Backdrafting Appliances: Causes and Fixes

September 10, 2013
September/October 2013
This online-only article is a supplement to the September/October 2013 print edition of Home Energy Magazine.
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Homeowners and contractors need to know that when they replace 70%- or 80%-efficient furnaces with 90%-efficient furnaces and leave standard atmospheric draft water heaters orphaned, old atmospheric water heaters may backdraft, creating CO problems for the occupants.

Non-fire-related CO poisoning is responsible for approximately 15,000 emergency department visits and nearly 500 deaths per year in the United States, according to CDC data from the National Poison Data System, 2011. My organization, Therma-View Infrared and Energy Consultants, conducted research on data provided by a third-party quality control inspection firm that inspected more than 5,000 homes that received energy retrofit measures through energy efficiency programs and found that

An example of an atmospheric draft water heater that is of concern and has the largest possibility of back drafting. (Pure Energy Coach)

As air is heated it becomes more buoyant and moves to the top of the building. This is called the stack effect. This puts the top of buildings under a higher pressure and the lower part of the building more negative (suction). This alone could cause the CAZ to go more negative that the chimney. Note the plastic at the top of this building is ballooned out while the bottom is being sucked in. (Pure Energy Coach)

This photo shows leaky filter slots that suck air from the CAZ. It also shows registers installed in return ductwork located in CAZ pulls most of the return air from the CAZ and not the house. This also causes comfort problems. (Pure Energy Coach)

Rust stains on the draft diverter of this atmospheric draft hot air furnace is a sure indication that the flue gasses are not going up and out of the building and entering the CAZ and into the home! (Pure Energy Coach)

An example of an atmospheric draft furnace. (Pure Energy Coach)

An example of a gas water heater commonly vented to the chimney. (Pure Energy Coach)

  • 60% of houses with 80% or lower-efficiency gas furnaces and natural-draft water heaters had inadequate draft pressure,

  • 50% of orphaned water heaters spilled flue gasses for longer than one minute upon completion of energy retrofit work to the building; and

  • 10% of the combustion appliances had dangerous levels of CO.

Let’s look at a few of the problems that, alone or in combination, can result in CO poisoning. First we need to understand how 70%, 80%, and 90% combustion appliances vent flue gases to the outside.

Many older homes with atmospherically vented appliances or existing 80% efficient (also known as mid-efficiency) gas appliances use the same chimney to vent their combined combustion gases out of the building. These chimneys were originally designed and sized to handle the combined by-products of combustion, namely, water vapor (H2O), carbon monoxide, (CO) carbon dioxide (C02) nitrogen oxides (NOx), other particulates, and the heat of the flue gases.

What makes 70% and 80% appliances draft, or remove these by-products, is the heat of the flue gases. This heat creates a negative pressure, or suction, inside the chimney, pulling the flue gases out of the building.

This suction is sometimes referred to as draft pressure. The draft pressure in the vent and chimney has to be more negative than the air pressure in the combustion appliance zone (CAZ). The CAZ is the area where the appliance is located, usually in the basement, but can be in the main body of the home if the home has no basement. If the CAZ is under more negative pressure, or suction, than the chimney, the combustion by-products will come back down the chimney, into the CAZ, and then into the building. When combustion gases come back down the chimney and into the building, it’s called backdrafting. If backdrafting continues and there is CO present in the flue gases, the occupants can get sick or even die.

High-efficiency or 90+% combustion furnaces and boilers do not vent flue gases through a standard chimney. Since these gases are relatively cool—about 100°F—they are vented through a sealed plastic vent pipe. All 90% appliances have built-in combustion blowers that draw air from the outside into the combustion chamber and then push the flue gases to the outside. The vent pressure in 90% furnaces would be positively pressurized with reference to the CAZ. Since there is a fan pushing the combustion gases out of the building, there is no chance that these appliances will backdraft. These appliances have pressure-sensing controls that shut down the unit if there is a loss of pressure in the vent, which, for example, may be caused when snow covers the vent pipe outside.

Why CAZs Are Depressurized

There are a few reasons why the CAZ may be under more negative pressure than the chimney and pull flue gases down the chimney and into the home.

Builders and remodeling and home performance contractors are making buildings tighter than ever before. This air sealing is a good thing. However, if done improperly, it can lead to serious problems with some types of combustion equipment.

Warm air, such as air in the CAZ during the winter, moves naturally to a colder area, such as unheated attic spaces, and then through the attic vents to the outside. This is called the stack effect. This air movement from the CAZ to the attic can cause suction on the CAZ, placing it under more negative pressure with reference to the rest of the building and the outside. This air from the CAZ moves through plumbing chases, electrical chases, HVAC duct chases, and unsealed building cavities. If air sealing of the CAZ is done at the perimeter of the CAZ (that is, at the exterior walls, rim joists, and band joists), which is usually easier than air sealing the top of the building, the CAZ often does not have enough air to create a draft in the chimney.

Other reasons the CAZ may experience a negative or suction pressure causing back drafting of appliances are:

  • Leaky return ductwork in the CAZ sucks air from the CAZ area and can suck flue gases down the chimney. This also makes homes heat and cool unevenly.

  • Grilles are sometimes installed in return ductwork to reduce a high heat rise across the appliance heat exchanger. This usually happens when the ductwork hasn’t been sized properly. The easiest way for the contractor to remedy the problem is to install a register grille into the return in the CAZ. This will provide all the air the appliance needs to reduce the high heat rise, and will prevent the combustion chamber and the heat exchanger from burning out early. Installing grilles in return ducts usually creates a big negative CAZ pressure in the CAZ.

  • Leaky filter slots at the furnace pulls air from the CAZ causing depressurization.

  • Clothes dryers pull air out of the building at the rate of approximately 150–200 CFM. This air must be replaced by makeup air. If the clothes dryer is located in the CAZ, the dryer can get the makeup air from down the chimney.

  • When homeowners or contractors replace 70% or 80% furnaces with 90+% furnaces, which no longer use the chimney, the water heater is left venting into the chimney alone. This is known as an orphaned water heater. The chimney is now sized too large for the flue gases. The draft temperature is so low in the water heater that it often can’t overcome the slug of cold air inside the chimney, and the flue gasses can't escape so they come back into the CAZ.
  • When homeowners use their bathroom exhaust fans, kitchen range hoods, dryers, attic fans, or anything else that pushes or pulls air out of the house, they are pulling air from the inside and exhausting it to the outside. If the building is closed up in the winter, or if the A/C is on and the house windows are closed in the summer, the replacement air will come from the easiest and closest hole in the building, which is usually the chimney. If the water heater comes on, this will cause it to backdraft.

How to be Safe: Conduct a Worst-Case CAZ Depressurization Test

Technicians need to create worst-case depressurization conditions in the CAZ and then measure the air pressure in the CAZ relative to the outside, using a pressure gauge. This means that the technician must configure the worst conditions in the building that the homeowner might create—conditions that would put the CAZ under the most negative pressure. In other words, how can the technician create the conditions in the home so that the most air is sucked out of the CAZ? This is done by turning on all of the exhaust devices and positioning interior doors. If the home has a forced-air heating system, the furnace fan will be part of the test. The furnace fan is needed to check for return duct leakage and unsealed furnace filter slot openings, and to measure their impact on the pressure in the CAZ. In all homes with forced-air systems, and especially in homes with a central return and supply registers in other rooms, closing the doors to the rooms with supply registers will increase air pressure in those rooms relative to the main body of the house. The main body of the house will be under more negative air pressure than the rooms with closed doors. When this happens, the connections in the building to the CAZ will cause more suction on the CAZ, causing it to be under more negative pressure with reference to the outside. It is very important to check the CAZ under summertime conditions, because water heaters draft poorly when outdoor temperatures are warm, and there is less stack effect in the chimney to help with the drafting of combustion appliances.

Unless HVAC, remodeling, and plumbing contractors who install and service atmospheric draft gas water heaters completely understand the house as a system, and learn how to conduct the worst-case CAZ depressurization test, they will be facing a huge liability. These tests can be a matter of life and death. The ACCA has identified this issue and has included guidance on how to conduct the depressurization test in its Standard 12.

The solution would be to train contractors to conduct the worst-case CAZ depressurization test at a minimum when starting a job and when finishing a job, and when replacing an 80% gas furnace with a 90% gas furnace.

The simplest test for homeowners to conduct on the CAZ can be done as follows. Close up the house to simulate winter conditions (all windows and exterior doors closed); turn on all appliances that suck air out of the building (range hoods, bath exhaust fans, dryers); and turn up the thermostat on the water heater until it comes on. Then hold a blown-out match or other smoke device at the draft hood to see if the smoke goes up the vent or back toward you. The smoke should go up the vent after a minute or two. If it doesn’t, open a window or door to the outdoors in the CAZ. If the smoke goes up the vent, you have a CAZ depressurization problem, which may be caused by a chimney chase open to the attic or another cause as mentioned above.

learn more

The depressurization test for the CAZ is described in ACCA Standard 12, “Existing Home Evaluation and Performance Improvement for Residential One- and Two-Family Dwellings and Townhouses Not More Than Three Stories Above Grade.”

Therma-View Energy and Pure Energy Coach LLC have a series of short videos showing the issues, tests, and solutions described in this article. For a list of these videos, go to and click on “Watch and Learn.”

How to Solve the Problem

There are a couple of solutions to this problem.

  1. Replace the atmospheric draft water heater with a power draft water heater or a tankless water heater. Or change the gas water heater to an electric water heater. The downside of this solution is that these options can be expensive.

  2. Add makeup air to the CAZ. This is simply done by installing a duct or grille from the outside to the CAZ. Another option is to add a so-called fan in a can. On call for heat, these devices energize the fan that draws outdoor air into the CAZ to aid in drafting flue gasses.

Homeowners and contractors need to be aware that when replacing 70% and 80% efficient combustion appliances, and when allowing atmospheric draft water heaters to remain, there is a chance of the water heater combustion gasses to coming back into the CAZ and home. The only way to be safe is to do the worst case depresssurization combustion appliance zone test or replace the atmospheric draft water heater at the same time.

Larry D. Armanda is the owner of Therma-View Infrared and Energy Consultants, based in Williamsport, Pennsylvania. A pioneer in the building science and home performance industry for over 30 years, he is certified as a Level I and Level II Infrared Thermographer.

In March 2012, Larry and A. Tamasin Sterner, president and chief coach of Pure Energy Coach, LLC, presented the effects of home depressurization on the CAZ to over 40 representatives from ACCA, DOE, the American Gas Association, the American Council for an Energy-Efficient Economy, and other organizations. To assist in their training for contractors and program managers, they built a 30-foot motor home, “COACH,” which they use as a mobile training lab.

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