Combustion Safety & Indoor Air Quality Diagnostics

Why Tightening a House Requires Combustion Safety Testing

Older, leakier homes rarely put a strain on combustion appliances. Enough outside air moves through gaps around windows, rim joists, and attic penetrations that a furnace, water heater, or fireplace almost never has to compete for the air it needs to draft properly. Once that leakage is closed up — weatherstripping added, top plates sealed, chases and can lights air-sealed — the passive air supply those appliances quietly depended on can disappear along with it.

When a combustion appliance can't draw enough makeup air, it can backdraft: instead of pulling exhaust up and out through the flue, the appliance pushes combustion byproducts, including carbon monoxide, back into the living space. This is the single most important safety check that gets skipped when air sealing is treated purely as an efficiency project. We treat combustion safety testing as a mandatory bookend to every sealing job — run before work begins to establish a baseline, and again after sealing is complete to confirm the finished house still breathes safely.

This is also why we never sign off on an air-sealing scope in isolation. A blower door number alone tells you how tight the envelope became — it says nothing about whether the appliances inside that envelope can still operate safely. Pairing the two tests is the only way to know both halves of the story.

Carbon Monoxide, Gas Leak & Appliance Draft Testing

Beyond the worst-case scenario, we take direct readings at the source. Every combustion appliance in the home — furnace, boiler, water heater, range, and any fireplace or fixed space heater — is checked individually for ambient carbon monoxide and spillage under both normal and depressurized conditions. Readings are taken at the draft hood, not just in the room air, so a marginal appliance doesn't get missed because the room happens to be well ventilated that day.

We also inspect every accessible gas line fitting — shutoff valves, union connections, and appliance connectors — for leaks, and verify draft strength at each flue connection to confirm exhaust gases are moving in the right direction at the right rate. Any appliance that fails a draft or spillage check is flagged immediately, with the specific reading and location documented in the report, before any sealing work is allowed to proceed further.

Turning a Hazard Into a Documented Pass

Every combustion safety report we issue documents pre- and post-sealing conditions side by side, giving you and your contractor a clear paper trail proving the finished work made the home safer, not riskier. If a test reveals a marginal or failing appliance, we flag it before sealing work proceeds, not after the crew has already packed up and the house is closed in.

That sequencing is non-negotiable in how we schedule work: air sealing and combustion safety testing happen together, never sealing first with a safety check added later as an afterthought. A house that trades a lower energy bill for an unmonitored hazard isn't a successful retrofit — the whole point of instrument-based testing is proving both outcomes at once: a lower ACH50 number and a clean combustion safety pass, documented in the same visit.

Combustion Safety & IAQ FAQs

Related Diagnostics

Combustion safety testing is one part of a full diagnostic picture.