Subcooling and Superheat

Discussion in 'HVAC' started by WyrTwister, Jun 15, 2018.

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  1. Jun 15, 2018 #1

    WyrTwister

    WyrTwister

    WyrTwister

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    It is hot !

    Found this on line . I did not write it . Good read concerning refrigerant levels for your A/C .

    Subcooling and Superheat: Superheroes of System Charging
    Don't always assume you have to "add refrigerant." Consider the three main causes of low suction pressure, and check superheat and subcooling to make the correct diagnosis

    Skip Egner | Aug 24, 2016

    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. 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.

    CAUSE #3: Low refrigerant.

    Yes, it's true! 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.

    Wyr
    God bless
     
    nealtw and kok328 like this.
  2. Jun 18, 2018 #2

    kok328

    kok328

    kok328

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    Thanks, good article. Now we can watch out for the "gas & go" technicians.
     
  3. Jun 21, 2018 #3

    Green8

    Green8

    Green8

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    The charge alone can have a substantial impact on system operation costs. A few percentage points of over-or-under-charge can raise electricity consumption significantly and shorten the life of the compressor.

    Basically, refrigerant serves three important functions in the operation of an air conditioning system.

    Heat transfer. The refrigerant moves heat from inside the house to the outside.

    Compressor cooling. It removes heat from the compressor motor windings.

    Compressor lubrication. It circulates oil to lubricate the compressor.

    All of these functions are important to the proper operation of the cooling system. All three affect both the efficiency of operation and the durability of the compressor.

    And, all of them are closely tied to the system's level of refrigerant charge. Here is what an under-or-over-charge can mean to you as the homeowner.

    Reduced comfort. An improperly charged system will not dehumidify the air properly.

    Poor heat transfer. An improperly charged system will not do its job of removing heat from inside the house and transferring it outside.

    Reduced efficiency. Poor heat transfer results in increased power consumption.
     
  4. Jun 21, 2018 #4

    Dennis Palmer

    Dennis Palmer

    Dennis Palmer

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    This is very useful information for me and many people out there. After reading your post, now I can analyse such AC issues and call the experts to fix at right time.
     
  5. Jun 21, 2018 #5

    WyrTwister

    WyrTwister

    WyrTwister

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    You can take an online open book course & exam to get your " first level " EPA " card . It cost $ 30 , if I recall correctly . Does not expire . Did this a few years ago .

    I already have the tooling . The EPA card allows me to purchase refrigerant .

    But I only work on my own , small systems .

    Wyr
    Bod bless
     

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