A/C and Heat Pump Preventive Maintenance

April 20, 2001
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Figure 1. When you check the blower wheel, look for excessive dirt, out-of-balance condition, and shiny spots where the fins are pressed onto the hub and where the hub presses onto the collar.
Now that summer is approaching, cooling preventive maintenance (PM) is keeping us busy. There has to be a different way to perform a PM for every different contractor out there.

Contractors have certain things that they routinely check, and individual contractors perform more or fewer repairs during a cooling PM. Good preventive maintenance can result in an abundance of chargeable work.

If while performing a PM the technician is in the frame of mind to locate as many problems as possible (as apposed to how much his paycheck is going to be this week), it can be a win-win situation for your company and the customer. Here’s a “pie-in-the-sky” list of preventive maintenance issues that can be considered.

  • Tighten all electrical connections.
  • Look for and repair burnt wire terminals.



    Figure 2. If the surface of the pulley is worn (concave) where it makes contact with the belt, it should be replaced.
    I always show the customer the burned wires to build perceived value into the maintenance. Always check the fused disconnect for overheated fuses, stabs or clips.

    I recommend that you always replace the fuses and disconnect when you find that there is any overheating. Also check for swollen or shrunken fuses. If the fuses are swollen or shrunken they have been wet. Locate the source of the water entry and fix it. Of course, don’t forget to check any seal-tight flexible conduit for leaks at the connections.

  • Check high and low voltage while the machine is operating to ensure there are no serious voltage drops in the wiring.
  • Check the crankcase heater for proper operation.
  • Crankcase heaters are often defective or were not reconnected during a previous compressor replacement. One way to check a crankcase heater is to use an amp meter. They draw about 0.1 to 0.3 A when they are operating.

    Depending on the vintage of the system you will have to use different strategies to check them. On really old systems, the crankcase heaters were often powered off the line side of the contactor so you can check them at any time. Some later systems powered crankcase heaters only when the compressor was not operating, so checking it while the system is operating is fruitless. Some crankcase heaters are operated by a compressor hot gas line thermostat.

    The compressor hot gas line must be cool to check the operation of the heater. Some heaters, called trickle crankcase heaters, operate by heating the start winding in the compressor. These heaters only operate when the compressor is off and they draw about 3 or 4 A. Don’t let this high amp draw alarm you; it is an anomaly specific to trickle heat and is of no concern.

    The most common crankcase heater used in the last number of years is called a PTC style crankcase heater. These are the ceramic heaters you see inserted into the side of a lot of compressors with the white thermo-paste goop on them. These crankcase heaters are self-regulating. Their resistance increases as their temperature increases.

    What this means to you is that if they are working properly they will have an almost undetectable amp draw. The way to check these heaters is to either check to see if the compressor crankcase is warm or to electrically disconnect them from the circuit and check their continuity. If they have resistance they are probably good.

  • Checking compressor amps is always a good procedure to ensure that there is not an extraordinary amp draw and to give you info to confirm you checked the compressor.
  • The important thing to remember is that unless the compressor amp draw is seriously out of whack, it doesn’t really tell you much about how the compressor is working. There is no way, in the field, to tell how much amp draw a compressor will have at any given load condition.

  • Check the compressor contactor for splattering of the silver coating on the points.
  • If you see small, solidified globules of metal, the contactor is doomed. It’s just a matter of time. This is another thing I like to show the customer so that they feel they are getting their money’s worth.

  • Check indoor motor bearings for wear and oil the motor.
  • Confirm motor amperage is not above the running load amp rating on the machine nameplate. This can be a tipoff of worn bearings, weak capacitor, or misapplication of the motor. If the motor is aftermarket, it’s important to use the RLA on the motor data plate. I also check for cracks adjacent to where the motor mount legs attach to the blower housing, as well as for loose motor mount grommets.

    Some machines use compression-type motor mounts to attach the motor bracket to the blower housing. They dry out and come loose.

    Another thing to check in the blower is the wheel itself. I look for excessive dirt, out of balance condition, and for shiny spots where the fins are pressed onto the hub and where the hub presses onto the collar (Figure 1). These shiny spots can indicate that the fins are starting to come loose from the hub. I also check the belt and drive assembly if there is one.

    I always look real close at the drive pulley (the one attached to the motor) for wear. If the surface of the pulley is worn (concave) where it makes contact with the belt, it should be replaced (Figure 2). Worn drive pulleys cause the belt to slip. When this goes undiagnosed, the belt gets tightened to diminish the belt slippage, which eventually causes the blower bearings to fail due to the belt being too tight.

  • Check the outdoor motor bearings and oil it when possible.
  • Check the motor amps to ensure they don’t exceed the RLA listed on the data plate. If they do, this could indicate worn bearings, a weak capacitor, or a misapplied motor. I also check the capacitor installation since these are often mounted poorly at the time of a motor replacement.

  • Check the condenser fan for cracks at the rivets that hold the blades to the hub (some people call this the “spider”).
  • Make sure the rain guard (if any) is tight on the motor shaft. When they become loose, they can make a noise that sounds like motor bearing failure.

  • Check the compressor run capacitor(s), also referred to as “that silver can thing.”


  • Even though a compressor seems to be starting properly, there is really no way — with everyday tools — to know with certainty if this is so. Capacitors don’t always fail all at once. Many times they fail as a result of a slow degradation. In other words, they begin to “bleed.” Their microfarad rating slowly drops until the stress put on the compressor and associated starting system causes them to overheat or some other component to fail.

    Publication date: 04/23/2001

    Figure 3. How to test a capacitor under load

    How To Check Capacities Under Load

    Unfortunately, some capacitor checkers we use in the field are ineffective at testing for a bleeding capacitor because there is no load on the capacitor at the time that it’s being tested.

    The good news is that there is a fairly reliable way to test a capacitor under load (Figure 3).

    While the compressor is running, measure the ac voltage across the run capacitor. You will be reading the voltage that the compressor is generating. The term for this is “back electromotive force.” Measure the amperage being drawn through the start wire between the capacitor and the compressor start terminal.

    Be sure to keep your amp meter away from the components in the control box — that could distort your reading. Use the voltage and amperage readings you’ve obtained in the following formula:

    Amps X 2,650 ÷ Voltage = Actual microfarads.

    If the solution to your test gives you a microfarad rating that is 5% below the capacitor’s labeled rating, be suspicious. If the results of your test show the capacitor to be 10% or more below the labeled rating on the capacitor, replace it.

    Of course, if the run capacitor is swollen or is leaking oil there is no need to test it. Just replace it. To check the start-assist components just place your amp meter on the wire on either side of the capacitor or solid-state start-assist device and watch for an amperage spike for about ¼ sec as the compressor starts. If there is no amperage spike when the compressor starts, more investigation is necessary.

  • Check to determine if the thermostat is calibrated properly, if there are any holes in the wall behind the thermostat and if the heat anticipator is adjusted properly.
  • I don’t mess with a thermostat unless the customer is unhappy with its performance. Remember that any adjustment to the thermostat or its installation will disturb the way the thermostat is operating and the customer’s perception of comfort in the conditioned space. Arbitrary adjustment of the thermostat can cause callbacks.



    The Condensate Trap

  • Check the condensate trap and drain line for restrictions, proper downhill run and dirty condensate pan. Also check that the machine is level.
  • Tipping the machine toward the drain is not an acceptable solution for drainage problems. It actually makes the problem worse by diminishing the amount of condensate the pan can hold. If the system has a condensate pump, check its operation.

    Here is a tip for spotting a machine with chronic condensate problems. Inspect the end of the evaporator coil where the return bends protrude from the tube sheet (the outer-most plate). If there is a rust line at about the same height as the top of the condensate tray, this indicates that there has been too much water standing in the tray.

    Possible causes include the trap design, of course; a clogged pipe; a dirty evaporator coil; a condensate line that runs uphill (I always get a kick out of that one); or a machine that is not level. On residential pull-through-type evaporators (negative pressure in the blower compartment), a trap will not work properly unless there is a drop between the inlet and the outlet of the trap of about 1 ¼ in. with a 1 ¼ -in. reservoir (Figure 4).

    There should also be a vent downstream of the trap to relieve any pressure that may build up in the drain. The vent should not terminate above the top of the condensate pan so that any overflow will not spill into the machine, where it will go unnoticed for a longer period of time.



    The Coils

  • Wash the outdoor coil.
  • The reason I say wash it instead of inspect it to determine if it is clean is because it is very difficult to tell if a coil is clean by inspection. The only way to tell if it is dirty is to clean it.

  • Check the indoor coil for dirt and debris.
  • Not many contractors include an indoor coil cleaning with their preventive maintenance. However, any machine that is more than 10 years old probably has a dirty indoor coil. Coil cleaning can be high-margin work if sold properly. Often the problem with generating this type of work from preventive maintenance work is that techs as a rule don’t like to clean coils, so they have a tendency to avoid selling the repair. This problem can be overcome through incentives or by using a dedicated coil-cleaning crew.

    Of course, another reason to clean coils is more technical. A dirty indoor coil will impact your evaluation of the system. A dirty evaporator coil raises the temperature split between the supply and return air, lowers the pressures, reduces capacity, changes the latent/sensible split of the machine, creates more condensate, increases run time — it affects everything.



    Figure 4. A trap will not work properly unless there is a drop between the inlet and the outlet of the trap of about 1 1/2 in. with a 1 1/2 in. reservoir.

    Refrigerant Charge in Mild Weather

  • Check the charge.
  • One of the things that often get checked on a cooling PM is the pressures. This is helpful not only to check the level of charge, but also to determine if there is a system failure such as a restriction or a compressor valve problem.

    In the heat of the summer, confirming proper system operation is relatively easy, but mild weather works against us. It is next to impossible to determine if a system is properly charged in mild weather because the a/c system is operating near or beyond its normal design parameters.

    Low outdoor ambient temps allow the condenser coil to dissipate heat picked up in the evaporator very rapidly. This “super efficiency” of the condenser coil causes a problem for determining overcharge, undercharge or control malfunction. If a machine is operating in an overcharged condition, it is very difficult to tell because excessive refrigerant in the system may not increase the refrigerant pressures a discernible amount. Even though the excess refrigerant is robbing the condenser of condensing space, there is still enough condensing space to remove the heat drawn out of the evaporator.

    Determining if a system is undercharged can also be difficult under low ambient conditions. When the load is low on an air conditioner, it requires much less refrigerant to extract heat from the evaporator. Consequently, a system with 80% of its charge can easily handle half-load cooling demands. This means that a system such as this would have a normal temperature split across the evaporator, and the refrigerant pressures would not necessarily reflect a low charge.

    I have personally seen many systems that had refrigerant pressures to factory specs but in reality were as much as 20% low on charge. In some cases this occurred in ambients as high as 95°F. My point here is that refrigerant pressures alone, even with charts, are inadequate for determining level of charge at any ambient, and the inaccuracy is exaggerated in low-load conditions.



    Art of Refrigerant Charge Evaluation

    So what can be done about this situation? I wish I could give you some hard-and-fast answers, but changing and evaluating the charge level in an air conditioner (properly) is almost an art form. There are a couple of tips I can share with you to optimize your efforts.

    The first thing to understand is that evaluating the charge of an air conditioner is not easy. Taking the charging process lightly can prevent a technician from being open to learning proper charging techniques, as well as understanding the ramifications of improper charging.

    When you put your gauges on an air conditioner, there are essentially two things you are trying to find out:

    1. Does the system have the proper amount of refrigerant?

    2. Is there anything in the system that is not functioning properly?

    Here are a couple of hints to do both these things. First thing to remember when evaluating a system in mild weather is to let the system run for awhile before proceeding. This is true at all times but is particularly necessary in mild weather.

    The first hint is somewhat helpful for evaluating the charge on expansion valve systems and finding system malfunctions. Many malfunctions, such as weak compressor valves and slight restrictions, can be masked by low ambient operation.

    Block the condenser airflow enough to get the system to operate at a high-side pressure that you might see on a hot day and then check the evaporator to see how it is performing. If you have pressure charts for the system, that makes it even easier. For instance, if I were checking a machine in 70° weather, I would block the condenser air until the machine was operating at a high-side pressure equivalent to a 100° ambient.

    Let’s say that the pressure chart for this machine says that at 100° it should have a 300-lb, high-side pressure. I would block enough air to achieve a steady-state, 300-lb pressure. Using the chart backward, I would look to see what the backpressure would be if the ambient were actually 100°. If the chart said the back pressure should be 75 lb, the back pressure of my system should be close to this. Don’t forget to take into account the indoor wetbulb temperature if the chart you are using incorporates the wetbulb options. If my back pressure were significantly higher or lower, this would tell me I had more investigation to do to evaluate this machine.

    When blocking the condenser air, you must be careful not to prevent subcooling in the condensing coil. This will cause the evaporator to freeze. The best way to block the air for this technique is to block it on the opposite side of the condenser fan from the coil itself. In other words, if you are blocking the air at the condenser coil, you’re doing it wrong.

    Remember, this test is only approximate and must be used judiciously. Try this technique experimentally for a couple of weeks, you will be surprised at what you learn.

    Another tool we have to check charge in low ambient is the fixed-orifice superheat chart. A fixed-orifice control is a capillary tube or an orifice control. One nice thing about fixed controls is that in low-load conditions they give us a fairly good indicator of how a machine is operating. By referring to a superheat chart for a particular system with a fixed orifice, we can tell what the superheat should be at any given load condition.

    This is particularly helpful because the superheat on a fixed-control system is sensitive to charge. This helps to overcome the very roadblocks that low ambient operation produces.

    What this boils down to is if you have a superheat chart for a fixed-control machine, you have a much better chance of determining if the charge is correct. A lot of systems nowadays have superheat charts in them, and many manufacturers will provide superheat information for their equipment.

    As you can tell, evaluating charge is a somewhat involved procedure, and this information is only the tip of the iceberg. Nevertheless, proper charging techniques are critical to the longevity, performance, and efficiency of all air conditioning systems.



    Figure 5. Cooling temperature splits.

    Cooling Temp Split

  • Check the temperature split.
  • The cooling temperature split changes depending on the outdoor and indoor temperature as well as the indoor wetbulb (water content of the indoor air) and the total airflow. Splits should be taken at the air handler to determine the performance of the machine, and at the vents to determine the performance of the ductwork system.

    Because there are so many variables that dictate the temperature split of a machine under any given load condition, it is important not to give temperature split undeserved diagnostic value. Figure 5 is a generic cooling temp split chart to give you a general idea what splits should be. I publish it here simply as a guide. Don’t take it too seriously. A system can appear to have a low temp split due to an unsealed return plenum.

    Over the years, this is an area of construction that has fallen into a black hole. Many plenums that are an integral part of the house infrastructure, usually underneath vertical air handlers, have never been sealed. Most technicians understand this, but what is not well understood is to what extent this is impacting the performance and efficiency of the customer’s equipment.

    When a return plenum is not sealed, the air handler draws air from the infrastructure of the residence. In most cases the majority of air comes from the attic. I have seen cases that were so severe that the equipment performance was reduced by 30%. In one case I was recently involved in, the air handler was drawing air from around the back of a fireplace and its flue.

    Determining whether or not a system has a leaky return plenum is simple and adds very little time to a PM procedure. While the system is running, measure the temperature of the air entering the return grille from the conditioned space. Compare this reading to the temperature of the area of the plenum where air is most likely to be drawn in from the attic. If the readings are different, there very likely is a leak in the plenum.

    Of course, this example assumes there is a difference between the temperature of the attic and the indoor air. There are many variations on this test that can be performed if necessary. One example is outlined below.

  • On heat pumps you should always manually advance the defrost timer while the machine is running in cooling mode, even though you are not necessarily checking the system for heating operation.
  • If the defrost temperature sensor is inappropriately calling for defrost, the outdoor fan will turn off every time the defrost timer calls for a defrost cycle, even though the machine is operating in cooling mode. This will cause a cooling problem. When you manually advance the defrost timer while the machine is operating in cooling, nothing should happen.

  • Check for leaks by investigating any oil spots that you may see on the piping or the coils.
  • Remember that oil near a schraeder access can be caused by previous access to the system. Old leak test solution can look like oil and mislead you. Make sure the O-rings that are in the schraeder caps are actually there. They tend to fall out when the cap is removed. All caps should be tightened after you remove your gauges because schraeder cores cannot be trusted to contain the refrigerant.

  • Check the ductwork.
  • Check for the obvious things like insulation integrity, large leaks, kinks, crushing, and air distribution. A quick way to determine if the ductwork has large leaks is to close all of the perimeter openings to the house such as windows, doors, fireplace damper, and attic access.

    With the blower operating, crack a window open slightly and check which way air is passing through the window. (Use smoke from a match.) If the smoke is pushed out, the return ductwork system may have a dominant leak. This means that the return ductwork leaks more than the supply. If the smoke is pulled in through the window, the system has a dominant supply leak. This only works on gross leaks.

  • Check the air handler or furnace for holes such as extra electrical openings or leaks in doors and panels due to sagging.
  • Sagging is usually caused by improper support of the air handler.

  • Check line sets on split systems.
  • Are they insulated properly? Is there any copper in contact with concrete or stucco? If there is, get the copper isolated from the concrete or a leak will occur. Is the line set installed properly? Is it buried in the ground without being sealed?

  • Check the air filter; clean or replace as needed.
  • As a business owner always looking for ways to increase my sales and margins, an issue that jumps out at me is the unsealed return plenums. There are a lot of these around. Anytime the infrastructure of the house is part of the return plenum, there is an opportunity for high-margin work that is easy to sell.

    It’s very easy for a customer to understand what it means if their air system is pulling air out of the attic and/or from the outside.

    The cost to you to repair it is relatively low because it can be done with somewhat unskilled labor, yet the value to the customer is very high. If you price the repair based on the value to the customer instead of pricing it based on your hourly rate, you make a good margin, add sales, and your customer is happy. Could the world be more perfect?

    Leonard is president of Total Tech HVACR Training, Phoenix, AZ. His firm specializes in service, installation, and application training for service technicians. He can be reached at 602-943-2517.

    Publication date: 04/23/2001

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