Safety Stop During a Depressurization Test With a Combustion Appliance
You are running a blower door depressurization test on a 1,500 square foot house with a natural-draft gas water heater in an interior closet and a gas furnace in the attached garage. The house is at -50 Pa and holding for a multipoint series. While the fan runs, your combustion analyzer in the water heater closet reads 180 ppm carbon monoxide in the flue, rising, and the draft gauge shows -1 Pa at the vent connector where you expect a healthy negative draft. The water heater burner is firing. The homeowner is standing in the kitchen with two small children, and the outdoor temperature is 20°F. Your test series still needs two more points to complete the curve.
1. State your immediate actions in order and justify each one.
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This is a stop-work condition. Act before collecting any more data.
- Stop the blower door and restore the house to neutral pressure immediately. Depressurization is the cause of the problem: at -50 Pa the house is pulling harder than the water heater's natural draft can overcome, which spills flue gases into the closet. Removing the depressurization removes the driving force.
- Shut off the water heater burner (and the furnace if it is firing) so no more CO is produced while you assess. Do not relight until the appliance has been checked.
- Ventilate and evacuate the immediate area. Open the closet and a nearby exterior door, keep the homeowner and children out of the kitchen and away from the closet, and let the space clear. 180 ppm in the flue with spillage into the space is an acute exposure risk, and children are more susceptible.
- Re-measure. After the house is neutral and the appliance is off, check ambient CO in the occupied space and re-check draft once the appliance is safely restarted under controlled conditions.
- Do not resume the multipoint series. The two missing points are irrelevant compared with an appliance that backdrafts under normal test conditions.
Justification: A natural-draft appliance in an interior closet is exactly the configuration that fails under depressurization. The -1 Pa draft reading and rising 180 ppm confirm spillage. Continuing the test would deliberately maintain the hazardous condition.
2. The homeowner says the house "passed" a safety check last year and asks why you are making a fuss. Explain the finding in plain terms and what it means for the house.
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Plain-language explanation: Your water heater needs to pull its exhaust up a chimney the way a straw pulls liquid. That only works if the house is at least as pressurized as the outside, or close to it. When I put the house under a strong negative pressure for the test, the house started sucking air - and it sucked exhaust from the water heater back into the closet instead of up the flue. My instruments measured 180 parts per million of carbon monoxide in that flue and showed the draft had reversed.
Why last year's check does not settle it: A safety check done under normal conditions may not have included a depressurization test, or may have been done when the closet door was open, the dryer was off, or the wind was different. The failure I found is triggered by a specific condition - a strong negative pressure in the house - and that condition also happens in real life: a big exhaust fan, a dryer, a range hood, or a windy day can all do it. So this is not just a test artifact.
What it means for the house: The water heater's venting is marginal and can reverse. That is a health risk, especially for children. It needs to be corrected - options include improving the combustion air supply to the closet, sealing the closet from the conditioned space and providing dedicated outdoor air, or replacing the natural-draft unit with a sealed-combustion or power-vented appliance. Until it is corrected, the appliance should not be operated with large exhaust fans running.
What I am not saying: I am not saying the house is unsafe every minute. I am saying there is a condition under which it becomes unsafe, and I found it.
3. What must go in the written report, and what should you do about completing the blower door test the client paid for?
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In the report:
- The safety finding, prominently - not buried in an appendix. State the flue CO reading (180 ppm and rising), the draft reading (-1 Pa at the vent connector), the test condition (-50 Pa house depressurization), and the appliance (natural-draft gas water heater in an interior closet).
- The action taken - test stopped, appliance shut off, area ventilated, occupants kept clear.
- The recommendation - appliance to be evaluated and corrected by a qualified technician before further use under depressurization; specific options such as dedicated combustion air, closet isolation, or replacement with a sealed-combustion unit.
- A clear statement that the blower door test is incomplete and why, with the number of points collected and the fact that the series was abandoned for safety.
- The ambient CO check result in the occupied space after ventilation.
- Client acknowledgment - a signed note that the finding was communicated verbally on site, with the date and time.
Completing the test: Do not resume the same day with the appliance in its current condition. Options are to return after the appliance is corrected and verified, or to run the test with the appliance positively shut off and locked out - but only if the shutoff is documented and the client understands the house is being tested in a non-typical state. If the appliance is shut off for the test, note that in the report so the result is not confused with a normal-operating-condition test.
Billing and expectations: Tell the client the test is incomplete for safety reasons and explain what is needed to finish it. Do not quietly bill for a full test that was not completed, and do not present a partial curve as a finished result.