Greetings,

Would anyone who has evaluated energy savings through the use of occupancy sensors controlling VAV boxes please share some experience, do's and don'ts, etc.

I believe the VAV boxes would have a minimum setback to 0% or 10% flow and possibly no reheat in the summertime. I have been trying to model this with little success. Is it possible to do using freeware or should I start constructing hourly reports. Thank you all for your assistance.

Kevin

Hello Kevin,

I have not yet modeled this but here is what I am planning to do:

1. Determine the total floor area of zones with occupancy sensor

control. Let's call it "Area X"

2. Estimate the amount of time that any one zone would be unoccupied.

For purposes of this explanation, assume it is 15% (per the ASHRAE 90.1-2004

Table G3.2).

3. Multiply "Area X" by 15% to determine an equivalent floor area that

would be unoccupied continuously. Let's call this "Area Y"

4. Look at all of your zones used to develop "Area X" and determine the

relative proportion of areas that have north, east, south, west exposures

and are in the core of the building. Let's call the proportion of area in

the north exposure "N"

5. Multiply "Area Y" by each of these different proportions (e.g. "Area

Y" x "N") to determine how much unoccupied space needs to be represented.

Review your list of "Area X" zones to find the ones with approximately the

correct floor area corresponding to the requirements for the different

exposures - these will be normally unoccupied spaces. Depending on how the

zones were initially modeled, you may need to split some zones to make

specialized "unoccupied" zones (this could get pretty time intensive unless

you anticipate having to do this when the model geometry is assembled). The

purpose of going through the hassle of identifying zones in each of the

different exposures is to model the impact of occupancy sensor control on

the average space.

6. Within the normally unoccupied zones, set the minimum airflow in the

Airside Systems/Zone/Airflow tab to 0 cfm and change the occupancy, plug,

lighting, heating, and cooling schedules to use an unoccupied schedule (use

the "FRACTION ALWAYS OFF YEAR", "COOLING SETBACK YEAR", and "HEATING SETBACK

YEAR" schedules you have already made).

It seems like this approach could potentially work and would be fairly

accurate for a building where the aggregate unoccupied periods are

distributed randomly throughout the typical workday. If there are separate

mechanical systems, the procedure above would need to be replicated for each

system. This approach would probably work for a typical office but may not

work for an operation with mandatory lunch hours. In reality, the typical

zone would probably not float all the way back to its unoccupied setpoint

since the unoccupied periods during normal building hours will be fairly

brief (<1 hr?). Assuming that some zones are always at setback temperatures

does decrease conduction gain/losses, however, this component of space load

is generally small relative to solar, equipment, and lighting loads during

occupied periods in non-residential spaces. The most significant

uncertainty in this approach is the guesstimate of the fraction of time that

the typical space will be unoccupied.

If there are periods when many zones are likely to be unoccupied at the same

time, things can get a bit more complicated. Specialized schedules can be

made for occupancy, plug, lighting, heating and cooling, and you would need

to use the minimum airflow schedule option in Airside Systems/Zone/Airflow.

Is this what others have done? Does it seem reasonable?

Eric