Navigation:  Air Handling > Demand-Based Ventilation (CO2) Control on Air Handlers
rev. 2009-01-03

Existing Conditions

The air-handling units provide heating, cooling and ventilation to the entire facility, blending fresh air and return air to a fixed mixed air temperature setpoint.  This provides temperature and humidity control in the space, but tends to bring in more ventilation air than needed, especially during partial occupancy periods.  Any outside air brought into the building has to be conditioned by heating or cooling equipment, so it is best to minimize it.

Retrofit Conditions

CO2 control on the air handlers would ensure that ventilation air volumes are matched to occupancy under all conditions.  CO2 sensors would be installed in occupied areas and return air ducts, as appropriate for each air handler.  ASHRAE Standard 62 recognizes that ambient (outdoor) CO2 levels can fluctuate, and that a better measure of ventilation requirements in the space is to use the difference between indoor and ambient levels.  An outside sensor would also be installed to determine the ambient CO2 levels.  The new sensors would be connected to the building automation system, which would be reprogrammed to maintain the indoor air CO2 levels at 700 ppm above ambient.

The outside air and relief dampers would be replaced with low leakage dampers.

Further Benefits

 

 

Application Details

Control outside air intake by sensing CO2, as a proxy for overall indoor air quality.

This is the the most energy-efficient way of controlling ventilation.

CO2 will not be evenly distributed within the space, so return air sensing alone will not capture the diversity of space conditions.  If room sensing is used, the sensors must be placed strategically to capture high-occupancy areas.

For those air handlers where CO2 measurement in the return duct is applicable, use the same CO2 sensor for ambient and return air sensing.  Use an electric/pneumatic solenoid to switch between reading sources every 15 minutes.  The logic is managed by the building automation system.  Run pneumatic tubing as needed from the source points to the solenoid switch.  This approach avoids loss of proper control due to sensor drift, which is very common with CO2 sensors.  The two comparative readings will drift together, so control function is maintained between sensor calibrations.

Issues and Concerns

This strategy does not take account of pollutants (volatile organic compounds or VOCs) generated by office equipment or off-gassed from furniture and construction materials.  It may be that dilution of these contaminants requires more fresh air than the occupants need for breathing.

Requires regular sensor calibration.

References

ASHRAE Standard 62, ventilation for indoor air quality.  ASHRAE Standard 90.1, energy efficiency for new building design

 

Analysis

Once the existing conditions for the building are established, isolate the consumption for the units to be retrofitted using the bin method.

EXISTING

Use (1.08 x CFM x dT x Time) to calculate the existing conditions for heating.

Use (1.08 x 1.5 x CFM x dT x Time  ÷ 3414 x (-1)) to calculate existing conditions for cooling

Use (fankW x 0.75 x Time) to calculate existing conditions for the fan motor consumption.

RETROFIT

The retrofit conditions will decrease the duty cycle of the units by 25%.

Use (1.08 x CFM x dT x Time x 0.75) to calculate the retrofit conditions for heating.

Use (1.08 x 1.5 x CFM x dT x Time x 0.75 ÷ 3414 x (-1)) to calculate the retrofit conditions for cooling.

Use (fankW x 0.75 x Time x 0.75) to calculate retrofit conditions for the fan motor consumption.

 

Subtract Retrofit from Existing to determine savings.

 

Example:

This building uses roof top units and perimeter baseboard electric heat, so a ratio was developed to account for the two heat sources.  Also, the natural gas calculations were in GJ.

1.08 x cfm x dT x %___ will isolate the heat loss from each source.

 

 Bin Analysis

 

CFM = 2000

First, Kitchen bin hours should be established.  This data should be collected from the raw data, and not as a percentage of 24 hours because the time of day factor for most cooler hours.

Next, The energy consumed should be calculated.  This includes heating, cooling, and fan consumption.  For the heating and cooling scenarios, create an if statement that states:

=If (1.08 x CFM x dT >0, 1.08 x CFM x dT, 0) which reads  =if (test, value if true, value if false)

For Cooling, use   dT< 0, then multiply the 'value if true' by (-1)

To calculate the natural gas heating INPUT in GJ:

1.08 x CFM x (Indoor Set Point °F - Outside Temperature °F) x Kitchen Bin Hours x 0.000001055(MBH/GJ) ÷ Boiler Plant Efficiency x %NGas

To calculate the electricity heating input:

1.08 x CFM x (Indoor Set Point °F - Outside Temperature °F) x Kitchen Bin Hours x 0.000293(Btuh/kWh) x %Electricity

To calculate the electrical cooling input:

1.5 x 1.08 x (Indoor Set Point °F - Outside Temperature °F) x Kitchen Bin Hours x 3414 x (-1)

To calculate the electricity used by the fan:

hp x 0.746 x 0.75 x Kitchen Bin Hours

 OR

CFM x Kitchen Bin Hours ÷ 1000

The Retrofit condition for Demand Based Ventilation uses 60% as an Average Air Flow.  The existing heating and cooling conditions can be multiplied by 0.6 for each bin hour to achieve Retrofit Conditions.  The fan, however, has an exponential factor for each type of fan.  The Building Opportunities Library - Variable Inlet Vanes to VSD Conversion describes the appropriate formulae for each type of fan.

This example used:

0.93 x Average Air Flow³ +0.07 x Existing Fan Consumption

 

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