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
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).
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:
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.
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.
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 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).
After surveying the home for combustion appliances and exhaust fans, the procedure goes like this:
- Seal the house: close all exterior windows and doors.
- Turn off all combustion appliances.
- Close all interior doors.
- 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.
- Turn on all the fans and measure the worst-case depressurization.
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.
- Peter Yost
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