In many of today’s buildings, owners install very high efficiency equipment, but the building utility bills don’t reflect the premium the owner paid. It cannot be stressed enough – proper control is essential to an energy efficient building. For proper control, control systems depend on getting accurate inputs from sensors. In that respect, properly calibrated sensors are essential to the energy performance of high performing buildings.
In the first chart are temperature sensor readings from a recent study on an institutional campus. The bold blue line represents the outdoor temperatures from the national weather service for this location. The remaining lines are the trends from the outdoor air temperature sensors from various buildings (sensors) on campus. Sensors should read within a few degrees of the NWS readings, so this is an issue for proper control of HVAC systems.
Traditionally, temperature sensors are touted as being accurate to within a degree or two, with calibrations that don’t drift much with time – this is sufficient and necessary for most temperature control applications. In this case, when compared to the national weather service, this campus’ sensors aren’t within the typical sensor error. This could be partially attributed to temperature microclimates (e.g. a sensor over a black roof vs. a sensor near a shaded pond), but not to the extent shown.
To make matters worse, the temperature sensor accuracy looks great in comparison to the relative humidity (RH) sensors. The second chart is for the RH sensors from the same air handling units on this campus.
A few of these RH sensors are less accurate than a broken clock, a couple more appear to give that broken clock a run for its money, and the rest leave plenty to be desired. Only two RH sensors deliver what could be remotely considered accurate information.
Consider that these sensors are used for controlling heating and cooling equipment, such as when to switch into and out of an economizer mode. Remember – please forgive another cliché – garbage in, garbage out.
Most buildings have these sensors installed and calibrated one time, if they’re calibrated at all. Generally, no one notices sensors need (re)calibration or an element needs replacement unless they’re obviously broken with alarms or if comfort conditions cannot be maintained.
There are a number of possible contributing factors to the performance woes of these sensors. Sensors located outdoors that are subjected to the full brunt of the weather seem to be more likely to fail than those that are indoors under more consistent conditions. Relative humidity sensors are also a young technology that is still improving.
The quality of a system’s sensors is important to verify over time. Sensor readings should be validated against actual weather conditions from time to time. A couple degrees variance either way isn’t cause for panic but a 20 degree variance may be!