Demand controlled ventilation:
how CO₂ control
cuts the AC bill

There is a genuine conflict at the centre of every air-conditioned building in a hot, humid country, and most buildings resolve it badly.

Ventilation requires bringing outdoor air in. Air conditioning requires that air to be cooled and — in the Indian climate, expensively — dehumidified. So every unit of fresh air you supply for air quality is a unit of cooling load you pay for. Ventilate generously and the electricity bill rises. Ventilate meanly and you get the headaches, drowsiness and complaints described in sick building syndrome.

Faced with that, a great many Indian buildings quietly choose the second option: somebody switches the fresh-air unit off, or wires the damper shut, and the electricity bill improves. The air-quality cost is invisible because nothing is measuring it.

Demand-controlled ventilation is the third answer, and it is the one the standards actually recommend.

Where the cost of fresh air comes from

Worth being precise, because the intuition that “a fan does not use much electricity” is correct and completely beside the point.

The fan motor is trivial. The cost is in conditioning the air the fan brings in. On a Delhi afternoon at 40°C, or a Mumbai afternoon at 32°C and 80% relative humidity, incoming outdoor air must be cooled to room temperature and have a large amount of moisture condensed out of it. In humid Indian conditions the latent load — the energy to remove water vapour — routinely exceeds the sensible cooling load for outdoor air. That is where the money goes.

So the real question is not whether to ventilate. It is how many hours per year you condition outdoor air that nobody needed.

Why fixed-rate ventilation over-supplies

Ventilation systems are sized using the ASHRAE 62.1 Ventilation Rate Procedure, which computes required outdoor air from the design occupancy of a space. That is correct and necessary: the system must be capable of handling a full room.

The problem is that a constant-volume system then delivers that peak rate whenever it is running, regardless of how many people are actually present. Consider how real spaces are used:

• A meeting room designed for 12 is empty most of the working day and holds four people for much of the rest.
• A banquet hall designed for 300 holds 300 for a few hours a week.
• A classroom designed for 40 is empty from mid-afternoon, and empty entirely through the holidays.
• An open-plan office designed for 100 typically holds 55 to 70, and far fewer on Fridays.
• A gym designed for peak evening load is nearly empty from 11 a.m. to 4 p.m.

Every hour those spaces run at design ventilation rate with a fraction of design occupancy is fresh air being conditioned for people who are not there.

What DCV does instead

Demand-controlled ventilation uses a measurement of actual occupancy to modulate the outdoor-air rate — and CO₂ is the standard proxy, because it is generated by people in direct proportion to how many of them there are and how hard they are working. Occupancy sensors detect presence; CO₂ measures how much presence, which is what the ventilation rate needs to respond to.

In its simplest and most robust form — the form that fits the overwhelming majority of Indian buildings — DCV is a two-threshold relay:

Fan ON when CO₂ rises above the upper threshold.
Fan OFF when it falls below the lower one.

The gap between the two thresholds is deliberate hysteresis: it stops the fan short-cycling around a single set point, which is hard on the motor and irritating to be near. Set ON at 1,200 ppm and OFF at 800 ppm and a meeting room ventilates hard during the meeting, coasts afterwards, and does nothing at all overnight.

Where the saving actually comes from

Three separate mechanisms, and it is worth distinguishing them because they apply differently to different buildings:

1. Unoccupied hours. The largest and most certain saving. A space generating no CO₂ needs no ventilation, and DCV supplies none. For a school this is evenings, weekends and holidays — well over half the year. For an office it is nights and weekends. For a banquet hall it is most of the week. No comfort trade-off exists here at all; the room is empty.

2. Partial occupancy. The steady, everyday saving. A 100-person floor holding 60 people needs roughly 60% of design outdoor air. Fixed-rate systems deliver 100%. DCV tracks it down.

3. Avoided over-ventilation at the shoulders. Early morning and late evening, when a few people are in a large space, fixed-rate ventilation is at its most wasteful in proportional terms.

How much this adds up to depends entirely on your occupancy profile, your climate and your system, and any vendor quoting you a single percentage without asking about those is guessing. The honest statement is: the saving is proportional to the fraction of running hours during which your spaces are below design occupancy — which, for meeting rooms, halls, classrooms and gyms, is most of them.

Estimating it for your own building

A rough method you can do on paper:

1. Take one space. Note its design occupancy and the hours per week the ventilation currently runs.
2. Estimate actual occupancy in each of those hours — honestly. Most people overestimate.
3. Compute the occupancy-weighted fraction: total person-hours ÷ (design occupancy × running hours).
4. That fraction is roughly the outdoor-air load DCV would deliver. One minus it is the fresh-air conditioning load you are currently paying for and do not need.

For a typical meeting room this fraction often comes out below 0.3. For a banquet hall it can be below 0.1. For a continuously staffed 24×7 control room it will be near 1.0 — and that is a space where DCV saves nothing, which is worth knowing before you buy anything.

The honest caveats

• DCV does not reduce peak load. When the room is full it ventilates at full rate, as it must. Your equipment still has to be sized for that. The saving is in energy consumed, not capacity installed.
• It only helps where occupancy varies. Constant-occupancy spaces gain little.
• CO₂ tracks people, not building emissions. The per-area term in ASHRAE 62.1 — off-gassing from carpets, furniture, finishes — does not fall when the room empties. In a newly fitted-out space, some base ventilation should continue regardless. This is why the standard treats DCV as a modulation of the occupancy-driven component, not a licence to shut ventilation off entirely.
• Sensor placement decides whether it works. A sensor in the return duct or beside the fresh-air inlet will control on the wrong number. Measure in the occupied zone.

The comparison that matters

For most Indian buildings the real alternative to DCV is not generous fixed-rate ventilation — it is the fresh-air fan being switched off. That is what is actually happening in a great many air-conditioned floors, and it saves the same energy DCV does while producing a building at 2,000 ppm every afternoon.

Demand-controlled ventilation gets you the same electricity bill as the switched-off fan for the hours when nobody is there, and proper air quality for the hours when they are. That is the whole proposition.