Maintaining Control Systems and Identifying Problems
March 10, 2008
Remember that automatic control systems are automatic in operation, but not self-repairing. Unless a building has a service contract with a reputable controls contractor to inspect and calibrate the HVAC control system every six months, it is very likely that the HVAC system is not providing maximum comfort conditions. Many things can go wrong with a pneumatic control system, for example - plugged orifices, jammed damper blades, air leaks, leaking valves, and disconnected linkages are common, as well as room thermostats which are damaged by occupants.
The person responsible for the building HVAC system (such as an HVAC technician or a building operator) should perform regularly scheduled inspections and routine maintenance on the control system. These can be carried out easily when routine checks for the central air handling system are being performed for such components as filters and coils.
Keep a log of all inspections, maintenance, and repairs of the automatic control system. You cannot depend on your memory about the last date a piece of equipment was maintained or repaired.
If you are not a trained controls technician, you are not qualified to make major changes in control sequences. However, you should be able to change set points (such as a thermostat setting) in order to check the operation of the HVAC system in different conditions.
PREVENTIVE MAINTENANCESet up and maintain a good preventive maintenance program. Follow the instructions in the operations and maintenance manual that should have been provided for the control system. If you do not have one, contact the manufacturer of the control system that is installed to obtain this information. Generally it is advisable to conduct a routine inspection and service of the automatic control system every three months. Routine checklists should be developed, and the service technician who performs the tasks should initial and date each one.
The following sections are a general guide to tasks that should be performed regularly.
Today, most new control systems are DDC (direct digital control) systems. However, you need to understand pneumatic controls because you will encounter many, many existing pneumatic systems. (Note: DDC systems will be discussed in an upcoming article.)
A pneumatic control system must have a clean, dry, reliable compressed air source. The orifices in controllers (such as thermostats) are very small and can easily be fouled by dirt or moisture. It is extremely costly to purge a system of any moisture that gets into it.
Maintain the air compressor in good working condition:
• Keep it clean, and keep the space around it clean and free of debris.
• Change oil as recommended.
• Replace V-belts if worn.
• Lubricate the motor as recommended.
• Check for oil leaks.
• Keep the air intake filter clean. If it is metal mesh, use kerosene to clean it but never use gasoline. It may be advisable to relocate the air intake to the outside air to provide a source of dry air (except in very humid climates).
• Keep the receiver free of moisture. Manually blow down the tank daily, or if it has an automatic drain valve, verify that it is functioning properly.
• Test the relief valve by manually opening it. Hold the relief valve open until the compressor starts (should be about 50 psig). Let the relief valve close and be sure it reseats properly. Observe the tank pressure when the compressor stops (should be about 80 psig).
• If there is a refrigerated after-drier for the main air supply, check that it is functioning properly.
• Observe the main air pressure gauge. It should indicate about 20 psig.
• Observe how frequently the compressor starts and stops automatically. It should run about a third or half of the time. If the run time is excessive, look for an undersized receiver or for leaks in the air lines. The pneumatic system must be kept airtight.
Check the thermostat to see if the set point is correct. Be sure the thermostat has not been damaged by someone trying to change the set point. The thermostat should be calibrated every six months. Calibration is best performed when the space is unoccupied.
See that the thermostat is properly located so that it senses the actual room temperature and is not affected by sun, lamps, hot or cold surfaces, etc.
The HVAC system cannot condition the space properly if the airflow is inadequate - or if the air entering the conditioned space is at the wrong temperature. This can happen if the exhaust, return, or outside air dampers are in poor mechanical condition. Even if the damper actuator is working properly, the damper may have a problem:
• The linkage between the actuator and the damper can become loose.
• Linkages may be out of adjustment so that the dampers are not mixing the correct proportions of outside and return air.
• Damper blades may jam.
• Damper gaskets may not be sealing tightly.
• Dirt and corrosion can make the damper too stiff for the actuator to move it.
• Dampers may not be returning to their NO (normally open) or NC (normally closed) positions.
To check the operation of the dampers, disconnect the pneumatic line to the actuator. This is generally plastic tubing pushed onto a bayonet fitting on the actuator. The easiest way to disconnect is to cut the tubing with diagonal pliers, up very close to the connection. Then check the operation of the dampers:
• When the pneumatic line is disconnected from the actuator, the damper should go to its normal position (either NO or NC). If it does not, the damper is not operating freely.
• Next, use a squeeze-bulb air pump to put 15 psi air pressure on the actuator. The damper should go to its off-normal position. (The top of the spring range of the actuator will probably be less than 15 psi.)
If the dampers are moving freely, but are not moving to their proper off-normal position under 15 psi, check the linkage between the actuator and the damper. The push rod of the actuator is bolted into a slot in the damper operating arm. If this connection is loose or has slipped out of position in the slot, the damper cannot be moved to the proper position. Lubricate damper linkage and bearings as needed.
If the damper is working properly, use a hacksaw to mark the end of the damper rod in order to indicate the position of the blades.
Never correct a problem of a malfunctioning automatic damper by manually blocking it in a fixed condition. Dampers must be able to function automatically as intended.
Malfunctioning control valves for the hot water return (HWR) and chilled water return (CWR) piping (or in the supply piping) mean that the hot water or the chilled water coils are not receiving water at the required flow rate. If this happens, there is no way the conditioned spaces are receiving air at the correct temperature. Inspect the control valves when you inspect the dampers.
Inspect each valve for any sign of corrosion on the valve stem or leakage around the packing gland. Next check the valve movement:
• Mark the valve stem with a felt tip marker so you can observe its movement.
• Disconnect the pneumatic line and observe if the valve moves to its normal position (either NO or NC).
• Apply 15 psi to the actuator with a hand pump. The valve should move to its off-normal position. (The spring range of the valve is likely to be less than 15 psi. The actual spring range can be observed and verified during this test.)
IDENTIFYING CAUSES OF PROBLEMSIf you are responsible for troubleshooting a building HVAC system, your first job is to identify the cause of a problem. Then you must use careful judgment to decide whether you can adjust, repair, or replace components to solve the problem or whether you should call in a specialist such as a:
• Controls technician
• Refrigeration service technician
• TAB (test, adjust, balance) technician for air and water
• Manufacturer’s representative
Observe Your Control System
When your car develops an unusual noise or vibration, you look for the cause and remedy it. Do the same with your HVAC control system.
Be alert for unusual operating conditions, such as abnormal noises, excessive vibrations, oil leaks, moisture in the system, and off-normal temperatures. If you are alert to these warnings, you can often make simple corrections that will avoid costly repairs or equipment replacement later on. A major breakdown in the automatic control system will likely result in unacceptable comfort conditions for the building occupants.
Know Normal Operating Values
You cannot know if conditions are abnormal unless you know the normal operating values for the HVAC system. These are the ranges of temperatures and flow for the air and water in both heating and cooling modes. For example, you cannot know if the water entering the hot water coil is hot enough unless you know what the normal range of temperatures should be. Study the contract and as-built drawings, operation and maintenance manuals, balance report, commissioning report if any, and records of previous system operating conditions. You need to know the range of normal operating values for these conditions:
• Air temperature in the mixed air plenum
• Air âˆ†P (pressure differential) across the filters
• Air temperature leaving the coil
• Water temperature entering the coil
• Water flow rate through the coil
• Pump gpm
• Cfm of air through the coil
• Temperature of supply air at the fan
• Cfm of supply air at the fan
• Air temperature at outlets
Acting on the Problem
Think before you act. What could be causing the abnormal condition? Is it related to the automatic control system or is it caused by some other component in the HVAC system?
For example, the entire building is too hot in the winter. This could be caused by low main air pressure at the compressor. This is because most control systems are designed to fail-safe to a heating condition (no air pressure causes full heating). This is why the hot water valves are normally open. If the whole building is too hot, check the operation of the air compressor, and check the main air line for a major leak.
Pay Attention to Complaints
Complaints from building occupants can be one of the more difficult problems you have to deal with. They can be annoying. They can be petty or silly. Yet, when taken overall, they are your best indicator of how well your HVAC system is performing. Always keep two things in mind:
• Your job is to provide comfortable and healthful indoor conditions for the building occupants.
• “Comfort” means different things to different people. For every 10 persons, there are probably 10 different notions of what is comfortable. Some persons are more sensitive to heat, cold, and air movement than others.
Here are some guidelines that will make complaints work for you:
• Use common sense and diplomacy. Every dissatisfied occupant should feel that you treated the problem seriously and will follow up on it.
• Keep a log of complaints, giving a description of the complaint, and showing the person, location, date, and time. The log will provide a history that will often indicate that there is a problem.
• Don’t regard complaints as annoyances. Look on them as indicators that will help you do your job better.
• A complaint from only one or two persons in a group can well be a problem within the conditioned space rather than with the HVAC system. It can be the location of work stations (for example, too close to an outlet or in a drafty area). It can also be such conditions as the addition of heat-producing equipment, poor location of the thermostat, or a change in occupancy patterns or work processes.
When you receive a complaint, don’t just remedy the symptom - look for the cause.
For example, you receive numerous complaints that a space is too cold. Since several persons have the same complaint, you can assume that the problem is real. Don’t just turn up the space thermostat. Changing the setting of the thermostat will not increase the output of a system that is already at 100 percent. The first step is to look in the space for causes of the problem. Are there drafts from open doors or windows? Has heat-producing equipment (such as a lamp or an office machine) been placed under the thermostat so that it is getting the signal that the room is too warm?
When you have identified a problem and ruled out any source in the occupied space, you have to check the mechanical system.
On any electrically powered machinery, always follow the Lock Out & Tag Out procedure. This means to de-energize the main switch, lock it with your padlock, and attach a tag with your name and the date and time. Also, it’s a good idea to test the circuit to be certain that it is dead. Switches can be wired incorrectly. Sometimes the wrong switch is locked and tagged.
This procedure protects you in two ways. Many machines, such as air compressors, start automatically. If a machine starts when you are working on it, you can lose fingers. A more common accident is that someone sees a switch off and flips it on, unaware that you are working on the machinery.
Look for Multiple Problems
Always keep in mind the “Law of Sixes.” This is an old saying that means that if there is a problem, there is likely to be more than one cause. For example, if the conditioned space is not receiving cool air, the cause may be in the control system. But, in addition, there may be some - or all - of the following problems:
• Chiller malfunction
• Dirty cooling coils
• Insufficient chilled water flow
• Warm chilled water temperature
When you find the cause of a problem, always test and check to be sure the problem is completely solved. You may still have the same problem from another cause.
Excerpted and reprinted from Control System Basics for HVAC Technicians by Leo A. Meyer, one of the books in the Indoor Environment Technician’s Library series published by LAMA Books.
Publication date: 03/10/2008