The Tight House That Started Setting Off the CO Alarm
A 1,600 ft² single-story ranch built in 1974. A 40-gallon natural-draft gas water heater sits in a closet opening directly onto the central hallway. The 80% atmospheric furnace is in the attached garage and its return pulls from that same hallway. A weatherization crew air sealed the attic floor and rim joists and replaced the windows; blower door went from 2,150 CFM50 to 1,480 CFM50. Two weeks later the owner reports headaches and a stale smell, soot streaking at the water heater's draft hood, and a CO alarm that sounded once in the hallway. A worst-case CAZ test with the dryer, both bath fans, and the kitchen fan running showed −4 Pa in the water heater closet relative to the hallway, −1 Pa draft in the flue, and 12 ppm CO at the closet door. No combustion testing was done before the work, and none since.
1. In house-as-a-system terms, how does the weatherization work connect to the CO alarm? Be specific about what the blower door numbers do and do not prove.
Read the worked answer
Tightening changed the pressure and dilution budget of the house, and the water heater was living on the margin of that budget.
- Before: at 2,150 CFM50 the envelope leaked enough that exhaust fans and stack effect were partly fed by random infiltration. Some of that air also washed the closet and diluted spillage.
- After: at 1,480 CFM50 the shell no longer backfills the fans, so the hallway - and the closet hanging off it - goes negative when the dryer and three fans run. The closet reads −4 Pa relative to the hallway, which means the closet is the combustion appliance zone and it is being starved.
- The flue at −1 Pa is a marginal, unreliable draft. A natural-draft appliance needs a stable negative draft in the vent connector with no spillage at the draft hood after several minutes of operation. −1 Pa in a space that is itself −4 Pa is a spillage signature, and the 12 ppm at the door corroborates it.
- What blower door does not prove: CFM50 is an envelope leakage number. It says nothing about CAZ depressurization, flue draft, or spillage. Those require a separate worst-case depressurization test and combustion analysis. The crew measured the envelope and never measured the zone the appliance breathes from - that is the system error, and it is why the 'before' numbers would not have predicted this either.
2. Rank your immediate actions, and name one thing you would specifically not do.
Read the worked answer
- Life safety first. Confirm working CO alarms on every level and near sleeping areas; if there is any doubt, the appliance gets shut off and tagged before anything else is discussed. Headaches plus a soot signature plus spillage is not a 'monitor it' situation.
- Isolate the appliance - shut the gas off at the water heater, tag it, and tell the owner in writing not to relight it.
- Finish the diagnostic the first crew skipped: worst-case CAZ depressurization with the furnace firing too, draft and spillage at the draft hood, and combustion analysis (CO air-free, O2, stack temp) before and after any corrective action.
- Look at the second appliance. The garage furnace returning from the hallway is a separate pathway - return ductwork running through a garage can pull garage air into the airstream, and the furnace shares the same starved pressure field.
- Then design the fix, not before.
What I would not do: I would not 'solve' it by cutting a transfer grille from the hallway into the closet, cracking a window, or removing the closet door and calling it combustion air. Those are uncontrolled, weather-dependent, and unverifiable. Combustion air has to be a designed, sized, permanent opening or a sealed-combustion appliance.
3. The owner refuses to replace the water heater. What are your real options, and what goes on the work order?
Read the worked answer
Ranked by how reliably they solve the problem, not by cost:
- Best: replace with a direct-vent (sealed combustion) or power-vent unit. Sealed combustion removes the CAZ from the house entirely and is the only option that does not depend on the owner maintaining a pressure balance.
- Workable: keep the atmospheric unit but give it a dedicated, sized combustion air duct from outdoors to the closet, plus a louvered door with adequate free area, and verify by re-testing worst-case depressurization and draft with everything running. If the numbers still fail, this option is off the table.
- Marginal: remove the closet door and open the space, which increases the volume but does not fix a house that goes negative under fan load.
- Not acceptable: relocating it into the garage or attic without redoing the CAZ analysis - you have just moved the problem.
Work order must contain: the measured worst-case CAZ pressure, flue draft, ambient and undiluted CO readings, the date and conditions, the CO alarm installation and test, the specific recommendation made, the owner's refusal in her own words, and her signature. Also record that the water heater remains tagged off. If the owner insists on restoring it without corrections, that is a documented refusal, not a completed job.