Walk in the door with your CO meter already on. Ambient CO is the first reading of the visit, not the last.
What a standard alarm is built to do
Residential CO alarms listed to UL 2034 are designed to sound based on concentration over time: roughly 60–240 minutes at 70 ppm, 10–50 minutes at 150 ppm, and 4–15 minutes at 400 ppm. They are also designed not to sound at low concentrations, to avoid false alarms. That is a deliberate trade-off — and it means a home can sit at a low, chronic CO level with a silent alarm on the wall.
BPI ambient action levels
ANSI/BPI-1200 lays out ambient CO tiers for building analysts. From 9 to 35 ppm: tell the occupant, open windows and doors, and recommend turning off likely sources. The response escalates from there, and at 70 ppm or greater the inspection stops and the building is evacuated. Whatever standard you work to, have a written threshold in your head before you need it.
Low-level monitors
A low-level monitor shows readings well below the UL 2034 alarm points. I install the NSI 6000, which displays CO starting at 5 ppm. It is not a replacement for listed alarms — it is the instrument that lets a homeowner see a developing problem before it becomes an emergency, and it gives me a history to look at when I arrive.
The conversation
If the alarm is sounding or anyone has symptoms, the conversation is simple: everyone outside, call 911 from outside. Everything else — finding the source, combustion testing, the heat exchanger — comes after responders clear the house. Keep those two messages separate so nobody waits on an HVAC appointment in an emergency.
Record the ambient reading in every report, even when it is zero. Zero is a result.