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In the past, the room air temperature was the only measurement required in a room. Over the years, studies have shown other parameters, such as humidity, carbon dioxide, ambient light intensity, and others, also affect the productivity and comfort of the occupants of a room. Increasing the number of parameters has led to a new breed of sensors that measure multiple parameters inside of a single enclosure. The most common combination sensors are temperature paired with humidity. However, in recent years, there has been an increase in combination units that include carbon dioxide in addition to temperature and humidity.

A minimum amount of outside fresh air is required in buildings to ensure that appropriate oxygen levels are maintained. This minimum amount is based on the maximum occupancy and size of the room for standard buildings without a building management system. Whenever rooms are occupied in these buildings, their HVAC systems provide the same amount of conditioned air to meet the maximum occupancy requirements. Frequently, however, the buildings are not at maximum occupancy and, therefore, a lot of wasted conditioned air is supplied to the building, which, in turn, wastes a lot of energy. Therefore, ASHRAE and other heating, ventilation, and air-conditioning industry trade organizations allow for the building management to reduce the amount conditioned air if it can be quantified that there are less people occupying the space.
The most common method of measuring the occupancy level is monitoring of the carbon dioxide level, which is proportional to the number of people in the space. Based on this, the HVAC system will output conditioned air to maintain a CO2 level of less than 1,000 ppm instead of putting out the minimum required conditioned air for the building design. If the level is reading too high, more conditioned air needs to be brought into the room. If the level is still reading too high after this, the outside air damper needs to be opened more to bring in more fresh air.
The major benefits of adopting a combination sensor instead of individual sensors are aesthetics, space savings, material cost savings, and installation labor cost savings. On the flip side there are some hidden costs that have to be considered before deciding on a combination sensor, such as replacement cost if only one of the sensors goes out and the effects the sensors have on each other.
Architects desire sensors to blend into the room. Having three sensors mounted next to each other on the wall makes that task much more difficult. Since many times the three sensors on the wall are made by different companies, they will have different colors, shapes, and sizes. Combination sensors address these aesthetic issues and take up less overall wall space.

Read more: Combination Sensors in HVAC: Doing More with Less