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Commercial · LaboratoriesLaboratory HVAC — Fume Hood Exhaust & Make-Up Air
Laboratory HVAC, fume hood exhaust and make-up air are one calculation: a fume hood exhausts conditioned air continuously, and every litre it removes must be replaced with tempered outdoor air while the lab stays slightly negative to the corridor. A 1.8-metre bench hood with the sash open moves roughly 500 litres per second, about half the total airflow of a typical 5-ton rooftop unit, so the building's existing system cannot feed it. We install and service the exhaust fans, make-up air and cooling for small testing and dental labs in commercial units.
Size the replacement air from the hoods, not from the floor area. Add the exhaust of every hood, snorkel and vented cabinet, then supply about 90% of that total as heated and cooled outdoor air, so the remaining 10% enters from the corridor and keeps the lab negative. Supply diffusers go well away from the hood face and discharge at low velocity. A draft across the sash faster than about half the face velocity pulls fumes out of the hood, which is how a badly placed make-up air outlet can make containment worse.
The standards are specific and worth naming in your tender. CSA Z316.5 covers fume hoods and their exhaust systems in Canada, hoods are normally set for a face velocity of about 0.4 to 0.5 metres per second, and containment is proven with the ASHRAE 110 tracer-gas test performed by an independent certifier. Our part is delivering the airflow those tests depend on: the right exhaust volume at the hood collar, stable room pressure and calm air at the sash. The certifier then tests and labels the hood.
Exhaust construction is different from ordinary ductwork. The fan sits on the roof so every metre of duct inside the building is under suction, the duct material is matched to the chemicals, joints are sealed, and the stack discharges straight up at speed, clear of air intakes and neighbouring units. Hood exhaust is never tied into washroom or general exhaust. In a leased commercial unit this means a new roof opening, so the landlord and the municipal building department are part of the job from the first drawing.
Small labs also carry more heat than their size suggests. Ovens, furnaces, autoclaves, incubators and ultra-low freezers reject heat year-round, so cooling often runs in February and needs a low-ambient kit to do it. Dental labs add burnout furnaces, model-trimmer dust and acrylic vapour that want local exhaust at the bench. New lab systems are normally built to an engineer's stamped drawings; we install to them or team with an engineer through commercial HVAC design-build, and the commercial HVAC hub lists related services.
Equipment we install and service
A roof-mounted, corrosion-resistant fan with a vertical discharge stack and a duct run in stainless steel, coated steel or PVC, chosen for what the hood handles. One fan per hood is simplest in a small lab, because a shared manifold needs controls to keep each hood's flow steady. Repairs to belts, bearings and motors are covered under commercial exhaust fan repair.
A gas-fired or heat-pump unit that supplies outdoor air heated in winter and cooled and dried in summer, interlocked with the hood exhaust. It is the largest energy cost in the lab, since every litre is heated from outdoor temperature and thrown away minutes later. See make-up air units for how sizing works.
A rooftop unit or split system sized for equipment heat, with low-ambient controls so it can cool in winter. Recirculated air stays within the lab zone and is never shared with offices or neighbouring tenants.
A hood airflow monitor that alarms on low face velocity, and a simple differential-pressure indicator at the lab door showing the room is negative to the corridor. Both give staff a reading they can check every morning without calling anyone.
What goes wrong in these buildings
- The hood alarm goes off every time the rooftop unit's fan ramps up.
- The lab door whistles and will not latch when both hoods are running.
- Our certifier failed the hood on face velocity and said it was the building, not the hood.
- Gas for this one unit costs more than the whole office next door.
- The landlord wants drawings before we cut the roof and our equipment arrives in six weeks.
- It is 27°C at the benches in January with the ovens on.
How it works
We list each hood and vented device with its required exhaust, check the existing rooftop unit, gas and electrical capacity, and read room pressure. Then we tell you plainly which parts are ours and which go to a certifier, a cleanroom firm or an engineer.
New exhaust and make-up air need a building permit and usually an engineer's design, plus landlord approval for roof work. The written quote separates exhaust, make-up air, cooling and controls. Cost is set by total exhaust airflow, duct material, the route to the roof and any gas or electrical upgrades; the pricing page gives general guidance, but lab work is always quoted per site.
After installation we set exhaust and supply airflows, verify the room is negative and record the numbers. The independent certifier then runs face-velocity and ASHRAE 110 testing and labels the hood. For repeat visits or a failed fan, message or call the shop.
Common questions
Can a fume hood run off the existing rooftop unit?
Almost never. A typical rooftop unit in a commercial bay brings in a small fraction of its airflow as outdoor air, and one bench hood can exhaust half of that unit's total. Without dedicated make-up air the lab goes strongly negative, doors become hard to open, the hood's face velocity drops, and gas appliances nearby can backdraft. The hood needs its own replacement air, interlocked with its fan.
Does closing the sash save energy?
On a variable-volume hood, yes: the controls cut exhaust as the sash closes. On a constant-volume bypass hood, which is what most small labs have, no. The hood draws the same airflow through its bypass opening with the sash down. Closing the sash is still the right habit for safety, but the heating bill only drops if the exhaust system is designed to turn down.
Do you certify fume hoods or biosafety cabinets?
No. Hood certification to ASHRAE 110 and biosafety cabinet certification are done by independent certifiers, and they should stay independent of whoever installed the airflow. Containment laboratories, cleanrooms and perchloric acid hoods with wash-down systems are also specialist work we refer out. Our scope is exhaust fans and duct, make-up air, cooling, controls and the balancing that prepares the room for the certifier.
How often should lab exhaust and make-up air be serviced?
Twice a year suits most small labs: fan belts, bearings and motor checked, the make-up air burner and safeties tested before winter, filters changed, and the airflow monitor compared against a measured reading. Hoods are typically certified once a year by the certifier, and it makes sense to book our visit shortly before theirs so a worn belt does not cost you a failed test.
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