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An economizer is only useful when outdoor air can do real cooling work. The trouble is that outdoor air is often helpful without being sufficient. On a mild day, it can carry part of a building's cooling load but not all of it. What the control system does in that in between condition is the difference between a non-integrated economizer and an integrated one, and it decides how much of the available free cooling a building actually captures.
The Old Either/Or Approach
A non-integrated economizer treats outdoor air as a switch. Below a changeover point, the economizer works alone and mechanical cooling stays off. Above that point, the economizer gives up and mechanical cooling takes over completely, with outdoor air pulled back to the minimum needed for ventilation.
The weakness is the gap between those two states. When outdoor air is cool enough to help but not cool enough to meet the whole load, the control sees an unmet demand and hands the entire job to the compressors. The partial free cooling that was available, sometimes a large share of the load, is simply thrown away.
What Integration Changes
An integrated economizer removes that gap. The economizer and mechanical cooling are allowed to run at the same time, with outdoor air doing as much of the work as it can and mechanical cooling supplying only the remainder.
This is not just a design preference. Energy standards, including ASHRAE 90.1, require economizers to be integrated with the mechanical cooling system and capable of providing partial cooling even when additional mechanical cooling is needed. The requirement is not new either. Compliance checklists tied to older editions of the standard already listed integration as a prescriptive item, though exceptions exist, and trade guidance notes, for example, that certain single zone systems may use mixed air temperature control instead.
Integration Is a Spectrum, not a Switch
In practice, "integrated" covers a range of behaviors. One commonly cited classification describes five levels.
Non-integrated. Economizer alone below changeover, mechanical cooling alone above it.
Time delay integration. The economizer runs alone for a set period. If the space is still not satisfied when that time expires, mechanical cooling joins in.
Alternating integration. The economizer serves as the first stage of cooling and mechanical cooling as the second stage, so the compressor starts when the thermostat calls for the second stage.
Partial integration. The economizer is paired with multi stage or variable speed mechanical cooling, with the dampers adjusting outdoor airflow as mechanical cooling comes on.
Full integration. The economizer works with a chilled water coil whose output can be modulated smoothly to any level, using a differential changeover between outdoor and return conditions.
The pattern across these levels is worth noticing. The more finely the mechanical cooling can modulate, the more completely the economizer can be blended with it. A fixed stage DX system can only add compressor capacity in coarse steps, while a modulating chilled water coil can fill in whatever small gap the economizer leaves, which connects directly to the chilled water versus DX comparison covered elsewhere in this library.
The Three Operating Modes in an Air Handling Unit
For an airside economizer with proper integration, operation typically moves through three modes as outdoor conditions change.
Modulated economizer mode. The outdoor and return air dampers adjust to blend the two airflows and match cooling output to the load, with no mechanical cooling running at all.
Integrated economizer mode. Once outdoor air is cool but the dampers are fully open to one hundred percent outdoor air and the load is still not met, mechanical cooling joins to supply the shortfall.
Mechanical cooling mode. When outdoor air passes the high limit shutoff discussed earlier in this series, the dampers return to minimum outdoor air and mechanical cooling carries the load.
These modes are not individually prescribed by the standard. They emerge from sensible sequencing, and the sequence also has to avoid wasteful simultaneous heating and cooling, which is one reason the control logic needs real attention.
The Same Idea at the Plant Level
Integration applies to waterside economizers in chilled water plants too. A waterside economizer, often a free cooling heat exchanger paired with a cooling tower, uses cold outdoor conditions to cool the chilled water without running a chiller.
In a non-integrated arrangement, the economizer is either carrying the load alone with the chiller off, or it is off. In an integrated arrangement, the free cooling heat exchanger precools the chilled water returning to the plant, so the chiller only has to remove what is left. Both operate together, and the chiller sees a reduced load. This is why integrated waterside economizers are valuable on days when the cooling tower can produce cool water but not cool enough to carry everything.
A practical detail worth knowing is that when a chiller is running at partial load in this arrangement, its condenser still needs the manufacturer's minimum flow to maintain head pressure control. Reducing condenser flow because the chiller is lightly loaded can create its own problems. Waterside economizer performance also depends heavily on the approach temperatures discussed in the approach temperature article, since a smaller approach means more hours when free cooling is available. The 2013 edition of the standard, for example, expects waterside economizers to be capable of carrying the full expected load at roughly fifty degrees dry bulb and forty-five degrees wet bulb or below.
Why Integration Is Not Free
If integration captures more free cooling, why would anyone choose a non-integrated sequence? Because blending two cooling sources introduces real control and equipment challenges.
The most important is what happens to DX equipment. When an economizer carries most of the load and mechanical cooling only supplies a small remainder, a DX system may be asked to run at a very low load, cycling on and off. That kind of operation can lead to unstable refrigerant system behavior and coil frosting, which is a known reason some designs deliberately avoid integrated operation in certain conditions. Later editions of the standard added provisions aimed at making integration workable with DX equipment, including limits on artificially loading the system and minimum cooling stages for certain single zone systems, depending on capacity. The refrigerant side of this problem gets its own treatment in the next article of this series.
Integration also demands more of the control system. Sequencing dampers, compressor stages, and changeover logic so they cooperate, instead of fighting each other, takes careful programming and commissioning, and sensor accuracy matters even more than in a simple either/or scheme.
The Practical Takeaway
A non-integrated economizer treats free cooling as all or nothing, which wastes the partial cooling that outdoor air can provide on mild days. An integrated economizer lets free cooling and mechanical cooling share the load, capturing that partial benefit, which is why standards require it in most cases. The degree of integration depends heavily on how finely the mechanical cooling can modulate, and the benefits come with real obligations, particularly for DX systems, where low load operation can undermine the very efficiency integration was meant to deliver.
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