The Ele(fan)t in the Room (Part 1): Optimization of HVAC for Vacancy
Fan energy is the highest single electric energy-consuming component in commercial buildings according to US EIA data. Fan energy is rarely optimized for part load and/or vacant space conditions. Use of Occupied Standby mode and optimization of Min Primary Airflow setpoints is essential to maximizing savings of HVAC systems in these conditions.
The national office vacancy rate is expected to increase to levels substantially higher than the pre-pandemic. According to one study, vacancy rates as much as 55% higher than pre-2020 rates by end of this decade (Note 1). What is happening to the HVAC when all these spaces are partly occupied?
The reduction in occupancy results in a lower cooling load which automatically reduces the chiller/compressor energy use. But what about fans; is there also an automatic reduction in fan energy to the same extent with higher vacancy rates?
The answer to this is: No, unless the right sequences are implemented, there is most often not a substantial reduction in fan energy.
Research has shown that air systems are generally considerably oversized and spend the largest share of their time operating at the terminal box minimum primary airflow setting (Note 5 ,8). Even when the space is vacant, these terminal boxes (VAVs and FPBs) deliver barely less, or even the same amount of airflow they do when occupied.
To capitalize on this growing opportunity for energy savings at the fans, a focus on the basic operating parameters of the terminal box is required, with optimization of the Zone Minimum Primary Air setpoints and incorporation of a terminal box mode known as “Occupied Standby Mode”. Both of these control approaches are required in the latest versions of ASHRAE 90.1 (and described by ASHRAE Guideline 36 in greater detail), but are rarely deployed to the extent required, or described by this guideline. Depending on your jurisdiction and the referenced version of 90.1, both may be code required by your code.
The savings opportunity for this is not minor. For a medium office building, research has shown that optimized minimum airflow control can save 16.1% of building site energy across a variety of climate zones when compared to typical box control (Note 3). This research was done pre-pandemic, with lower vacancy rates. The savings would be even higher now with higher vacancy rates.
This is a major opportunity with low implementation costs and an extremely high rate of return. Both for new construction and for existing spaces with system retrofits, in particular as we head into the new office hybrid work paradigm.
Fans: The Biggest Consumer of Electric Energy
Fan energy is the highest electric energy-consuming component in commercial buildings according to the most recent US Energy Information Administration (EIA) survey data, surpassing chillers, pumps, lighting, DX compressors, office equipment, cooking, refrigeration, and computers (Note 2). Fan energy falls behind only heating in overall commercial building energy consumption and far exceed the energy required for cooling/heating ventilation air.
There are a handful of control optimization opportunities that target reduction in fan energy. All of these are described in detail in ASHRAE G36 (Note 7). Three of the main opportunities are:
· Zone Min Primary Air Optimization and Occupied Standby Mode
· Supply Air Temperature Reset
· Static Pressure Reset
Research showed savings of total building site energy for a medium office building with the use of G36 minimum airflow control of 16.1%, supply air temperature reset savings of 6.6%, and static pressure reset savings of 3.6%, across different US climate zones (Note 3).
This post focuses on the savings from optimization of the minimum primary airflow setting at the terminal box (aka “Vmin” or “Vpz-min”) and implementation of a mode now required by the latest versions of ASHRAE 90.1, “Occupied Standby Mode”. which was added in ASHRAE 90.1-2019 section 6.5.3.8.
Why do fans consume so much energy?
Air is not an efficient medium for energy transfer. Air's specific heat is much less than that of water and far less than that of refrigerant. From a volumetric flow standpoint, and therefore an energy standpoint, air is the least efficient component of the HVAC energy transfer process. Think of how much larger the ducts leaving an Air Handling Unit are than the CHW lines, or the combined supply duct area compared to the chiller refrigerant expansion device cross-section. We take this discrepancy for granted, and because air is needed for ventilation of the space and some amount of airflow is required when the space is occupied, don’t often question if the fans are moving too much air for what is required by the space. Fans are the giant electricity-consuming elephant in the commercial space.
Fan energy savings opportunity
Fan power reduces with a near cubic relationship to a reduction in airflow (Note 4), so, a slight reduction in airflow saves a much, much larger amount of fan power. Identifying a control strategy that targets airflow and operating pressure minimizes the largest energy consumer and is of the lowest-hanging fruit opportunities for control optimization. Furthermore, targeting savings with these measures also optimizes the ventilation heating/cooling load, bringing it into compliance with current code requirements.
ASHRAE RP-1515 analyzed the frequency that typical VAV boxes operate at their minimum primary airflow and found that VAV boxes often spend the majority of their time at minimum position.
VAV Mins – Too Damn High
Engineers oversize equipment (What?!). It happens at every level of the mechanical systems for the built environment. Engineers call it a safety factor. In some cases, it’s prudent. For terminal boxes, in some cases, it may be prudent (likely not), but if the minimum airflow that the box is allowed to control is not aggressively set, it has a major negative impact both on energy and occupant comfort. Often the terminal box zone minimum primary air is not specified by the design engineer and/or is arbitrarily set between 25-50% of the max, for an already oversized VAV box! For a terminal box that is substantially oversized, this could mean that the box delivers close to the full actual requirement for design flow all the time. Nuts!
How often does this happen, and what is the impact to terminal box operation? ASHRAE commissioned a study specifically to analyze this. ASHRAE RP-1515 demonstrated the pervasiveness of the oversizing issue (Note 5,8). The figure here shows the relative frequency that VAV boxes across a number of climate zones operate at. It reveals that VAV boxes spend the majority of their time at minimum position. The red line is for a typical 30% min VAV box and the blue is for the newer “dual max” control logic. Both spend their highest percentage of time at their minimums. At these minimum positions, the occupant is either overcooled or if reheat is present, the air is reheated, wasting fan energy, cooling energy, and reheat energy. The higher airflow also means higher ventilation rates with is another contributor to wasted cooling and heating energy.
Tackling this waste has been demonstrated to save 16.1% of total building site energy! (Note 3 , 8)
Step 1: Optimize Zone Minimum Primary Air (Vmin)
The expected peak occupancy of a space is often somewhat arbitrarily determined by an engineer counting chairs on an architectural floorplan. This is what is used to determine both the peak occupancy and minimum airflow rate for ventilation. Does this reflect the actual occupancy of your office space?
Additionally, current Mechanical and Energy Codes for many jurisdictions (including NY and NJ) reference the ASHRAE 62.1 Indoor Air Quality Procedure which requires the calculation of Vmin (not an artificially high rule of thumb Vmin). Energy codes, following 90.1-2016 or more recent versions set the default maximum allowable Vmin at 20% for many spaces, unless the Ventilation Rate Procedure calculation demonstrates otherwise. The NYC code specifically allows for a 62.1 Ventilation Rate Procedure that allows for a new simplified Vmin calculation, that is often less than 20%.
If your VAV mins are generally above 20%, consider this as your first step. There is no hardware cost associated with this measure!
Occupied Standby Mode is an important tool, along with optimization of Minimum Primary Airflow Setpoints for maximizing performance of HVAC for lightly occupied and intermittently vacant spaces. It has also been a requirement of ASHRAE 90.1 since 2019.
Step 2: Occupied Standby Mode
ASHRAE 90.1-2019 section 6.5.3.8 added a requirement for an “occupied standby” mode for all spaces where lighting vacancy control is also required. This means that where hardware is already required for lighting control, the terminal box goes into an “occupied standby mode” when the occupancy sensor indicates that the space has been vacant for 5 minutes, during normal operating hours (i.e. during lunchtime on a normally occupied weekday). In this mode, two things occur, which are described in greater detail in ASHRAE G36:
1 (Most importantly) Vmin goes to 0 CFM (Note 6)
2 The space temperature setpoint is also changed by +/-1°F after 5 mins.
The change to a 0 CFM Vmin is indiscernible by the occupant when they return to the space. The exception to this is spaces that have no reheat, in which case this will improve the occupant’s comfort! The 1°F reset is intentionally mild to prevent complaints (and can be overridden if there are complaints). For BAS retrofits, the occupancy sensor can be integrated into the replacement thermostat at a minor incremental cost. For open offices, the detection limits of occupancy sensors can be pushed further than with lighting control sensors, since the occupied standby mode has a 5-minute dwell timer and the impact to the occupant is practically indiscernible.
Full Solution: G36 BAS Retrofit
Programming custom code for occupied standby is possible, but in reality best done with the use of the replacement of the VAV box controller that has built-in G36 code.
I talked at length about my support for ASHRAE G36 in a previous post. The major control vendors have been developing the code since 2018 and are implementing the de-bugged code for the G36 sequences. When the terminal box controller has G36 code built-in, it is simply a matter of configuring the parameters. No custom programming! Better yet, with G36 the engineer does not have to specify Vmin, the G36 controller automatically calculates this value. Furthermore, the G36 sequence automatically does the Ventilation Rate Procedure (VRP) calculation that is required by code in many jurisdictions (including NY and NJ), and all other code-required sequences automatically. There are many other reasons why a standardized approach to controls for HVAC across the industry is wise and important, but the use of Occupied Standby Mode and automatic calculation of Vmin is one shining example of the opportunity.
Consider a BAS G36 Retrofit for your system. Often, utility and government rebates are available to fund a substantial portion of this upgrade to your legacy controls system.
Notes and References:
4 The fan affinity laws describe the relationship between power, pressure, flow rate, and speed for a variable flow centrifugal fan/pump system. In a typical real-world VAV system, where VAV boxes respond to duct supply air pressure by opening and closing dampers and the fan efficiency does not remain constant, the affinity laws are an approximation, but the order of magnitude impact is close. For example, a 20% decrease in airflow or increase in air temperature difference results in a ~50% reduction in fan power.
6 Vmin does not always go exactly to 0 CFM. G36 describes the procedure for determining the minimum controllable CFM based on the capabilities of the VAV box. Often this is between 8-12% of design max CFM.
7 ASHRAE G36 has a multitude of optimized control sequences including those that also impact fan energy such as, such as Optimized Start/Stop and Demand Controlled Ventilation, but the above three have proven to be particularly effective.