There are many times when we do an air conditioning service call, we notice that the thermostat in the home is set for 72 degrees or lower. Sometimes as low as 68! Believe it or not, this can actually be a sign of an improperly sized, installed, or commissioned piece of equipment. Or even possibly, an issue with the home itself. 

 

If its a matter of comfort, to some extent, its understandable, some people will claim that they sleep better when its colder. This is ok, but there is an inherent risk involved with the equipment or application if we go beyond manufacture or industry design. Those risks include:

 

• Inefficient performance
• improper humidity removal 
• life expectancy of the equipment
• Possible uncomfortable environment

 

 

If we utilize a basic understanding of air conditioning, it starts to become more clear. We all know (or at least should know by now) that the target coil temperature we try to achieve by design is 40*F. This is represented by our suction line pressure and referencing the PT chart. EX: R22- 68.6 psi, R410A- 118.8 psi, R32- 121 psi, R454B- 107 psi. Now we include the manufacture Design Temperature Differential of the evaporator itself of 35*F colder than the return air. Ex: Room temp (75*F) - DTD of evap (35*F) = 40*F. 

 

Now lets just say that the home owner says that the home "feels" better when they set thermostat for 68*F. If we subtract the DTD (Evaporator Design Temperature Differential) of 35*F, from the space temperature of 68*F, we would see that our coil temperature is running approximately 33*F, which is extremely close to freezing. Our suction pressure reading would also seem to be low, R410A EX: 33*/ 103.1 psi. We now are technically running beyond design conditions. NOTE: System performance is very difficult to determine when return air temps are below 70*F. 

 

In this scenario, we quite possibly have a system that is too large for the home and short-cycling, or not dehumidifying the home correctly. This could also be an airflow issue or poor duct design as well. Or, there could be an issue with the home itself regarding improper ventilation.

 

In cases like this, a technician will see the "low Suction pressure" and feel the need to adjust the charge by adding refrigerant, and actually over-charging it, causing it to run less efficient. Its extremely important to understand design numbers so we can accurately trouble shoot a system and solve the real problem.  

 

Most of your customers will have different comfort levels, and we do have the ability to at least get close to those expectations, but we also have design parameters to follow by the manufactures, as to not damage the equipment or provide a less than efficient system. 

 

Below are examples of industry design targets. 

 

A real HVAC trouble-shooter is a Marksman, always on target. 

 

 

🏢 By Manufacturer (Typical Coil Design Temp)

Manufacturer	Design Coil Temp (Saturation)	Notes
Carrier/Bryant	~40°F	Typical for R-410A
Trane/American Standard	~40°F	Designed for latent and sensible removal
Lennox	~40°F	Some systems designed for slightly higher SHR
Goodman/Amana	~40°F	Common across budget-tier models
Rheem/Ruud	~38°F – 42°F	May run slightly lower for higher latent loads
York/Luxaire/Coleman	~40°F	Similar to Carrier/ICP ranges

 

 

 Why ~40°F?

• At 40°F, the evaporator is:

  • Cold enough to dehumidify (remove latent heat)

  • Warm enough to avoid frost under normal indoor conditions

• Air entering the coil is typically 75°F–80°F, so this ensures a 20°F delta T, which matches typical design performance.

 

🏢 Manufacturer-Specific Information

While manufacturers like Carrier, Trane, and Lennox may not explicitly state the 35°F difference in their publicly available documentation, their system designs and performance data align with this guideline.​

• Carrier:

  • In a JustAnswer discussion, a technician mentions: "Assuming proper airflow, the indoor coil surface temperature will be about 35 degrees below the indoor temperature." ​JustAnswer+1JustAnswer+1

• Trane:

  • Trane's engineering documents discuss coil selection and optimization, emphasizing the importance of appropriate temperature differences for efficient operation. ​Trane

• Lennox:

  • Lennox's installation instructions for their C35/CX35 series coils provide detailed guidance on coil installation and operation, which, while not explicitly stating the 35°F difference, are consistent with industry standards. ​Lennox Tech+2Lennox Tech+2Lennox+2

 

 

📘 Industry References Supporting the 35°F Design Temperature Difference

1. HVAC School:

   • Bryan Orr notes that a good rule of thumb for suction pressure is 35°F saturation below indoor ambient ±5°F, meaning that if the return air temperature is 75°F, the evaporator coil's saturation temperature should be around 40°F. ​HVAC School

2. HVAC Know It All:

   • The site explains that the evaporator temperature difference (TD) is typically 35°F for air conditioning systems. For example, if the return air is 75°F and the saturated suction temperature is 40°F, the TD is 35°F. ​HVAC Know It All

3. HVAC School – Important Terms:

   • The article states that the target evaporator temperature should be based on the return temperature, with a 35°F difference being standard for air conditioning systems. ​HVAC School

4. HVAC-Talk Forum:

   • A discussion on the forum mentions that in air conditioning, the evaporator is around 30–35°F below the return air temperature, aligning with standard design practices. ​Heating Help: The Wall

 

 

 

   

Understanding the 35°F Design Temperature Difference

• Purpose: This 35°F difference ensures efficient heat exchange, allowing the evaporator coil to absorb sufficient heat from the return air to cool and dehumidify the space effectively.​

• Example: If the return air temperature is 75°F, designing the evaporator coil to operate at a saturation temperature of 40°F achieves the desired 35°F difference.​

• Considerations:

  • Airflow: Proper airflow is crucial. Reduced airflow can lead to lower evaporator temperatures, potentially causing the coil to freeze.

  • Humidity: High indoor humidity levels may require adjustments to the design temperature difference to ensure adequate dehumidification.​