| For proper ventilation, whether in a classroom or an office, the continuous
introduction and exhaust of outside air is a necessity during occupied times.
Previous Comfort Zone articles have discussed many different devices that meet
most code jurisdiction requirements. As a reminder, here is a list of the
frequently used methods. So what are some popular methods to control these
ventilation devices?
Standard HVAC Operation
Normally HVAC unit-mounted ventilation devices come wired into the HVAC unit's
blower relay. When the HVAC unit's blower is energized, so is the ventilation
device. Therefore any standard thermostat with a manual fan switch is capable
of turning "on" the ventilation device during occupied times. However the
occupant is in control of this method.
If the occupant believes that the unit is too noisy, or there is too much air
turbulence, they will turn off the continuous ventilation and/or HVAC unit.
This is one of the top reasons for poor IAQ: occupant control of poorly
designed HVAC systems resulting in too much noise and too much turbulence (air
changes.)
Light Switch
The ventilation components are energized when the occupants arrive in the
morning and turn on the lights. This method requires the use of several high
and low voltage relays/contactors, both in the ceiling and in the HVAC unit. If
the lights are on during occupied times, so is the ventilation device. Any
overnight off-gassing of furniture, cleaning materials, etc. will not be
exhausted from the room until the light switch is turned on. Pre-ventilation is
not possible with this method, as this is occupant initiated.
Digital Clock Timer
Commercial Electronic clock timers may be used to control ventilation devices
and/or the entire HVAC system. Available in 24 hour, 7 day, or 365 day, these
timers can be programmed with a daily, weekly or yearly schedule. Similar to
residential type timers used to control household appliances and Christmas
lights, these commercial electronic timers come in many different types.
Pre-conditioning and pre-ventilating is possible, as these are time-based.
Mechanical "Twist" Timer:
This is a simple spring-wound countdown timer. When the occupants arrive, they
have to "twist" the timer to the number of hours they will be in the
conditioned space; 6, 8, 10 hours, etc. These spring countdown timers can be
purchased in any time amount, with or without a hold feature. Purchase without
a hold feature is suggested, as the occupant may twist the dial to the hold
position, losing the value of the timer. This timer may also be used to run the
HVAC unit as well as the ventilation device. Do not use this timer to control
the heating system in areas that freeze in buildings plumbed with water, as the
timer shuts the HVAC unit off completely. No pre-conditioning/pre-ventilation
is possible with this method, as this is occupant initiated.
Thermostat with a Programmable Ventilation Option
Many digital/electronic thermostats have a separate function to control the
ventilation cycle (separate from the heating/cooling function.) These cycles
are programmed into the stat by the end user based on time of day occupancy.
The thermostat uses the fan terminal or an auxiliary terminal to energize
whichever ventilation components are being used. Pre-ventilating and
pre-conditioning of the room is possible as this is a time-based control
method. Thermostat programming remains an industry problem due to user
fiddling, power outages and dead batteries.
CO2 Sensors
CO2 is a very stable trace gas. When exhaled by the occupants, CO2 can then be
used to control ventilation systems. Many codes/standards require either a
minimum ventilation rate of 15 cfm per person, intake and exhaust, continuous
ventilation during occupied times, or a maximum CO2 of 1000 parts per million
(ppm) (ASHRAE) as the set point.
CO2 sensors can open the ventilation device and turn on the blower motor
independent of the heating and cooling function in most HVAC units with the
addition of some optional relays. The objective of using CO2 sensors is to
limit ventilation (energy use) to the times when the room requires it. If only
one occupant is in the room, chances are no ventilation will be required. Add
20-30 more occupants, and the CO2 level will raise quickly beyond the 1000 ppm
set point. Pre-ventilation does not occur with this method. Post occupancy
ventilation will occur above the CO2 sensor set point.
Occupancy Sensors
Usually a passive infrared device is chosen, because unlike standard infrared
sensors, passive infrared takes a warm-blooded body to energize. Any movement
triggers standard infrared sensors, energizing the unit. If the airflow starts
to move a banner in a room, the HVAC unit would never shut off, because the
occupancy sensor continues to sense the banner moving. Standard infrared
sensors are similar to a front porch sensor that turns on when the trees move.
Select passive infrared sensors for HVAC applications.
Passive infrared sensors are available as a sensor-only or with options that can
control the heating/cooling, ventilation, and lights, etc. These are very cost
effective in room locations where the expense of trenching/cabling is cost
prohibitive. Occupancy sensors usually energize the ventilation devices to the
maximum occupancy position immediately after the first occupant enters the
room. The objective is to ventilate, even if the room has only one occupant.
This philosophy proposes that ventilation is not only meant to introduce
outside air into the room, but to also exhaust stale air, harmful off-gassed
fumes, pesticides, and cleaning materials, and to do this even when the CO2 has
not reached 1000 ppm.
Occupancy and CO2 sensor methodology are usually mutually exclusive. Facility
Directors are divided equally on which of the two is best. Pre-conditioning
and/or pre-ventilation may occur with some passive infrared occupancy sensors,
but most inexpensive models do not offer these options. Some passive infrared
sensors are self re-programming after power outages.
Energy Management System
These include hard-wired and modem-controlled systems that allow the HVAC unit,
and/or ventilation devices, lights, etc. to be remotely monitored and
controlled with analog or digital data. Known as EMS Systems, they are able to
control HVAC systems on user-defined control strategies. Installed sensors
provide input data to a remote computer terminal, which allows the program to
sense, control and report many different HVAC and ventilation conditions. This
is, by far, the most advanced controls method, and also the most expensive. All
components must be hard-wired or modem-connected to the central computer
terminal. Some wireless systems are becoming popular, but have range drawbacks
associated with them. Pre-conditioning/pre-ventilation is possible with this
method.
We have taken a brief refresher course in some of the more common ventilation
control options. Each method will require field wiring and added HVAC unit
controls for optimal ventilation device operation. As always, consult your
mechanical engineer or HVAC supplier for additional information on this
subject. Stay in control and well-ventilated until we meet again in . . . The
Comfort Zone.
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