided, exhaust fans of this strength should not be used in homes with chimneys.
In homes with the potential for back- drafting, a simple test can be conducted to determine the likelihood of problems:
Perform the test with the air handler both on and off, since unbalanced airflows can be a significant factor. If smoke spills into the room for more than 30 seconds at any combustion appliance, the home has a potential backdrafting problem that requires attention.
A more scientific procedure for determining backdrafting potential, using a pressure gauge, can be found in Step 7 of the “Recommended Procedures for Safety Inspection” in Appendix H of the National Fuel Gas Code.
Traditional open fireplaces and older leaky woodstoves burn very inefficiently and produce hundreds of chemical compounds, including carbon monoxide, organic gases, particulates, and some of the same cancer-causing agents found in tobacco smoke. Minor spillage of these pollutants occurs regularly, primarily when starting or stoking the fire. However, the larger concern is when the fire smolders late at night, producing high levels of CO and a weak draft. Backdrafting at this time can be dangerous or even fatal.
Another problem, particularly with fireplaces, is created when the fire is roaring and drawing up to 400 cfm of combustion air. At this point, its voracious appetite for air can cause backdrafting in other combustion appliances such as a gas water heater. Also, the need to reheat all the makeup air drags down the fireplace’s heating efficiency to less than 15% and, if the fireplace is allowed to smolder all night, it becomes a net heat loser.
Woodstove efficiency has improved dramatically in response to EPA emissions standards (begun in 1988 and updated in 1990), which apply to most freestanding wood stoves and to fireplace inserts with air-supply controls and tight-fitting doors. To meet these standards, manufacturers use either a catalytic converter, similar to the ones used in cars, or a reengineered firebox.
The new fireboxes have primary and secondary combustion zones capable of reaching system efficiencies of 60% or more and reducing combustion air intake to as little as 10 cfm. If installed with an outdoor air supply, these can be successfully de- coupled from household air pressures.
While many fireplaces are fitted with glass doors, and some have outside air intakes, nearly all of the glass doors leak air. Even with low levels of depressurization, these fireplaces can still backdraft, and the fireplace’s outdoor air supply might become the makeup air for the kitchen range hood or other exhaust fans, drawing fireplace fumes along with it. The best solution is an airtight fireplace insert.
-- Adapted with permission from Best Practices Guide to Residential Construction.
Continue reading at SPILL SWITCH & COMBUSTION AIR TESTS to detect flue gas spillage that may be due to backdrafting, or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.
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(Feb 7, 2015) Lori said:
we are having a major back draft issue at our place of business. we recently had to remove workers and clients out of the building because of high Carbon monoxide levels, this has happened 3 times over the past 2 weeks. We have changed nothing in the building, we have had 4 different professionals in to check the problem and NO ONE can find a reason!! We have no idea what to do...Please help
Thanks for asking, Lori but gee: I surely can't be smarter than 4 different on-site professionals who will have seen a lot more than I.
Basically you're looking at
- inadequate combustion air
- inadequate draft or a blocked flue
- improper heater operation
Has anyone inspected the chimney flue for leaks or blockages?
Certainly I would not occupy such a building until you think the problem has been identified and fixed, and even then I'd be darn sure that there are properly located and installed carbon monoxide CO detectors and smoke detectors.
(Feb 17, 2015) Charles Miller said:
I have a similar problem with a home furnace that gets it's combustion air from the garage. There is inadequate combustion air. Apparently, opening the garage door a smidgeon allows more combustion air inside although it also creates a negative pressure in the house that cancels the improvement. This is a real problem we've had for years, with an old furnace and a new one.
My skin turns blue. I have a cough at night that drives me crazy. WHen I leave the house in the morning it takes about an hour so that I can breathe. Technicians are clueless. Any input from anyone would be valued. firstname.lastname@example.org
Watch Out: for safety, be certain that you have properly located and installed working carbon monoxide detectors and smoke detectors. CO poisoning can easily be fatal. You can die.
Get everyone out of the building immediately if you think that you are being exposed to carbon monoxide.
Then you can call for heating system inspection and repair by a trained expert.
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