The Range Hood That Backdrafted the Water Heater
Why this matters
A kitchen remodel finished in October. In late November, two hours into holiday cooking with the new hood on high, the CO alarm at the basement stairs went off. Nobody on the remodel crew touched a gas appliance, and the remodeler's first instinct was that the alarm or the old water heater was the problem. Both were reporting, not causing. The hood, the air sealing the owners had done that summer, and an atmospheric water heater in the basement had combined into a house that pulled outdoor air down the water heater's flue whenever the hood ran hard. This case follows what happened in order: getting the family safe, keeping the house safe until the fix, then working out why.
First, the house: what happened in the first hour
Everyone out. The owner did the right thing: the family left the house, called 911 from the driveway, and waited. The fire department metered elevated CO in the basement, ventilated the house, and asked the gas utility to come out. The utility found the water heater spilling, shut it off at its gas valve and tagged it. That tag is the containment. It stays until a qualified gas technician clears the appliance, and nobody from the remodel shop removes it, relights the pilot, or tells the owner it is probably fine.
Why this order. CO gives no warning a person can sense, and its symptoms (headache, nausea, sleepiness) look like a long day of cooking. An alarm that sounds during a high-load event is to be treated as real until a meter says otherwise, and the meter belongs to the responders, not to whoever happens to have one in the truck. See related: How a House Pulls Flue Gas Back Down a Vent.
The interim the shop set. When the remodeler got the call the next morning, they told the owners three things in writing: the water heater stays off until the gas tech clears it; do not run the hood above its lowest speed until the makeup air question is settled; keep the CO alarms where they are and add one outside the bedrooms if there is not one there. Cold showers for a few days is the cost of the tag, and the owners were told it was the right cost.
The house before the remodel
The details mattered, so the shop collected them before anyone tested anything:
- Two-story house, about 2,400 sq ft, 8 ft ceilings, so roughly 19,200 cubic feet of conditioned volume including the basement.
- Air sealing in July. An energy auditor's report: 11 air changes per hour at 50 Pa (ACH50) before, 4.2 ACH50 after attic and rim joist sealing. See related: Blower Door Numbers Explained for Other Trades.
- Water heater: 40-gallon atmospheric gas, draft hood, vented into the old masonry chimney. The furnace had been replaced years earlier with a sealed-combustion unit vented in plastic pipe, which left the water heater alone in a flue sized for two appliances.
- The new hood: an island hood rated 900 cfm. Rated flow is measured at little or no static pressure, and a real installation with a duct run, a wall cap and a tight house delivers less. The old hood had been a 250 cfm under-cabinet fan.
- No makeup air was in the remodel scope or the mechanical permit.
What the gas technician measured
The remodeler did not run a combustion test. That is a gas technician's work, and the shop called the HVAC contractor it subs gas work to. With the owners out of the kitchen, the tech set the house to the condition of the event and read the basement where the water heater sits (the combustion appliance zone, or CAZ; see related: The Combustion Appliance Zone and the Door That Closes It):
| Condition | Basement pressure, with reference to outdoors | Water heater draft |
|---|---|---|
| All fans off | about -1 Pa | established within about a minute of firing |
| Hood on high | about -11 Pa | spilled continuously at the draft hood rim |
| Hood on high, kitchen window open 6 in | about -3 Pa | established, slower |
The tech's verdict was that the water heater could not vent against the house with the hood running, and that the hood depressurized the house far past the few pascals a natural-draft appliance in that situation can tolerate under the testing standard the tech works to. The window line is not a fix. It is the diagnosis: give the hood a path for its air, and the house stops taking that air from the flue.
SECTION: KITCHEN OVER BASEMENT, HOOD ON HIGH
hood fan --> exhaust out the wall cap
^
| kitchen: house pulled to about -11 Pa
----+-------------------------------- first floor
^ air rises from the basement to
| replace what the hood throws out
|
basement chimney flue
| v | outdoor air drawn
draft hood | v | DOWN the flue
+----+ <---------+ | instead of flue
| WH | spill: flue gas | gas going up
| | pushed out at |
+----+ the hood rim |
--------------------------------------- slab
Why it happened now and not in July
The owners asked the obvious question: the water heater had been fine for twelve years. The answer is two changes multiplying each other.
Tightening makes the same fan pull harder. A house's leakage follows a curve, and a common working form is flow equals a coefficient times pressure raised to about 0.65 (the house's own exponent is on the blower door report; 0.65 is a typical value). From the auditor's numbers:
Volume 19,200 cu ft
Before: 11 ACH50 x 19,200 / 60 = 3,520 cfm at 50 Pa
After: 4.2 ACH50 x 19,200 / 60 = 1,344 cfm at 50 Pa
50 Pa ^ 0.65 = 12.7
Coefficient before 3,520 / 12.7 = 277
Coefficient after 1,344 / 12.7 = 106
Pressure to pass 500 cfm of hood air:
P = (500 / C) ^ (1 / 0.65)
Before: (500 / 277) ^ 1.54 = about 2.5 Pa
After: (500 / 106) ^ 1.54 = about 10.9 Pa
The 500 cfm is an estimate of what a 900 cfm-rated hood actually moves in that installation, not a measurement, chosen because it reproduces the -11 Pa the gas tech read. On the same hood, the air sealing alone raised the depressurization about 4.4 times (10.9 divided by 2.5). Recheck from the other end: 1,344 is 0.38 of 3,520, and 1 divided by 0.38, raised to the 1.54, is about 4.4. The two routes agree.
The hood got bigger. The old 250 cfm fan, in the tightened house, would have needed (250 / 106) raised to 1.54, about 3.7 Pa. Marginal for an orphaned water heater on a cold flue, but a different world from 11.
Either change alone might have been survivable. The remodel stacked the bigger fan on top of the tighter house, and the scope never asked about either. See related: Why Air Sealing Changes What a House Needs for Ventilation.
Who owned which part of the fix
The remodeler owned makeup air. The IRC requires makeup air for a kitchen exhaust hood capable of more than 400 cfm, in substance: an equal flow brought in, with a damper that opens automatically when the hood runs (M1503.6 in the 2021 edition, as adopted by your jurisdiction; earlier editions number it differently, and the provision has local amendments in some places, so check the current edition). A 900 cfm hood is over that line by its rating, whatever it actually delivers. The remodeler sold and installed the hood, so the makeup air was the remodeler's scope from the start and it was the remodeler who paid to add it. The HVAC contractor designed and installed it: a ducted outdoor air inlet with a motorized damper interlocked to the hood, discharging near the kitchen, with heat added so winter air does not dump into the room cold. See related: Kitchen Hood Makeup Air, which covers the sizing and the choice of tempering.
The HVAC contractor owned the water heater. An orphaned atmospheric water heater in an oversized masonry chimney is a weak venter even without a hood, because the flue is too big and too cold to build draft quickly. The owners chose to replace it with a direct-vent unit, which takes combustion air from outdoors and vents through its own sealed pipe, so it no longer competes with the house for air at all. Relining the chimney to size was the other option the tech offered.
The owners owned the interim and the alarms. They kept the hood at low speed, kept the window cracked when it ran, and added a CO alarm outside the bedrooms.
The return to service. Before the tag came off, the gas tech ran the same test again with the new water heater and the makeup air in place: hood on high, the basement read about -2 Pa against outdoors, and the new direct-vent water heater was unaffected by it. The tech also confirmed the makeup air damper opened every time the hood started and closed when it stopped, because a damper that sticks shut puts the house back where it was. Only then did the gas tech clear the appliance and the shop close the job.
The hood check the shop added to every remodel
The remodeler's lesson was not about this house. It was that nobody on a kitchen job had been asking the questions that would have caught it. The intake checklist for any job that installs or upsizes a kitchen exhaust now carries four lines:
- Hood rated capacity. Over 400 cfm triggers makeup air in the scope and the mechanical permit, in jurisdictions that have adopted that provision.
- Every fuel-burning appliance in the house and how it vents. Any draft hood or natural-draft appliance, anywhere in the house, gets a combustion safety test by a qualified gas technician after the hood goes in, at the hood's highest speed.
- Recent air sealing, insulation, or window replacement. If the house was tightened in the last few years, the existing appliances were last commissioned in a different house.
- CO alarm coverage, recorded at the start of the job, with the owner told where alarms are missing.
The checklist costs a few minutes at the walkthrough. The salesperson who ran it on the next job found an atmospheric water heater and a homeowner who wanted a 1,200 cfm hood, and wrote makeup air into the quote before anyone ordered anything.
What would have pointed somewhere else
A spill that persists with the hood off. Then the problem is the vent itself: a blocked flue, a disconnected connector, a cracked liner. The hood would be a coincidence, and the chimney or the connector is the first thing the gas tech opens.
An alarm that sounds with no cooking and no fans. A failing or end-of-life alarm chirps rather than sounding its full alarm pattern; a full alarm in a quiet house is a reason to look at every combustion source, the attached garage included. See related: The Car Warm-Up That Set Off the CO Alarm Upstairs.
A sealed-combustion water heater from the start. Then the hood would still have pulled the house negative, and the complaint would have shown up as a whistling door, a smoking fireplace, or radon, not CO. The makeup air requirement would generally still apply; check whether your adopted edition or local amendment carries any exception.
References
- International Residential Code, 2021 edition, M1503.6 (makeup air for kitchen exhaust over 400 cfm), as adopted by your jurisdiction
- See related: Kitchen Hood Makeup Air
- See related: How a House Pulls Flue Gas Back Down a Vent
- See related: The Combustion Appliance Zone and the Door That Closes It
- See related: Counting Every Exhaust Device in the House