As an energy engineer of 35 years’ experience, I have seen a lot of wasteful operation of HVAC systems. It can be tough for design engineers to truly optimize HVAC system design when so often schedules slip, deadlines loom and projects go over budget.
Many buildings are less efficient than they are supposed to be
Often a design or a building as a whole will be touted as efficient for the technology that is contains: heat pumps, VFR systems and / or “intelligent” controls, etc. Getting controls operating correctly, and setting things up right, AND so that they do not get over-ridden after the place is occupied, is often something that is
- More difficult to insure
- More challenging for the engineering staff
- Tougher for non-professionals to see and understand
Commissioning often happens too soon
The commissioning process is often less effective because it happens too soon. As someone who has done a lot of commissioning, and worked for a place that specialized in this, I see this as a chronic problem. Buildings are rarely completely ready on time, and more often than not the Cx agent is in there while the controls contractor is still finishing up, and the testing and balancing is being done at the same time. Time and again I watched this process result in
- The Cx agent burning through al the hours they have in the budget
- A huge corrective action log being delivered – of course, because the place was not ready yet
- Many problems and issues persisting during building operation, with maintenance staff left to deal with them (often by over-riding energy savings controls), and / or with people oblivious to the inefficient operation
All of this sadly because the powers that be did not want to hear that the building was not ready for commissioning, and make a simple adjustment to the schedule. Unless regulations or government forbid it, a delay in Cx usually need not delay the occupancy of the building. Most functional testing can take place with little or no disruption to normal operations, and Cx can be what it should be: a verification that things are working correctly.
A systematic big-picture approach to HVAC controls and optimization
Many designers (and energy engineers such as myself) sort of fly by the seat of their pants when it comes to optimizing efficiency (and reliability) for HVAC systems. They will look at one thing and then another without using a set of lists and an organized approach Oftentimes this is manifested in incomplete and / or poorly written sequence of controls, and other shortcomings. Use of a systematic approach, starting at a high level, can help to make this process organized and complete. Certain principals can be applied to this process to help optimize efficiency and performance of air-side HVAC systems.
Create and refer to an appropriate basis of design
The basis of design for a conditioned area can be used to form an efficient strategy for control. The design of HVAC systems should include consideration of how to control conditions of the air delivered to the space by:
- volume
- temperature
- percentage of outdoor air
- how that air is conditioning and treated
Single space served by many HVAC units
For spaces that are served by many HVAC units, control the group of units together, staging and coordinating their operation to save energy and meet loads. So many times I have seen many rooftop units serving one large commercial, industrial or storage space. With each unit on its own thermostat, differences in setpoints and sometimes thermostat calibration can lead to heating fighting the cooling. This type of egregious energy waste can produce the big energy savings numbers that an energy auditor likes to display!
Obviously using a decently large (5 degree F or greater) deadband between heating and cooling can help with this. However, the proper way to handle it is to have some form of central control over these separate thermostats. Too bad an energy management system costs so much. At $1500 per point, and with a minimal number of points per HVAC unit for:
- Space temperature
- Heating setpoints for occupied and unoccupied operation
- Cooling setpoints for occupied and unoccupied operation
- Heating command
- Cooling command
- Fan command
We get a rough cost of $9000 to control one unit, which an inexpensive programmable thermostat could cost less than $40! If not a real EMS, communicating thermostats can be an affordable option to avoid trouble with HVAC units (especially rooftops) working against one another.
A control that will duty cycle or rotate the operation of multiple HVAC units can be a good idea, so one or a few do not always take the lead and run all the time. This can help avoid maintenance problems and uneven wear on equipment. I witnessed an industrial space that humidified to meet manufacturing requirements, and while most of the rooftops that served it were operating normally, two had cooling coils that were blocks of ice because they just never shut off.
Remediate spot loads and unusual conditions locally is possible
Many HVAC systems use extra energy because they are set up to handle spot loads that may be either intermittent or limited to a specific location. For efficient operation, consider:
- Exhaust of conditioned air from targeted areas to help optimize efficiency and allow for smoother more trouble-free building operation
- Critical zone reset, with sensors throughout areas, to allow HVAC setpoints to be adjusted real-time to only what is needed
- Use of specialized systems where appropriate may save energy and increase reliability