The Case for Checking the Charge Without Using Gauges

Jim Bergmann and I recorded a podcast for HVAC School that covered when and how to check the refrigerant circuit without connecting gauges. Listener Joe Reinhard listened several times and wrote up this summary of what he gained from the episode. I edited it lightly, but most of this is his work. Thank you so much, Joe!

Keep in mind that when we make Fahrenheit to Celsius conversions, we use K (Kelvin) to show temperature differences like splits and DTD, and we use °C (degrees Celsius) to show measured temperatures.


Following mostly from two 45-50 minute podcasts from https://hvacrschool.com/checking-charge-without-gauges-podcast/ discussions between Bryan Orr HVACR School.com, expert tech, teacher, & business owner, and Jim Bergmann, renowned HVAC-R expert & teacher, from Redfish instruments and the MeasureQuick app, providing a detailed explanation of why techs should not connect gauges & hoses to a system just to check refrigerant charge (in many cases).

Why Not Connect?

The benefits of NOT connecting gauges during every visit for HVAC-R business owners, technicians, and clients include:

  1.  Non-invasive measurements with only temperature data taken. You would take temperature data the exact same way you would check if a typical refrigerator were operating properly, which has no ports to attach hoses and gauges.  
  2. Just measuring DTDs (Design Temperature Differences) and line set piping temperatures are non-invasive, involve less liability both for the system and technician safety, and demonstrates technical knowledge and best practices.   
  3. It is better for the refrigeration system and the environment (“green”) since it saves R22 and R410A released to atmosphere.
  4. You save time at the site, so techs can concentrate on better and more preventative maintenance (PM) of the airflow system (including condensate drainage) and PM checking electrical characteristics of various control components (capacitors, contactors, sequencers, etc.).
  5. Eliminate more callbacks and potential premature system cooling (and heating for heat pumps) performance problems and failures due to cross-contamination, moisture contamination, and lost refrigerant.
  6. Saves the customer money on refrigerant added due to connection losses.


Term Definitions 

  • Evaporator DTD (design temperature sifference) is the designed difference between the evaporator coil saturation/boiling temperature as measured on the suction gauge and the return air temperature. 35°F  (1.66°C) of difference is considered normal for a typical system set at 400 CFM (679.6 m3/h) per ton airflow. Oversized evaporator coils and increased airflow above 400 CFM (679.6 m3/h) per ton will result in lower DTD, and lower airflow with smaller coils will result in higher DTD.
  • Condenser CTOA (condensing temperature over ambient) is the temperature difference between the condensing coil saturation/condensing temperature measured on the liquid line high side gauge and the outdoor temperature. This difference will vary depending on the efficiency of the system/efficiency of the condenser coil.

6 – 9 SEER Equipment (Very Old) = 30° CTOA

10 -12 SEER Equipment = 25° CTOA

13 – 14 SEER Equipment = 20° CTOA

15 SEER+ Equipment = 15° CTOA

  • Delta T (evaporator split) is the temperature difference between the return and supply air. Delta T will vary quite a bit depending on airflow and indoor relative humidity. The chart shown below is designed for a 400 CFM (679.6 m3/h) per ton system. Lower airflow will result in a higher delta T, and higher airflow will result in a lower delta T. That is why Jim Bergmann does not prefer delta T as a firm diagnostic or commissioning tool but rather as an approximation of airflow.
  • Target superheat on a TXV system is dictated by the design of the TXV. Usually, target superheat on a TXV system will be 5°F- 15°F (2.75K – 8.25K) at the outlet of the evaporator where the TXV bulb is located. On a piston system, the target superheat is calculated using a superheat chart and measuring and plotting the outdoor dry bulb temperature and the indoor wet-bulb temperature.
  • Target Subcooling on a TXV system will be listed by the manufacturer but is generally between 8° – 14° (4.4K – 7.7K) subcool. Subcooling will vary quite a bit on fixed orifice systems, but 5°-20° (2.75K – 11K) is a common range.

DTDs (Design Temperature Difference) of the coils, after a system is newly commissioned or first-time assessed with gauges, should not change over the life of the sealed refrigeration system once a system has been charged correctly unless one or more of the following has developed:  

  1. Airflow restriction with dirt buildup as the main cause–dirty outdoor coil, dirty indoor coil, dirty filter, dirty blower blades/inside the housing, return/supply duct restrictions, blower motor speed or operation problems, and if the homeowner installs a so-called high efficiency, nothing-gets-thru-including-air filter.
  2. Critical component failure.
  3. Refrigerant flow restriction.  

So, after the first-time visit performance assessment or a new system is commissioned, subsequent system checkups or maintenance visits should be performed without connecting gauges.    

The following risks, problems, and liabilities occur and eventually develop when technicians attach gauge hoses every time to check the system refrigerant characteristics versus just using measured system temperatures and knowledge of return/supply air TD, evaporator/condenser split, and refrigerant P/Ts. Not attaching hoses & gauges to systems without good reason is actually correct practice, and the following could be avoided or greatly minimized:

  1. Techs are inducing system contamination if, prior to connecting the hoses, the techs did not use dry nitrogen to purge air, moisture, or old refrigerant out of their hoses & manifold from the prior system the gauges were attached. Perhaps the prior system had a different refrigerant that may or may not have been contaminated with non-condensables and other refrigerants.
  2. Were the hoses on the gauges left open to the atmosphere in the back of the truck used for the prior R410A system? If so, the coating of POE (polyester) refrigerant oil (highly hygroscopic) would have absorbed moisture, which, if not correctly purged with dry nitrogen, would contaminate systems by inputting moisture. That, in turn, will cause TXV and liquid filter-drier freeze-ups (blockages), cause contaminated refrigerant (making R22 recycle subject to high fees and fines), and cause acids which will attack and corrode compressor surfaces (copper plating), valves, and windings. Hoses should always be tightly connected to the manifold parking ports to prevent moisture contamination.
  3. Are techs properly & carefully disconnecting gauge hoses while the system is running? If not, perhaps a service call back will shortly occur since every time hoses are connected and disconnected, some refrigerant is lost. If the liquid hose is not charged back through the manifold and suction hose, several ounces or more in the liquid hose are lost if techs inadvertently or on purpose blow or dump refrigerant by not properly disconnecting gauge hoses while the system is running. That occurs if techs are inexperienced, decide not to take the time, or are not equipped with low-loss-ball-valve hose end fittings to slowly, carefully, after purging hoses if needed, charge from the liquid hose (holds 7x the R410A as the vapor or suction line; 10X for R22) through the gauge manifold into the vapor or suction hose back into the running system. If this procedure is not done correctly, air and moisture can enter the system. After one, two, or three years of visits, techs can be chasing “leaks” created by multiple connects/disconnects.  
  4. Caps no longer inadvertently left off on Schrader valve ports leading to leaks.
  5. Reduced safety issues for techs since there is a lower chance of getting refrigerant in eyes, frozen-fingers, and lost refrigerant to the atmosphere.

Data to record during first-time system performance assessments and new system commissioning using refrigerant gauges so that benchmarks exist to compare to future checkup visits but without attaching gauge hoses if no observed or reported system problem reasons:  

  1. TD or temperature difference between the return air dry-bulb (DB) and supply air DB. TD level depends on the sensible & latent heat content of the inside air—higher TD for low RH% (relative humidity), lower TD for high RH%.  20°F (11K)  TD is good if a system is operating properly at 75°F (23.88°C), 50% RH, and set for 400 CFM/(679.6 m3/h) ton. If reduced CFM/(m3/h) ton, TD increases, but if RH% increases, the TD decreases back-and-forth so the TD can range 16°F – 24°F (8.8K – 13.2K) or more in extreme cases (see the delta T chart).    
  2. Evaporator DTD (Design Temperature Difference), also called “split,” is the temperature difference between the return air dry-bulb (DB) temp and the refrigerant saturation temp of the coil – either 35°F (1.66°C) at 400 CFM/(679.6 m3/h) ton to 525 CFM/(891.98 m3/h)ton or 40°F (4.44°C) at 350 CFM/(594.65 m3/h) ton.  
  3. Evaporator outlet SLT (suction line temperature) and SH (superheat) On a TXV system, the superheat ranges from 5°F (2.75K) to 15°F (8.25K), depending on factory setting +/– 5°F (2.75K) of 10°F (5.5K). Fixed-bore or piston reading depends on the inside heat load, return air WB, and outside air DB temperatures.
  4. TESP (total external static pressure) inches WC of the air handler between the non-turbulent point in the return plenum before a clean filter and in the non-turbulent area in the supply plenum. With caution, drill 3/8”-1/2” holes. When you've finished, cover them with vinyl or plastic professional-looking plugs. On a furnace, drill above the filter for the return reading and between the furnace and the coil for the supply reading. Note if the coil was wet or dry since TESP changes. 
  5. Pressure drop  “WC across the filter.
  6. Pressure drop  “WC across the evaporator coil; note if the coil is wet or dry, and plug holes.
  7. Indoor blower motor (IBM) running load amps (RLAs) compared to nameplate rated or full load amps (FLA) with the panels on.
  8. SLT and SH at the condenser (compressor inlet). SH within +/– 5°F (2.75K)  is acceptable. For a TXV, superheat average 10°F (5.5K) plus an additional 1-3°F (.55K – 1.65K) of SH the suction/vapor line absorbs (as measured). For a fixed-bore or piston metering device at the indoor coil, a total “target SH” is determined by outdoor DB and indoor WB temps.  
  9. Condenser DTD or split is the temperature difference between the refrigerant saturation temperature and the DB temperature of the air entering at the middle of the coil. As SEER increases, condenser surface areas are larger but are limited by diminishing heat transfer capability as the temperature difference between the outdoor air and the coil temperature decrease.  
  10. LLT (liquid line temp) and SC (subcooling) at the condenser outlet. SC within +/– 3 °F (1.65K) is acceptable. For example. 85°F (29.4°C) ambient, 13 SEER with a 20°F (11K) DTD split, and 10°F (5.5K) subcool nameplate, the LLT = 95°F (35°C)  = 85°F (29.44°C) outdoor + 20°F (11K) CTOA  – 10°F (5.5K) SC).
  11. Compressor and OFM running load amps (RLA) compared to nameplate rated and full load amps (FLA), respectively.  
  12. Measured suction temperature differential between the suction line leaving the evaporator and entering the compressor in °F. So, if the suction line is 50°F (10°C) inside and 53°F (11.66°C) outside, there would be a 3°F (1.65K) temperature rise.
  13. Measured liquid temperature differential between the liquid line leaving the condenser and entering the metering device in °F. So if the liquid line is 95°F (35°C) outside and 92°F (33.33°C) inside, there would be a 3°F (1.65K) temperature drop.

Again, benchmarked DTDs, SHs, SC, and ESPs should not change during the life of the system unless one or more of the following has developed:  

  1. Airflow restriction
  2. Component failure
  3. Refrigerant flow restriction  

The following list shows data to record during follow-up seasonal checkup visits and to compare to benchmark data. See if problems have or are developing and if they show improvement after any services are performed, which offers value to clients/customers paying for the service call or membership fee. Service could be simple as a filter change, coil cleaning, and blower maintenance. However, since you have more time for PM, also identify potential electrical failures and inform clients. Offer them the choice to fix the issues now or later.

  1. TD between the return air dry-bulb (DB) and supply air DB. This value should be in the 16-24°F (8.8K-13.2K) range, depending on the sensible and latent heat content of inside air (see chart).  
  2. Evaporator outlet SLT.  If the metering device is a TXV, this value should be within +/– 5°F (2.75K) of the benchmark reading. Fixed-bore or piston readings depend on the inside heat load, return air WB, and outside air DB temp. A more practical way to determine SLT is at the outdoor coil suction/vapor line.
  3. TESP (total external static pressure) “WC of the air handler, and note if you have a wet or dry coil.
  4. Static pressure drop “WC across the filter and re-plug holes (or visually inspect/replace).
  5. Static pressure drop “WC across the evaporator coil; note if you have a wet or dry coil, and re-plug holes.
  6. SLT at the condenser (compressor inlet). For an indoor TXV, this value should be within +/– 5°F (2.75K) of the benchmark reading. For a fixed-bore or piston indoor coil metering device, determine the total “target SH” from outdoor DB and indoor WB temperatures.  
  7. LLT (liquid line temp) and SC (subcooling) at the condenser outlet. LLT using SEER-rating split should be within +/– 3°F (1.65K) of benchmark reading. Outdoor air temperature + CTOA based on system efficiency – subcooling = target liquid line temperature.

Other notes:

Always use pre-tested, calibrated (as possible) digital thermometers to measure air temperatures and line set pipe temperatures or insulated temperature sensors. Do not depend on the space thermostat to accurately represent inside air temperatures; there could be return duct leakage, bypass ducts not dampered correctly, and air handler cabinet leaks (e.g., holes/gaps at indoor coil line set inlet affecting the return air temperature).  

Airflow through/across evaporator and condenser coils will only decrease and not “magically” increase. The primary reason is dirt accumulation on airflow components (e.g., coil fins, indoor filter, indoor blower blades, and outdoor fan blades). Other reasons include leaky air handler cabinets from gaps at the return and supply duct connections, holes at the line set inlet to evaporator cabinet, and a bypass duct with no damper to close off airflow between supply and return in cooling mode.

Systems should not be benchmarked with a wet condenser coil or if the LLT is at or below the outdoor ambient air DB temperature.

Use a battery or cord-operated leaf blower to dry out the coil for 5-10 minutes.  

The only action that increases airflow is increasing the fan or blower RPM or speed. If the suction line is supposed to be 54°F (12.22°C) [40°F (4.44°C) coil + 10°F (5.5K) SH if TXV + ~2°F SH added to the vapor line length] but is actually 47-48°F (25.85K-26.4K), then you would look for indoor airflow restriction issues. The evaporator is like a boiling pot of water, but it's a sealed system. If the burner heat is turned up, pressures and temperatures increase. If there is more than an additional 24°F (1.1K-2.2K) superheat at the compressor inlet, it's probably better to insulate the vapor line.

Maximum inlet temperature of the suction line at the compressor inlet should be below 65°F (18.33°C). If not, the compressor will have the potential to overheat, and oil breakdown can occur due to excessive discharge superheat/temperature.

A TXV is designed to maintain 5-15°F (2.75K-8.25K) superheat [10°F (5.5K) given +/- 5°F (2.75K) range]. Still, it only maintains the temperature at the evaporator outlet or where the sensing bulb is located on the suction line. Some SH is added to the suction line before the refrigerant gets to the compressor inlet. However, if the line set is located in a 145°F (62.77°C) attic and the vapor line is not well insulated, a significant SH gain will be seen at the compressor inlet. The vapor line needs good insulation, as it does for heat pumps in heating mode (e.g., with thicker tubing insulation and/or using a foil-bubble wrap or “Reflectix” attached with foil tape, since it reflects IR heat).

Summary of the Jim Bergmann/Bryan Orr Podcast on checking the charge without using gauges by Joe Reinhard

P.S. – As mentioned in the podcast, the Testo 605i and the 115i make a great pairing to check a system in the way described above

You can now do ALL of these calculations easily with the MeasureQuick app at MeasureQuick.com/downloadnow.

5 responses to “The Case for Checking the Charge Without Using Gauges”

  1. You quantify what I did by feel. You wrote it well. I’m going to try to make a check out form to accompany this for sharing. Thank you

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