Reliable and Efficient HVAC Systems

A chapter from my upcoming book Reliability is Green

HVAC Systems

Maybe the most commonly used asset around the world Heating Ventilating and Air Conditioning (HVAC) systems are in most cases ripe for few changes in design and maintenance practices that will have a direct impact on the reliability and energy efficiency of the asset. As a general rule HVAC systems are installed, balanced on start up, and then ignored. Over time we make what we view as minor changes to the system based on the ever changing location of office space and the humans sense of it being too hot or too cold to work. Over time we collect small space heaters and fans to help compensate for individual preferences and in many cases the maintenance people will install dummy thermostats in hopes that as each office person makes an adjustment up or down the act of making a change will have a placebo effect thus saving them from having to actually make changes.

In fifteen years I have never performed a RCM Analysis on a HVAC system, unless we are talking about a clean room environment the HVAC system typically doesn’t rank too high when it comes to asset criticality ranking. Because of this we are now looking at a system that will be ignored in terms of routine maintenance. Like any other asset made up of mechanical and electrical components if it is ignored long enough the efficiency will degrade overtime and ultimately result in system shutdown. This being said does that mean run to failure is the most cost efficient maintenance strategy? Could improving the reliability and energy efficiency of these assets result in a return on investment for taking the time to properly balance and maintain the system? Let’s take a look.

When I first began research energy efficiency improvements for HVAC system I began with the Energy Star Program a joint effort from EPA (Environmental Protection Agency) and US Department of Energy, they have loads of information available on the web including a 170 page PDF on Building Upgrades seems the Energy Star answer to energy efficiency is to replace your old, outdated and inefficient building assets with newer and more energy efficient products. The document is thorough starting with how to build a business case for upgrading your lighting, insulation, controls and HVAC equipment. While there are several pages of great information here I wanted to trim this down to a few pages of useful information on how to get the most out of making your building HVAC more energy efficient and reliable.

Taking the simple mans approach let’s starting with the outside air intake and working our way through the typical system to the point where air is delivered to the work area and returned to be mixed again with outside air. The typical HVAC system begins with a outside air damper that regulates the amount of outside air taken in by the system, this outside air is now mixed with return air and filtered to remove dirt, dust, bugs or other solid particles. Once the air has been filtered it enters a centrifugal fan and is pushed through a series of coils where the air is heated or cooled depending on the season and desired work condition temperature. At this point depending on your location and environment, the air may be humidified or dehumidified and pushed through the ductwork to various locations at your plant. Within the ductwork we will have a number of dampers and registers to balance the air flow to various locations. To keep the air at a regulated temperature we must have a constant flow of air back to air intake, this is accomplished through return air registers or building exhaust again depending on your work environment. In the manufacturing world we like to view the HVAC system as a simple process and as a result it receives very little attention until it is not working. To make the system more reliable and energy efficient let’s break it down to it basic functions.

1. Heating
2. Cooling
3. Humidifying
4. Dehumidifying
5. Ventilation

While we have what looks like five simple systems there are a number of different ways that each can be accomplished and this is why there a limited number of companies who offer plans or services on how to make your specific system more energy efficient. The best place to start in looking to make your system more reliable and energy efficient is by developing a complete maintenance strategy. The best way to develop a complete maintenance strategy is to perform a reliability centered maintenance analysis and of course, I would recommend you use RCM Blitz™ to complete that analysis. While I started this chapter stating I have never had a company request a RCM Analysis on their HVAC system I would guess that most companies have no idea how much money they spend or waste when it comes to HVAC. While I have looked at several sources in writing this chapter the average company spends up to 35% of their energy on HVAC.

Using a RCM approach lets define the main function of your typical HVAC system;

Main Function: To be able to provide XXX CFM of air at a temperature of 68-70°F, at a humidity of 30 to 55 percent while meeting all Health, Safety and Environmental standards.
We should also view the main function with the assumption that we want to provide these standards while we are utilizing the facility. If we are working 24 hours a day for 5 days a week then we need to provide this main function all of that time. In order to develop our maintenance strategy we need to know how the main function can fail, in RCM terms this is known as functional failure. Understanding the functional failures will help us to identify potential failure modes and then develop mitigating maintenance tasks to ensure reliability and energy efficiency.

Functional Failures

1. Unable to supply air at all
2. Unable to supply XXX CFM of air
3. Unable to supply air at a temperature of 68-70°F
4. Unable to supply air at a humidity of 30-55%
5. Unable to maintain Health, Safety or Environmental Standards

The next step of the RCM process would be to discover all the failure modes and effects for the HVAC starting with the failure modes associated with being unable to supply air at all and working through all the failure modes for each functional failure. The objective of the analysis is to then develop tasks to mitigate each failure mode by:

1. Eliminating the failure mode
2. Reducing the probability of failure
3. Reducing the consequences of the failure

The outcome of finished product would be a combination of Preventive Maintenance tasks (PM’s), Predictive Maintenance tasks, (PdM) Failure Finding tasks, Redesigns and Consequence Reduction tasks that ensure the reliability of the asset. Without specific knowledge of the context in which we plan to operate the HVAC system I described above prevents me from completing a thorough RCM analysis of this system. In the spirit of offering basis suggestions of how to improve reliability and energy efficiency I would offer the following suggestions for improving your HVAC systems.
HVAC System Operation
1. Optimize your system by having it balanced by a professional at least once each year ensure OSHA exchange of air requirements are being maintained
2. Use a variable speed drive for your HVAC system to improve efficiency
3. Automate system controls and make sure your have separate controls for separate zones
4. Control who has the ability to change temperature control settings
5. Set the thermostat higher in the cooling season and lower in the heating season
6. Control all outside air sources (Doors and windows)
7. Minimize exhaust and make-up air – Again each facility will be different depending on workplace requirements for personnel, equipment needs as well as local, state and federal requirements.
8. Only use the system when and where it is needed
9. Reduce your system requirements in the off hours
10. Only heat or cool space this is being occupied (Conference Rooms)

HVAC Maintenance

1. Replace HEPA filters based on differential pressure instead of time. HEPA filters are expensive enough to begin with and as they become fouled the amount of energy used to pull the air through the filter increases significantly.
2. Replace lower cost front end air filters based on time or condition
3. Perform a annual PM inspection of the air intake to ensure they are operable (The specifics of PM will vary depending on the type of damper and control)
4. Clean heating and cooling coil surfaces twice each year (Heating Coils, Cooling Coils, Heat Exchangers, Evaporators and Condensers) (Spring/Fall)
5. Inspect ducts for leaks, open inspection ports – seal all leaks and ensure inspection doors are closed and sealed
6. Inspect drive motor belts for evidence of cracking or fraying replace if noted
7. Ensure belt drive has been precision aligned at installation and replacement
8. Clean and balance the fan – Ensure your fan has been precision balanced when it is installed, inspect the fan for cleanliness on a annual basis. Provided one does a sufficient job of replacing filters there should be no build up on this fan.
9. Calibrate thermostats and control loops on a annual basis
10. Maintain steam traps utilizing Ultrasonic leak detection methods on a monthly basis in the heating season
11. Ensure heating and cooling is controlled to +/- 2°F
12. Test refrigerant lines for leaks and repair if noted (Calculate efficiency of your present chillers and check to see if a more energy efficient unit can be installed)

Boiler or Fuel Fired Heating Systems
I will use caution in discussing these systems as local, state and federal regulations are usually very clear in the level of maintenance that needs to be performed on these systems. The good news here is most required maintenance plans include the steps necessary to ensure both reliability and efficiency. I can’t stress enough the importance of proper maintenance when it comes to these systems yet in drafting this chapter I am reminded of dozens of potential failures my RCM teams have discovered in various furnace RCM analyses. These included gas supply valves installed in a “Fails Open” condition, Oxygen sensors that were removed or disabled, intake dampers pinned in position, intake screens removed. The list goes on and on yet for some reason these systems are often treated as less important than our manufacturing equipment. My recommendation when it comes to performing maintenance or making upgrades to improve the efficiency of your boiler of furnace heating system, consult with a company who specializes in this equipment and use their assistance to perform a RCM analysis based on the context in which you plan to operate this equipment.

Resources:
US Department of Energy http://www1.eere.energy.gov/buildings/
North Carolina Division of Pollution Prevention and Environmental Assistance http://www.p2pays.org/ref/26/25985.pdf