How to Screen for Spillage After You Add an Exhaust Fan

Why this matters

A new range hood, a bigger bath fan or a second dryer pulls air out of the house, and the house has to replace that air from somewhere. One of the easiest places for it to come from is backward down the vent of an atmospheric water heater or furnace, which is a pipe already open to the outdoors with a gap at the bottom. When that happens the appliance spills its flue gas, carbon monoxide included, into the house. The trade that installed the fan is rarely a gas technician, and that is fine: the job here is not to test the appliance, it is to find out whether your installation just made it spill under ordinary conditions, before you leave. This card is that screen. Why a house pulls flue gas down a vent belongs to How a House Pulls Flue Gas Back Down a Vent; what to do about a house that has lost part of its combustion safety margin belongs to Combustion Safety After a Partial Conversion.

Stop rules, before and during

• Gas odor at any point: everyone leaves the house, nobody touches a light switch, appliance control or phone inside, and you call the gas utility or 911 from outside.
• Your personal CO monitor climbs above the background you noted at arrival: the screen has failed. Turn off every exhaust device you turned on, open the nearest exterior doors, and follow the failed-screen steps below. Do not resume the screen.
• Your monitor reaches 9 ppm in living space: stop the test, shut off the appliances and fans you turned on, open doors and windows, and do not restart anything until a gas-qualified tech has found the source. The 9 ppm line is a shop default, not a regulation; set yours no higher.
• A CO alarm sounds, or the reading keeps rising after you ventilate: everyone leaves the house and you call the fire department or gas utility from outside.
• Soot, scorching or melted plastic at the draft hood, or a vent connector that is disconnected, rusted through or sloping downhill toward the chimney: do not run the screen at all. The appliance already has a problem. Route it to a qualified gas technician and tell the customer in writing.
• Any smoke pushed out of the draft hood during the screen: that is a fail. Follow the steps for a failed screen below and stop there.

What this screen is, and what it is not

It is a check under the conditions the household actually creates: windows and doors shut, every exhaust device running, the appliance firing. It is not a worst-case depressurization test, which a qualified gas technician runs by setting interior doors to their worst positions, measuring the appliance zone pressure with a manometer, timing the draft and measuring CO in the flue products. Do not attempt that test, adjust a burner, or open a gas valve. A pass on this screen means your installation did not make the appliance spill under ordinary conditions on the day you checked. It does not certify the appliance.

Which appliances it applies to. Atmospheric appliances, the ones with a draft hood (an open-sided collar where the vent connector meets the appliance), are the ones that spill. That is most tank gas water heaters and older furnaces and boilers. Sealed-combustion and direct-vent appliances draw combustion air through their own pipe and are far less susceptible; they have no draft hood to check. A fan-assisted furnace without a draft hood is not screened this way, but a water heater sharing its vent is. A fireplace or wood stove can also be pulled backward by exhaust; do not light a fire to test it, and route the question to a chimney professional.

The steps

1. Find every combustion appliance and its vent before you switch anything on. Walk the basement, utility closets, garage and attic. Note each appliance's type, whether it has a draft hood, and which room it sits in. Skip it and you screen the water heater and never notice the atmospheric boiler in the next room on the same chimney.

2. Put on a personal CO monitor and note the background reading. Write the number down. Skip it and you have no baseline to tell a rise from a reading the house always has.

3. Note the CO alarms. Look for a CO alarm on the level of the appliance and outside sleeping areas, and read the manufacture date on its back. Do not press its test button during the screen; you want it free to sound. A residential CO alarm listed to UL 2034 is built to stay quiet at low levels and short exposures so it does not nuisance-trip, so silence during a few-minute screen proves nothing. It is a backstop; your personal monitor is the instrument. Skip it and a missing alarm goes unmentioned on the day it mattered most.

4. Close the house as the household lives in it. Exterior doors and windows shut, fireplace damper as you found it, interior doors (including the door to the appliance room) in their usual positions. Write the positions down. Skip it and an open window relieves the house and you pass a screen the family will fail every winter evening.

5. Turn on every exhaust device. Your new device on its highest setting, the range hood on high, every bath fan, and the dryer on an air-only setting. Never run a whole-house fan with the house closed; it is designed to run with windows open and can pull a closed house hard enough to backdraft anything in it. Skip it and you screen your fan alone when the spill happens with your fan plus the dryer.

6. Fire the appliance with its normal control. Open a hot water tap for the water heater, or turn the thermostat up for a furnace or boiler. Use only the controls a homeowner would use. Wait until the main burner is lit.

7. At one minute after the burner lights, puff smoke at the draft hood relief opening, all the way around. Hold the smoke pencil at the edge of the opening, not inside it, and work around every side; spillage often starts at one edge first. Pulled up into the hood on every side is a pass for that check. Pushed out on any side is a spill. Check again at about two minutes. Skip the second check and a vent that starts weakly and then reverses as the house settles gets passed. See related: How to Use a Smoke Pencil to Read Air Direction.

8. Turn the exhaust off, then confirm the appliance drafts on its own. With the exhaust devices off and the appliance still firing, puff at the draft hood again after a minute. Then close the hot tap or return the thermostat to its original setting, and watch the burner shut off normally. Skip it and you do not know whether you left the appliance able to vent at all.

If the screen fails

Turn every exhaust device off, open exterior doors near the appliance, and recheck the draft hood with smoke a minute later. If it now draws on every side, let the appliance finish its cycle and return its control to where you found it. If it still spills with the exhaust off, turn the appliance down with its normal control (thermostat down, water heater to vacation or pilot), keep the doors open, and treat it as an appliance or vent problem, not a fan problem. Either way the job stops: tell the customer in writing what you saw, leave your new device switched off and labeled with their agreement, and call a qualified gas technician the same day.

Worked example: a 300 cfm hood that failed

A handyman replaces a recirculating range hood with a ducted under-cabinet hood rated at 300 cfm. At 300 cfm it sits under the 400 cfm point above which IRC M1503.6 (2021 edition, as adopted) requires makeup air, so no makeup air was required or installed. See related: Kitchen Hood Makeup Air. The house has an atmospheric gas water heater in a basement utility room whose door the family keeps shut, and a sealed-combustion furnace beside it.

The record, as written on the job:

• Appliances: water heater, atmospheric with draft hood, utility room, basement. Furnace: sealed combustion, not screened. No fireplace.
• CO alarms: one, upstairs hall, dated 7 years ago. None in the basement.
• Personal CO monitor background at arrival: 0 ppm.
• Positions: windows and exterior doors shut, utility room door shut (as normally kept), other interior doors open.
• Exhaust on 10:20: new hood high, 2 bath fans, dryer on air.
• Water heater fired 10:21 (hot tap open in kitchen).
• Check at 10:22: smoke pulled in on the back and both sides, pushed out along the front edge. Spill. Screen failed.
• Personal monitor in the utility room: 3 ppm at 10:23.

The failed-screen steps, as taken:

• 10:23: all exhaust devices off, utility room door opened, basement exterior door opened.
• 10:24: with exhaust off, smoke at the draft hood pulled in on all four sides at one minute after the change. Personal monitor back to 0 ppm by 10:27.
• 10:28: hot tap closed, burner shut off normally.
• The customer was told, and the job ticket says, that with the household's normal exhaust running the water heater spilled flue gas, that the new hood should not be used while the water heater could fire until a gas technician has assessed it, and that a CO alarm belongs in the basement. With the customer's agreement the hood's circuit was switched off and labeled. A qualified gas technician was called the same day.

What the handyman did not do, and should not have: run the hood alone to see if it passed by itself, prop the utility room door to find a combination that worked, or adjust anything on the water heater. Each of those is the diagnostic test that belongs to the gas technician, and running combinations yourself means repeating the spill.

What the reading actually shows. It does not show the hood is too big or the water heater is faulty. It shows the house cannot supply the hood, two fans and a dryer with a closed utility room door and still let the water heater vent. The fixes the gas tech may weigh include makeup air, a combustion air supply to the utility room, a different water heater, or a change in how the room door is kept. Choosing among them is their call.

What a pass does not prove

• It was a mild day. In cold weather the lower floor runs more negative from the stack effect, so a basement appliance that passed in October can spill in January. Note the outdoor temperature on the record.
• The vent was already warm. A vent that has been firing all morning drafts better than a cold one. If the appliance had just run before you started, say so.
• The doors will not always be where they were. A family that shuts the utility room in winter and leaves it open in summer gets two different houses.

Any of these is a reason to recommend a full combustion safety test by a qualified gas technician even after a pass, especially where the exhaust you added is large or the appliance is old.

What goes on the job record

Appliance list with vent types, CO alarm locations and dates, the CO background, door and window positions, the exhaust devices run, the times of each check and the smoke result on each side, the outdoor temperature, how the house was restored, and, on a fail, who was called and what the customer was told in writing. That record is what the gas technician starts from, and it is the shop's evidence that the screen was done.

References

• International Residential Code, 2021 edition, M1503.6 (makeup air for kitchen exhaust hoods), as adopted by your jurisdiction
• UL 2034, the listing standard for residential CO alarms, which sets their response at low and short exposures
• See related: How a House Pulls Flue Gas Back Down a Vent; Combustion Safety After a Partial Conversion
• See related: Counting Every Exhaust Device in the House; Carbon Monoxide Testing + Response Reference
• See related: The Range Hood That Backdrafted the Water Heater