Range Hood and Downdraft Venting Service

Purpose

A range hood is a duct with a fan on the end of it. The blower is rarely the fault and the duct almost always is, so a hood call answered by cleaning the filters and selling a motor comes straight back with the same complaint.

The second reason this is a standing instruction is the part nobody sees. A hood strong enough to work is strong enough to pull a tight kitchen negative, and a negative kitchen can reverse the draft on an atmospherically vented water heater in the same room. That puts combustion products where the family cooks, so a spillage and carbon monoxide check belongs in this procedure's acceptance, not in somebody else's trade.

Scope

Residential and light commercial cooking exhaust served by a wall-mount, under-cabinet, island or downdraft hood, including remote and in-line blowers: filters, blower and speed control, lighting, backdraft dampers at the hood and at the cap, the duct run and its termination, and the combustion-safety check that goes with running it.

It does not cover commercial hooded cooking with a fire suppression system and a grease duct, which is the commercial kitchen appliance service call SOP, the cooking appliance itself (the gas range burner and igniter service standard, or the induction and glass cooktop service standard), or lifting a hood out of finished cabinetry, which the built-in appliance removal SOP owns.

Roles and the handoff between them

The safety gate before the blower is touched

Shut the cooking appliance at its own controls and confirm it is off before anything goes over the cooktop. A dropped filter onto a live burner is a grease fire at head height. On any gas odor at any point, everybody leaves the building immediately, no switch touched, no light operated, no phone used inside, and the utility is called from outside.

Isolate the hood circuit and prove dead before the blower or light housing is opened. Open the branch breaker, lock and tag it, and prove dead at the hood's junction, meter proved on a known live source before and after per NFPA 70E-2021, 120.5, work practices per 29 CFR 1910.333(b)(2) for qualified persons under 29 CFR 1910.332 and 1910.399. A cord-connected hood gets unplugged with the plug kept in sight. If it will not prove dead, the isolation goes to an electrician.

Degreaser is a chemical exposure with more than one route. Alkaline cleaners injure on skin and eye contact, the spray aerosol is an inhalation route, and a hand that touched a soaked filter is an ingestion route at the next break. Follow the product label, which is the binding instruction: gloves and eye protection, no spraying into a running blower, wash before eating, and on a splash to the eye flush 15 minutes and get medical care. A label naming a respirator makes it a condition of use, and a respirator is a control only under a written program per 29 CFR 1910.134.

Wear a personal carbon monoxide monitor for every step where a burner runs. Our default action level is 35 ppm at breathing height, matching the NIOSH recommended 8 hour exposure limit. At that reading, shut the burners and the hood, open a window and a door, clear the kitchen, and do not restart until the spillage cause is found.

The procedure

1. Trace the duct end to end before any part is quoted. Establish ducted or recirculating, diameter, material, elbow count, flex length and where it terminates. Acceptance: a route sketch on the ticket, the termination physically found outdoors, and the diameter compared against the minimum the installation instructions state. Wrong looks like a duct disappearing into a soffit nobody follows. Stop rule: a duct terminating in an attic, soffit, crawl space or wall cavity is the finding; report it, do not sell a blower. Hazard: in attics and crawls, loose granular fill of unknown type is not disturbed, swept or walked through, because some vermiculite fill contains asbestos; back out and have it identified.

2. Isolate the hood, lock and tag, and prove dead at the hood junction. Acceptance: 0 V line to neutral and line to ground, meter proved live either side, or the cord unplugged and in sight. Wrong looks like a hood "off at the switch" with a live line still landing in the housing. Stop rule: a circuit that will not prove dead goes to an electrician. Hazard: the light and blower housings share a live compartment above the tech's head, so the cover comes off after the isolation is proved.

3. Pull, degrease and refit the filters, and inspect the first accessible duct length. Acceptance: filters free of standing grease and seated with no gap at the frame, first duct length free of heavy deposit. Wrong looks like a filter refitted backwards, leaving an open bypass at the frame. Stop rule: heavy grease beyond the first accessible length is a duct cleaning scope, quoted rather than reached into. Hazard: this step uses degreaser under the gate above, so gloves and eye protection are on and spray never goes into the blower housing.

4. Verify both backdraft dampers move freely and close. Check the damper at the blower outlet and the one at the wall or roof cap by hand. Acceptance: each opens under light finger pressure and falls fully closed under its own weight, with no paint bridge, bent blade or lint-packed screen. Wrong looks like a cap damper painted shut by a siding crew, which reads as a dead blower from inside. Stop rule: a damper that will not close is replaced before flow is measured, because the measurement is meaningless through a stuck one. Hazard: the exterior cap is reached from a ladder, set and secured before anything is carried up.

5. Measure the flow the installation delivers and compare it to the rating, not to the sound. Traverse the duct with a vane anemometer and convert velocity to flow using the duct's own area. On a downdraft add a capture check at the rear burner, because a downdraft fights the plume and can move air without capturing anything. Acceptance: measured flow within the fraction of rated flow the manufacturer's static pressure curve supports for this length, elbow count and diameter, that rating being a bench figure no installed duct delivers in full. Wrong looks like "it sounds strong", which is turbulence, not capture. Stop rule: flow well under rating on an undersized duct is a duct finding, and no blower is ordered. Hazard: the cooktop stays off and the anemometer goes at the duct, never over a burner.

6. Run the worst case for the house and check every atmospherically vented appliance in the space. Close exterior doors and windows, run any other exhaust fan and the dryer, hood on high, then fire the water heater or furnace and watch its draft diverter. Acceptance: no spillage at the diverter within the interval its instructions state after burner start, and CO below the 35 ppm action level at breathing height. Wrong looks like a match flame at the diverter pulled outward, or a monitor climbing while the hood runs. Stop rule: spillage or a monitor at the action level means hood and appliance both off, room ventilated, and a makeup air referral to a gas contractor before the hood is released to high. Hazard: this step deliberately creates the condition that puts combustion products in the room, so the monitor is on the tech, the test is short, and nobody puts their face in the plume.

7. Restore everything you turned off or opened, then verify the hood and re-verify the house. Refit filters and covers, remove the lock and tag, close the breaker, and return every appliance and fan to the position it was found in. Acceptance: every speed and the light operate, dampers open on start and close on stop, each appliance you shut is relit and drafting, and the step 6 check repeats clean as left. Wrong looks like a water heater left with its gas valve down after the draft test. Stop rule: an appliance that will not relight or draft is left off at its own control and goes to the gas contractor. Hazard: this is where power and fuel go back into a kitchen with the family in it, so the relight is watched to a stable flame and the monitor stays on until the last appliance is proved.

The record this produces

• Duct: diameter, material, elbow count, flex length, termination, and the minimum diameter the instructions require.
• Isolation: the prove-dead readings at the hood junction, or the unplugged confirmation.
• Dampers: each of the two, opens freely and closes fully, noted separately.
• Flow: measured velocity, the duct area used, the computed flow, and rated flow beside it.
• Combustion check: which appliances were tested, spillage at each, and the highest CO reading against the 35 ppm action level.
• Restore: every appliance and fan returned to its found position, each confirmed.

The duct sketch is what saves the next visit. Without it the next tech re-traces the run from the beginning.

One run of this standard, filled in

A 36 in wall-mount hood over a gas range in a tight townhouse, rated 600 cfm. Complaint: smoke fills the kitchen on anything seared.

• Step 1: 6 in round rigid duct, two 90 degree elbows, a 3 ft flex section at the blower, terminating at a sidewall cap. The instructions call for 8 in round minimum. Route sketched, and the recorded size is already a finding.
• Step 2: breaker open, locked and tagged. Hood junction reads 0 V line to neutral and 0 V line to ground, meter proved either side. Pass.
• Step 3: filters heavily loaded, degreased, refitted with no frame gap. First 3 ft of duct lightly coated only. Pass.
• Step 4: blower outlet damper opens under a finger and falls shut. Sidewall cap damper the same, screen clear. Pass.
• Step 5: traverse in the 6 in duct averages 1,150 fpm. A 6 in round duct is 0.196 sq ft, so 1,150 times 0.196 is 225 cfm. Against the rated 600 cfm that is 37.5 percent of the rating, on a duct under the 8 in minimum the instructions state. Fail. Stop rule taken: duct finding, no blower ordered, the quote is the duct correction.
• Step 6: run anyway, because the house has to be characterised before the correction. Doors and windows shut, dryer running, hood on high, water heater fired. No spillage within the interval its instructions state, CO peaked at 4 ppm, well under the 35 ppm action level. Pass, and the reason matters: at 225 cfm the house is nowhere near the 400 cfm makeup air trigger the IRC and IMC carry at M1503.6 and 505.2, in the edition the jurisdiction has adopted.
• Step 7: filters and covers back, lock and tag off, breaker closed, dryer returned to off, water heater left drafting. Both speeds and the light operate, dampers cycle, spillage and CO check repeated clean. Pass.

The correction is where the two findings meet. Taking the duct to 8 in round moves installed flow toward the rating, and the rating is 600 cfm, above the 400 cfm trigger. So the duct correction and the makeup air provision are one scope, quoted together, and the step 6 check reruns after it. A shop that fixes only the duct hands the customer a hood that finally works and a water heater that now spills.

References

• The hood's installation instructions, which own the minimum duct diameter, maximum equivalent length, rated flow and the static it was rated at.
• The 400 cfm makeup air trigger at IRC M1503.6 and IMC 505.2, in the edition your authority having jurisdiction has adopted, which binds the installation and reaches the shop through the permit.
• NFPA 70E-2021, 120.5 for live-dead-live proving, with work practices at 29 CFR 1910.333(b)(2) for qualified persons under 29 CFR 1910.332 and 1910.399.
• The NIOSH recommended exposure limit for carbon monoxide, source of the 35 ppm action level here, and the label for whichever degreaser the van carries.
• See related: the gas range burner and igniter service standard, and the built-in appliance removal SOP.