Show All ItemsIntroduction:How Air Conditioners Work:Step 1: What You Will Need and What You Should KnowShow All ItemsWHAT YOU'LL NEED:PURCHASING GUIDE::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. Subcooling on systems that use a thermostatic expansion valve (TXV) should be approximately 10F to 18F. Higher subcooling indicates excess refrigerant backing up in the condenser. On 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. what ac unit is the bestSubtracting the two numbers gives 36F. how much does it cost to replace ac unitHigh superheat and high subcooling indicates a problem with the metering device. price of an ac unitKeep 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." For other uses, see Coulomb (disambiguation). The coulomb (unit symbol: C) is the International System of Units (SI) unit of electric charge. It is the charge (symbol: Q or q) transported by a constant current of one ampere in one second: Thus, it is also the amount of excess charge on a capacitor of one farad charged to a potential difference of one volt: It is equivalent to the charge of approximately 6.242×1018 (1.036×10−5 mol) protons, and −1 C is equivalent to the charge of approximately 6.242×1018 electrons. This SI unit is named after Charles-Augustin de Coulomb. As with every International System of Units (SI) unit named for a person, the first letter of its symbol is upper case (C). However, when an SI unit is spelled out in English, it should always begin with a lower case letter (coulomb)—except in a situation where any word in that position would be capitalized, such as at the beginning of a sentence or in material using title case.

Note that "degree Celsius" conforms to this rule because the "d" is lowercase.— Based on The International System of Units, section 5.2. The SI system defines the coulomb in terms of the ampere and second: 1 C = 1 A × 1 s.[3] The second is defined in terms of a frequency naturally emitted by caesium atoms.[4] The ampere is defined using Ampère's force law;[5] the definition relies in part on the mass of the international prototype kilogram, a metal cylinder housed in France.[6] In practice, the watt balance is used to measure amperes with the highest possible accuracy. Since the charge of one electron is known to be about −1.6021766208(98)×10−19 C,[7] −1 C can also be considered the charge of roughly 6.241509×1018 electrons (or +1 C the charge of that many positrons or protons), where the number is the reciprocal of 1.602177×10−19. The proposed redefinition of the ampere and other SI base units would have the effect of fixing the numerical value of the fundamental charge to an explicit constant expressed in coulombs, and therefore it would implicitly fix the value of the coulomb when expressed as a multiple of the fundamental charge (the numerical values of those quantities are the multiplicative inverses of each other).

The elementary charge, the charge of a proton (equivalently, the negative of the charge of an electron), is approximately 1.6021766208(98)×10−19 C[7]. In SI, the elementary charge in coulombs is an approximate value: no experiment can be infinitely accurate. However, in other unit systems, the elementary charge has an exact value by definition, and other charges are ultimately measured relative to the elementary charge.[8] For example, in conventional electrical units, the values of the Josephson constant KJ and von Klitzing constant RK are exact defined values (written KJ-90 and RK-90), and it follows that the elementary charge e = 2/(KJRK) is also an exact defined value in this unit system.[8] Specifically, e90 = (2×10−9)/(25812.807 × 483597.9) C exactly.[8] SI itself may someday change its definitions in a similar way.[8] For example, one possible proposed redefinition is "the ampere...is [defined] such that the value of the elementary charge e (charge on a proton) is exactly 1.602176487×10−19 coulombs",[9] (in which the numeric value is the 2006 CODATA recommended value, since superseded).