Quick answer: Free cooling uses cool outdoor air or water to remove data center heat so the compressors in chillers or CRAC units can run less or switch off. There are three main types: direct air-side (filtered outdoor air goes into the white space), indirect air-side (outdoor air cools indoor air through a heat exchanger without mixing), and water-side (cool outdoor conditions chill the water loop through a heat exchanger). The savings depend on how many hours a year the climate allows it. Direct air-side brings the most savings but also outdoor humidity and contamination into the room, so it needs humidity and filtration control.
Quick answer
- Direct air-side: cheapest to run in cool, dry climates; manage humidity, dew point and pollutants.
- Indirect air-side: keeps outdoor air out; smaller savings, simpler humidity control.
- Water-side: works with chilled-water plants; check water use and treatment.
The three types compared
| Direct air-side economizer | Indirect air-side (air-to-air exchanger) | Water-side economizer | |
|---|---|---|---|
| How it works | Filtered outdoor air is supplied to the IT space; hot return air is exhausted or mixed | Outdoor air and indoor air pass through a heat exchanger, sometimes with evaporative cooling on the outdoor side | A heat exchanger lets cool condenser water chill the chilled-water loop, bypassing the chiller |
| Outdoor air in the room | Yes | No (except leakage) | No |
| Humidity impact | Indoor humidity follows outdoor air unless treated | Indoor moisture largely isolated | No direct impact |
| Main limits | Outdoor temperature, humidity, pollutants, smoke | Heat-exchanger approach temperature, fan power | Wet-bulb or dry-bulb temperature, water use |
| Typical fit | New facilities in cool, dry or moderate climates | Sites with poor outdoor air quality or humid climates | Existing chilled-water data centers |
ENERGY STAR describes air-side economizers as bringing outdoor air to the servers and exhausting warm air, with mixing and filtration as needed, and water-side economizers as cooling the chilled-water loop through a separate heat exchanger (ENERGY STAR, air-side economizers; ENERGY STAR, water-side economizers).
When free cooling runs
Free cooling is a mode, not a separate building. The control system switches between:
- Full free cooling: outdoor conditions alone meet the supply temperature.
- Partial free cooling: outdoor air or water pre-cools, and the chillers or DX units trim the rest.
- Mechanical cooling: outdoor conditions are too warm or (for direct air-side) too humid or polluted.
Raising the IT inlet temperature within the allowed range increases free-cooling hours, often the biggest single lever. The US DOE's Federal Energy Management Program covers economizers and other energy-efficient data center design practices (DOE FEMP, Best practices guide).
Humidity limits with free cooling
ASHRAE's thermal guidelines for data processing environments give a recommended inlet temperature of about 18–27 °C for most IT equipment, with humidity limits expressed as a dew-point range (about −9 °C to 15 °C) and an upper RH limit. Equipment classes have wider "allowable" ranges. Check the edition and class your equipment vendors use.
What this means for each type:
| Type | Humidity risk | Control |
|---|---|---|
| Direct air-side | Humid outdoor air raises dew point; cold, dry air lowers it; rain, fog and smoke events | Switch to recirculation when outdoor dew point is out of range; dehumidify or humidify the make-up air; filter |
| Indirect air-side | Low; indoor moisture comes from leakage and people | Seal the building; small dehumidifier for make-up air |
| Water-side | None directly | Normal CRAH humidity control |
Contamination matters as much as humidity: ASHRAE's guidance on gaseous and particulate contamination warns that corrosive gases combined with high humidity accelerate corrosion of electronics (ASHRAE, Gaseous and particulate contamination guidelines). Direct air-side systems near industry, coasts or wildfire areas need good filtration and a fast switch to recirculation.
Free cooling beyond data centers
Free cooling is any cooling that uses a cold ambient source instead of a compressor. The same idea is used in industry:
- Free-cooling chillers: an air-cooled chiller with an extra coil (dry cooler) that pre-cools the returning water or glycol whenever outdoor air is colder than the return. Below a certain outdoor temperature the compressors switch off entirely; in between, the coil and compressors share the load ("hybrid" or partial free cooling).
- Dry coolers and cooling towers: for process water, machine cooling and plastics or food plants with year-round cooling loads.
- Evaporative free cooling: water evaporation on the outdoor side lowers the air temperature towards the wet bulb, extending free-cooling hours in dry climates (more water use).
- Enclosure free cooling: fans or air-to-air exchangers cooling electrical cabinets with ambient air; see fan-filter vs sealed cooling for the humidity risk.
Estimating the savings (illustrative)
- IT load 1,000 kW; chiller plant uses about 0.25 kW of electricity per kW of cooling in mechanical mode.
- The climate allows full free cooling for 3,000 hours a year and partial (half load on compressors) for 2,000 hours.
- Saved compressor energy ≈ 1,000 × 0.25 × (3,000 + 0.5 × 2,000) ≈ 1,000 MWh a year, minus the extra fan or pump energy for the economizer.
Use hourly weather data for your site, the supply temperature you can accept, and the heat exchanger's approach temperature to count real hours.
Adding free cooling to an existing data center
- Check the cooling plant: chilled-water plants can usually add a water-side economizer (plate heat exchanger) between the condenser and chilled-water loops.
- Raise supply temperatures within the ASHRAE range; every degree adds free-cooling hours.
- Improve airflow: hot and cold aisle containment so warmer supply air still keeps inlets in range.
- Controls: automatic switching between full, partial and mechanical modes, with dew-point limits for any outdoor air.
- Direct air-side retrofits need space for dampers, filters and relief air; often only practical in new builds or major refits.
Benefits and trade-offs
| Benefit | Trade-off |
|---|---|
| Lower compressor energy and peak demand | Fans, pumps and controls still use power |
| Lower PUE in suitable climates | Savings depend on the local climate hours |
| Less chiller wear | Direct air-side needs filtration and humidity management |
| Water-side uses existing chilled-water plant | Cooling towers use water; evaporative indirect units also use water (DOE FEMP, cooling water efficiency) |
Related: DOE FEMP, cooling water efficiency.
Where dehumidification fits
Free cooling handles heat; humidity needs its own control:
- In direct air-side systems, treating the outdoor air for dew point when it is humid keeps the white space within range and extends the hours the economizer can run.
- In sealed rooms, a small dedicated dehumidifier controls moisture from leakage and people, independent of the cooling mode.
- For battery rooms and energy storage next to data centers, the humidity problem is different; see data center vs BESS humidity.
Our ceiling-mounted and ducted dehumidifiers and desiccant dehumidifiers can treat make-up air or room air.
FAQ
Frequently asked questions
What is a free-cooling chiller?
A chiller with an extra air-to-water coil that cools the return water whenever outdoor air is cold enough, switching compressors off fully or partly.
What is the difference between direct and indirect free cooling?
Direct brings filtered outdoor air into the room; indirect cools room air through a heat exchanger so outdoor air never enters.
What is free cooling in a data center?
Using cool outdoor air or water to remove heat so mechanical cooling can be reduced or switched off.
What is free cooling in HVAC?
Using cool outdoor air or water to cool a building or process instead of running compressors, through an economizer, a dry cooler or a free-cooling chiller.
What is refrigerant free cooling?
A system where refrigerant circulates between an indoor coil and an outdoor condenser by natural circulation (thermosiphon) or a small pump when it is cold outside, so the compressor can stop.
Can evaporative cooling be used for free cooling?
Yes: evaporative or adiabatic coolers extend free-cooling hours in dry climates by cooling the air or water below the dry-bulb temperature, at the cost of water use; see indirect evaporative cooling.
Is free cooling really free?
No. Fans, pumps, filtration and controls still use energy, but much less than compressors.
Which free cooling type is best?
Direct air-side saves the most in cool, dry, clean-air climates. Indirect air-side suits polluted or humid sites. Water-side suits existing chilled-water plants.
Does free cooling affect humidity?
Direct air-side brings outdoor humidity into the room and needs dew-point control. Indirect and water-side systems keep outdoor air out.
Sources
- ENERGY STAR, Use an air-side economizer; Consider water-side economizers.
- US DOE FEMP, Best practices guide for energy-efficient data center design; Cooling water efficiency opportunities.
- ASHRAE TC 9.9, Thermal guidelines for data processing environments; Gaseous and particulate contamination guidelines.
