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Frequently Asked Questions
What makes a make-up air unit different on a roof?
It handles outside air only.
Nothing is recirculated at any point.
At design conditions it works on real weather.
The units beside it see a tempered mixture.
That difference drives the whole specification.
Why is freeze protection a live concern?
The coil sees ambient rather than a mixture.
A water or glycol coil is genuinely at risk.
Return air is not there to temper anything.
Low-limit protection becomes a requirement.
A recirculating unit rarely faces this.
What arrangements protect the coil?
Glycol concentration matched to the climate.
Face-and-bypass dampers on the coil face.
Low-limit thermostats that stop the unit.
Drain-down provisions where they suit the design.
Several are usually used together.
Why does turndown matter more than peak capacity?
Design conditions occupy very few hours.
Most of the year needs a fraction of capacity.
A unit that cannot modulate will cycle.
Cycling overshoots and delivers uncomfortable air.
Minimum stable capacity deserves close attention.
What should be checked on the heating section?
The minimum firing rate, not only the maximum.
A wide modulation range suits variable weather.
Staged heating gives coarser control than modulation.
Shoulder seasons expose a narrow range quickly.
Ask for the turndown ratio explicitly.
Why do duct runs and insulation matter here?
Because the unit serves several areas at once.
Ventilation air is distributed, not thermostatically zoned.
Runs are longer than a single-zone unit needs.
Uninsulated duct loses the conditioning it just paid for.
Discharge setpoint then affects the whole building.
How does it sit alongside the other rooftop units?
As one machine among several on the same roof.
It supplements them rather than replacing them.
Structural provision is planned with the rest.
So are electrical and gas services.
Access for service is arranged for all of them together.