Furnace Maintenance

MERV (Minimum Efficiency Reporting Value) comes from ASHRAE Standard 52.2, the test method that rates general-ventilation air-cleaning devices by how well they remove particles of specific sizes — not by brand claims or marketing language. The EPA recommends choosing a filter rated MERV 13 or higher where a system can accommodate it, noting that filters rated MERV 13 and above are required to demonstrate at least 50% removal efficiency for the smallest particles tested under that standard.

Higher isn't automatically better for every furnace, though. A filter's MERV rating and the static pressure it adds to the system are linked — a point covered in more depth on this site's furnace installation page — so the right MERV for a given furnace also depends on the filter rack it's installed in.

Change frequency depends more on filter type than on the calendar. A thin, 1-inch fiberglass or pleated filter loads up faster and generally needs checking monthly during the heating season, with a change roughly every one to three months depending on the rating. A deeper 4-to-5-inch media cabinet filter has far more surface area to load particles onto before it restricts airflow, and manufacturers of that format commonly rate them for six to twelve months between changes.

Pets, dust from renovation work, a home with more foot traffic, and simply running the furnace more in a cold winter all shorten that interval. The EPA's general guidance is straightforward: check filters regularly and replace them as needed rather than on a fixed schedule alone, since a visibly gray or clogged filter is already restricting airflow before a calendar reminder would tell you to change it.

A gas furnace and an oil furnace share plenty of maintenance items — filter, blower, safety controls, heat exchanger inspection — but the combustion side of the service is genuinely different equipment and genuinely different failure modes.

Combustion analysis with a calibrated analyzer is the common thread on both fuels: it measures carbon monoxide output and combustion efficiency directly, rather than assuming a furnace is burning cleanly because it sounds fine. Oil systems need more of that attention more often. The National Oilheat Research Alliance (NORA), the industry's technical and training body for liquid-fuel heating, organizes its service training around exactly this combustion-focused work — nozzle and chamber condition, draft and venting, and documented service procedures — because an oil burner's spray pattern degrades with nozzle wear and its combustion chamber accumulates soot in a way a gas burner simply doesn't.

• Gas furnace tune-up: burner and ignition inspection, combustion analysis for CO and efficiency, gas manifold pressure check, heat exchanger inspection, blower and safety-control check
• Oil furnace tune-up: nozzle replacement, oil filter change, electrode and ignition inspection, combustion analysis including a draft and smoke/soot check, heat exchanger and chamber inspection, blower and safety-control check

Maine law (25 M.R.S. §2468) requires a carbon monoxide detector in each area within, or giving access to, bedrooms in the residential occupancies it covers, positioned according to the manufacturer's instructions for that detector. Depending on the type of building, the law also requires a dual power source — tied to the building's electrical service with battery backup, or a detector using a sealed, non-replaceable 10-year battery.

The U.S. Consumer Product Safety Commission's general guidance goes further than the legal minimum: install a CO alarm on every level of the home and outside every sleeping area, not just near bedrooms in a single part of the house, and use interconnected alarms where possible so that one sounding triggers all of them. Combining the two — the legal placement near bedrooms and CPSC's every-level guidance — covers a house more completely than either one alone.