Like you and I, buildings also have to “breathe”.
In North America, most, if not all highrise residential apartment and condominium buildings accomplish this breathing with a mechanical method to introduce fresh outdoor air into the building and remove exhaust air.  It is important to understand why ventilation is necessary, how it is done and today’s options for retrofit for increased energy efficiency.
A sample building is later analyzed for the energy savings potential available from selected retrofit methods.  The table below shows the results from the sample building energy savings analysis.

Why is Ventilation so Important?

1)  Occupant Ventilation:
A supply of fresh outdoor air (or “makeup air” as it is more commonly known) is necessary under various buildings codes and to ensure occupant health.  

2)  Corridor Pressurization:
Residential towers can contain many-hundreds of suites, each o­ne occupied by people of differing ages, backgrounds and work schedules.  People go about their day cooking, showering and smoking, producing odours that don’t always match the desires of their neighbors.  The makeup air is supplied directly to the corridor, producing a small positive pressure relative to the suites.   The makeup air travels into the suites typically through a small gap under the main entrance door, effectively containing any odours to the suite.

3)  Building Pressurization:
The laws of physics create all kinds of interesting phenomena at different scales.  A single storey building o­n a windless day will be relatively stagnant no matter how many windows and doors are open.  However, a building of similar footprint and 10 or even 50+ stories tall will create its own interior convective wind system.
For those residing in the northern United States and the majority of Canada, this internal convective system is greatly amplified in the winter when interior temperatures are typically 70 °F (21 °C) and exterior temperatures are anywhere from 32 ° to -30 °F (0 °C to -28 °C).  Because the warmer air in the building is more buoyant (lighter) than the colder air outside, the warm air in the building is trying to rise up and out of the building, also known as stack-effect; this is the same process that allows combustion gasses from a fireplace to exit up through a chimney.
This stack-effect results in warm air from the building trying to escape from the upper levels through occupant suites through cracks and open windows and balcony doors, elevator shafts with rooftop mechanical rooms, and roof access doors.
Without proper building pressurization, stack effect can create the following conditions:

- For those in lower suites, cold air will infiltrate around their windows, balcony doors, or could even be drawn in through exhaust ductwork through their kitchen hood and washrooms.  This can result in temperature control problems, drafts and even loud whistling noises.

- Similarly mentioned above for Corridor Pressurization, this produces potentially objectionable odours for occupants as air from some suites will enter the corridors and move around the building where it is not desired.

- Moisture from outside is drawn through the building envelope and could potentially cause building envelope problems including but not limited to visual defects, insulation degration and even mould.

Makeup Air
The makeup air is often introduced to the building by way of o­ne or more makeup air units (MAU) typically located o­n the roof or in a mechanical room.  A makeup air unit consists of a fan, and depending o­n geographic location, can have a furnace/heating coil for heating during winter in cold climates, and/or a cooling coil for cooling/dehumidification in the summer for hot/humid climates.
More efficient systems include energy recovery wheels that reuse energy from the exhaust air to pre-condition incoming makeup air, significantly reducing energy consumption costs to condition the makeup air.

Exhaust Air
Exhaust air removes spent air, odours and moisture from cooking and washrooms, and exhaust air also helps to maintain the corridor and building pressurization, by “pulling” the air out.  Exhaust methods vary, but are typically performed by o­ne of the following:

1) Distributed exhaust fans:  o­ne exhaust fan for each suite or in each kitchen/washroom in the suites. 

2) Central Exhaust:  Consists of o­ne or more large exhaust fans o­n the roof or other mechanical room(s).

The pros and cons of each approach vary depending o­n geography, occupant type, capital cost budgets and anticipated maintenance requirements.

A lot of highrise construction has occurred in the last 50 years, and surprisingly, many buildings still contain their original mechanical equipment.  Depending o­n the original equipment and duct layout, various options exist for retrofit and are briefly outlined below.

1) Right Sizing:  Existing airflows may be too high, or unbalanced creating unnecessarily higher energy consumption and/or drafts.  Air is flowing out of the diffusers in the corridors, and the exhaust fans o­n the roof are whirring away, but just how much air is being moved?  While anyone is justifiably nervous making changes to something that works, it is possible that there is too much airflow and there is room to reduce the airflow volume without sacrificing comfort.

2) Higher Efficiency Equipment:  In colder climates, a lot of energy is used to heat incoming makeup air, and is typically performed with natural gas furnaces located in the makeup air unit.  Older single stage furnaces are mid-efficient, and the frequent cycling produces additional inefficiencies and potentially interior comfort control problems, not to mention additional maintenance from the cyclic thermal expansion effects of the furnace components.  Newer packaged makeup air systems provide high-efficiency, and even condensing furnaces with modulating control to provide makeup air at a constant temperature.

3)  Variable Speed Fans:  Not everyone is cooking or showering all day long, however, many makeup air and exhaust systems are running full speed 24 hours a day.  A simple variable frequency drive (VFD) can be placed o­n the supply fan for the makeup air unit(s) that can allow a slight decrease in fan speed during non-peak times, again without sacrificing occupant comfort.  A quick evaluation of the makeup air system is required to ensure it is capable of handling reduced airflows.

4)  Energy Recover Ventilators:  In the winter, all of that exhaust air exiting your building is nice and warm, say at 70 °F (21 °C), something any engineer or contractor is familiar with huddling around a rooftop exhaust fan warming up their hands.  All of that warm exhaust air is replaced at a minimum 1:1 with cold dry makeup air from outside that must be heated.  In cold climates, this can add up to a significant cost to the electricity or natural gas bill.
Heat Recovery Ventilators and Energy Recovery Ventilators take heat from the exhaust air and transfer it to the incoming makeup air, saving many thousands of dollars a year o­n energy costs.  What about odours you ask?  Not a worry, these systems are designed so that potentially objectionable odours, including tobacco smoke, do not enter the makeup air stream.
There are dozens of heat recovery and energy recovery ventilator solutions available in a wide range of sizes and configurations.  Additional conditioning is often required such as heating to top up the temperature in the winter and/or cooling/dehumidification for the summer.

• Located: Toronto, o­ntario
• 100 suites
• Each suite is currently receiving 125 cubic feet per minute (cfm) of outdoor air.
• Makeup Air Unit:
• Total outdoor air ventilation required is 12,500 cfm, provided by o­ne makeup air unit.
• Assume a 10 hp (7.46 kW) motor for the makeup air unit fan.
• Assume an older single stage gas furnace with a seasonal efficiency of 70%.
• Operates 24 hours a day, 365 days a year.
• Exhaust Fan:
• Assume total exhaust from the building is also 12,500 cfm provided by o­ne large fan.
• Assume a 15 hp (11.19 kW) motor for the central exhaust fan.
• Operates 24 hours a day, 365 days a year.
• Utility Rates:
• Assume electricity cost of $ 0.10 per kWh.
• Assume natural gas cost of $ 0.35 per m3.

Let’s say the existing system is not balanced properly, and is supplying too much air, we o­nly need 100 cubic feet per minute (cfm) per suite.  Then the supply fan can be rebalanced to 10,000 cfm, which would actually reduce electricity consumption even with the same motor, or a smaller motor could be installed.  If we could reduce the supply fan motor to 7.5 hp and the exhaust fan motor to 10 hp, then the new annual electricity and natural gas consumption is:

Electricity Consumption:   114,362 kWh  ($ 11,436 per year)

Natural Gas Consumption:  81,064 m3 per year  ($ 28,372 per year)

A 70 % efficient, single stage furnace is old technology, and rarely used anymore for larger makeup air units.  Now let’s install a new makeup air unit with a condensing furnace achieving 90% combustion efficiency.

Electricity Consumption:   114,362 kWh  ($ 11,436 per year)

Natural Gas Consumption:  63,050 m3 per year  ($ 22,068 per year)

Now since people are not cooking and showering every second of every day, there is the potential to reduce the amount ventilation during non-peak periods.  Assume that peak times are for 3 hours in the morning, 3 hours around lunch, and 4 hours at night for a total of 10 hours of peak ventilation required, leaving 14 hours where total airflow could be reduced.  Let’s say we reduce this airflow by 30%.  This provides electricity and natural gas savings.

Electricity Consumption:   80,339 kWh  ($ 8,034 per year)

Natural Gas Consumption:  52,016 m3 per year  ($ 18,206 per year)

There is a lot of heat in that exhaust air that can be transferred to the cold incoming makeup air.  A basic heat recovery ventilator (HRV) can capture about 50% of this heat, therefor the natural gas furnace is still needed to top up the temperature.  Depending o­n your climate, natural gas consumption savings can be quite significant.  Electricity consumption will actually increase slightly due to slightly larger fan motors required to overcome the static pressure requirement of the heat exchanger or heat wheel.

Electricity Consumption:   114,770 kWh  ($ 11,477 per year)

Natural Gas Consumption:  26,008 m3 per year  ($ 9,103 per year)