Depressurization - What Is It?


As homes become increasingly more air-tight, one issue that needs to be discussed is depressurization.  Essentially, depressurization is a negative pressure that develops when an exhaust device (fan) is turned on in a home.  As the exhaust fan pushes air outside, the inside pressure begins to drop. The degree of the pressure drop is determined by the size of the house, the air tightness of the house and the size and/or number of exhaust fans running.

Source: HRAI - Residential Mechanical Ventilation


The two large main culprits responsible for creating concerning negative pressures in homes are the kitchen range and clothing dryer.  Exhaust fans that expel less then 150 cfm (bathroom fans) generally do not pose a health or building envelope risk. When using an air exchanger (HRV or ERV), they are intended to operate at a balanced state and therefore do not depressurize while ventilating.


Creating a negative pressure in a home is especially dangerous when there is a wood burning fireplace present. Wood stoves or fireplaces require indoor air (oxygen) to burn.  When the house is at a lower pressure than outside, the chimney can easily back-draft bringing smoke or CO back into the house. Bringing smoke, or even worse fire, back into a house is very dangerous.

Similarly having naturally drafted combustion appliances in a depressurized home creates the same dangers. These ‘spillage susceptible’ combustion appliances include standard water heaters, mid-efficient furnaces or boilers, and gas pizza overs as they all vent up through a “B” vent  chimney.  Each pose the same risk of back-drafting carbon monoxide back into the home when depressurized.



Depressurization - So What?


The risks from having a house in a negative pressure include higher utility costs, poor indoor air quality, increased risk of mold building up inside the wall and in extreme cases can lead to asphyxiation from the smoke or carbon monoxide back-drafting down the chimneys.

For example, if someone were to use their fireplace and range hood, there could be risk of asphyxiation if there were no protections against depressurization. When the range hood is turned on, the chimney becomes that path of least resistance for replacement air when the house is drawn into a depressurized state. It takes literally seconds to bring smoke or even flames back into a house when a fire is burning.

Even if your house does not include a spillage susceptible combustion appliance, the outside air would be pulled inside through the various leaks in your building envelope. In winter, the cold outside air can condense inside your walls where it changes state. This can and does lead to long-term mold issues or even worse structural integrity issues like rotting wood.

Managing depressurization is done by installing a make-up air (MUA) system, which provides replacement air from the large exhaust devices at the time they are used, keeping the indoor and outdoor pressure balanced. While this resolves the complications of depressurization, bringing in unconditioned air from the outside will raise your heating and cooling bills, not to mention the cost of the make-up air system in the first place.

Decoding Depressurization


The discussion of the acceptability of a ‘large exhaust device’ is one that is often overlooked or disregarded by building officials and brings up some notable concerns. First, a section from the  Ontario Building Code (OBC) that references depressurization or makeup air parameters:

Part 9 of the Building Code applies to residential buildings and is pretty clear that if a solid fuel appliance (wood fireplace, wood stove) or a natural draft appliance (mid efficient furnace, standard efficient boiler or even a pizza oven) is present within a home the provision for a make-up air system is necessary to protect the safety of the occupants. Further, if radon is of concern then again it should be managed.

What does managed mean? In the simplest terms for all CFM being exhausted, less a small provision for infiltration, a make-up air system must be installed and interlocked to the large exhaust device. When the range turns on, so does a make-up air fan to simultaneously re-pressurize the home to prevent any form of depressurization.  

Part 6 of the Building Code is enforceable when 5 bedrooms or more are present in a home, or if the home is 6400sqft or larger. Within that section it notes that the large exhaust fan should not adversely affect “other exhaust devices” which would include HRV’s or furnace venting. To me this again would suggest that make-up air is required in all cases.

Large Range Hoods - Are They Worth It?


We often come across clients who are excited to be getting a new large range hood, but are unaware of the implications of managing them with a make-up air system. To manage the depressurization of a 600cfm range hood one would have to install a make-up air (MUA) system and interlock it to the kitchen range. This could be challenge to wire since the kitchen may already be finished. When the range turns on, so does a make-up air fan to simultaneously re-pressurize the home to prevent any form of depressurization.


If it could be installed and wired it would be located in one of the basement mechanical rooms with suitable air transfer grills from the mechanical room to the basement area. A MUA system would ‘likely’ cost $2800-$3600 and use a lot of electricity when running in the winter. To give you some more information of the energy used with an electric MUA heater in the winter.

Here is a simple temperature rise calculation that shows how much heat is being exhausted along with the 600cfm of air.

BTU/h = cfm x 1.08 x DeltaT

Where Btu/h is the amount of heat and the DeltaT is the temperature difference from outside to inside, in this case it’s in Fahrenheit (-25C = -13F), which is Ottawa’s outdoor design temperature. Which means when it’s -13F outside the 600cfm of exhaust air must be reheated to 72F (22C) inside….which equals a 85F temperature rise.

Example: BTU/h = 600cfm x 1.08 x 85F = 55,080 Btu/h.

To put this amount of heat into perspective, that is more than enough heat to heat the average house. Granted the fan only runs 30 mins at a time…it’s still a lot of heat. I know people love the idea of having large range hoods but the economics of their installation is hardly justifiable. Sadly, the retailers do not divulge these facts and even more concerning is the manufacturers of these fans do not express the liability associated with possible asphyxiation when a wood stove or fire place is present and a make-up air system is not installed. It takes literally seconds to bring smoke or even flames back into a house when a fire is burning.


Depressurization - The Solution


In the case where you must have a large exhaust device, here are some tips to minimize the implications due to depressurization. The first recommendation would be to eliminate any spillage susceptible combustion appliance (eg. fireplace) as this resolves all issues. If this is not an option, then, it might be possible to duct the required HRV from the kitchen (and bathrooms) and NOT install a kitchen range fan vented to outside. The HRV would then be the kitchen exhaust. However, this is only an option if the range is electric and not propane.  

If the kitchen range is propane then TSSA requires a traditional kitchen exhaust fan. If the range is electric and the HRV is managing the kitchen ventilation then only the clothing dryer has to be managed. This can sometimes be achieved with an inexpensive Passive Fresh Air Intake rather than an expensive power makeup air system.  

In order to properly size a make up air system, we can perform an HRAI W-3 calculation to manage depressurization in your home.