The Non-Invasive System Test (NIST™) is a method of accuracy verifying proper operation of an air conditioning or refrigeration system without the need to install the gauges or pressure transducers/probes.
This method uses information from the system profile or a table of operating pressures provided by the manufacturer to estimate the suction and liquid pressures and determine target for the actual suction and liquid line temperatures to determine if the system charge is correct. In order for non-invasive test mode to work properly, make sure that the filters are clean, condenser and evaporator are clean, and the blower is clean.
System airflow should set. The NIST method works best if the system was ‘”benchmarked” in the past for the most accurate profile but will provide reasonably accurate results if the system is profiled properly and the system is clean.
If this method of determining proper operation is used industry wide, 1000’s of pounds of refrigerant could be saved per year in hose losses, and the compromise of refrigerant charge and associated efficiency losses will be eliminated by service technicians and energy raters.
Entering the NIST Mode for Testing
Required Conditions for Testing:
The return air wet bulb shall be above 50°F (10°C) and the return air dry bulb temperature between 70°F (21°C) and 84°F (29°C). The system and the temperature sensors must be stable before measurements are recorded.
Procedure for Testing & Measurement
Retrieve the system profile that is available by geotagging the location and pulling down the required information to complete the non-invasive test.
Note: If the system has never been tested prior, the NIST can be used, but it is not as accurate of an indicator of correct charge as an invasive test (hooking up pressure transducers or gauges) is for initial measurements. Once the system is tested and the system information is benchmarked, recorded and stored, the NIST is as accurate and reliable as an invasive test method. It is recommended that an invasive test (attaching gauges) is performed only once, preferably during initial commissioning, or retro commissioning, during the entire life of the system. This non-invasive method of checking charge keeps the “sealed system” sealed, preventing refrigerant loss, contamination, and minimizing potential leaks. System pressure should not be checked as part of regular service.
If a current benchmark is not available, as a starting point, from the “System Information” home screen, select the following.
1.) Refrigerant type
2.) Nominal or design airflow
3.) SEER range and evaporator style
4.) Metering device type
Note: Do not install pressure measurement probes for non-invasive testing. The measureQuick application will automatically enter and operate in the NIST mode if no pressure is detected.
Start the air conditioner. The air conditioning equipment shall be in operation no less than 15 minutes and/or line temperatures stable before any temperature measurements are recorded. Indoor and outdoor measurements must be recorded within 5 minutes of each other, so they are taken under similar load conditions.
Care must be taken for proper temperature measurement assuring that the temperature sensors has complete contact with the pipe surface being measured.
Probe Placement for NIST Testing
Return Air Temperature Dry Bulb (RADB) and Return Air Relative Humidity (RARH) shall be measured in the return air stream, out of line of sight of the evaporator coil in the return air. Wireless probes can be inserted into the vent or placed inside the duct.
Supply Air Temperature Dry Bulb (SADB) and Supply Air Relative Humidity (SARH) shall be measured in the supply air stream, out of line of sight of the evaporator coil, in the supply air, preferably 6-10′ away from the evaporator in the closest supply air register. Wireless probes can be inserted into the vent of placed inside the duct.
Outdoor Air Temperature (OAT) shall be measured in the inlet air condenser air stream out of line of sight of the sun near the middle of the condenser.
Liquid Line Temperature (LLT) shall be measured at the condensing unit within 6″ of the service valve. The sensor clock position is not important.
Suction Line Temperature (SLT) shall be measured at the condensing unit, within 6″ of the service valve. Pipe temperature shall be measured with the temperature measurement sensor at the 2:00 or 10:00 position on the suction line.
TESP shall be measured according to manufacturer’s instructions.
Electrical Measurements shall be made in accordance with standard practices and can be made phase to phase to ground as needed or required.
Calculations & Diagnostic Conclusions
The measureQuick in NIST mode will display orange ghost needles showing where pressures should be for the current load and ambient conditions, it will calculate and measure for TXV systems.
• Suction line temperature, target and actual
• Liquid line temperature, target and actual
And for fixed orifice systems
• Suction line temperature, target and actual
• Approach temperature, target and actual
These line temperatures are derived from the current load conditions and the superheat, subcooling, or approach targets. For a complete NIST, electrical measurements can be added.
If at any time pressures are detected due to pressure probes or a manifold being installed, the actual pressures will be displayed alongside the target temperatures for approximately 10 seconds.
• SLT High
• SLT Low
• LLT High
• LLT Low
• Approach High
• Approach Low
• Temperature Split High
• Temperature Split Low
• Total Capacity High: Higher than normal total capacities are typically seen when the indoor load is higher than typical equipment design. High relative humidity or high space temperatures can push equipment to operate in excess of its rated capacity. Once the load decreases, the system capacity will typically fall within the design criteria. Higher than normal total capacity can also be the result of bad or incorrect data input into the application, resulting in error.
• Total Capacity Low: There are many causes of low capacity. Low total capacity is typically seen during low load, low airflow, a dirty or restrictive filter, low refrigerant charge, high refrigerant charge, a dirty condenser, non-condesibles, a condenser that is recirculating exhaust air, a blocked filter dryer, liquid line restriction, suction line restriction, component sizing issue, or inefficient compressor. Most if not all deficiencies in operation will result in reduced total capacity. Low total capacity can also be a function of high airflow where we see normal sensible capacity, but low latent capacity due to the higher coil temperatures or excessive velocity across the coil.
• Sensible Capacity High: A higher than normal sensible capacity is typically the result of high airflow or higher than normal load.
• Sensible Capacity Low: A low sensible capacity is a result of low airflow or normal than lower load. Low sensible capacity can also result from low airflow, a dirty or restrictive filter, low refrigerant charge, high refrigerant charge, a dirty condenser, non-condensibles, a condenser that is recirculating exhaust air, a blocked filter dryer, liquid line restriction, suction line restriction, component sizing issue, or inefficient compressor. Most, if not all deficiencies in operation will result in reduced sensible capacity.
• Latent Capacity High: A high latent capacity can be the result of a hight latent load (high humidity or wet bulb) or more typically the result of low airflow.
• Latent Capacity Low: A low latent capacity can be the result of low latent load (low relative humidity or wet bulb temperatures) or the result of, a dirty or restrictive filter, low refrigerant charge, high refrigerant charge, a dirty condenser, non-condensibles, a condenser that is recirculating exhaust air, a blocked filter dryer, liquid line restriction, suction line restriction, component sizing issue, or inefficient compressor, This is also very typical when airflow is higher than design conditions for the climate zone the equipment is installed in.
• EER High: High EER’s are typically the result of higher than normal suction pressures coupled with low outdoor ambient temperatures. Under these conditions, the equipment is operating more efficiently due to the lower compression ratios of the compressor. High EER’s can also be the result of incorrect capacity or electrical measurements.
• EER Low: Low EER’s are the result of inefficient equipment operation. They can be the result of high electrical consumption or low total capacity, or both. Verify capacity and consult the manufacturer’s service facts to determine the probable cause.
• TESP High: A high total external static pressure is typically the result of an airflow restriction. The restriction can be in the supply or return side of the duct system. Both sides should be carefully evaluated. Highly restrictive filters, a dirty evaporator coil or undersized ducts are the most typical causes.
• TESP Low: Low total external static pressures are not typical but can occur on 90+ furnaces when there is a dirty or restricted secondary heat exchanger. They can also be the result of a dirty blower, or an inefficient blower resulting from a failing capacitor.
• Fan Watt Draw High: High fan watt draws are typically seen with elevated TESP (total external static pressure). Look for dirty or highly restrictive filters, duct restrictions, zone systems with closed dampers, or undersized ducts.
• Fan Watt Draw Low: Low fan watt draws are typically seen with low TESP (total external static pressure). Look for an open bypass damper.
• Airflow High: As airflow is calculated, if the calculated value is high, it is typically due to an incorrect blower speed setting, air bypassing around the evaporator, or improper probe placement. High airflow will result in higher sensible cooling, and low latent cooling resulting in lower levels of dehumidification.
• Airflow Low: Low airflow is typically due to a dirty or restricted filter, a dirty blower wheel, a dirty evaporator coil or incorrect fan speed or fan dip with settings. In many cases, undersized duct systems can also result in low airflow problems. Verify that the system is clean, fan speed or dip switch settings are correct, and the total external static pressure is within the acceptable range.
• ∆h High: Typically, the design change in enthalpy (total heat) for comfort Collings will be 6.66 Btu/lb. or air across the evaporator coil at 400 CFM/ton. A high change in enthalpy is normally the result of fewer pounds of air moving across the coil due to either low airflow or low air density. Standard air density is .075 lbs./Ft3. In higher elevations, the actual CFM or ACFM must be higher than the ACFM at sea-level due to the lower air density at elevation.
• ∆h Low: Typically, the design change in enthalpy (total heat) for comfort cooling will be 6.66 Btu/lb or air across the evaporator coil at 400 CFM/ton. A low change in enthalpy is typically the result in a decrease in air conditioning capacity due to either an issue with heat absorption at the evaporator or heat rejection at condenser coil. There are many causes of low change in enthalpy. A low change in enthalpy is typically seen during low load, high airflow, low refrigerant charge, high refrigerant charge, a dirty condenser, non-condensibles, a condenser that is recirculating exhaust air, a blocked filter dryer, liquid line restriction, suction line restriction, component sizing issue, or inefficient compressor. Most, if not all deficiencies in operation will result in reduced change in enthalpy.
Other measureQuick Measurements
• Airflow (SCFM & ACFM)
• Target temperature split
• Actual temperature split
• Total BTUH
• Tons of cooling
• Sensible BTUH
• Latent BTUH
• Sensible Heat Ration
• Approximate SEER
• Fan watt draw
• Ambient environmental condtions
• Bypass factor
• Dew points
• Air Density
The NIST has integrated diagnostics to assist in identifying issues that cause the measured temperatures to fall outside of calculated targets. If a troubleshooting flag appears, follow the system prompts to attempt to resolve the issue before attaching gauges to the system, and only attach gauges after all other options have been exhausted.
Most, if not all faults can be cleared by performing system maintenance including:
• Cleaning the condenser coil
• Cleaning/replacing the system air filter
• Cleaning return air registers
• Cleaning the evaporator coil
• Cleaning the system blower wheel
• Replacing/adjusting belts on belt drive equipment
• Repairing duct or cabinet leaks
• Removing supply or return obstructions
• Replacing any failing electrical components like capacitors that can affect motor operation.
• Increasing airflow
• Decreasing airflow