Kitchen and Bath Exhaust Interaction Survey

Purpose

Exhaust devices get sold and installed one at a time and then operate as a group. A shop that only ever looks at the fan it was called about will report a bath fan as fine while the house has a 600 cfm hood, a dryer and no makeup air path, which is the combination that pulls a water heater backward on a cold still evening.

The other thing it fixes is the gap between rated and delivered. A label figure is measured on a test rig at a stated resistance. What the room gets depends on the duct somebody actually ran, and the difference is routinely half.

Scope

Covers the survey visit on residential and small light commercial buildings: device inventory, termination inspection, per-device airflow measurement at the grille, range hood capture geometry, per-configuration house pressure, the makeup air determination, restoration and findings.

Does not cover the worst-case depressurization and spillage test or the combustion air sizing calculation, which both belong to the combustion air verification SOP that this survey hands off to. Does not cover installing a hood or a makeup air system, or commercial kitchen hoods under fire suppression, which is a different trade.

Roles and responsibilities

Procedure

1. Inventory every exhaust device with its rating and its duct route. Include range hood or downdraft, every bath and laundry fan, the dryer, and any whole-house or attic fan. Acceptance: each device listed with rated cfm from its label or certified rating, plus duct diameter, material, approximate length and termination type. What wrong looks like: an inventory that stops at the fan the customer called about. Stop rule: a device with no findable rating is listed as unrated rather than guessed at. Hazard: none at this step, a walk-through with a notepad.

2. Inspect every termination from outside before measuring anything. Acceptance: with the device running the damper opens fully, with it off the damper closes, the discharge is to outdoors, and the screen or louver is clear of lint and grease. What wrong looks like: a bath fan discharging into a soffit or attic, a moisture finding that outranks every airflow number on the sheet. Stop rule: a stuck damper is cleared before the flow reading, since measuring through it describes the blockage, not the fan. Hazard: ladder and roof work, so the roof access SOP governs it, and grease-laden terminations are cleaned with the fan off and gloves on rather than reached into while running.

3. Measure delivered airflow at each grille, one device at a time. Acceptance, two gates: measured flow at least 70 percent of rated cfm, and at or above the local exhaust rate required by the ventilation standard your jurisdiction has adopted, which for most residential work is ASHRAE 62.2 in the edition your building code names; the 2019 edition sets intermittent local exhaust at 100 cfm for a kitchen and 50 cfm for a bathroom, with lower continuous alternatives. What wrong looks like: reporting the label figure because the fan is audibly moving air. Stop rule: below either gate, the device is not reported as adequate and the duct is traced before anything is quoted. Hazard: the flow hood is held rather than balanced on a toilet tank or a cooktop.

4. Check the range hood on capture geometry, not only on flow. Acceptance: the hood covers the cooking surface with at least the manufacturer's overhang, mounting height inside the manufacturer's stated range, and a plume from a steaming kettle on a back burner drawn in rather than rolling out at the front. What wrong looks like: a hood mounted high enough to look good and too high to catch anything, which measures its rated flow and still leaves grease on the ceiling. Stop rule: a hood outside its mounting range is a finding even where the airflow passes. Hazard: the capture check uses kettle steam, not an open flame or a smoke pellet, over a cooktop that stays off apart from the kettle.

5. Record house pressure per configuration, and do not add the readings together. Take house-to-outdoor pressure with each device alone, then with everything running, house closed. Acceptance: one figure per configuration in Pascals, each labeled with what was running. Pressures do not sum: devices reading minus 6.5, minus 2.0 and minus 4.0 alone will not read minus 12.5 together, because each one's own flow falls as the house goes negative. What wrong looks like: an all-on figure calculated rather than measured. Stop rule: an all-on reading past the limit for any vented appliance hands off to the combustion air verification SOP before this survey closes. Hazard: the all-on configuration is the depressurized condition, so where a vented appliance is present the zone stays clear of people and pets and a CO monitor runs while it is held.

6. Determine whether makeup air is required, from the adopted code and the rated capacity. Acceptance: the threshold from the code your jurisdiction has adopted, commonly stated at 400 cfm in the IRC and IMC family, compared against the kitchen exhaust system's rated capacity, with a note on whether your authority applies it to rated or measured flow, since that varies. What wrong looks like: a 600 cfm hood with no makeup air path and nobody writing it down. Stop rule: a required makeup air path that does not exist is a written finding whether or not the customer wants it addressed. Hazard: none at this step, a code lookup at the truck.

7. Restore every device and prove the house is back where you found it. Switch off everything you ran, confirm every damper closed, reopen doors and windows, and where a vented appliance is present fire it under normal conditions with the CO monitor in that zone. Acceptance: all dampers closed, house pressure back within a Pa of the pre-survey reading, appliance drafting with no spillage after its establishment time, 0 ppm CO. Hazard: this step hands a normal-operating house back to the family, so the appliance is watched while it fires, and any spillage or CO ends it with the gas valve closed, the zone opened to outdoor air, the appliance tagged and the service manager phoned from the property. Stop rule: a damper that will not close is reported, never left for a homeowner to notice in January.

8. Write the findings as a ranked list with the numbers beside them. Acceptance: a list where each finding names a device, a measured value and the gate it failed, ordered by what it costs the building, with moisture and combustion findings above airflow shortfalls. What wrong looks like: a findings list that reads as a quote, with the biggest item first because it is the biggest item. Stop rule: a combustion or moisture finding stops any quoting on that visit. Hazard: none at this step, a conversation at the kitchen table.

When the site does not match

Downdraft cooktop rather than a hood: the step 4 capture check does not apply and the flow reading moves to the downdraft's own grille, but step 6 still runs on rated capacity. Dryer that cannot be run: record it as untested and say so, because it is often the largest single exhaust in the house. Recirculating hood: steps 2, 3 and 6 do not apply, and the finding is that the kitchen has no local exhaust at all, written that way rather than as a pass.

The record this produces

The survey sheet carries: every device with rated cfm, duct size, material, length and termination type; the termination result per device, including damper open and closed; measured cfm per device with both gate results; hood overhang, mounting height and capture observation; house pressure per configuration with what was running in each; the makeup air threshold with its code edition and the rated capacity it was compared against; the restoration result including the appliance draft check; and the ranked findings.

Two people use it. The estimator, who can size a makeup air path or a duct correction off measured flows instead of label figures. And the next tech asked to add a hood, a fan or a ventilator to this house, because the all-on pressure reading is the number that says whether there is any room left.

Worked pass: 2,200 square foot house, island hood, two bath fans, gas water heater

Natural draft water heater in the basement. Customer complaint is grease film on the kitchen ceiling.

Step 1. Island hood rated 600 cfm on 6 in duct, about 22 ft to a sidewall cap. Two bath fans rated 80 cfm each. Electric dryer on 4 in duct.

Step 2. Kitchen cap found with a spring damper glued half shut with grease, and the 6 in duct reduced to a 4 in cap. Damper cleaned and a 6 in cap fitted before any flow reading was taken. Bath terminations open and close correctly and discharge to outdoors.

Step 3. Before the cap work, the hood measured 285 cfm; 285 divided by 600 is 0.475, so 47.5 percent of rated, under the 70 percent gate. Step 3 fails and takes its stop rule: the hood is not reported as adequate and the duct route is traced, which is what found the reduction. After the cap change the hood measures 430 cfm; 430 divided by 600 is 0.717, so 71.7 percent, above the 70 percent gate, and well above the 100 cfm kitchen local exhaust rate. Bath fans measure 62 and 58 cfm against 80 rated, which is 77.5 and 72.5 percent, both above the gate and both above the 50 cfm bathroom rate.

Step 4. Hood overhang covers the front burners with the manufacturer's stated overhang, mounting height 32 in above the cooktop against a stated range of 30 to 36 in. Kettle plume on the back burner is drawn in cleanly. Pass, and the grease film is explained by the 285 cfm the hood was actually delivering before today.

Step 5. House closed. Hood alone on high: minus 6.5 Pa. Both bath fans alone: minus 2.0 Pa. Dryer alone: minus 4.0 Pa. All four together: minus 9.5 Pa, measured, not the minus 12.5 the individual figures would suggest if they added, which they do not. Minus 9.5 Pa is past the limit for the natural draft water heater, so this survey hands off to the combustion air verification SOP, booked before leaving.

Step 6. Kitchen exhaust rated capacity is 600 cfm, above the commonly cited 400 cfm threshold in the adopted code family, and there is no makeup air path. Written as a finding with the edition noted.

Step 7. All devices off, every damper confirmed closed from outside, house pressure back to minus 1.5 Pa against a minus 1.5 Pa pre-survey reading. Water heater fired under normal conditions, no spillage at 60 seconds, 0 ppm CO.

Step 8. Findings ranked: first, no makeup air path for a 600 cfm rated hood in a house with a natural draft appliance, with the minus 9.5 Pa all-on reading beside it; second, the kitchen duct reduction, corrected today, with 285 cfm before and 430 cfm after; third, bath fans passing but near the low end of their ratings.

References

• ASHRAE 62.2, in the edition your jurisdiction has adopted through its building code, for local exhaust rates in kitchens and bathrooms
• The mechanical or residential code your jurisdiction has adopted, in the IRC and IMC family, for the kitchen exhaust makeup air threshold and how your authority applies it
• Manufacturer's installation instructions for the hood, for overhang, mounting height range and duct sizing
• See related: the combustion air verification SOP, which owns the worst-case depressurization and spillage test this survey hands off to, and the roof access and fall protection SOP for the termination inspection