Using demand-controlled ventilation in HVAC
Mechanical engineers should consider the many factors that go into designing buildings for indoor air quality and indoor environmental quality
- Understand what indoor environmental quality is.
- Learn how to save energy by using demand-controlled ventilation.
- Gather information about what thermal comfort is.
- Know how the occupant interface experience affects thermal comfort and indoor environmental quality.
A recent article published in The Washington Post by Christopher Ingraham clearly explained “Why crowded meetings and conference rooms make you so, so tired.” It had a concise description of carbon dioxide levels and their effect on occupant comfort and performance.
A graph of a live meeting showed how quickly the CO2 in a crowded conference room went from 800 to 1,000 parts per million, the threshold at which ASHRAE Standard 62.1-2016: Ventilation for Acceptable Indoor Air Quality states occupants first start to feel stuffy and sleepy.
Indoor environmental quality
IEQ includes everything from room color and ergonomic layout, to how well the pest control is done. For many, it is defined by the U.S. Green Building Council’s LEED rating system and comes down to a few main topics:
- Thermal comfort.
For the heating, ventilation and air conditioning engineer, there are two ASHRAE compliance standards: ASHRAE Standard 55: Thermal Environmental Conditions for Human Occupancy, and ASHRAE Standard 62.1.
What is a high-performance building?
According to Title IV – Energy Savings in Buildings and Industry in the Energy Independence and Security Act of 2007, a high-performance building is “a building that integrates and optimizes on a life cycle basis all major high-performance attributes, including energy conservation, environment, safety, security, durability, accessibility, cost-benefit, productivity, sustainability, functionality and operational considerations.”
The convergence of making a building both comfortable and energy–efficient has been a challenge for several years. As designers are discovering, traditional HVAC designs don’t make it easy in many parts of the country, especially those with higher levels of cooling requirements, as traditional HVAC designs are developed based on an average of climate conditions across the country.
One of the most popular ways to meet ASHRAE 62.1 requirements and conserve energy is through DCV. This method allows the engineer to decrease the amount of ventilation in a space if it can be demonstrated there are either no people in the space or not enough people to justify the “cubic feet per minute rate per person.” The intent of this method is to match the provided ventilation rate with actual occupancy (demand), maintaining indoor air quality without overventilating.
The ASHRAE 62.1-2016 rules for implementing DCV are found in section 6.2.7:
22.214.171.124. DCV shall be permitted as an optional means of dynamic reset. Exception: CO2-based DCV shall not be applied in zones with indoor sources of CO2 other than occupants or with CO2 removal mechanisms, such as gaseous air cleaners.
126.96.36.199.1 For DVC zones in the occupied mode, breathing zone outdoor airflow (Vbz) shall be reset in response to current population.
188.8.131.52.2 For DVC zones in the occupied mode, breathing zone outdoor airflow (Vbz) shall be no less than the building component (Ra x Az) of the DCV zone. Note: Examples of reset methods or devices include population counters, carbon dioxide sensors, timers, occupancy schedules or occupancy sensors.
Tables 1 and 2 represent the potential savings in effective cubic feet per minute per person reduction.
With this in mind, how do we determine the number of individuals in the breathing zones? While there are many different ways to accomplish DCV sequences, here are two basic examples:
CO2 control for:
- Open offices.
- Meeting rooms.
- Other transient spaces.
Occupancy sensor control for:
- Private offices.
- Enclosed limited purpose rooms.
Both start on the premise that if the schedule is “occupied,” but no one is in the space, primary air is reduced to the minimum square foot requirement.
Using outside air
The most precise way to control outside air is by having a dedicated source, such as a dedicated outside air unit, feeding only outside air to the terminal zones. The terminal zones are sequenced as follows:
- Primary air is reduced to a “minimum setpoint.” This is the ASHRAE 62.1 cubic feet per minute per square foot value only (0.06 cfm/square foot) for the office space category.
- If CO2 increases above the outside air CO2 level by a differential, primary air is incrementally increased back to the design airflow rate.
- If the occupancy sensor is activated, primary air is returned back to the design airflow rate.
As an option in private offices, primary air can be shut off completely until the occupancy sensor is activated.
Two examples of when you may need additional primary air include temperature override and dewpoint override. In temperature override, you must stop the DCV reset and return to normal design airflow if the cooling setpoint cannot be maintained with the sensible coil. In the case of dewpoint override, you can stop the DCV reset and return to normal design airflow if the space dewpoint approaches the sensible chilled water temperature (58°F), if using a sensible chilled water coil.
There are a few factors to consider when controlling outside air. First, you should be aware if there are very low airflow rates on the primary air and whether they can be controlled accurately below 20% of box rated flow. Secondly, one needs to consider the availability and cost of points for occupancy sensor, CO2 or dewpoint calculation.
When considering DCV at the air handler with a single-zone air handling unit, outside air can be measured and controlled directly at the outside air inlet (see Figure 3). CO2 can be measured for the zone in the return air duct. Outside air is controlled to the minimum zone cubic feet per minute per square foot ventilation rate. If return air CO2 increases above the outside air CO2 by a differential of 700 ppm (or 1,100 ppm for outdoor air with acceptable CO2 concentrations), outside air is increased back to the design airflow rate Vbz (Ra + Rp).
Variable air volume systems
Multiple-zone VAV systems with direct digital controls of individual zone boxes reporting to a central control panel may include means to automatically reduce outdoor air intake flow below design rates in response to changes in system ventilation efficiency as defined by Appendix A of ASHRAE Standard 62.1-2016.
A few things to note about this scenario:
- Outside air can be measured and controlled directly at the outside air inlet.
- CO2 and occupancy are measured for each zone at the zone.
- Zones send their occupied status to the AHU based on CO2 differential.
- There are two outside air cubic feet per minute flow setpoints.
- Outside air is controlled to the measured minimum zone cubic feet per minute per square foot ventilation rate Ra.
- Outside air is increased back up to the design airflow rate Vbz (Ra + Rp) measured maximum.
Typical sequence of operations for this AHU/VAV system scenario:
- The ventilation outside air damper will modulate to maintain the minimum outside air design setpoint value once the unit is enabled to run.
- The minimum outside air cubic feet per minute will be increased on a trim and respond setpoint optimization sequence: each zone associated with the AHU will be capable of registering a vote for more ventilation air. Upon a demand for one or more CO2 monitored zones, the minimum outside air cubic feet per minute will be allowed to gradually increase up to the “design maximum” ventilation rate.
- As the CO2 in the monitored zones decreases, minimum outside air cubic feet per minute will be decreased back to the scheduled “minimum” ventilation rate.
- The following represents the trim and respond formula to be calculated once every five minutes (adj.):
OA cfm = [(max cfm stpt – min cfm stpt)/20] * (votes)] + last cfm value
When the votes go to zero, then the cubic feet per minute will be trimmed back to minimum once every five minutes (adj.):
OA cfm = last cfm value – (max cfm value – min cfm stpt)/*20
For engineers concerned about a ventilation increase that is too gradual, this factor in the default formula can be changed and lowered to produce a faster and more dramatic response to CO2 changes. This is best determined in the field during system commissioning.
Mechanical engineers should consider the cost of CO2 sensors, aesthetics and reliability/calibration of these sensors. When considering CO2 sensors, know that:
- Most control system manufacturers have CO2 options built into their zone sensors, which helps bring the cost down and improve the “look.”
- CO2 sensors are easy to maintain and calibrate if you understand how they self-calibrate.
- Physical destruction is the most common problem.
- Building automation system service agreements are highly recommended.
The use of a separate outside air CO2 sensor is not recommended for a few reasons. First, if the sensor does need to be calibrated or otherwise fails, you not only cause problems with a zone but with your entire building. Second and most importantly, it is simply not needed. Ambient CO2levels in the atmosphere are currently at 414 to 430 ppm worldwide.
Dewpoint monitoring and control
You may notice that several of these methods use sensible chilled–water coils. As a result, condensation at the zone may not be desired, and if you are in a situation where you may be condensing at the zone, you might find the zone outside the ASHRAE 55 standard for thermal comfort as well. For both of these reasons, controlling to a dewpoint is more desirable than basic relative humidity control.
The general ASHRAE 55 requirement is to maintain humidity that corresponds to a dewpoint temperature at or below 62.2°F. Because sensible chilled water systems run at temperatures of 58°F, monitoring and maintaining a dewpoint control at 54°F (at a zone air temperature of 74°F) for the space will meet both requirements.
It is recommended to space dewpoint sensors throughout the floorplate, approximately one per 10,000 square feet. If a section of floor has a separate environmental system or is shut off tightly from other spaces, the area should have a separate monitoring point.
Thermal comfort of occupants
The requirements of ASHRAE Standard 55-2017 are various and include:
- Thermal radiation.
- Air speed.
There are many personal factors as well that need to be taken into account when designing spaces. The standard does not cover other nonthermal environmental factors such as air quality, lighting or acoustics. While the standard is complex and beyond the scope of this article, in the context of the high–performance building, occupant control over their comfort settings should be more accessible. The sophisticated technologies that allow us to perform algorithms like the DCV sequences also allow the designer and facility owner to allow occupants to customize their environmental experience.
Occupant interface experience
Attention needs to be paid to the occupant interface experience. How can an occupant adjust not only the temperature setpoints, but other factors in the environment such as lighting? This is done through unified room controls.
Unified room controls can:
- Sense thermal comfort at the person, not the corner of the room.
- Get rid of the wall warts.
- Control lighting with the HVAC.
- Allow an occupant a single point of control.
When it comes to indoor comfort, sensing temperature and the control of HVAC in general, the traditional methods have limitations that can be overcome. Sensing temperature at the wall with a traditional thermostat or electronic sensor is no longer required. Proper occupant comfort is sensed at the occupant, not at the wall. Many modern ceiling devices, even lighting fixtures, are available that can measure the temperature in the middle of the room, where the occupants spend most of their time. This is just one convergence of lighting technology with HVAC control technology.
Smartphone apps are the second convergence of lighting technology and HVAC control technology; they are unified room controls.
DCV is a key component in achieving both ASHRAE 62.1 requirements and saving energy. In many cases, it can improve energy efficiency or gain points in a rating system like LEED.
Additionally, new technologies are replacing the traditional concept of room controls with light switches and thermostats on the wall. Designers and consultants can embrace unified room controls, which give the occupant an enhanced interface experience.