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June 13, 2018
The second in a series of three articles on technical issues around blower door testing.
The first article in this series dealt with concerns around the ‘retirement’ of the CGSB standard for blower door testing. For thirty years, Canadian energy modelling software and analysis has relied on CGSB test results as a base for performance. The retirement of this standard leaves us with some important questions: will it impact code compliance? What’s the best path to follow - replace or revamp?
This article deals with a problem that relates to code, another withdrawn CGSB standard, is confirmed by a blower door test, and is caused by fashionable trends in home design.
In Canada, depressurization testing is showing that many houses with large capacity range hoods are exceedingly good at exceeding depressurization limits. Large capacity, commercial grade range hood fans and spice kitchens (more on this later) were a hot topic in provincial reports at CHBA’s national Technical Research Committee meeting in October 2017, and April 2018, and not in a good way.
Homes are becoming more airtight. As houses become more airtight, they are more prone to depressurization. Depressurization happens when any exhaust device (fan) is turned on in a home. As the exhaust fan pushes air outside, the inside pressure begins to drop. Voilà, negative pressure.
The kitchen range hood and the clothes dryer are typically the large exhaust devices that are responsible for creating negative pressure regimes in houses.
Important Note: heat recovery ventilators (HRVs) and enthalpy recovery ventilators (ERVs) are balanced ventilation systems, meaning they bring in the same amount of supply air as they exhaust. They do not cause depressurization, nor do they factor into depressurization problems.
What’s the problem with depressurization?
A negative pressure regime is dangerous when there is one or more ‘combustion spillage susceptible appliance’ in the house. This could be a naturally drafted gas furnace, boiler, or water heater, a naturally drafted oil furnace or boiler, or a wood burning stove or fireplace, among others.
Any wood burning appliance that uses a chimney, or gas appliance that is vented through a “B” vent chimney is a spillage susceptible unit, that is, it poses a risk of backdrafting carbon monoxide and other combustion products into the living space when the house is depressurized.
When the range hood is turned on and the house is under negative pressure, a chimney or a ‘B’ vent becomes the path of least resistance for replacement air. Your house literally sucks air from wherever it can. If there are no spillage susceptible combustion appliances present, a house that is under negative pressure will pull outside air in through the building envelope, where it can condense inside the wall cavities. This can lead to mold issues and eventually to structural problems.
Having the house under negative pressure for any length of time carries risk. On top of concerns about backdrafting, risks include:
- Higher utility costs
- Poor indoor air quality
- Moisture and mold build-up inside walls
How to Test for Depressurization?
Glad you asked.
You use a blower door.
Depressurization testing protocols in Canada and the US differ, but essentially, the goal is to use the blower door (or the manometer) to determine the worst-case scenario depressurization level in the house. This is done by turning on the various exhaust devices in the house and measuring the pressure differences (very simplified explanation).
In the US, this is done using Combustion Appliance Zone (CAZ) testing protocols. In Canada, the now-withdrawn CGSB standard 51.71-95, The Spillage Test Method to Determine the Potential for Pressure-Induced Spillage from Vented, Fuel-Fired, Space Heating Appliances, Water Heaters and Fireplaces, is referenced in the code.
Whichever standard or protocol you’re using, three key factors determine the extent of the pressure drop and the hazard to to the occupants:
- House size
- Airtightness level
- Size and number of exhaust devices running
I’m a Big Fan of Airtight Houses
Generally speaking, the tighter the house the more likely it is that depressurization will be a problem. The closer the house gets to a Net Zero Energy, Passive House or other performance target that relies on a well-insulated, well-sealed building envelope, the more challenging it becomes to battle depressurization.
A drop of 10 Pa of air pressure is enough to backdraft fireplaces and gas water heaters. That’s the equivalent of the pressure due to the direct impact of a gentle breeze (~9 mph or 14 km/h). To get a sense of how much air movement can generate 10 Pa of air pressure: a breeze of 9 mph translates into 792 ft/min. A 1000 cfm range hood fan pushing air through an 8 inch duct has a velocity of 714 ft/min.
The good news is that many high-performance houses are moving to sealed combustion units, taking the problem of the open chimney out of the equation.
The bad news is that depressurization still occurs when the exhaust devices are oversized, and it will still result in indoor air quality problems and/or moisture problems within wall systems without a makeup air system, and higher heating and cooling costs with one.
Is Bigger Really Better?
To put the exhaust ventilation needs of a house in perspective, the Home Ventilating Institute (HVI) recommends 40 cfm per foot of cooktop for a residential kitchen, meaning that most homes will need a range hood of less than 150 cfm. The Canadian ventilation standard, CSA F-326, uses this same size calculation for range hood sizing, and in the US, 100 CFM is the minimum required by ASHRAE Standard 62.2. That’s a far cry from the 600 to 2400 cfm capacity of the commercial-grade hoods that are being installed in many new houses.
Way back in the 1990’s I read an article in Home Energy Magazine (I think) that described these big fans as the culinary equivalent of a SUV.
So What the Heck is a Spice Kitchen?
So now you have two culinary SUVs in a house, intermittently pulling anywhere from 600 to 5000 cfm from the house. In comparison, a generic 80,000 Btu 90% efficient gas furnace with a variable speed motor might typically push 1200-2200 cfm.
No Depressurization - The Ultimate Solution
Here’s how to never have a problem with depressurization:
- Minimize or eliminate large exhaust devices
- Eliminate any and all spillage susceptible combustion appliance
This ‘ultimate solution’ is impractical and impossible in many houses, both new and existing. Obviously, a singular focus on avoiding depressurization misses the mark on healthy indoor environment and controlled ventilation needs. But it’s good to have clear goalposts.
Depressurization can be managed with the installation of a make-up air (MUA) system to provide replacement air when large exhaust devices are used. This keeps the indoor and outdoor pressure balanced but raises heating and cooling bills by bringing in unconditioned air from the outside. Not doing this puts you in violation of code.
Part 126.96.36.199 of the National Building Code states that if a natural draft appliance (furnace, boiler) or a solid fuel appliance (woodstove, fireplace) is part of the house, a make-up air system is required to protect the safety of the occupants from depressurization. The make up air system is sized to allow the same amount of air into the house as is being exhausted, and must be interlocked to the large exhaust device(s).
(Side note: if radon is present, it needs to be managed as well, but separately from the makeup air system).
In Canada, essentially, any exhaust device operating at a higher airflow rate than the ‘normal operating exhaust capacity’ for the dwelling must include make-up air. In the US, the building code requires any range hood over 400 cfm to include makeup air.
A makeup air system with a preheater, sized for a 600-1000 cfm range hood could run anywhere from $3,000 to $5,000 to install. It also comes with an electrical cost for operation. The preheater doesn’t compensate for the total cost of the heat that is pulled out of the house when it runs (NBC requires makeup air that gets dumped into living spaces to be ‘tempered’ to 12°C).
Although you can see that a MUA system is reasonable solution to a problem, it doesn’t address the problem itself. Because...
In an all-electric, high-performance house with little air leakage, there is no problem with depressurization that will lead to backdrafting, but there is still a major problem with a humongous exhaust fan:
It. Won’t. Work.
It’s going to suck. Literally. It will depressurize the house and yet will be unable to exhaust the cooking smells because there is not enough make up air to allow it to do it’s job. So the house gets stale and stinky. If you open a window, you’ve provided make up air and the fan can work (you could do this in a house with a spillage-susceptible appliance, too). Unlike a mechanically controlled MUA system, however, you have no option for tempering the air.
Someday I’ll be keen enough to create a model of how much running a big kitchen exhaust or two with a properly sized makeup air system costs in cold climate.
I’ll post an update when I do.