Running HVAC Systems Effectively
Keep Hvac Systems Running at Their Best
Basic Operation Overview
Heating, ventilation, and air conditioning (HVAC) equipment come in various forms such as ground or air-source heat pumps, air or water cooled towers, chilled beam cooling, and radiant cooling/heating, to name a few. Most systems operate on the same principles and cycles, but technology specifics differ for condensers, evaporators, and compressors. These differences simply change the method in which heat is absorbed, compressed, and released, along with a corresponding difference in system efficiency. A good reverse cycle air conditioning system can provide both cooling and heating in locations with a mild climate.
Starting with a run through of system basics will help understand how best to run them optimally and what can be done to improve their operation. Having much in common with refrigeration systems, a reverse cycle A/C system consists of four main components; an evaporator heat exchanger (HX) for providing cooling to the space, a compressor to push the warmer refrigerant to a higher pressure, the condenser HX where it releases heat to the non-conditioned outside space, and an expansion valve where the refrigerant passes through to lower pressure on its way back to the evaporator.
For more details on the vapour compression cycle visit the quora page.
General Operational Recommendations
A/C usage makes up a large portion of your electricity costs, so it pays to make sure you’re doing the most you can to keep it running smoothly and efficiently, while maximising lifetime.
Keep HVAC turned off as much as possible during shoulder seasons and avoid situations where both heating and cooling are running simultaneously. During this period of time, make use of natural cooling methods such as open windows supplemented with mechanical fans.
Using A/C in areas open to the outdoors keeps units running continuously, which drains on electricity and increases equipment wear.
Schedule equipment using a building management system (BMS) or programmable thermostats to match operating hours and avoid unnecessary usage overnight. For hospitality, adjust schedules and set points according to the needs of customers rather than staff.
Run A/C only during normal operating hours and not for minimal staff occupancy. In such a case as for cleaners or early arrival/late stay of employees, instead make use of mechanical fans and natural ventilation/lighting. This will significantly reduce energy requirements on site, reduce maintenance requirements, and extend equipment lifetime.
Utilise metering and monitoring on A/C systems as a way of identifying when loads are operating abnormally, out of schedule, and to verify that best practice is being followed. Even viewing main meter data benchmarked against other sites in the same industry can give a good idea of good or bad A/C performance.
Maintaining Your System
A good maintenance strategy is key to avoiding hidden issues over time which degrade energy efficiency. This means regular, thorough, preventative maintenance, not just service calls in response to occupant complaints. The latter category these often results in quick comfort fixes at the expense of higher ongoing energy costs.
Preventative maintenance might be cut during budgeting exercises, but cutting corners on HVAC equipment is a short-sighted approach that often leads to concealed, costly issues. Examples include faulty chilled or hot water valves which result in simultaneous heating and cooling, a costly method of maintaining set points as the heating system fights against cooling. Variable speed drive (VSD) sensors can fail and prevent fan speeds from changing to deliver the savings expected from this technology. Variable air volume (VAV) mixing boxes can go out of calibration resulting in freezing conditions, fixed by enabling continuous use of reheat systems (e.g. electric duct heaters) even through Summer. When a maintenance services provider is called out in response to diminished cooling and heating capacities, they may bypass time clocks to keep the plant operating out of hours to try and meet the required set points.
To get the most out of maintenance providers, they should have incentives not only to “put out fires” highlighted by the end user, but be empowered to look further underneath for underlying issues affecting energy and water wastage. It’s important to ensure that A/C units continue to deliver close to their stated cooling capacity rating in both flow rates and temperatures.
This means collaboration between the facility manager and the contractor in understanding the assets, considering the age and type of equipment as well as controls and settings available such as a BMS. The building owner should also be aware of the need to allocate resources in order to implement high performance maintenance to transform a neglected building or retain environmental certifications. For a more detailed look at the whys and hows of implementing effective maintenance strategies, see pages 14-15 of the AIRAH’s HVAC best practices.
To achieve the best outcomes, responsibilities for involved parties such as the facilities manager, building owner, occupants/tenants, and the maintenance provider are outlined on pages 23-26 of AIRAH’s best practice manual.
Upgrades & Considering a New System
Stay aware of governmental incentives which may help to help offset costs for upgrade projects. Metering on your HVAC equipment first lets you see the current consumption of your units, guide which improvements to make, and then to track savings achieved by initiatives.
Once equipment has become inefficient, obsolete, and unreliable due to age and/or use, it becomes likely that it will be more cost effective to replace than continue to run. New systems bring with them modern technologies across the board for vast improvements in efficiency of heat exchangers, compressors, refrigerants, system design, controls, settings, and other efficient peripherals to tweak the system. Advancements include VSDs on compressors, high efficiency motors, pumps and fans, pipe insulation, electronic expansion valves, improved heat exchanger design for condensers and evaporators, fault detection, and alarm systems. Read more about variability upgrades in our article, Efficiency Tweaks for Your A/C, covering VSDs, VAVs, and VRFs.
Upgrading with VSDs, electronic expansion valves, insulation, and heat exchangers boost efficiency
While many of these advancements are available as drop-in upgrades, replacing out-of-date units in their entirety will provide better efficiency increases in a more cost effective package. This is especially true for the long-term where existing systems are more than 15 years old or still use R12, R22, or R502 refrigerant.
More important factors to consider when maintaining, upgrading, or replacing various aspects of HVAC systems such as AHUs, chillers, boilers, VAVs, pumps, BMS, etc are discussed in greater detail on pages 36-60 of AIRAH’s Guide to Best Practice Maintenance & Operation of HVAC Systems for Energy Efficiency.