Re: GBlist: Demand controlled ventilation

Tom Anderson Mechanical Systems Design (
Thu, 27 Mar 1997 08:08:09 -0500

Marc J. Rosenbaum wrote:
> Does anyone have experience with demand-controlled ventilation in
> commercial/institutional buildings? I am working on a classroom building for a
> law school. The occupancy of a classroom can vary widely so we are considering
> DCV. I have heard CO2 sensors need frequent recalibration - if so, not a good
> strategy for a small institution, where the maintenance people tend to fight
> fires rather than leisurely take care of routine maintenance. What other means
> are there to determine how many people are in the space? Does anyone have real
> building experience here?
> Marc Rosenbaum
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I design commercial institutional HVAC systems and have successfully applied DCV, but not with CO2 sensors.

CO2 calibration/ accuracy, understandability by facility staff are one reason I have not advised my clients to
use them. CO2 is a surragate for IAQ. IAQ contamination from VOC's and outgassing, etc. would not be
detected, of course. Use good industrial grade sensor, if you do. It will cost > $1500. (US), but much more
reliable than the commercial $ 400. to $600. models. There is still an outstanding question about accpered
methodology for calibration procedures. Yes, they drift and must be calibrated 1-2 times ayear, last I heard.

My understanding is CO2 cannot be used in place of code mandated minimum fresh ventilation air (not 100%
positive on this). The new ASHRAE standard 62 addresses this I hear tell, but have not personnally looked
into the review version of the standard yet.

There are many potential pitfals, as with any new application.

The most successful projects of mine, and the most commonly overlooked opportunity, are using occupancy
sensors for DCV. You simply connect the room hvac equipment to the occ sensor aux contacts or DDC control

Thus, anytime the room is occupied, even unscheduled weekend/evening activities, ventilation is automatially
established. Ventilation closes when occ sensor indicates vacancy, after time delay for purging.

For a single classroom with a typical unit ventilator, it works well. I do NOT reset the space temperature
during regular working hours.... to avoid too hot or cold of a room. Provide a time delay to enable
ventilation, thus avoided short cycling of ventilation mode when someone walks in and out of the room quickly.

For central HVAC systems, they work well. I use them for Variable Air Volume systems (VAV). When a room is
not occupied, the local VAV terminal DDC controller "talks" to the air handler and "tells" it how many people
are in the room. The number of occupants is programmed by the user. The fan then sums up the total number of
people on the system from all the other VAV boxes. Then, total people X cfm per person determines the volume
of fresh air needed, and mixing dampers do their thing.

Combining occ sensors WITH an occupancy schedule allows a room, normally scheduled (per DDC front end PC) for
regular working hours to go into a "standby" mode when occ sensor says it is vacant. Room temperature
setpoints are maintained, air flow shut off, but airflow will be enabled by the VAV controller to maintain
temperature setpoints only.

Also, a zone consisting of say three rooms, each with occ sensor and space temperature allows the controller
to get an average all three sensors, for more accurate zone control. The space temperature is ignored (not
averaged) if the room is vacant. There are many other acpects to this I could talk about....

In my opinion, this creates a true "Intelligent Building"s. Not to be confused with control vendors
marketing pitches, where all they really due is mimic old time "dumb" sepquences of control ( very common in
this industry). You get better comfort and big time energy savings. Figure about a 50% annual energy
reduction, compared to constant volume reheat.

Tom Hartman of the Seattle area has applied this methodology for years. His
firm is The Hartman Company. See his DDC book and also HPAC magazine
has reprints of his articles. Also ASHRAE has tech articles of his.

Mr. Hartman is one of a minority of Mechanical Engineers (HVAC) who also have a strong creative right brain
approach, unlike most in this industry, who have right brain deficiencies in direct proportion to the left
brain analytical skills.

For the most reliable occupancy sensors, I stick with Watt Stopper.

Tom Anderson

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