LCBS CONNECT SOLUTION
31-00118EFS—01
10
Minimum Ventilation
Let's examine the minimum ventilation requirement first. The notion of minimum ventilation is now driven by building
codes and is tied to providing "fresh air" to building dwellers. The code typically provides a guideline that is driven by
number of people in a building space, multiplied by CFM per person. This is further mitigated by the operation going in
the building, but that's simply too complex for this paper.
A Simple Example
We are retrofitting the controls on a five (5) ton rooftop unit in a commercial application. We are assuming that there will
be an average of 20 people in the space served by this unit. Our local code dictates that we need to provide 5 CFM of
"fresh air" per person to the building occupants. Our customer would like to make sure there is a rich supply of fresh air in
their building space, so we'll increase this to 10 CFM per hour. Simply put, this requires us to ventilate the building space
at a rate of 200 CFM per hour. The rooftop unit that we've installed has the capability to supply 2,000 CFM air per hour
and further, has the capability to ventilate our space at that rate. In conclusion, we need to ventilate the space at a
maximum rate of 200 CFM per hour. Divide 200 CFM by 2,000 CFM and we'll need to provide minimum ventilation rate of
10%. Don't panic! Hysteresis that occurs from damper and actuator linkage, plus the relatively poor accuracy of OEM
dampers will make it easy for you to estimate 10% damper position. Use the LCBS Connect service mode to help you set
the 10% air flow target.
Naturally, when folks leave the building at the end of the day, we aren't required to provide any fresh air and we close the
outdoor air dampers, shut off the system fan and operate at "unoccupied set points" that result in customer energy cost
avoidance.
Enable Economizer Function
There are literally millions of economizers installed in North America that are integrated through a simple integration to a
time of day signal and call for cooling.
How it works. A digital "enable economize" signal must be configured on the LCBS Connect controller. Again, typically this
"dry" digital output can be used to carry 24 VAC to a packaged economizer system. By definition, during building
unoccupied schedule, the contact is open, disabling the economizer. This typically allows a spring return actuator to close
to 100% position.
NOTICE
Opportunity to upgrade your customer's system and provide trusted advisor service!
Using the "integrated"
techniques described above, you will be able to 1] improve economizer operation 2] provide full remote access of
settings, configuration information, 3] permit access to interesting and useful graphics, trending, and service
mode capability and 4] enable full Title 24, economizer analytics that will allow you to provide heroic service to key
service customers.
Honeywell Integrated Economizer Control Described
In addition to making sure that fresh air is provided to building occupants, the economizer function consists of two more
basic control loops.
First is a "mixed air control loop."
This is a direct acting control loop that is enabled by a call for
cooling and positions an outdoor air damper actuator based on mixed air temperature and set point.
Second is a
changeover or "high limit" control function.
In our industry, these terms are sometimes used interchangeably. This
control loop senses outdoor air temperature, or outdoor air temperature and humidity (enthalpy). If this loop senses high
temperature or high enthalpy, the control system drives the outdoor air damper actuator to minimum position. The high
limit controls make sure that warm or moist air is prohibited from getting into the building space.
There is another changeover strategy that is associated with the economizer function called "differential" control. On a
call for cooling, if the system return air sensor senses a temperature less than the system outdoor air sensor, and a call for
cooling exists,
the economizer is disabled.
The test is simple… if there is a call for cooling and return air is cooler than
outdoor air, I'm going to use return air to provide "free cooling." Likewise, on a call for cooling, if the outdoor air
temperature is less that the return air temperature,
the system economizer continues to be enabled.
This is called
"differential temperature changeover." There is an equivalent "differential enthalpy changeover" described in this
document. Applied properly, differential control can provide substantial customer energy cost avoidance.