Title 24 Economizer Requirements
Section 140.4(e) requires an economizer on air conditioners and heat pumps with a design total mechanical cooling capacity over 33,000 Btu/hr. The threshold is on design capacity, and it applies to heat pumps as much as to air conditioners.
The threshold, precisely
Over 33,000 Btu/hr of design total mechanical cooling capacity. Below it, no economizer requirement from this provision. Above it, one applies — and because the test is on design capacity rather than physical size, a modest-looking unit can cross the line. It is read off the equipment schedule, not estimated from tonnage in conversation.
Heat pumps are named too
The provision covers air conditioners and heat pumps. Choosing a heat pump for the heating side does not remove the cooling-side obligation, which is the single most common misconception about this requirement.
What an economizer does, and why the code wants one
It uses outdoor air for cooling when conditions allow, instead of running mechanical cooling. In a climate with long mild shoulder seasons that is a large amount of avoided compressor work, which is why the requirement attaches at a fairly modest capacity rather than only to large plant.
The acceptance test covers more than the economizer
The 2025 form NRCA-MCH-05-A covers economizer controls together with dedicated outdoor air systems and heat and energy recovery ventilation. So a project with an economizer and an HRV is not necessarily producing two separate certificates — but it is producing a test performed by a technician certified for mechanical acceptance testing.
One form is required for each system demonstrating compliance, so four rooftop units with economizers produce four.
On VAV systems the controls go further
For variable-air-volume systems the prescriptive route follows ASHRAE Guideline 36, which specifies an air economizer under direct digital control alongside static pressure setpoint reset and supply air temperature reset. On those systems the economizer is part of a defined control sequence rather than a standalone device.
Where it goes wrong
Sequences, not hardware. An economizer physically present but not controlling as the sequence requires fails the test, in the same way that outdoor air controlled by a fixed minimum damper position fails the outdoor air test. The equipment is the easy part.