When system airflow is poor, or the designer omits a proper cutout around the sensor, the system thermal time constant can suffer. This results in system temperature lagging the actual ambient air temperature significantly. For an instantaneous change in temperature, this will appear as in Figure 4-1.

This kind of theoretical instantaneous change in temperature is not realistic in HVAC systems after installation. Most systems will see gradual or cycling changes in temperature in response to heating and cooling action. The response in Figure 4-1 should look like the familiar exponential response of a first-order RC circuit to a step change in voltage. This style of RC circuit is commonly used as an analogue for thermal behavior in ICs and electrical circuits. This concept is described in more detail in Optimizing Placement and Routing for Humidity Sensors.

In a scenario where the ambient temperature is steadily increasing (or decreasing), a system with slow thermal response will exhibit transient error where system temperature lags behind ambient temperature, as shown in Figure 4-3. In cycling environments like in Figure 4-4, the transient error may never settle to zero if the system thermal mass is too large.