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Combustion Safety and the Worst-Case Depressurization Test

Make sure that if all the exhaust fans in a home are running, you don't end up with dangerous exhaust inside the living space

Posted on Mar 30 2010 by Peter Yost

The problem
Vented combustion appliances—gas and fuel oil water heaters, furnaces, boilers, even fireplaces—need to exhaust all of their combustion by-products, all the time, outside the home. An especially dangerous combustion exhaust component is carbon monoxide because it is odorless, colorless, tasteless and quite toxic. If you have no combustion appliances or they are sealed-combustion, no worries. But if any of these appliances is atmospherically-vented or has induced draft or even if they are power-vented, we need to test to see if conditions in the home can create backdraftingIndoor air quality problem in which potentially dangerous combustion gases escape into the house instead of going up the chimney. (backdrafting is combustion exhaust rolling out of the intended exhaust pathway and into the home).

The test
There is actually a series of combustion safety tests but we are going to focus on the simplest and most often the first test, the Worst Case DepressurizationSituation that occurs within a house when the indoor air pressure is lower than that outdoors. Exhaust fans, including bath and kitchen fans, or a clothes dryer can cause depressurization, and it may in turn cause back drafting as well as increased levels of radon within the home. Test, also known as the worst-case CAZ—combustion appliance zone—test. So what is the worst case? Essentially, you turn on every device in the home that can create negative pressure in the room or space in which the combustion appliance is located and compare that pressure to the outside. Here is a list of these devices:

• Bath exhaust fans – typically pull somewhere between 25 and 100 cubic feet per minute (cfm) out of the house)
• Kitchen hood (if it really vents to the outside) – usually around 100 – 150 cfm but downdrafting or commercial types can exhaust more than 1500 cfm!
• Clothes dryer – usually around 150 cfm
• Laundry room exhaust fan – about the same as a bath fan
• Attic exhaust fan – see below
HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. air handler – see below

There are three things to note about this list:

  1. Attic exhaust fans: Aren’t these just to cool off the attic and the attic is outside of the conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. ? They are, but if the ceiling plane of the top floor is not airtight, attic exhaust fans can end up depressurizing a home’s interior.

  2. Air handlers: They just move air around the INSIDE of the home, right? Well, that is what they are SUPPOSED to do, but if there are ducts that run through unconditioned space that leak—or if there is unbalanced delivery/leakage between the supply and return side of the system that expresses itself in the space with a combustion appliance—the air handler can de-pressurize the CAZ. So, we need to test worst case with and without the air handler on, if the home has a forced-air HVAC system.

  3. Whole-house attic ventilation fans: These are mighty large exhaust fans; shouldn’t they be on the list? While these fans can certainly depressurize a home big time, they are not included in the CAZ test. It is assumed that since these fans are for cooling by air movement, enough windows are open during operation of the fan that depressurization and subsequent backdrafting are not an issue.

The equipment
The test requires just two pieces of equipment—a manometer (pressure gauge, pictured above) and a smoke source (several different types, but the “toy” smoke generator pictured is the most fun).

The procedure
After surveying the home for combustion appliances and exhaust fans, the procedure goes like this:

  1. Seal the house: close all exterior windows and doors.
  2. Turn off all combustion appliances.
  3. Close all interior doors.
  4. Measure the baseline pressure. This is the the pressure relationship between the combustion space and the outside with no exhaust fans on and should be no more than a pascal or two. NOTE: Wind can really make it difficult to get good readings. Sometimes turning fans off and on is necessary to get decent readings.
  5. Turn on all the fans and measure the worst-case depressurization.

The results

Here is a representative chart (from the CMHC Chimney Safety User’s Manual Reference #4) for safe limits for the CAZ test.

Combustion Appliance Chimney or Flue Height (ft) Unlined Chimneys on Exterior Walls Metal Lined, Insulated or Interior Chimneys
Gas Furnace 13 or less - 5 Pa - 5 Pa
Gas Boiler 14 - 20 - 5 Pa - 6 Pa
Gas Water Heater More than 20 - 5 Pa - 7 Pa
Oil Furnace 13 or less - 4 Pa - 4 Pa
Oil Boiler 14 - 20 - 4 Pa - 5 Pa
Oil Water Heater More than 20 - 4 Pa - 6 Pa
Fireplace N/A - 3 Pa - 4 Pa
Wood Stove N/A -10 Pa -10 Pa
Induced Draft Appliance N/A -15 Pa -15 Pa

If you get test results that exceed these limits, there are only really four options:

1. Replace the appliance with a power-vented or sealed combustionCombustion system for space heating or water heating in which outside combustion air is fed directly into the combustion chamber and flue gasses are exhausted directly outside. unit.
2. Switch out to a non-combustion appliance.
3. Isolate the combustion space (build a room with dedicated outside air to the room with the combustion appliance)
4. Ensure operation of exhaust fans that avoids worst-case depressurization, backed up by a CO monitor in the combustion zone.

In the first picture above, the manometer is reading -6.1 Pa during the worst-case depressurization test on my own home. This is with the clothes dryer, two bath exhaust, and a kitchen exhaust hood running and an average base line pressure reading of - 2.9 Pa (bit of a windy day but not gusting too badly). That's a CAZ test of -3.2 Pa, well within the safety limits for a natural draft oil boiler with a center chimney more than 20 feet tall. We do keep a CO monitor in the oil boiler combustion zone as a bit of a belt-and-suspenders approach.

For more information on all of the combustion safety tests, visit the Building Performance Institute website.

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  1. Peter Yost

May 13, 2010 1:53 PM ET

Worst Case Safety Limits
by Naomi Davies

A most helpful supplement to using the BPI Standards alone.
Thank you

Jun 16, 2010 7:41 PM ET

Dealing with wind
by Frank Richter

Wind can effect the baseline reading if the exterior hose is exposed to wind. Placing the end of the hose inside a large soda bottle can protect it from the wind while still yielding an accurate outside reading. If the reading fluctuates wildly, wind might be the culprit.

Jun 17, 2010 5:44 AM ET

Dealing with wind and pressure readings
by Peter Yost

Hi Frank -

You are right; wind is quite often the culprit when outside pressure readings are erratic. I have tried shielding the hose end with a cardboard box with mixed results, but the large soda bottle is a slick idea. Will have to try that--thanks.

Jun 18, 2010 7:07 PM ET

When one door closes, another door opens
by Ed Voytovich

Simply closing all the interior doors does not necessarily create the worst case. One certainly shouldn't close the doors leading to rooms where there are exhaust fans, dryers, or what have you. There's really no way to predict the effect of the air handling system on building pressures without trying various combinations of door configuration to achieve the worst case. If there are no cold air returns in the bedroom, or if a particular supply duct is leaky to the outside, or if the dresser is in front of the register, then the pressures are affected.

The large bottle to attempt to control the effects of wind won't work because the bottle is "connected" to the ambient pressure by the opening through which the manometer hose passes. We can't fool Mother Nature! I'd recommend the long-term averaging setting on the manometer.

I once overheard a building performance contractor from a mountainous area in the Carolinas complain at the Energy Conservatory booth at a conference that their manometers were no good because they don't work when the wind blows. The contractor had it backwards: it's not that they work poorly, it is exactly because they work so well that digital manometers are all over the place in the wind.

If somebody told you it was going to be easy, I hope you didn't believe them!

Nov 12, 2010 6:19 PM ET

"Airtight Woodstove" Defined?
by Todd VanOsdol

There is a large difference between the depressurization limits for open fireplaces (-3pa) and "airtight" woodstoves (-10pa). However, there is also a large grey area between these two extremes of solid-fuel appliances that have varying levels of airtightness. We often see many woodstoves that are hardly airtight and also run into fireplaces that seal up fairly well. Is there a standard by which we determine whether to use -3pa or -10pa as an acceptable amount of depressurization for a particular solid-fuel appliance?

Nov 13, 2010 7:24 AM ET

Standard for depressurization limits for solid fuel appliance
by Peter Yost

Good question Todd. But I am not aware of any standard and I think your field experience may be the best answer we have. If we can "slide" the depressurization from around -3 to -10 Pa by adding exhaust loads one at a time, checking for backdrafting as we go, then that might be the best we can do.

But maybe there is a standard and others will know of it. Any takers?

Dec 7, 2010 2:35 PM ET

re: "Airtight Woodstove" Defined?
by David Whte

to make things more complex, the report does introduce the term "open wood stove" in a previous table, but does not mention open wood stoves in the table of depressurization limits.

i called CMHC, who do not do technical Q&A on the report. i tried looking up the report authors (Scanada-Sheltair Consortium), and they appear to still exist in some form 22 years after the report was written, but their website ( did not load.

anyone have insight/info on this? it's a big jump between 3 Pa and 10 Pa.

Apr 7, 2011 7:34 AM ET

Edited Apr 7, 2011 7:35 AM ET.

Don't Overlook Central Vacuum - Worst Case
by Mike Guertin

Add Central Vac systems to Peter's list of exhaust equipment to turn on. Some systems pull 200+ CFM and can depressurize the CAZ above the limits. Some systems are exhausted internally and some externally so you may have to fiddle with door positions to reach the greatest depressurization.

Apr 7, 2011 9:15 AM ET

Central Vac and depressurization
by Peter Yost

Wow - good one Mike, and I can't believe I missed this one. Good thing your work is "exhaustive." :-)

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