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Check out the below infographic about what an HRV is and the various ways that they can connect to your heating system.

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, my suggestion would be 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.

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.  When the range turns on, so does a make-up air fan to simultaneously re-pressurize the home to prevent any form of depressurization.  This could be challenge to wire since the kitchen may already be finished.

The basic principal is that for every cfm that is exhausted outside.......the house will naturally re-pressurize bringing in outside air.  If that air is cold then the furnace has to re-heat 100% of that exhausted air.   I find the below method the easiest to understand......calculate it in KW's first then transfer to Btu/h which is how furnaces are sized.  The other reason its in KW is that Makeup Air heaters are usually small electric heaters thus sized in KW.

Temperature Rise Calculation

Equipment Load =  1.08 * TempRise F * CFM

(TempRise = the difference between the inside temperature and the outside temp)

If it's -13F (-25C) outside and it's 72F (22C) inside then the Temperature Rise is

72 - (-13) = 85 degrees F,

and Equipment Load equals to....

1.08 * 85F * 1000cfm =

91,800 BTU/h

In kilowatts, that converts to...

91,800 BTU/h ÷ 3412 =

27KW

So, when its -25C outside and the inside t-stat is 22C......the 1000cfm of exhaust requires 91,800 BTU/h to heat up.   This doesn't include the actual heat loss of the house when it's that cold.  To put 91,800 BTU/h of heat that is enough heat to heat about a 4000sqft house when its -25C.   The formulas can be sliced and diced into full metric or full imperial.....but I find the above the easiest to understand.

The risks from having a house in a negative pressure include increased 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 smoke or carbon monoxide backdrafting down the chimneys.

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 reference 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 makeup 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 makeup air system must be installed and interlocked to the large exhaust device.  When the range turns on…..so does a makeup air fan to simultaneously re-pressurize the home……or not allow any form of depressurization.  See two attached document for illustration purposes.

Part 6 – which is enforceable when 5 bedrooms or more are present in a home….or the home is 6400sqft or larger…..or there is a solid fuel appliance present…..see attached for the verbiage of it.  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 makeup air is required is all cases…..which fall under Part 6 of the OBC.

As homes become increasingly 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 airtightness of the house and the size and/or number of exhaust fans running.

Source: HRAI - Residential Mechanical Ventilation

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