HVAC economizers, and why so many of them are broken

What an economizer actually does

When the air outside is cool enough to help, the economizer opens a damper and brings that air in rather than running the compressor. The building gets cooled by the weather instead of by electricity.

New Jersey's climate makes this worth a great deal. A large share of the cooling hours in spring and fall happen on days when outside air is already cool enough to do most of the work, and on those days a functioning economizer means the compressor barely runs.

The control logic decides when outside air is suitable, using temperature or a combination of temperature and humidity. When it works, it is the closest thing to free cooling a commercial building has.

Why the failure is invisible

A seized economizer damper does not break the building. The unit still cools, because it falls back on mechanical cooling and runs the compressor instead. Nobody is uncomfortable, nobody calls anyone, and no alarm goes off.

That is exactly what makes it expensive. Every other significant HVAC failure announces itself — a space gets warm, a tenant complains, a technician is dispatched. This one produces a bill that is higher than it should be, month after month, with nothing to attribute it to.

The linkages seize, the actuators fail, the sensors drift, or somebody disconnected the whole thing years ago during a service call because it was producing nuisance faults. All four are common, and none of them generate a work order.

How we check it during a survey

It is a physical inspection: whether the dampers still modulate through their full travel, whether the actuator drives them, and whether the sensors are reading plausibly. It takes minutes per unit and is one of the first things looked at.

The reason it comes so early in a survey is that it changes the energy model substantially. A rooftop unit that has been running mechanical cooling on mild days for a decade has a much worse baseline than its nameplate suggests — and a worse baseline means a larger modeled saving, which means a larger incentive.

It is one of the clearest cases where the poor condition of existing equipment works in the building's favor when a project is assessed.

Repair or replace

On a unit with useful life left, an economizer is a repair. On a unit that is already past its service life, it is one more reason the replacement models well. We do not perform repairs — if that is the answer, a service contractor is who you want.

The honest split is by age and condition. If the rest of the unit is sound, fixing the economizer is the cheap, correct answer and there is no project here. If the unit is twenty years old, the seized economizer is not a standalone problem, it is a symptom of a machine at the end of its life — and it is worth folding into the replacement rather than spending money on a component of something that is going to be removed.