Air
Home air equipment: purifiers, humidifiers, furnace filters, CO and smoke alarms, space heaters, smart thermostats, air quality monitors and radon. Size it to the room, match the filter, compare the units worth a shortlist.
Every page under this heading is about the equipment that treats the air in a room: what size a purifier or a humidifier needs to be, which filter fits the model you already own, and which units are worth a shortlist. Ratings are cited by the standard that defines them.
Air equipment calculators: inputs you already have, one formula, one answer, with the formula printed so the number can be checked.
6 calculators
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.
25 guides
Air purifier and humidifier parts: a model number in, the filter that fits it out, with the cross-fit alternatives and the maker document that says so.
5 parts
Air purifiers and humidifiers compared on published, certified ratings: what each unit is rated for, what it is not, and which one a given room needs.
21 reviews
Space heaters 11 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.
Size
Space heater size calculator: watts per sq ft, BTU
Watts = length x width x 10 W per sq ft at an 8 ft ceiling (Dimplex, Stelpro), 12.5 at 10 ft. A 10 x 14 ft room needs 1,400 W: one 1,500 W heater.
Size
Space heater running cost calculator: per hour, month
kWh = watts / 1,000 x hours x duty cycle; cost = kWh x rate. A 1,500 W heater on 8 hours a day at EIA's 18.31 cents is 12 kWh, $2.20 a day, $65.92 a month.
Compare
Best garage heater: 240 V, 120 V and gas by spec
Maker sheets compared: Dura Heat EUH4000 240 V 20 A NEMA 6-20P 500 sq ft; King KBP2406 5,700 W 23.8 A UL 2021; EUH1500 120 V 12.5 A; vented Big Maxx 50,000 BTU.
Compare
Best baseboard heater: Cadet, King, Fahrenheat
Maker sheets compared: Cadet 6F1500 72 in 1,500 W 6.3 A 5,120 BTU UL; 3F750W 750 W 3.1 A; King 3K2407 cULus; plug-in Fahrenheat FBE15002 120 V 12.5 A.
Compare
Best oil-filled radiator heater: 5 picks by spec
Maker pages compared: Dreo 153 600/900/1500 W, 7 fins, 41-95 F, 17.9 lb; Dreo 310 alone prints ETL listed; De'Longhi EW7707CM 23.37 lb, no tip-over switch.
Compare
Best electric fireplace heater: picks by spec sheet
Maker manuals compared: Touchstone Sideline 28, 12 A plug-in only; PuraFlame Western 750/1500 W, 4600 BTU; Dimplex IgniteXL 2,500 W, 8,530 BTU on 240 V, UL file.
Compare
Best space heater: 1500 W picks compared by spec
Maker sheets compared: Dreo Atom One 1000-1500 W, 41-95 F thermostat, 37.5 dB; Vornado MVH tip-over switch, ETL Listed, 5-year warranty; Lasko 751320 prints none.
Air purifiers 11 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.
Size
Air purifier running cost calculator: filters + kWh
Yearly cost = filter price x 12 / interval months + watts x hours x 365 / 1,000 x rate. A 56 W unit at 24 h is 490.56 kWh, $89.82 at EIA's 18.31 cents (July 2026).
Size
Air purifier CADR calculator: room size and ACH
AHAM's rule: smoke CADR at least two-thirds of the room's square feet at an 8 ft ceiling (room size = 1.55 x CADR); ACH = CADR x 60 / volume. 180 sq ft: 120 cfm.
Match
Air Purifier Replacement Filter by Model
Levoit Core 300-P takes Core 300-RF (6 to 8 months), Coway AP-1512HH and 200M share set 3304899, Winix 5500-2 takes Filter H 116130, HPA300 holds 3 HRF-R.
Compare
Best air purifier for wildfire smoke by smoke CADR
For wildfire smoke AHAM asks for a smoke CADR equal to the room's sq ft: Levoit Core 600S-P 391, Blueair 311i Max 283, Winix 5510 253. ENERGY STAR and CARB listed.
Compare
Best air purifier for pet hair and odor by spec
Levoit Vital 200S-P: AHAM smoke CADR 250, washable pre-filter for hair, 6 to 12 month filter. Core P350 Pet Care 140 but brush-only pre-filter; Blueair 211i Max 435.
Compare
Best air purifier for a large room by AHAM CADR
AHAM smoke CADR x 1.55 sizes a large-room purifier: Levoit Core 600S-P 391 for 606 sq ft, Blueair 211i Max 435 for 674 sq ft, ENERGY STAR and CARB listed.
Compare
Best air purifier for bedroom and living room by CADR
AHAM, ENERGY STAR and CARB listings compared: Levoit Core 300 smoke CADR 143 for a 222 sq ft bedroom, Blueair 211i Max 435 for 674 sq ft, CARB types listed.
Humidifiers 9 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.
Size
Indoor humidity by outdoor temperature calculator
CEE's winter table: indoor RH 35% at 20F outside, 30% at 10F, 25% at 0F, 20% at -10F, 15% at -20F. Calculator interpolates the table and checks dew point.
Match
Humidifier Filter by Model: Wick and Panel Numbers
Honeywell HCM-350 takes Filter A HAC-504 (30 to 60 days), HEV615/620 take Filter T, Aprilaire 600/700 take Panel 35 yearly, the 400 takes Panel 45 twice a season.
Compare
Best hygrometer for indoor humidity, by maker spec
Maker specs compared: TempPro TP50 +/-2% at 30 to 80% RH on one AAA; Govee H5075 +/-3% RH, 20 days of records, CSV; AcuRite 01083M +/-2% from 10 to 90% RH.
Compare
Best large room humidifier, by maker spec sheet
Maker sheets compared: AIRCARE MA1201 3,600 sq ft and 3.6 gal, Levoit Superior 6000S 1,500 mL/h, Vornado EVDC500 3.5 gal per 24 h. No AHAM rating printed anywhere.
Compare
Best whole-house humidifier, by maker spec sheet
Maker sheets compared: Aprilaire 600 17 GPD bypass, 5-year warranty; Honeywell HE280 17 GPD, 1 year; Aprilaire 800 steam 11.5 to 34.6 GPD by voltage and amps.
Compare
Best bedroom humidifier, by maker spec sheet
Maker sheets compared: Vornado EV100 1 gal per 24 h, 40 to 60% humidistat, wick 4 to 8 weeks; Levoit LV600HH 300 mL/h, 280 W, Auto Mode 55 to 68% RH, past EPA's 50%.
Thermostats and heat pumps 7 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.
Size
Thermostat setback savings calculator: DOE 10% rule
DOE: as much as 10% a year by setting back 7-10 F for 8 hours a day; ENERGY STAR smart thermostat average 8%. On $1,600 of heating and cooling: $160 or $128.
Compare
Best smart thermostat: C-wire, ENERGY STAR compared
ENERGY STAR prints 8% savings, about $50 a year. ecobee Premium ships its Power Extender Kit and a sensor; Nest 4th gen prints 3 heat stages; Sensi needs a C wire.
Furnace filters 6 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).
Match
Furnace Filter Sizes: Nominal vs Actual, by Maker
A 16x25x1 measures 15.5 x 24.5 x 0.75 in at Nordic Pure; a nominal 20x25 media filter has five actual sizes; the AprilAire 410 is 16.52 x 29.38 x 4 in.
Match
4 in Media Filter by Cabinet: AprilAire, Honeywell
AprilAire 1210/2210/3210/4200 take the 210 or 213 (20.6 x 26.63 x 4 in actual); 1410/2410/3410/4400 the 410 or 413; Honeywell F100/F200 the FC100A1037 or FC200E1037.
Compare
Filtrete 1500 vs 1900 vs MERV 11-13: 1-inch filters
Filtrete 1500 (MERV 12) and 1900 (MERV 13) vs Honeywell MERV 11 and 13 in 16x25x1 and 20x25x1: 3M's MPR chart, no Honeywell FPR equivalence, in-stock packs.
Smoke and CO alarms 5 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.
Match
Hardwired Smoke Detector Replacement by Brand
Kidde 20-9003 fits Kidde and Firex harnesses only; First Alert SMI100-AC replaces the BRK 9120B, no rewiring; USI names no cross-brand fit; 10-year limit.
Compare
Best smoke detector, by maker spec and NFPA rule
Kidde 20SD10 for battery-only homes, 20SA10 for hardwired, both listed to UL 217:2024 Ed.10; First Alert dual sensor runner-up; NFPA placement and the 10-year rule.
Compare
Best carbon monoxide detector, by maker spec sheet
Maker sheets compared: Kidde KN-COPP-3 plug-in, 30-999 ppm display, 10-year life; Kidde COB10 sealed 10-year battery; First Alert CO605, 7 years; same UL 2034 bands.
Radon 4 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.
Compare
Best radon mitigation fan: fan curves compared
Maker fan curves compared: RadonAway RP145 34-66 W, 81 CFM at 1.0 in. WC; GP501c 3.8 in. WC for clay; Festa Maverick 116 CFM. EPA says use a qualified mitigator.
Compare
Best radon detector: monitors compared by spec
Maker specs compared: Airthings Corentium Home 2 shows 24h to long-term averages on 2 AA for 3 years; RadonEye RD200 reads in 10 minutes. EPA fixes at 4 pCi/L.
Air quality monitors 2 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.
Compare
Best air quality monitor for PM2.5 and CO2, by spec
Maker specs compared: Aranet4 NDIR CO2 0-9999 ppm, 4-year battery; Airthings View Plus adds laser PM2.5 0-1000 ug/m3 and radon 0-500 pCi/L; Temtop exports CSV.
Dehumidifiers 2 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.
Compare
Best bathroom exhaust fan, by maker spec sheet
HVI sizes a bathroom at 1 CFM per sq ft up to 100 sq ft. Panasonic FV-0511VF1 prints 110 CFM, 0.8 sones and 10.2 W at 0.1 in. w.g.; Broan BEL8 80 CFM, 1.5 sones.