Combustion Air Verification in a Tight House
Purpose
Free area on a grille is a design input, not proof. A utility room can carry openings that satisfy the fuel gas code on paper and still pull an atmospherically vented water heater backward the moment a dryer and a range hood run in a house that has been air sealed since it was built. The only acceptance that means anything is a measured pressure in the appliance zone with the house at its worst case, and an observed draft at the appliance while it is that negative.
Written down, that measurement is also the shop's defense: it says the appliance drafted under conditions worse than the family will ever create.
Scope
Covers verification on residential and small light commercial buildings with one or more fuel-burning appliances inside the thermal boundary: safety screening, appliance inventory, the code calculation, baseline and worst-case pressure measurement, the spillage and CO observation, and restoration.
Does not cover designing or building the remedy, the appliance repair itself, chimney relining, or the full IAQ assessment visit, which runs a version of this test inside a broader survey. Where an ERV or exhaust device is being added, this procedure runs before that install, not after.
Procedure safety screen, before anything else
Walk in with a personal CO monitor already running, clipped at breathing height, before the door opens. If anyone smells gas, everyone leaves the building immediately, no switch is touched, no light is turned on, no phone is used inside, and the call to the gas utility or fire service is made from outside. If the monitor shows a sustained reading in the tens of ppm, the same evacuation applies. Neither of those is a reason to start testing; they are a reason to be outside.
Roles and responsibilities
| Role | Owns | Hands off |
|---|---|---|
| Dispatcher | Booking this as its own visit, asking how many exhaust devices the house has | Passes the appliance list from the last service so the tech knows what is in the zone |
| Technician | The safety screen, steps 1 to 8, every pressure reading, the decision to red-tag | Phones the service manager from the property on any spillage or CO finding |
| Service manager | Whether the finding becomes a proposal, a utility referral, or a red tag that stands | Passes any red tag to the office the same day with the appliance and reading named |
| Office | Filing the worst-case reading as the building's record | Passes it forward so any later ventilation install is checked against it |
Procedure
1. Inventory every fuel-burning appliance in the zone with its input rating and venting type. Acceptance: each appliance listed with input in Btu per hour off its rating plate and its type as natural draft, fan-assisted, power vented or direct vent, since the pressure limit differs by type. What wrong looks like: a list built from memory, missing a gas dryer or a fireplace. Stop rule: an unreadable plate means manufacturer's data, not an estimate. Hazard: this means kneeling beside operating appliances, so the CO monitor stays on your person and hot vent connectors are not touched.
2. Define the combustion appliance zone and how it connects to the rest of the house. Acceptance: the zone described, with the position of every door that separates it and any return duct or transfer grille tying it to the house. What wrong looks like: treating a utility closet as its own zone when a return leak makes it part of the whole house. Stop rule: open return seams inside the zone are a finding recorded before testing continues. Hazard: none at this step, observation with a flashlight.
3. Run the code calculation for combustion air and record it as an input, not a verdict. Use the fuel gas code your jurisdiction has adopted, NFPA 54, also published as ANSI Z223.1, or the IFGC where that is the adopted document. In current editions the standard indoor-air method calls for 50 cubic feet per 1,000 Btu per hour of combined input, and the two-permanent-openings method sizes each opening direct to outdoors at 1 square inch per 4,000 Btu per hour, with a different rate for horizontal ducts. Acceptance: required volume or free area computed, and existing free area measured with the grille's free area percentage applied rather than its gross size. What wrong looks like: calling an 8 by 8 grille 64 square inches when its louvers and screen pass a fraction of that. Stop rule: none here, since passing this calculation does not release the appliance, the test does. Hazard: none at this step, arithmetic at the truck.
4. Take the baseline pressure in the zone with respect to outdoors, everything off. House closed, exhaust off, air handler off, interior doors normal. Acceptance: a baseline in Pascals written with the wind condition, because a gusty day moves this number. What wrong looks like: a baseline taken with the dryer running, which hides the effect you are hunting. Stop rule: wind swinging the reading more than a couple of Pa means reschedule, not average. Hazard: the reference hose runs outdoors through a door, so route it where nobody trips and no door closes on a finger.
5. Set the house to its worst case and record what you turned on. Close the house, run every exhaust device at once, meaning range hood, bath fans, dryer and any whole-house fan, then run the air handler and try interior doors both ways to find the position that makes the zone most negative. Acceptance: a written list of which devices run and which doors are closed, beside the reading, because that configuration is what the number means. What wrong looks like: a worst-case number with no configuration beside it, which cannot be reproduced. Stop rule: a device that will not run is noted as untested, not assumed harmless. Hazard: you are deliberately creating the condition that makes an appliance spill, so occupants and pets stay out of the zone and the test is never left running while you are elsewhere.
6. Record the worst-case depressurization and compare it against the limit for that appliance type. Acceptance: the most negative reading found, in Pascals, checked against the depressurization limits table in the protocol your program certifies against, commonly BPI-1200. That table is per appliance type and the natural draft limit is commonly stated at minus 5 Pa, so pull it rather than working from one remembered figure. What wrong looks like: one limit applied to a house with both a natural draft water heater and a direct vent furnace, which sit far apart. Stop rule: exceeding the limit is a finding whether or not the appliance spills. Hazard: the negative condition is still running, so the same rule applies about who is in the zone.
7. Fire the appliance under worst case and watch for spillage with the CO monitor in the zone. Acceptance: no spillage at the draft hood or barometric damper after the appliance's normal establishment time, and no CO rise in the zone. Hazard, the sharpest here: you are running a combustion appliance in the condition most likely to make it vent indoors, so the zone stays clear of people and pets, you stay with it, and the run is aborted at the first CO rise by closing the gas valve and opening the zone to outdoor air. Stop rule: sustained spillage or any CO rise ends the test, the appliance is shut off at its gas valve and tagged, the occupant is told plainly it is not to be used, and the service manager is phoned from the property.
8. Restore the house and prove the appliance drafts normally again before you go. Turn off every exhaust device you ran, reopen the doors, return the air handler and appliance controls to the settings you found, and remove the reference hose. Acceptance: the appliance fires under normal conditions with no spillage after its establishment time, 0 ppm CO in the zone, and zone pressure back within a Pa of the step 4 baseline. What wrong looks like: a house left with a bath fan running and a water heater set to a temperature nobody chose. Stop rule: an appliance that spills under normal conditions is red-tagged whatever the worst-case test showed. Hazard: this step returns fuel and normal operation to a house with people in it, so the occupant is walked to the appliance and told what was changed.
When the site does not match
Occupant will not allow the dryer or range hood to run: the test proceeds with what is permitted and the sheet records what was excluded, because a worst case missing the biggest exhaust device is not one. Appliance out of gas or locked out: steps 1 to 6 run, the sheet is marked incomplete at step 7, and a return visit is booked rather than a verbal all-clear. Zone is a vented crawlspace or garage: without a stable boundary the test is reported as not performed.
The record this produces
The verification sheet carries: arrival CO; every appliance with input, venting type and location; the zone description with door positions; the code calculation with its edition, required figure and measured free area; baseline pressure with wind; the worst-case pressure with the full list of devices running and doors closed; the limit compared against and where it came from; the spillage and CO result at each appliance; and the step 8 restored result.
Two people use it. Whoever quotes the remedy, who needs the worst-case number and its configuration to size anything. And the next tech proposing a range hood or ventilator here, because adding exhaust to a building with a marginal reading on file is how a shop creates the event it was hired to prevent.
Worked pass: 1,900 square foot house, 40,000 Btu/h water heater and 80,000 Btu/h furnace
Utility room in the basement, 12 by 10 with an 8 ft ceiling. Water heater is natural draft, furnace is fan-assisted. Two grilles to outdoors, 8 by 8 nominal, metal louver with insect screen.
Safety screen. CO monitor reads 0 ppm on entry, no gas odor. Step 1. Water heater 40,000 Btu/h natural draft, furnace 80,000 Btu/h fan-assisted, combined input 120,000 Btu/h. Step 2. Zone is the utility room behind a solid door to the finished basement, no return duct inside it, return seams checked and tight.
Step 3. Standard indoor-air method: 50 cubic feet per 1,000 Btu/h times 120 is 6,000 cubic feet required. Room volume is 12 times 10 times 8, or 960 cubic feet, so indoor air alone does not qualify and the openings carry the load. Two openings direct to outdoors: 120,000 divided by 4,000 is 30 square inches each. Each grille is 64 square inches gross at 60 percent free area, which is 38.4 square inches, above the required 30. The openings satisfy the calculation. Recorded as an input, not a verdict.
Step 4. Baseline with everything off and doors normal: minus 1.0 Pa, wind light and steady. Step 5. Worst case set: range hood on high, two bath fans, dryer on, air handler running, utility room door closed, configuration written out in full. Step 6. Most negative reading found is minus 8.0 Pa with that door closed. Against a limit commonly stated at minus 5 Pa for the natural draft water heater, that is 3.0 Pa past the limit.
Step 7. Water heater fired under worst case. Smoke at the draft hood rolls out and is still rolling at 60 seconds. CO monitor rises off zero in the zone. Step 7 fails and takes its stop rule. Run aborted, gas valve closed, zone opened to outdoor air, appliance tagged, occupant told plainly it is not to be used, service manager phoned from the property.
Step 8. Exhaust off, doors reopened, air handler to auto, hose removed. Zone pressure back to minus 1.2 Pa, within 1 Pa of the minus 1.0 baseline. Furnace fires under normal conditions, no spillage, 0 ppm CO. The water heater stays tagged and off, recorded as the reason step 8 could not be completed for that appliance.
Retest after the remedy, a passive makeup air path plus relocating the dryer termination: the same worst-case configuration measures minus 3.5 Pa, and the water heater fires with no spillage at 60 seconds and 0 ppm CO. Tag removed, both readings kept on file.
References
- NFPA 54, also published as ANSI Z223.1, or the IFGC, in whichever edition your jurisdiction has adopted, for the combustion air methods and opening sizes
- BPI-1200 or the equivalent field protocol your program certifies against, in its adopted edition, for worst-case depressurization limits by appliance type
- Appliance manufacturer's installation instructions, which can impose combustion air requirements beyond the code minimum
- 29 CFR 1910.1000, Table Z-1, whose 50 ppm eight-hour limit for carbon monoxide runs to you as an employee rather than to the homeowner beside you, whose protection is a listed CO alarm and your ordinary duty of care
- See related: the kitchen and bath exhaust interaction survey and the CO alarm response visit SOP