Refrigerant charge quantity for air conditioners & heat pumps:

This air conditioning repair article series discusses the the diagnosis and correction of abnormal air conditioner refrigerant line pressures as a means for evaluating the condition of the air conditioner compressor motor, which in turn, is a step in how we evaluate and correct lost or reduced air conditioner cooling capacity.

We explain how overcharging or undercharging of refrigerant in an air conditioner or heat pump is detected and we list the effects of overcharged or undercharged refrigerant.

Effects of Overcharging the Refrigerant Level in an Air Conditioner, Heat Pump, or other refrigeration equipment

Effects of too much refrigerant in the system - overcharging, over-metering, or other high refrigerant pressure situations:  normally we want the low side pressure to be as low as possible for refrigeration systems.

Excessive refrigerant in the system raises the system operating pressure and temperature and actually reduces the cooling ability of the system.

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Overcharging of the refrigerant in a closed refrigerant system such as a typical residential air conditioner, heat pump, or refrigerator, where no liquid refrigerant receiver is included, has the following effects:

• Improper operating pressures - too high: Refrigerant pressure will be increased on both the high side and low side of the system
• More expensive operation: The system may still be able to change state, but at a higher temperature than intended, thus in an air conditioner, it won't cool as effectively nor as efficiently as with the proper charge
• Effects of refrigerant floodback or of liquid slugging the compressor motor: if liquid refrigerant enters moving parts of the compressor motor, the motor is likely to be destroyed.
  
  Watch out: overcharging sufficient to cause liquid slugging can quickly destroy the HVACR compressor motor. The compressor motor's moving parts (piston & cylinder, scroll spirals, or rotary vane pump) expect to be compressing a gas and can't handle liquids. HVAC technicians refer to these conditions as liquid slugging. Details are at REFRIGERANT FLOODBACK, LIQUID SLUGGING
  
  Liquid slugging is cited at AIR CONDITIONING & HEAT PUMP NOISES where we discuss A/C or heat pump compressor motor noises.

Air Conditioner or Heat Pump Compressor Motor or other Electric Motor Runs Backwards?

An electric motor may start running backwards due to a failed start capacitor, a reaction to high refrigerant pressure (for refrigeration motors), or other electrical events.

High refrigerant pressure in an A/C unit or heat pump or some other refrigeration motors can cause the equipment to run backwards. Cooling compressors, heat pump compressors and some other electrical motors can run backwards too: well pumps, fans, even an A/C or heat pump compressor.

If the compressor motor is a scroll-type design and in some cases if the motor is a rotary vane type design high refrigerant pressure combined with a loss of electricity can, on return of power, start the motor backwards. Scroll type compressor motors use an anti-restart control (basically a timer) to prevent the compressor from starting to run backwards.

A scroll-type compressor (and possibly a rotary vane A/C compressor motor) can start to run backwards if the motor is suddenly stopped (due to brief power loss or because you foolishly switched its electrical power "off" and back "on" quickly. In that condition, high refrigerant pressure on the outlet side of the compressor motor gives it a backwards push. When power is restored that backwards push gets the motor running backwards.

And as we discuss at CAPACITORS for HARD STARTING MOTORS, a bad start capacitor can let a motor run backwards. We've also found cases of water well pump motors running backwards after a lightning strike.

And at Questions & Answers about compressor/condenser unit fans we include a field report of a backwards running condenser unit fan.

More Effects of Overcharging A/C Refrigerant In a Cooling or Refrigeration System

Reader Question: What happens to an overcharged cooling system with too much refrigerant?

Could you describe what will happen with an over charged system? - S.F.

Reply: Here are the effects of overcharging with refrigerant

An expert inspection of any individual cooling or air conditioning system is likely to discover things that we miss by email discussions. That said, here are some things to consider:

• The most serious problem is damage to the A/C compressor. Damage could occur to the compressor piston or to the refrigerant control flow valve (A/C compressors use a reed type valve that seals the compressor high side outlet) - imagine sending a slug of liquid into any reciprocating piston engine or valve that is designed to move a gas - the piston comes up and slams into liquid - and is destroyed.

• Overcharging a residential A/C system or refrigerator can damage or even put the system out of operation - because these systems don't have a receiver, too much refrigerant risks sending a slug of liquid refrigerant+oil into the cap tube, the metering device, clogging it and leading to failure
• Overcharging a commercial system with a receiver - it depends. A small overcharge just rests in the receiver but a large one can result in Thermal Expansion Valve (TEV) damage or TEV freeze-up, leading to loss of system cooling entirely.
• We want just enough air conditioner or refrigerator refrigerant to cool the coil; too little can lead to coil frost-up (suction side pressure too low) and too much refrigerant can lead to cooling coil running warm over part of its surface (suction side pressure too high).
• Refrigerant overcharging can also result in liquid refrigerant running too far into the evaporator coil, leading to a too-low suction side pressure and a reduction in cooling capacity
• In other cases overcharging with a refrigerant can lead to unexpected condensation or frosting on some of the system parts and components both in the air handler and in the compressor/condenser unit. In HVAC school, with gauges in place and a means of adding or removing refrigerant we could watch the frost line advance or recede on a cooling coil from this effect.
• Some sources also cite higher compressor amps draw but that claim needs more research
• Watch out: an air conditioner or heat pump system that has been over-charged with refrigerant can also show up the problem as short cycling, rapid on-off cycling of the compressor motor. This happens when excess refrigerant is blocking proper operation of the thermostatic expansion valvce (TEV) or capillary tube or other refrigerant metering control.

Effects of Undercharging the Refrigerant Level in an Air Conditioner, Heat Pump, or other refrigeration equipment

Effects of too little refrigerant in the system are described in detail separately at THERMOSTATIC EXPANSION VALVES

How do We Measure the Actual Quantity of Refrigerant in an Existing HVAC System

Reader question: How can i measure the quantity of refrigerant in the system?

How can i measure the quantity of refrigerant in the system? is there any instrument that can be use to measure this? said:

How can i measure or ascertain the quantity of refrigerant in the system? is there any instrument that can be use to measure

1. Please kindly assist me on how to ascertain or measure the refrigerant quantity in a system.

2. Is there any instrument for this measurement?

3. Any relationship between an air-conditioners rated current and the refrigerant quantity?

4. Can the Pressure-Temperature chart be used to ascertain refrigerant quantity?

- S.A. 4/1/2014

Reply:

On your question of does the electrical current determine the amount of refrigerant in the system, No the rated current - if you mean the equipment amps draw - does not determine the proper refrigerant charge quantity, though an overcharge or undercharge that is causing improper equipment operation or even damage might be reflected in the current draw by the compressor.

I don't know a way to precisely measure the liquid volume of what's in an individual HVAC system, as there are technical demands that I will explain here, but you can get an idea of quantity by looking at pressures, temperatures, ambient temperatures around the unit and tables published by the manufacturers.

You'd think we could just calculate the gas volume and liquid volume at any given time and temperature and pressure - theoretically that's right. But because each HVAC system has some unique installation details such as exact lengths of refrigerant piping (depending on where components are installed and how far apart they are located), the actual physical volume of liquids and gases that are in the system are not trivial to compute.

Consider that at any moment in the operation of a piece of refrigeration equipment the refrigerant inside the device exists in two places - on the high side and on the low side of the system, under high pressure as a high pressure gas, or as a liquid, and on the low side as a gas. Just how much of the refrigerant is on either side of the metering valve varies. But an accurate total of the refrigerant quantity would have to add both of these together and convert them to a common measure (liquid or gas volume with adjustments for temperatures and pressures).

A different data point is at the time of installation: the installer can use a charging cylinder to measure the accurate quantity of refrigerant that was introduced into the system - again with some adjustments for temperature. That's where the charging pressure-temperature chart comes in handy. If the manufacturer says install N liquid ounces of refrigerant X the technician has to consider temperature and possibly pressure when measuring refrigerant volumes.

A refrigerant charging cylinder provides a chart built into the device for making an accurate charge. Electronic refrigerant charging equipment does the same thing, more precisely, using circuitry and temperature (and perhaps pressure) data sensors built into the equipment.

These difficulties explain why on small systems that need a very accurate refrigerant charge, the technician often has to approach the job by first evacuating the system completely and next installing an accurately-measured charge of refrigerant.

Two times that the HVACR refrigerant charge is accurately known

There is no simple instrument that reads how much refrigerant by volume (gas, liquid, or more accurately a combination of the two) in a refrigeration system. There are just two times that we know accurately how much refrigerant is inside of the equipment:

1. When the equipment is new, has not been installed, or has been fully evacuated - then the answer is normally zero though to be more precise, in an evacuated system that previously contained refrigerant some molecules remain.
2. When the equipment has been evacuated and then given a measured refrigerant charge. For small systems with no receiver the refrigerant charge has to be very precise. For larger HVACR systems a receiver is included in the design to act as a buffer or accumulator of liquid refrigerant so that the system will work properly with a refrigerant charge quantity that is less precise.

Now if an air conditioner, refrigerator or similar HVACR device has not leaked, we would reasonably assume that the amount of refrigerant in the system is exactly what was originally installed. This is the case for thousands of HVACR appliances that run for years without leaking.

Since your wrote again that you still didn't see a clear answer to your question as you posed it, in sum, the short answer is no.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-06-14 - by (mod) -

I'm not an authority, IL but I'd start by checking the TEV operation. If a TEV is jamming it can cause high head pressure.

On 2020-06-13 by I Loeb

High Pressure switch open on Carrier heat pump; starts & then cuts out. How should I approach identifying the cause?

How should I approach identifying the cause?

On 2020-05-07 - by (mod) -

The excess refrigerant is retrieved into a refrigerant collection system since it is no longer permitted to discharge to the environment.

On 2020-05-06 by Tony Hopkins

How to fix a overcharged unit

On 2019-11-21 by Mikael

Hi,
We have an Aermec Heat Pump on a site. There is two compressor by circuit.
After defrosting, one compressor still in function, and aroud 1 minute later, the second compressor starts.
The hot glycol is passing, and the fan are on. When the second compressor start, the hi pressure increase very quickly and the unit go in fault.
Is refrigerant overcharge can explain this problem? Or maybe the expansion valve close too quickly to ajust the sub?

Thanks you,

On 2018-08-11 - by (mod) -

You might, depending on other devices being present or absent, send liquid refrigerant into the compressor motor, damaging or destroying it - this is called liquid slugging.

On 2018-08-11 by Md. Abdul Awal

refrigerant r600a charging excess what happen

Question:

(Sept 6, 2014) Anonymous said:
so very hard work . but this is my course R.A.C

(Apr 28, 2015) Robert Muela said:
my low side is 10 my hi side is 200 why

Reply:

Robert, see REFRIGERANT PRESSURE READINGS & CHARTS and then also see REFRIGERANT DIAGNOSTIC FAQS. We expect the low side to be much lower pressure than the high side when the compressor is running.

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Continue reading at UNDER CHARGED REFRIGERANT, EFFECTS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see REFRIGERANT OVERCHARGE FAQs - questions and answers posted originally on this page.

Or see these

Recommended Articles

• GAUGE, REFRIGERATION PRESSURE TEST
• HVAC COMPRESSION RATIOS
• REFRIGERANT CHARGING PROCEDURE
• REFRIGERANT GASES & PIPING - home
  • CAR / TRUCK AC PRESSURE SWITCH
  • FROST BUILD-UP on AIR CONDITIONER COILS
  • GAUGE, REFRIGERATION PRESSURE TEST
  • REFRIGERANT CHARGING PROCEDURE
    • OVER CHARGED of REFRIGERANT, EFFECTS
    • UNDER CHARGED REFRIGERANT, EFFECTS
  • REFRIGERANT DRIERS & FILTERS
  • REFRIGERANT FLOODBACK, LIQUID SLUGGING
  • REFRIGERANT LEAK DETECTION
  • REFRIGERANT LEAK REPAIR
  • REFRIGERANT PRESSURE DIAGNOSTICS
  • REFRIGERANT CHARGE QUANTITY
  • REFRIGERANT QUANTITY on DATA TAG
  • REFRIGERANT HIGH HEAD PRESSURE DIAGNOSIS
  • REFRIGERANT LOW HEAD PRESSURE DIAGNOSIS
  • REFRIGERANT PRESSURE READINGS & CHARTS
  • REFRIGERANT PROBLEM TYPES
  • REFRIGERANT SIGHT GLASS

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