How the two differ in kind
A building that needs cooling while it is cool outside is a common situation — an office full of people, computers and lights on a mild day, a server room in any weather, a shop with a lit display. Mechanical cooling handles it by running a compressor. An ECONOMISER handles it by opening a damper and using outdoor air directly, modulating the mix of outdoor and return air to hold the supply temperature the system needs.
When it works, it is close to free: a fan moving air costs a small fraction of a compressor removing the same heat, and the hours available can be a large part of the year in a temperate climate. It is required by many energy codes on systems above a certain capacity for exactly that reason.
The question is when outdoor air is actually the cheaper option, and this is where the intuitive test misleads. The comparison is not outdoor temperature against indoor temperature; it is the total ENTHALPY of the two air streams — their heat and their moisture content together. A cooling coil has to remove both, and removing moisture is the expensive half.
So in a humid climate, cool but nearly saturated outdoor air can carry MORE total energy than warmer but drier return air. A dry-bulb economiser, which compares temperatures only, sees the lower temperature and opens the damper — and the coil then has to dehumidify a large volume of outdoor air, increasing the load the economiser was installed to reduce. That is why enthalpy control, or a dry-bulb changeover point set conservatively for the local climate, matters more in a humid location than the economiser itself does.
The second thing to know is that economisers fail silently. A stuck damper or a drifted sensor leaves the unit either not economising when it could — invisible, because nothing is uncomfortable — or economising when it should not, which can mean heating and cooling at once. Studies of installed systems repeatedly find a substantial proportion not functioning.
The factors that actually differ
Show
| Economiser (free cooling) | Mechanical cooling | |
|---|---|---|
| How it cools | By bringing in outdoor air instead of recirculating, when outdoor air is cooler in useful terms. | By moving heat with a refrigeration cycle, regardless of outdoor conditions. |
| Energy cost | Fan power only — a small fraction of compressor power for the same cooling. | The compressor, which is the dominant cooling cost. |
| The right comparison | Total ENTHALPY of outdoor air against return air, not temperature. | Not applicable — it works in any condition. |
| Humid climates | The problem case. Dry-bulb control can increase the load by admitting cool, saturated air the coil must dehumidify. | Handles the moisture, which is what it is for. |
| Availability | Hours-limited. It works when the weather permits and not otherwise. | Always. |
| Air quality side effect | Large volumes of outdoor air, which is generally good for ventilation — and brings whatever is outside, so filtration matters. | None; the air is recirculated. |
| Failure mode | Silent. A stuck damper or a drifted sensor means either lost savings nobody notices or simultaneous heating and cooling. | Obvious. The space gets warm and somebody complains. |
| What it needs | Outdoor and return air dampers, actuators, sensors, a controller, a relief or exhaust path for the air being displaced, and commissioning. | The refrigeration plant, which exists anyway. |
| Relief air | Essential and often overlooked — bringing in a large volume of outdoor air pressurises the building unless an equal volume can leave. | Not applicable. |
| Relationship | Reduces the hours the compressor runs; it does not replace it. | Still required for the design condition and for every hour the economiser cannot cover. |
Which one, and when
Choose economiser (free cooling) when…
- The climate has many hours when outdoor air is genuinely cooler in enthalpy terms — temperate and dry climates especially.
- The building has significant internal gains and needs cooling on mild days.
- A code requires it, which above a certain system capacity it commonly does.
- There is a commissioning and maintenance regime, without which the savings quietly disappear.
Choose mechanical cooling when…
- The design condition, which no economiser covers — it is what the plant is sized for.
- Hot or humid weather, when outdoor air carries more energy than return air.
- Spaces where outdoor air cannot simply be admitted — cleanrooms, some laboratories, tightly controlled environments.
- Where there is no relief path for the displaced air and none can be created.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Why is temperature the wrong test?
- Because a cooling coil removes heat and moisture, and the energy in the moisture is a large part of the total. Air's total energy content — its enthalpy — combines its temperature with the water vapour it carries, and the latent heat in that vapour is substantial. So comparing only temperatures ignores half the question. In a humid climate the consequence is concrete: outdoor air at a lower temperature but near saturation can hold MORE total energy than warmer, drier return air, so a system that admits it because it is cooler has just increased the load on the coil, which must now condense water out of a large volume of outdoor air. That is why enthalpy-based changeover is specified in humid climates, and why dry-bulb changeover points there are set conservatively low rather than at a temperature that merely looks cool.
- What is a changeover point?
- The condition at which the control switches from economising to mechanical cooling, and choosing it is most of the design. The simplest form is fixed dry-bulb: above a set outdoor temperature, the damper closes. Differential dry-bulb compares outdoor with return temperature instead, which adapts better. Fixed and differential enthalpy versions do the same comparisons on total energy and are what a humid climate needs. Each is progressively more accurate and more dependent on sensors that must stay calibrated — an enthalpy sensor that has drifted makes worse decisions than a conservative fixed dry-bulb setting. Which is appropriate depends on the climate, and energy codes often prescribe it by climate zone for that reason. The failure worth avoiding is a generic setting applied in a climate it was not chosen for.
- Why do so many economisers not work?
- Because their failures are invisible and nothing forces anyone to look. A damper actuator seizes, a linkage comes loose, an outdoor sensor drifts or sits in direct sun, a controller is left in a manual position after a service — and the building stays comfortable throughout, because the mechanical cooling simply runs more. The only symptom is a higher energy bill, which is attributed to the weather. Field studies of installed rooftop economisers have repeatedly found a large fraction not functioning correctly, and it is one of the better-documented gaps between design intent and operation in building services. The responses are the unglamorous ones: commission the economiser specifically rather than assuming it works, check the damper travel and the sensors on a schedule, and monitor for the signature of a failure — mechanical cooling running in mild weather.
- What is simultaneous heating and cooling?
- Exactly what it sounds like, and an economiser fault is one of the commonest causes. If a damper is stuck open in cold weather, the unit brings in outdoor air that is far below the supply temperature needed, and the heating coil then warms it back up — so the building is heating air it deliberately made cold, continuously, all winter. The reverse happens when a damper is stuck closed in mild weather and the compressor runs when free cooling was available. Both are pure waste, both are silent, and the first can be very expensive. The diagnostic signature is heating and cooling stages operating in the same period, which building management systems can be set to alarm on, and which is one of the highest-value checks available on an air handling system.
- Does the building need somewhere for the air to go?
- Yes, and it is the part most often under-designed. An economiser at full outdoor air is moving a large volume into the building, and unless an equal volume can leave, the building pressurises — which forces air out through every gap, makes doors difficult to open, can reverse the draught on any atmospherically vented appliance, and eventually limits the flow the fan can deliver. The provision is a relief or exhaust path sized for the full economiser flow: barometric relief dampers, a powered relief fan, or a return fan arrangement, controlled to maintain a slight positive pressure rather than an uncontrolled one. Systems retrofitted with economisers are the usual offenders, because the original design never had to shift that volume and the relief path was sized for the minimum outdoor air rate.
- Does an economiser help air quality?
- Substantially, as a side effect, and it is an under-counted benefit. Economising means running at far more than the minimum outdoor air rate for as many hours as the weather allows, which dilutes carbon dioxide, odours and indoor pollutants well beyond what the ventilation requirement alone would provide. That is a real benefit in occupied spaces and one of the reasons economisers are favoured in codes beyond their energy case. The qualification is that outdoor air brings what is in it: filtration has to be adequate for the volume being admitted, which is much greater than at minimum position, and in locations with poor outdoor air quality — near heavy traffic, during a wildfire smoke event, in high pollen season — the control strategy needs a way to override economising, which not all installations have.
- Is this the same as a water-side economiser?
- The same idea applied on the water side rather than the air side. An air-side economiser brings outdoor air into the building. A water-side economiser uses the cooling tower to produce chilled water directly when outdoor conditions allow, bypassing the chiller — either through a plate heat exchanger between the condenser and chilled water loops, or by a strainer-cycle arrangement. It suits buildings where bringing in large volumes of outdoor air is impractical or undesirable, including many data centres and laboratories, and it depends on wet-bulb temperature rather than dry-bulb, since a cooling tower works evaporatively. The economics are similar in shape: fan and pump power instead of compressor power, available for the hours the weather permits, with the chiller still required for the design condition.
- Does it replace the cooling plant?
- No, and expecting it to is a sizing error. An economiser reduces the HOURS the compressor runs; it does nothing about the design condition, which by definition is the hot weather when outdoor air is useless as a cooling medium. The plant is therefore sized for the design load exactly as it would be without one, and the economiser's value is entirely in operating energy rather than in capital. There is one related design interaction worth knowing: because an economiser lets a system meet mild-weather loads without the compressor, it changes the part-load profile the plant sees, which matters for equipment that performs badly at very low load. That is an argument for staged or variable-capacity plant alongside an economiser, not an argument for a smaller one.