Indoor Air Quality

Cooling only removes moisture while it runs, and it runs when the thermostat asks for temperature. In a mild damp stretch — most of June, a rainy week in July — the house never gets warm enough for a long cooling cycle, so humidity climbs while temperature stays fine. A ducted dehumidifier runs on humidity instead and dries the air without overcooling the room. That is why it fixes basements a bigger air conditioner never did.

Roughly 30 to 40 relative humidity indoors, and lower as it gets colder outside. The ceiling is set by your windows, not by comfort: once the indoor dew point rises above the temperature of the coldest glass in the house, water condenses there. Old single-pane windows in a Montclair colonial will run wet at a level a new build shrugs off. That is what an outdoor reset control is for, and why a humidifier installed without one causes the problem it was bought to fix.

Only if the system can breathe through it. Rating and restriction climb together, and a one-inch slot has a fixed face area to work with. Push the rating up in that slot and the pressure drop can exceed what the blower was designed against, which drops the coil toward freezing in summer and raises temperature rise in winter. The fix is not a worse filter — it is more filter. A four-inch cabinet gets you finer media and a lower pressure drop at once.

A one-inch pleat, every one to three months, and closer to one if there are dogs, a renovation underway or a return in a dusty basement. A four-inch media cartridge, once or twice a year. Rather than trusting the calendar, hold the old one up to a light: if you cannot see through it, it was overdue. Filters left past their life do not just stop filtering, they restrict.

An energy recovery ventilator is a small balanced fan unit with a core in the middle. It exhausts stale indoor air and brings in the same volume of outdoor air, passing them through the core so the outgoing air gives up most of its heat and some of its moisture to the incoming air. You need one when the house is tight enough that it no longer ventilates itself by accident: new construction, a deep retrofit, a condo, or any house where the air feels stuffy and the windows fog when everyone is home. In a leaky older house, ERV installation is money spent before the cheaper work.

Partly, depending on the trigger. Filtration at the air handler helps with what is airborne — pollen tracked in, dander, dust the blower stirs up. It does little for what lives in carpet or bedding, or for moisture-driven triggers, which are a humidity problem. Start with a media cabinet, get humidity into range, then reassess.

Because cold outdoor air holds almost no moisture, and your house is exchanging its air with it whether you asked or not. Heating that air does not add water, so the relative humidity collapses. The leakier the house, the drier it gets. Static shocks, gapping floorboards and morning nosebleeds all point the same way. A humidifier treats the symptom well; air sealing treats the cause.