Testing Refrigerant Charge Without Gauges

By Bryan Orr

First, checking the charge without gauges is a balancing act, a trade-off. We get more accurate readings when we connect gauges but we may also: 

1. Lose refrigerant.

2. Risk contaminating the system with moisture and air.

3. Risk leaving a leak at the Schrader cores and caps. 

It’s important to clarify that charging and testing the refrigerant charge are two different things. You must attach a gauge manifold or probes and charge using a scale and manufacturer specs. Checking a system that has already been charged is a bit different and the decision should be made to connect gauges only when it makes sense to do so. 

There is no test procedure that replaces a well trained and aware technician using their senses to detect issues. Test measurements are important but they don’t take the place of general awareness. 

Like everything, your best INITIAL diagnosis tools are your hands, eyes and ears. Look for dirt buildup, oil spots, listen for abnormal sounds, feel the lines and condenser discharge air when approaching the condenser, check for dirty blower wheels, evaporators, filters, and grilles when approaching the furnace or fan coil.

• Look for wire and refrigerant tube rub-outs. 

• Look inside drain cleanout tees and in pans for buildup. 

• Look inside condensers for wires laying on the tubing, pay attention to disconnects that are loose, belts and sheaves that are worn, high voltage connections that are getting discolored, capacitors that are bulging or leaking, electrical whips that are coming apart, control cables that are nicked or bare, air handlers that are sagging or out of level, moisture on ducts and broken or damaged line insulation. 

• If you’re working in refrigeration, look for icicles hanging down, broken insulation on drains and suction lines, ANYTHING dirty and damaged doors and door seals. 

There is no test procedure that replaces a well trained and aware technician using their senses to detect issues. Test measurements are important but they don’t take the place of general awareness. 

Think about your refrigerator at home. It’s a sealed refrigerant circuit and if it had an issue the last thing you would do would be to add piercing valves and “Invade” the refrigerant circuit to test. 

This means that we only connect gauges when there is a good reason to do so, such as:

1. We have not touched the unit recently and want to make sure it is operating 100% (on air conditioning only, in small refrigeration you still don’t connect in this case)

2. We made a significant repair that may impact the operation

3. We need to “set” a charge because the system is newly started or we made a refrigerant circuit repair.

4. Your non-invasive measurements tell you are out of range or a problem may exist.

On a system that has been properly commissioned you will have prior readings to go off of. 

Benchmarks like DTD (Evaporator Saturated Temp to return air design temperature difference), CTOA (Condensing Temperature Over Ambient), Subcool and Superheat (on a TXV system) and Static Pressures will stay fairly stable across most operating conditions. 

Readings like suction pressure, head pressure and superheat and subcooling on a fixed metering device system and air temperature split will vary with load conditions. 

If you have system historical data you can often use it to learn about the system and its history before you begin taking readings. 

Keep in mind that this “non-invasive” process works best during normal operating conditions between 75°F to 95°F outdoors and 70°F to  80°F indoors at around 50% indoor RH.

In this method, we are essentially using a combination of line and air temperature measurements to diagnose system performance instead of connecting gauges. This makes liquid line, suction line, discharge line and indoor air measurements your primary indications of performance.  

When checking a unitary air conditioning system without gauges, do it in the following steps (these are subject to change and adjustment based on historical benchmarks, abnormal conditions and manufacturer specs):

1. Visually inspect the unit for all the above-listed items AND note if the metering device is a piston or a TXV

2. Measure the outdoor temperature in the shade entering the condenser

3. ADD the CTOA (Condensing Temperature Over Ambient) based on the SEER rating, age or prior benchmarks   

4. Subtract the nameplate subcooling or 10 degrees if there is no nameplate.

5. Compare to the liquid line temperature. If +/- 3°F on a TXV system or +/- 5° on a Piston the liquid temperature is in range during normal operating load. 

6. You may also check the air temperature split entering & leaving the condenser fan and it will usually be about ½ of the target CTOA (Condensing Temperature Over Ambient). So on a VERY old system with a CTOA of 30°F the condenser discharge air will generally be 15°F +/- 3°F and on a brand new high SEER unit with a CTOA of 15°F it will be 7.5°F +/- 3°F (This is a rule of thumb so a prior benchmark on the system would be more accurate)

7. Also, note how much warmer the liquid line is than the outdoor temperature. It should be between 3°F and 20°F warmer than the outdoor temperature. If it is above or below that range, connect gauges. 

8. Measure the suction line temperature outside. If it is at or above 65°F the compressor is in danger of overheating / oil breakdown. If the suction line is 40°F or below the evaporator is in danger of freezing. Stop and connect gauges.

9. Go inside and check the wet bulb and dry bulb temperature at the air handler/furnace inlet (return right before the inside unit or in the filter tray, cabinet make sure to keep the sensor out of “line of site” from the evap coil).

10. Take the return dry bulb (DB) and subtract 35°F (DTD), this is your target coil temperature difference with normal airflow (400 CFM per ton) at typical indoor humidity conditions. 

11. If the system has a TXV add in 10 degrees for superheat if you don’t have prior benchmarks, if it is a fixed orifice (piston) then add in the target superheat based on a superheat chart or using a superheat calculator app (such as the free HVAC School app). This gives you a target suction line temperature at the evaporator. 

12. Compare the target suction line temperature to the actual suction line temperature at the evaporator if it is within +/- 5°F it is within range. Outside of that range connect gauges.

13. Compare the indoor suction temp to the outdoor suction temp. 1°F of change per 20’ of lineset is allowable as a rule of thumb.

14. Compare the indoor liquid line temp to the outdoor liquid line temp. 1°F change per 30’ of lineset is allowable as a rule of thumb. 

15. Check temp drop across all exposed line filter driers. Recommend replacement if there is a drop of 3°F or more across a filter drier and perform further testing if you get even 1°F of drop with the same temperature clamp.

16. Use a Delta T chart to calculate target evaporator air temperature split like this one if the split is within +/- 3°F then it is within range. If higher then check for airflow issues and blower settings. If lower then connect gauges. 

17. When checking an RTU (Rooftop Unit) or residential package unit you will often have easy access to the compressor, in this case check the suction temperature entering the compressor and the discharge temperature leaving the compressor. The suction temperature should be above 35°F and below 65°F entering the compressor and the discharge line temperature should be below 220°F and above 150°F (typically) on a properly functioning package unit. 

18. Check motor amperage against manufacturer rating plates or part data plates if the compressor, blower or condensing fan motor are aftermarket

19. Confirm that the voltage imbalance on a 3 phase system does not exceed 2% 

20. When applicable check TESP and Static Pressure Drop across coil and filters against benchmarks.

The MeasureQuick app is free and has a non-invasive testing mode that can help with some of these calculations if you find the math confusing. 

Using duct psychrometers in the return and supply can further improve your testing and will allow you to easily calculate the enthalpy split (should generally be between 6.5 and 7 btu/lb on a system that is functioning well with proper airflow) as well as total system capacity.  

This process is not theory or a diagnosis guideline, it is a practical process for verifying operation for a range of common air conditioning equipment. If you find readings that are outside of the guidelines listed you will need to connect gauges and further diagnose the system. These guidelines do not apply to ductless or VRF systems and are not a replacement for manufacturer specifications. 

The goal as always is to serve customers with technical excellence and “do no harm” in the process. Non-invasive testing without gauges is one tool in the arsenal.

BEWARE of These Common Measurement Mistakes 

• Reading air temperatures in sunlight. If the sun is shining on a probe it will always read too high due to radiant heat.

• Reading air temperature in a place that is “line of site” to a cold or hot surface like a coil, heat strips, heat exchanger, etc..  It is always best to have a probe in an area shielded from other hot or cold surfaces.  

• Reading line temps in a dirty or uneven area of the tubing. The sensor on your temp clamp must have full, flat, tight contact to the line being read

• Trusting tools without testing and calibration. All tools require proper care and maintenance and must and can be tested. They can either be tested against other tools or against a known constant (like the freezing temp of water) or they can be calibrated by a lab. Know your tools and learn how to test them.