Failing the Depressurization Test: When a House Sucks

Updated September 2024

This article deals with a problem that relates to code, is confirmed by a blower door test, and is caused by fashionable trends in home design.

In Canada, depressurization testing shows 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) are a hot topic, and not in a good way.

What is Depressurization?

Homes are becoming more airtight.

As houses become more airtight, they are more prone to depressurization. Depressurization happens when any exhaust device (fan) or exhaust fans are 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.

Dangers of a Depressurizing House

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
• 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. This means it poses a risk of backdrafting carbon monoxide and other combustion products into the home's interior when the house is depressurized.

When the kitchen range hood or the clothes dryer 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.

Depressurization Risks

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 Perform Depressurization Testing?

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 also includes measuring the combustion appliance zone (CAZ).

Why do we care? Mostly because houses have furnaces, boilers, and cooktops that are susceptible to combustion spillage.

During a depressurization test, all exhaust devices, including clothes dryers, are turned onto accurately assess the impact on the air supply necessary for the proper operation of combustion appliances.

Three key factors determine the extent of the pressure drop and the hazard to the occupants:

1. House size
2. Airtightness level
3. Size and number of exhaust devices running

Airtight Houses: Good or Bad

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.

Here's the thing: a drop of 10 Pa of air pressure is enough to cause backdrafting in fireplaces and naturally-vented gas water heaters. That's the equivalent 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. Electrification, using heat pumps for space and water heating, in other words, doing away with combustion in houses all together is gaining traction throughout North America and other cold climate zones.

The bad news is that depressurization still occurs when the exhaust devices are oversized. Depressurization will still result in indoor air quality problems and/or moisture problems within wall systems without a makeup air system. And a makeup air system will lead to higher heating and cooling costs.

Ventilation Requirements - 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. The most common sizes for cooktops are 32" and 36". That means 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. The minimum (ASHRAE Standard 62.2) is 100 CFM.

Thhe commercial-grade hoods that are being installed in many new houses offer 6x - 24x the minimum. In this case, bigger is not better.

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. 

(Sidebar: Home Energy Magazine had somewhere north of 30 years of articles. Where's the archive?)

So What the Heck is a Spice Kitchen?

A spice kitchen is a second kitchen, usually adjacent to the main kitchen, used for cooking foods with strong odours. To serve its purpose, a spice kitchen needs two key items: a high capacity exhaust hood over the cooktop or range, and a door.

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 moves 1200-2200 cfm.

No Depressurization - The Ultimate Solution

Here's how to never have a problem with depressurization:

1. Minimize or eliminate large exhaust devices
2. 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 a healthy indoor environment and controlled ventilation needs. But it's good to have clear goalposts.

How to Fix Negative Pressure in Your House

You can manage depressurization 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.

In Canada, Part 9.32.3.8 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, any exhaust device operating at a higher airflow rate than the ‘normal operating exhaust capacity' for the dwelling, which is determined by the number of bedrooms in the house, 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).

Makeup Air System is Not Enough

Although you can see that a MUA system is a 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 won't do a good job of exhausting the cooking smells because there is not enough make up air to allow it to do its 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.

Depending on how much you cook, and how cold it is outside, opening a window regularly could be an easy solution. Energy Vanguard's Allison Bailes brings it down to open the damned window. Allison is also in Georgia. If you're in Canada or cold climate states, it could impact your heating bills to open a window if you cook a lot during the winter. Certainly it's going to impact your comfort.

For this cold climate building science nerd, who also enjoys doing a lot of cooking, it feels counterintuitive to focus on the envelope and then open a window.

Someday I'll be keen enough to create a couple of energy cost models for ya, in a range of climates:

1. Running a big kitchen exhaust or two with a properly sized makeup air system
2. Open the damned window and have a 600cfm fan pulling sub-zero air into the main living space for 15 minutes

I'll post an update when I do.