Here's a common scenario. You go on a service call, put your gauges on a condensing unit, and find that the suction pressure is low. What do you do? In too many cases, the answer is "add refrigerant." But doesn't it seem like a good idea to confirm that low refrigerant is the problem before you start adding refrigerant? That's why checking superheat and subcooling is so important. Let's go back to the beginning. You go on a service call and find low suction pressure. However, this time you consider the three main causes of low suction pressure, and check superheat and subcooling to make the correct diagnosis. CAUSE #1: Insufficient heat getting to evaporator. This can be caused by low air flow (dirty filter, slipping belt, undersized or restricted ductwork, dust and dirt buildup on blower wheel) or a dirty or plugged evaporator coil. Checking superheat will indicate if the low suction is caused by insufficient heat getting to the evaporator. To check superheat, attach a thermometer designed to take pipe temperature to the suction line.

Don't use an infrared thermometer for this task. Then take the suction pressure and convert it to temperature on a temperature/pressure chart. Subtract the two numbers to get superheat. For example, 68 psi suction pressure on a R-22 system converts to 40F. Let's say the suction line temperature is 50F. Subtracting the two numbers gives us 10F of superheat. Superheat for most systems should be approximately 10F measured at the evaporator; 20F to 25F near the compressor. If the suction pressure is 45 psi, (which converts to 22F) and the suction temp is 32F, the system still has 10F of superheat. The fact that these readings are normal indicates the low suction pressure is not caused by low refrigerant, but insufficient heat getting to the evaporator. CAUSE #2: Defective, plugged, or undersized metering device. Let's say a system has 45 psi suction pressure (converts to 22F) and 68F suction line temperature, the superheat is 46F (68 minus 22). This indicates low refrigerant in the evaporator.

However, before adding refrigerant, check the subcooling to be sure the problem isn't caused by a defective, plugged, or undersized metering device. While superheat indicates how much refrigerant is in the evaporator (high superheat indicates not enough, low superheat indicates too much), subcooling gives an indication of how much refrigerant is in the condenser. buy air conditioning unit londonSubcooling on systems that use a thermostatic expansion valve (TXV) should be approximately 10F to 18F. ac window unit insideHigher subcooling indicates excess refrigerant backing up in the condenser. central air conditioner unit fansOn TXV systems with high superheat, be sure to check the subcooling as refrigerant is added. If the superheat doesn't change, and the subcooling increases, the problem is with the metering device.

In the case of a TXV, it's likely that the powerhead needs to be replaced. To check subcooling, attach a thermometer to the liquid line near the condenser. Take the head pressure and convert it to temperature on a temperature/pressure chart. Subtract the two numbers to get the subcooling. For example, 275 psi head pressure on an R-22 system converts to 124F. The liquid line temperature is 88F. Subtracting the two numbers gives 36F. High superheat and high subcooling indicates a problem with the metering device. Keep in mind that subcooling won't increase on systems with a liquid line receiver, as extra liquid will fill the receiver instead of backing up in the condenser. Receivers are rare on air conditioning systems, but very common on small refrigeration systems such as walk-in coolers and freezers. If a system with a receiver has high superheat and the liquid line sight glass is full of liquid (no bubbles), check the metering device. If the sight glass has bubbles, the system could be low on refrigerant, or the liquid line filter/dryer could be plugged.

Your clue here is that a noticeable temperature drop across a liquid line filter/dryer indicates it's plugged. There are indeed some cases where low suction pressure is going to be caused by low refrigerant. If the superheat is high and the subcooling is low, the refrigerant charge is probably low. Just keep in mind two things here: first, find and fix the leak. Second, monitor both superheat and subcooling as you add the refrigerant, to prevent overcharging. Skip Egner is a technician with CS Service Experts, Ft. Myers, FL. He has been in the HVAC industry for 30 years, and in 2006 won the North American Technician Excellence (NATE) Certified Technician Competition-at HVAC Comfortech. He can be reached at 239/768-2665. As a "Top Tech," here's what Egner says about NATE: "A service technician needs to have a high mechanical aptitude and the ability to understand complex mechanical systems. NATE certification is important to let the customer know that the technician working on their system has been trained and tested, and is competent to solve the problem."

HVAC-02, Air Conditioning Troubleshooting and Repair Since I'm constantly receiving questions on 944 air conditioning systems, I figured it's time to come up with come helpful troubleshooting tips. Working on the 944 A/C system is no different than working on any other automotive A/C system. They have all the same components which function in the same manner as any other A/C system. First, if you're having a problem with the A/C system on your 944, there are two ways you can approach solving the problem. The first is to just start replacing components and hope you get lucky by replacing the faulty component the first time. The other method involves doing some actual testing to narrow down the cause of the problem. In order to properly test the A/C system and narrow down the possible causes of your problem, you're going to have to invest in a set of A/C gauges. Before we begin with the actual troubleshooting, to properly troubleshoot the A/C system, it's extremely helpful to understand how the automobile A/C cycle works.

So, we'll begin by discussing automotive air conditioning theory of operation. There are two basic types of automotive air conditioning systems. The type of system is determined by the device used to control the expansion of the high pressure, high temperature liquid to a low pressure, low temperature liquid. One system uses and expansion valve while the other uses an orifice tube. 944s use an expansion valve system. We'll discuss the operation of both. The first component in the A/C cycle is the compressor. The compressor takes low temperature refrigerant gas and compresses it into a high pressure, high temperature gas. The refrigerant is then sent to the condenser which sits in front of the radiator. The condenser removes some of the heat from the refrigerant which causes the refrigerant to change phase from a hot gas to a warm liquid. In the expansion valve A/C system, the warm liquid is then passed through a receiver-drier which removes moisture (via a desiccant in the drier) from the refrigerant to maximize the efficiency of the heat exchange capability of the refrigerant.

On 944s, the receiver-drier is a small canister (probably with a sightglass on top) located near behind the driver's side headlight (LHD cars). No phase change occurs as the refrigerant passes through the receiver-drier. From there, the refrigerant is then passed through the expansion valve. The expansion valve has a sensing line that is attached to the suction of the A/C compressor. The expansion valve senses the pressure at the suction of the A/C compressor and modulates to maintain the pressure at the compressor suction. On 944s, the expansion valve is located near in the passenger's footwell (LHD cars) near the center console next to the firewall. In the orifice tube system, the warm liquid refrigerant passes directly from the condenser to the orifice tube. The orifice tube is a fixed expansion device. The pressure drop across the orifice tube is proportional to the pressure into the orifice tube. So, the orifice tube system maintains the pressure at the suction of the A/C compressor by cycling the compressor on and off via a clutch.

This is the type of system typically used by GM vehicles. The expansion of the warm liquid through the expansion valve or orifice tube causes a pressure drop and hence a temperature drop in the refrigerant. So, out of the expansion device we get a low temperature liquid refrigerant. The next component in the A/C system is the evaporator. Air from the passenger's compartment passes across one side of the coils in the evaporator. The low temperature liquid refrigerant passes through the other side of the coils. The refrigerant removes heat from the air in the passenger's compartment and returns to the A/C compressor suction as a low temperature gas (phase change). In the orifice tube system, there is an accumulator located between the evaporator and the A/C compressor suction. The accumulator has moisture removal desiccant just like the receiver-drier used in the expansion valve system. However, it also collects any un-evaporated refrigerant to prevent liquid lock of the compressor.