Guides
Air equipment guides: a question about a rating, a filter or a deal in, the number that decides it out, with the standard cited by name.
Each guide starts from a rating on a box or a price on a listing and ends at the number that decides whether it fits your room. The standard behind the rating is cited by name.
Thermostats and heat pumps 5 pages
Diagnose
Heat pump blowing cold air: defrost or fault?
Carrier prints 85–100°F as normal heat pump supply air and a 5–10 minute defrost; Lennox 5–15 minutes every 30–90. Ice over 1/4 inch after defrost is a fault.
Diagnose
Aux heat vs emergency heat on a heat pump
AUX HEAT is automatic backup running with the heat pump; EM HEAT is a manual switch that locks the compressor out. Google says aux costs about 2 to 5 times as much.
Diagnose
Thermostat wire colors and terminal letters
Google Nest Help: R is power, C common, W heat, Y cooling, G fan, O/B heat pump changeover. Colour is a habit: Google prints only blue for C and orange for O/B.
Diagnose
Smart thermostat Black Friday deal check: wire first
A smart thermostat deal is real only if your system is 24 V with a C wire or a maker's power kit, and the exact model number is on the ENERGY STAR list.
Diagnose
Smart thermostat C wire: do you need one?
Google: a C wire only powers the thermostat. Nest, ecobee and Honeywell work on 24 V systems without one via a spare wire or a Y/G adapter. 120 or 240 V means no.
Space heaters 4 pages
Diagnose
Can you leave a space heater on overnight? No
NFPA, CPSC, USFA and ESFI all say turn a space heater off when leaving the room or going to sleep; Lasko's manual says not unattended. Keep 3 feet clear.
Diagnose
Oil-filled radiator vs ceramic heater: which room
At 1,500 W both make the same heat: Missouri DNR calls electric resistance heaters 100 percent efficient. Choose by fan noise, weight and tip-over switch.
Diagnose
Space heater tripping breaker: watts, amps, fixes
A 1,500 W space heater draws 12.5 A at 120 V, above the 12 A (80 percent) figure ESFI gives for a 15 A circuit. Give it its own circuit or run it at 900 W.
Diagnose
Space heater Black Friday deal check: safety first
The four heaters here all top out at 1500 W, so a discount buys no extra heat. Check the ETL or UL mark, tip-over switch and warranty first.
Air purifiers 4 pages
Diagnose
How often to replace an air purifier filter
Levoit Core 300 every 6 to 8 months, Coway and Winix HEPA 12 months, Blueair 6 to 9, Honeywell HEPA 12 and pre-filter 3, Shark up to 5 years. Timer vs sensor.
Diagnose
True HEPA vs HEPA-type: what each label means
DOE defines HEPA as 99.97% at 0.3 micrometres; EN 1822 H13 is 99.95% at MPPS. 'HEPA-type' and '99% HEPA' have no standard. How to read the box.
Diagnose
Does my air purifier make ozone? CARB list check
Look the model up on CARB's list: Mechanical means no ozone or ions; Electronic means tested at or below 0.050 ppm. EPA names ionizers, ESPs, plasma and uncoated UV.
Diagnose
Air purifier Black Friday deal check: CADR first
An air purifier deal is real only if its smoke CADR is at least two-thirds of the room's square feet (AHAM). The box room size is the 1-air-change figure.
Humidifiers 3 pages
Diagnose
How to clean a humidifier: pink film, vinegar, bleach
Pink film in a humidifier is residue the maker steps remove: brush, a 15 to 20 minute vinegar soak, then 1 tsp bleach per gallon. EPA: clean every 3 days.
Diagnose
Condensation inside windows in winter: cause and fix
Water on the room side of the glass means indoor RH is above the CEE ceiling: under 40% at 20-40F outside, under 30% at 0-10F. Between the panes is a failed seal.
Diagnose
Humidifier Black Friday deal check: output and water
Black Friday is November 27, 2026. EPA says ultrasonic units disperse tap-water minerals as white dust. Check output first.
Furnace filters 3 pages
Diagnose
Furnace filter arrow direction: which way it points
The furnace filter arrow points toward the blower, the way air flows: up in an upflow, down in a downflow, into the wall at a return grille (Trane, Carrier, Lennox).
Diagnose
MERV vs MPR vs FPR: what each filter rating means
MERV comes from ASHRAE 52.2 tests of 0.3 to 10 um particles in three ranges; MPR is 3M's 0.3 to 1 um scale; FPR has no standard. No conversion exists.
Diagnose
How often to change a furnace filter, by depth
A 1-inch furnace filter is changed every 30 to 90 days (Carrier, Trane, ENERGY STAR); a 4- or 5-inch media filter every 6 to 12 months (Honeywell, AprilAire).
Smoke and CO alarms 2 pages
Diagnose
Smoke detector chirping: battery or end of life?
Chirping every hour? No manual prints that: 1 chirp every 30-60 s is a low battery. Still chirping with a new battery means a stale error or end of life.
Diagnose
Carbon monoxide detector beeping: what it means
Four beeps is a CO alarm on Kidde, First Alert and X-Sense units: get to fresh air and call 911. One chirp a minute is low battery; 2 to 5 a minute is end of life.
Radon 2 pages
Diagnose
Radon manometer U-tube reading: what it means
The U-tube shows suction in inches of water column, never radon. Even levels mean the fan stopped. Compare to the installer's sticker and retest every 2 years.
Diagnose
Radon level in pCi/L: what the number means
EPA says fix the home at 4 pCi/L (150 Bq/m3) and consider fixing between 2 and 4 pCi/L; WHO's reference level is 100 Bq/m3. One short-term result is not the verdict.
Air quality monitors 1 pages
Diagnose
Air quality monitor PM2.5 and CO2 reading explained
EPA's AQI calls 0.0 to 9.0 ug/m3 PM2.5 Good and 9.1 to 35.4 Moderate on a 24-hour basis; an instant monitor reading is not an AQI. No EPA indoor CO2 limit exists.
Dehumidifiers 1 pages
Diagnose
Humidifier vs dehumidifier: which do I need?
EPA says keep indoor RH below 60%, ideally 30 to 50%. Under 30% in winter means a humidifier, above 60% means a dehumidifier, and 30 to 50% means neither.