By Greg Jourdan
Commercial buildings utilize four common types of mechanical systems to accomplish air-conditioning and heating within the building:
- Central systems with built-up air-handling units (AHUs) with chilled water and hot water or steam heat.
- Rooftop units (RTUs), packaged HVAC units typically located on the roof of the building.
- Split AC and heating systems, which are similar to home airconditioning applications. The indoor unit is comprised of an evaporator and central air-handler, which is inside the building; the outdoor condensing unit is outside the building.
- Split heat pump systems, which are similar to a split AC and heating system (except that they utilize a reversing valve for providing mechanical heat from the compressor during the winter months with an auxiliary heat source for cold-climate conditions). In the summer, these units work just like a standard air-conditioner.
This article will focus on troubleshooting the refrigeration and central AHU air-supply systems, utilized in most commercial buildings. The systems that work on basic vapor compressor principles (composed of the evaporator, the condenser, the compressor, and the metering device) will be discussed.
The main component of the vapor compression system is the compressor, designed to pump cool refrigerant gas from the evaporator into the condenser. The compressor design can either be a piston, a centrifugal, or a rotary screw type. The size of the building and the HVAC system will dictate the type of compressor. Regardless of the compressor type, the compressor raises the temperature and pressure of the low superheated cool gas, forcing it into the condenser. The compressor should never pump liquid. This will not only damage the compressor, but can also create a potential safety hazard. The refrigeration compressor needs to be properly maintained and requires periodic inspection and testing. Unfortunately, the compressor is often ignored until it malfunctions or stops running, at which time it gets replaced and the system is back up and running - temporarily. Oftentimes, the culprit is not the compressor, but a system failure or design problem with accessory equipment that killed the compressor prematurely.
Federal EPA regulations require that anyone who accesses a stationary refrigeration system have a Section 608 license. Be sure to obtain this license or hire a qualified contractor to perform this service if you need to access the refrigeration side of the system.
A quick check of a system's components not only helps to diagnose troubles, but also allows you to anticipate failures by regularly monitoring critical temperatures. Use an infrared thermometer to survey:
- Compressor head temperatures.
- Compressor oil sump temperatures.
- Evaporator coil and suction line temperatures.
- Discharge line temperatures.
- Condenser coil and liquid line temperatures.
- Fan motor and pump motor temperatures.
With an infrared or surface thermometer, you can quickly survey a refrigeration system by scanning the temperatures of various components (see illustration). To minimize and prevent burns, a non-contact infrared tool or surface thermometer is faster and safer than touching with your hands.
By keeping careful records, it's possible to detect trends that indicate impending failure. This allows you to keep the system in top condition and avoid costly failures. IR instruments read best when measuring an object with a dull (not shiny) surface. If the surface is shiny, dull it with black marker, non-gloss paint, masking tape, etc.
Recording a Temperature Overnight
To check refrigeration-system performance, it is useful to record temperatures in the refrigerated space, allowing you to detect problems that may go unnoticed with a single system check. For instance, in a conditioned space, it is important to ensure that temperature variations are minimized. Temperature variations may result from changes in load or ambient conditions that occur over periods of time, so constant monitoring is required. By recording minimum and maximum temperatures in key locations over time, you can be sure that air circulation and refrigeration capacity meet application requirements. Digital recording thermometers allow you to record minimum and maximum temperatures over extended periods of time. Temperature values can be viewed at any time by pressing the "view" button (recording still continues). If the "hold" button is pushed, the recorded MIN/MAX values are saved and recording stops. The data is saved until the user selects a different input or turns off the instrument. When selecting a digital recording thermometer, look for a model that can record hundreds of temperature samples so that you get precise measurement. Other helpful options include a time-stamp feature, operator interval settings, and dual channels to record two temperatures at the same time. With this type of device, you can record temperature difference across a coil for extended periods of time. This feature is especially handy for troubleshooting erratic problem areas when time limitations don't allow you to wait until the problem occurs.
Troubleshooting Compressor Electrical Motor Faults
A clamp meter is a great tool for troubleshooting electrical motor faults - especially meters designed to accurately measure both AC voltage and AC current. These meters allow current to be measured without breaking into the electrical circuit. A compressor failure is often caused by an electrical fault. To check the compressor for electrical problems, check voltage at the contactor (starter) terminals, followed by de-energized tests at the compressor: Use an insulation tester to check resistance on windings and check from each winding to ground.
Troubleshooting Compressor Electrical Motor Failures Caused by Refrigeration- System Problems
Occasionally, defective compressors with electrical winding failures are diagnosed as being caused by an electrical-system problem. To quickly verify whether electrical is at fault, use an infrared thermometer to scan connectors, wiring, and circuit breakers while equipment is operational (if possible). Be sure to check compressor and pump motor amps to verify that they were within the manufacturer's design criteria. Any loose connections or overloaded circuits will appear as abnormal temperatures; however, mechanical-system failure or inferior installation and service practices often cause compressor electrical problems. These problems include:
- Poor piping practices, resulting in oil not adequately returning to the compressor during the run cycle.
- High discharge temperatures that create acids in the oil.
- Insufficient airflows across the evaporator and condenser coils.
- Extremely low suction pressures.
- Liquid refrigerant flooding back into the compressor.
Diagnosing these refrigeration-system problems and avoiding compressor failure can be done effectively using digital multi-meters (DMMs), clamp meters, digital thermometers, pipe clamps, infrared thermometer and refrigeration gauges, or pressure/vacuum modules.
Compressor bearings can fail or lock up due to poor piping practices, which causes oil clogging and results in insufficient oil return to the compressor. If the bearings don't lock up and continue to wear during these conditions, the rotor will lower into the stator housing, shorting out the windings. To diagnose this problem, measure the compressor amps: They should not exceed the manufacturer's full load ratings. Worn bearings will cause higher-thannormal amps. You can also scan the bearings with an IR thermometer and look for abnormally high temperatures. Inspect the oil level via the compressor sight glass. If there is no sight glass, use your infrared thermometer to measure the sump of the compressor housing. The oil level can be detected with the temperature probe. The sump temperature will be different on the compressor housing at the oil level. Whenever an oil problem exists due to poor piping practices, the correct remedy is to fix the piping, not to continue to add more oil to the system.
High discharge temperatures are caused by high head pressures or high superheat. The compressor discharge line can be measured quickly using the infrared thermometer on a dull section of pipe. Measure the discharge pressure at the gauge panel on the equipment. If a gauge panel is not provided with the equipment, utilize a set of refrigeration manifold gauges or use a pressure/vacuum module. Convert the refrigerant pressure to temperature and compare it to the ambient air temperature. If there is a temperature difference greater than 20 to 30 degrees F., there is either non-condensible gas in the system or restricted airflow across the condenser. If you are doing this test on water-cooled condenser, you should expect to find a 10- to 15- degree F. temperature difference between the inlet and outlet water temperatures. Temperature differences will vary due to size and application; refer to the original manufacturer's design specifications to determine optimum efficiencies.
Check for insufficient airflows across the evaporator using a digital thermometer. Place a bead thermocouple on the discharge side of the coil and on the return side of the coil. Record the temperature difference on the air-conditioning unit; expect about an 18- to 22-degree F. temperature difference. On refrigerated chilled water units, expect about a 10- to 15-degree F. temperature difference. Temperature differences may vary depending upon initial design and humidity requirements.
Extremely low suction pressures can be checked using the panel-mounted gauge set or use refrigeration gauges or a pressure/vacuum module and your DMM. Record your suction pressure at the compressor. Convert the refrigerant pressure to temperature using a pressure temperature (PT) chart. Measure the return air temperature before the evaporator. Compare the refrigerant temperature to the desired evaporator return-air temperature. On air-conditioning units, expect about a 35- to 40-degree F. temperature difference; for refrigerated chilled water units, expect about a 10- to 20-degree F. temperature difference.
Check for liquid refrigerant flooding back to the compressor by determining the superheat using your low side refrigeration pressure. Check suction pressure and convert the refrigerant pressure to temperature using your PT chart. Measure the suction line pipe temperature. Compare the difference of the two temperatures; if there is no temperature difference, then you  07 Buildings 83 are bringing back liquid to the compressor. If there is a temperature difference of between 10 and 20 degrees F., then you have normal superheat and you are not slugging the compressor with unwanted liquid.
Maintenance and Troubleshooting the Chillers and Cooling Systems
Good maintenance often requires simple (but labor-intensive) procedures and a PM checklist for the maintenance staff. Here are some minimum general guidelines:
- Refrigerant leaks should be fixed as soon as possible, according to law.
- The condenser and evaporator coils should be cleaned regularly and checked for debris that could block airflow or water flow.
- In terms of general maintenance considerations, keep water strainers and filters clean, check oil heaters, follow manufacturer-recommended inspections for routine overhaul procedures, and time frames. With an electrical megger, meg-ohm test all large motors annually or as recommended by the manufacturer.
- Circulating pumps should be checked quickly by simply measuring the differential pressure across the inlet and discharge of the pump.
- Regarding chiller maintenance, check refrigerant quantity, system pressures and temperatures, water-flow rates, and ensure a third-part oil analysis on compressor lubricants. Clean water side of vessels when pressure drop across the vessel exceeds minimum requirements. Good chiller maintenance may require bringing in the factory reps annually to perform non-destructive testing such as eddy current testing, pressure drops, and GPM water-flow analysis.
- To optimize controls, check set point vs. control point. Set point is the desired condition, but control point is the actual condition. Calibrate controls as needed.
- Remember to verify minimum condensing pressures being implemented to minimize compressor energy.
Maintenance for the Cooling Tower
Following are basic maintenance procedures for cooling towers:
- The sump and exposed water can freeze during cold-weather operation; electric sump heaters and controls needed to be inspected for operation.
- Cooling tower maintenance includes checking the water flow across the tower, checking for proper water levels, inspecting for clogged screens, and physically checking the spray nozzles for proper water distribution. Remember to also check pumps, fans, drives, and freeze protection.
- Proper water treatment includes testing for pH, TDS (Total Dissolved Solids), and conductivity levels, and checking for corrosion and scaling on the surface of the heat exchangers. Additives such as algaecides and biocides are often added as needed to neutralize pH levels and mineral contents. Algae build-up and slime have detrimental heat-transfer impact to the condenser tube bundles, which reduces water flow and heat-transfer capacity. Confirm proper water treatment with an experienced water-chemical expert that you trust.
- Remember to drain and clean the tower and spray system and sump for cold-weather operation before each fall.
Troubleshooting the Air Side of the AHU
Fan-system maintenance is important in maintaining peak operating efficiency of the HVAC system. It should include:
- Inspection of the filtration system and air distribution.
- Inspection of belts and drive systems on a regular basis.
- Changing filters on a fixed schedule or as indicated by filter-monitoring instruments.
- Sealing the leaks in air-distribution systems to maximize delivered air quantity and quality to each space or zone.
- The regular removal of grates and vacuum heat registers to remove dust and debris.
- Checking ducts yearly for loose connections and holes.
Develop and follow a preventive-maintenance plan that includes maintenance schedules. Activities in the plan should include:
- Inspection of outside and dampers for nearby sources of contamination.
- Ensuring that air dampers are clear of obstruction.
- Regularly replacing or cleaning air filters.
- Cleaning and inspecting drain pans.
- Inspecting and cleaning heating and cooling coils.
- Inspecting fan motors and belts.
- Regularly inspecting and cleaning airhumidification equipment and controls.
- Inspecting and cleaning air-distribution pathways and variable air volume (VAV) boxes.
Air-Conditioner and Heat Pump Basic Maintenance
Follow these simple guidelines to ensure efficient operation:
- Install a programmable or DDC (direct digital controlled) thermostat designed specifically to optimize heat-pump operation. Smart thermostats minimize the use of back-up heat and give you convenient program options.
- Set the temperature to adjust no more than 10-degrees F. warmer at night when you're away from the office.
- Don't make frequent thermostat changes. This can cause the back-up heat to come on, which can increase heating costs. It can also cause the system to turn off and on, and damage the compressor.
- Check air filters once a month. Clean or replace them when dirty.
- Don't block air returns/supplies with furniture or other items.
- Keep the outdoor unit free of leaves, dirt, and foliage.
Heat Pump Basic Troubleshooting
Make certain that the thermostat is set higher than the actual room temperature to create a call for heat or cooling as needed. If the thermostat is programmable, keep the batteries fresh.
Test for power to the air-handler by moving the fan switch from "auto" to "on." If the blower runs, the air-handler is functional. If nothing happens, check the circuit breakers on the air-handler cabinet and the breakers or fuses in the main panel. If any breakers are tripped, reset them once. If they trip again, don't reset them. Because controls are internally mounted and high-voltage wiring is exposed, only a qualified electrical service tech should open panels.
If the air-handler runs constantly, but cannot satisfy the thermostat setting, it is possible the back-up heat is running, but the condenser is not. Some condensers have the high-pressure cutout switch externally accessible. Look for a button sticking out of the cabinet in the vicinity of the refrigerant pipes and press it. If the machine starts up, the head pressure got too high, possibly from turning on and off too quickly, too much or too little refrigerant, or an electrical interruption.
Maintenance and Troubleshooting the RTU
The most important component to check on any RTU is the filter. Keep air filters clean to give the proper airflow across the furnace in the winter and evaporator in the summer. If airflow is reduced because of a dirty filter, the discharge air temperature will rise. This reduces the heat-transfer capability of the heat exchanger and causes premature failure. In addition:
- Check operating and safety controls for fail-safe mode. Make sure all components in the system are working as designed.
- Damper motor operation should be checked under normal operating conditions.
- Check the amp draw of the compressor and fans to catch problems early.
- On gas-heating RTUs, make burner adjustments for proper fuel-to-air ratios. Check flame condition and for carbon monoxide (CO) leaking through a worn or cracked heat exchanger. This should be done monthly as a minimum, including on-site constant-monitoring sensors and alarms.
- Lubricate fan motors according to the manufacturer's recommendations.
- Inspect V-belts at least quarterly for proper tension and excessive wear.
Greg Jourdan is an HVACR professor and BOC instructor at Wenatchee, WA-based Wenatchee Valley College.