What role do HVAC systems have in winter pest prevention?

It's easy to assume 'HVAC and pests' means something about the ductwork itself, or about the unit somehow attracting insects. The actual mechanism is simpler and more mundane: any HVAC system with an outdoor component — a central air conditioner, a heat pump — requires lines and conduit to physically pass through the building envelope, and that penetration is a hole by definition. Installers seal it, almost universally, with expanding foam, because foam is fast, cheap, and effective at the time of installation. The problem is what happens to foam over five or ten New Hampshire winters: it compresses, shrinks, and cracks, and a material that was once a snug seal becomes a gap with foam debris in it — which, notably, is itself soft enough to gnaw through if it hasn't already opened up on its own.

The durable version of this fix is the same principle as sealing any other gap: pack the opening with copper or steel mesh, then seal over it, so there's a hard material underneath rather than just foam. This is a straightforward DIY task for the wall or foundation penetration itself — the lines are usually accessible from outside near the outdoor unit.

Where this differs from a standard door or window gap is the outdoor unit's own vents and intake openings, which need airflow to function. Building science guidance specifically cautions against mesh finer than about half an inch by half an inch on these openings, because anything tighter can measurably restrict airflow and hurt the system's efficiency (Building America/PNNL). That's a case where excluding pests and preserving mechanical performance are in tension, and the fix is choosing the right mesh size for that specific opening rather than defaulting to the finer quarter-inch mesh used elsewhere in the house.

This is one of the few pages in this cluster where the industry has a genuine incentive to oversell, so it's worth naming plainly. An 'HVAC pest-prevention package,' an annual 'system pest-proofing' add-on, or duct fogging sold as a standalone service should all prompt the same question: what specific gap is being sealed, and with what material? The durable fix here is mechanical — metal mesh and sealant at a physical penetration — not a recurring chemical treatment applied inside ductwork. Pesticide circulated through an HVAC system is not a substitute for closing the hole, and any pitch that treats it as one is selling activity rather than exclusion.