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Frequently Asked Questions
What happens without make-up air?
The building finds it anywhere it can.
Air comes in through doors and gaps.
Doors become hard to open.
Extraction hoods stop capturing properly.
Combustion appliances can spill.
That last one is a safety matter.
What separates the options in this group?
How far the incoming air is tempered.
Non-tempered is filtered outside air only.
Indirect gas-fired heats behind a heat exchanger.
Direct gas-fired burns in the airstream itself.
Evaporative cooling handles the summer case.
The scale runs from nothing to fully conditioned.
What is the dividing question in the family?
Whether combustion products enter the supply air.
Direct-fired units put them into the space.
That is why they are so efficient.
It is also why their use is restricted.
Indirect-fired units flue them outside.
Everything else follows from that choice.
How is a unit sized?
Against the exhaust it is replacing.
Total extraction sets the requirement.
Slightly less make-up keeps the space negative.
That suits kitchens and process areas.
Slightly more pressurises against infiltration.
Decide the intended pressure deliberately.
Should the space be positive or negative?
It follows what the space contains.
Kitchens stay slightly negative to contain odours.
Clean areas stay positive to exclude contamination.
Neutral is rarely the right answer.
The balance decides where air actually moves.
State it as a design requirement.
Where does the unit usually sit?
On the roof, close to what it serves.
That keeps ductwork short and cheap.
It needs a curb and structural support.
Gas supply and flue routing follow.
Intake position must avoid exhaust discharge.
Short-circuiting is a common siting error.
What controls does it need?
Interlock with the exhaust it replaces.
Running exhaust without make-up defeats both.
Modulating temperature control saves energy.
Freeze protection matters in cold climates.
Filter pressure monitoring is worth having.
Confirm interlocks at commissioning.