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Tag: air changes per hour

  • How Long Should You Run an Air Purifier? 24/7, Overnight, or Only When AQI Is High

    During smog season, run your air purifier whenever someone is in the room, which for a bedroom means all night and for a living room means most of the day. Pollution keeps leaking in, so a purifier switched off for a few hours lets the room drift back towards the outdoor level. The easy way to do this without wasting power is Auto mode, which raises and lowers the fan as the room’s air changes.

    The Short Answer

    SituationHow long to run it
    Outdoor AQI Poor, Very Poor or Severe (common October to January in north India)Continuously in occupied rooms, on Auto. All night in bedrooms.
    Outdoor AQI ModerateAuto mode, mostly in the evening and overnight when pollution tends to build.
    Outdoor AQI Good or SatisfactoryVentilate instead. Run the purifier after cooking, incense or cleaning, or on Auto so it wakes up only when needed.
    Someone with asthma or allergies in the roomLean towards continuous running in their bedroom through the season.

    The reason to keep it running is simple: a purifier does not clean a room once and leave it clean. Fine particles keep arriving through gaps around doors and windows, and from cooking, incense and people moving about. When the purifier stops, the room starts heading back towards the outdoor level.

    How Long It Takes to Clean a Room

    The clean-up time depends on air changes per hour (ACH): how many times the purifier can pass a volume of air equal to the whole room through its filter in an hour. You get it by dividing the purifier’s CADR (in m³/h) by the room’s volume (floor area times ceiling height, in m³).

    • At 3 ACH, a volume equal to the room’s air passes through the filter about three times an hour.
    • At 6 ACH, twice as often, so the room clears roughly twice as fast.
    • A larger room, a higher ceiling, or an open door to the rest of the house all lower the effective ACH.

    One air change does not mean the room is fully clean. Clean air from the purifier mixes with the dirty air already in the room, so each pass removes a share of the particles, not all of them. The level falls quickly at first and then more slowly, which is why a purifier needs a few air changes, not one, to bring a polluted room down, and why it has to keep running to hold it there.

    For example, Boon Airvana delivers a CADR of 380 m³/h. In a typical 12 × 12 ft bedroom that is more than nine air changes an hour. Across about 484 sq ft with a 3 m ceiling, it is about 2.8 air changes an hour. Our air purifier size guide and the room size calculator work this out for your room.

    Rule of thumb: the higher the air changes per hour, the faster the room clears and the more easily it stays clean. A correctly sized purifier on a lower fan speed often does more than an undersized one on maximum.

    Doors and Windows: Closed or Open?

    Close them while the purifier is working, if outdoor air is polluted. An open window on a smoggy day keeps bringing in fresh particles, and an open door turns a bedroom into a bedroom plus the hallway plus the living room, which spreads the purifier’s work over a much larger volume.

    You do not need to seal the room airtight. Normal gaps are fine; the purifier is sized to cope with them. Two caveats:

    • Carbon dioxide builds up in a closed room with people in it. A purifier does not remove CO₂ or add oxygen. Air the room briefly when outdoor AQI is at its lowest point in the day, then close up again and let the purifier recover the room.
    • Never close up a room with a gas heater, angithi, coal or charcoal burning. A purifier does nothing for carbon monoxide. See what air purifiers cannot remove.

    Where you put the purifier matters too: away from walls and curtains, with clear space around the intake. Our guide on where to place an air purifier covers Indian rooms, fans and ACs.

    Boon Airvana’s Auto mode follows the room’s real-time AQI, Sleep mode runs quietly through the night, and Eco mode cycles off and on as the air settles or worsens.

    See Boon Airvana →

    Running It While You Sleep

    Yes, run it overnight. You spend more continuous hours in the bedroom than anywhere else, the door is usually shut, and in many north Indian cities pollution tends to build up in the evening and night. A bedroom purifier that runs from bedtime to morning does more for most people than one that runs in the living room during the day.

    Three tips for nights:

    1. Start it before bedtime, so the room is already clean when you lie down.
    2. Use a sleep mode with a dimmed display and a quiet fan, so nobody is tempted to switch it off.
    3. Keep the door shut. An open door roughly undoes the sizing you did.

    Auto and Eco Modes: the Practical Answer

    Most people do not want to decide every hour whether the purifier should be on. That is what sensor-led modes are for:

    • Auto mode reads the room’s particle level and raises the fan when pollution rises (cooking, incense, a door opened) and lowers it when the air is clean. It runs continuously but only works hard when needed.
    • Eco mode (on Airvana) goes a step further: it cycles the purifier off and on as the AQI settles or rises, to save electricity.
    • Scheduling and timers help where routines are fixed, for example starting the bedroom purifier an hour before bedtime. Airvana’s app also has an Auto-learn mode that learns your routine and starts purifying before you get home.

    Only rely on Auto if the purifier has a real particle sensor. A purifier that guesses, or one with a vague dust sensor, may lower the fan while the fine particles are still high.

    Electricity and Filter Life

    Running a purifier 24/7 does use more electricity than running it for a few hours, but at lower fan speeds most purifiers draw far less than at full speed, which is why Auto and Eco modes matter. The actual cost depends on the model’s power draw, the speed it spends most time at and your tariff; our air purifier running cost guide shows how to work it out.

    The bigger running cost is usually the filter. A filter that cleans more air catches more particles and fills faster, so running a purifier through a Delhi winter uses up a filter faster than running it in a clean month. Three habits help:

    • Wash or vacuum the pre-filter regularly (Airvana’s pre-filter is washable). It catches hair and dust that would otherwise clog the main filter.
    • Change the main filter when the reminder says it is due, not on a fixed calendar. On Airvana the reminder depends on the air it has actually cleaned.
    • Do not switch the purifier off to “save the filter” on smoggy days. The filter is there to be used; an unused filter in a polluted room protects nobody.

    See air purifier maintenance and filter care for the full routine.

    Frequently Asked Questions

    Should I run my air purifier 24/7?

    In smog season, running it continuously in occupied rooms makes sense, because outdoor particles keep leaking in and the room drifts back to the outdoor level once the purifier stops. Use Auto mode so the fan only works hard when pollution rises. When outdoor AQI is Good or Satisfactory, ventilating the home is often enough.

    How long does an air purifier take to clean a room?

    It depends on air changes per hour: the purifier’s CADR divided by the room’s volume. At 3 air changes an hour, a volume equal to the room’s air passes through the filter about three times an hour; at 6, twice as often. Each pass removes a share of the particles, so a few air changes are needed to bring a polluted room down.

    Should doors and windows be closed when using an air purifier?

    Yes, when outdoor air is polluted. An open window brings in fresh particles and an open door spreads the purifier over a larger space. You do not need to seal the room airtight. Air the room briefly when outdoor AQI is lowest to clear carbon dioxide, and never close up a room with a gas heater, angithi or charcoal burning.

    Is it safe to run an air purifier all night while sleeping?

    Yes, a HEPA air purifier is designed to run for long periods, and the bedroom at night is where it does the most good. Start it before bedtime, keep the door shut and use a sleep mode so the noise and display do not disturb you. Avoid purifiers that produce ozone, especially in bedrooms.

    How much electricity does an air purifier use if run 24 hours?

    It depends on the model’s power draw at each fan speed, how long it spends at each speed, and your electricity tariff. Purifiers draw much less on low speeds than at full speed, so Auto and Eco modes keep 24-hour running costs down. Check the model’s rated power and use our running cost guide to work out your monthly figure.

    Does running an air purifier all the time wear out the filter faster?

    Yes, a filter that cleans more air catches more particles and fills sooner, especially in a smoggy winter. That is the filter doing its job. Wash the pre-filter regularly to protect the main filter, and replace the main filter when the purifier’s reminder says it is due rather than on a fixed calendar.

    Set it once and let the room decide. Boon Airvana’s Auto mode follows the live AQI, and Eco mode cycles off and on to save electricity when the air is clean.

    See Boon Airvana →

  • Air Purifier for a Large Room or Living Room: How to Size It Properly

    A large room needs a purifier sized by CADR, not by the coverage figure on the box. For a living room or hall, aim for at least three and ideally four to five air changes an hour: at a 3 m ceiling that is roughly 375 to 625 m³/h for a 450 sq ft room. Beyond about 500 sq ft, two units placed apart usually work better than one big one.

    Why Coverage Claims Mislead

    “Covers up to 700 sq ft” sounds like a promise. It is really a calculation with two hidden assumptions: the ceiling height, and how many times an hour the purifier is expected to clean the room’s air. Some coverage claims assume one air change an hour or less. That might keep a room tidy on a clean day; it will not keep up with Indian winter smog, cooking, incense or a door that opens every few minutes.

    The honest number is CADR, in m³/h. With CADR and your room’s size you can work out the real result yourself. Our air purifier size guide covers the basics for bedrooms; this guide is about the bigger, more open spaces where most buyers under-size.

    Air Changes per Hour, Explained

    Air changes per hour (ACH) is how many times the purifier passes a room’s full volume of air through its filter each hour. The formula is simple:

    CADR (m³/h) = floor area (m²) × ceiling height (m) × ACH. To convert, 1 sq ft = 0.0929 m².

    As a guide for Indian homes:

    • About 3 ACH: a reasonable floor for a living room on moderate days.
    • 4 to 5 ACH: what you want during smoggy months, when AQI sits in the Poor to Severe bands, or in homes with allergy sufferers.
    • Below 2 ACH: the purifier is fighting a losing battle against every door opening and gap under the balcony door.

    Worked Examples: 300, 450 and 600 sq ft

    All three assume a 3 m (about 10 ft) ceiling. If your ceiling is lower, the numbers come down in proportion.

    RoomAreaVolume3 ACH4 ACH5 ACH
    300 sq ft (large bedroom, small living room)27.9 m²83.6 m³251 m³/h334 m³/h418 m³/h
    450 sq ft (typical living-dining)41.8 m²125.4 m³376 m³/h502 m³/h627 m³/h
    600 sq ft (open-plan hall)55.7 m²167.2 m³502 m³/h669 m³/h836 m³/h

    Take the 450 sq ft row: 450 × 0.0929 = 41.8 m²; × 3 m = 125.4 m³; × 4 ACH = about 502 m³/h. A purifier rated at 250 m³/h would manage only two air changes an hour in that room, whatever its box says about coverage.

    Two practical notes. First, CADR is measured at the highest fan speed, and few people run a purifier on maximum all evening. Some headroom lets the purifier keep up at a lower, quieter speed. Second, a double-height ceiling or a room open to a staircase adds volume that the floor area hides.

    Enter your room’s length, width and ceiling height to get the CADR it needs.

    Size Your Room →

    When Two Units Beat One

    A single purifier cleans air best close to itself. In a large or L-shaped room, the far end can lag behind, especially where furniture, a dining table or a partition breaks up the airflow. Two units placed apart often clean the whole space more evenly than one bigger unit in a corner. Two units are usually the better choice when:

    • the space is larger than about 500 sq ft, or has a double-height section;
    • it is L-shaped or split into living and dining zones;
    • you want to move one unit into a bedroom at night and leave the other in the hall;
    • the room needs more CADR than any single unit in your budget offers.

    One large unit can be simpler in a square, uncluttered room with one main seating area. Either way, add up the CADR of all units and check it against the table above.

    Placement in Open-Plan Homes

    Placement can matter as much as CADR in a big space. Keep the purifier off the wall so it can draw air freely, away from curtains that block the intake, and closer to where people sit than to the main door. If the kitchen opens into the hall, keep the purifier away from the cooktop itself; a chimney handles cooking fumes at source. Keep balcony doors and windows closed while it runs on bad-air days, or the purifier ends up cleaning the neighbourhood. Our guide on where to place an air purifier in an Indian home covers fans, ACs and sliding windows in detail.

    Check your city’s current air on the AQI by city page; on Poor to Severe days, aim for the higher end of the ACH range.

    Where Boon Airvana Fits

    Boon Airvana has a CADR of 380 m³/h. At a 3 m ceiling that covers about 484 sq ft at roughly 2.8 air changes an hour. Against the worked examples:

    • 300 sq ft: about 4.5 air changes an hour. A comfortable fit.
    • 450 sq ft: about 3 air changes an hour. Adequate on moderate days; on the worst smog days, a second unit or a closed-off space helps.
    • 600 sq ft: about 2.3 air changes an hour from one unit. Two Airvanas placed apart (760 m³/h combined) bring it to about 4.5.

    Its dual-suction design draws air in from both sides, through two filters, which suits a purifier standing away from the wall in an open room. The AtmoScan sensor shows PM2.5 as a number, so you can see whether the far end of the room is keeping up when you move the unit. Airvana is ₹14,900 at the time of writing.

    Frequently Asked Questions

    What CADR do I need for a 450 sq ft living room?

    At a 3 m ceiling, a 450 sq ft room holds about 125 cubic metres of air. For three air changes an hour you need about 376 m³/h of CADR; for four, about 502 m³/h; for five, about 627 m³/h. Aim for four to five during smoggy months. If one unit cannot reach that, two units placed apart can.

    Can one air purifier clean a hall?

    It can if its CADR is high enough for the hall’s volume and the space is reasonably open. In a hall larger than about 500 sq ft, or one that is L-shaped or split into living and dining zones, two units placed apart usually clean the space more evenly than one unit in a corner. Check combined CADR against the room’s needs.

    Why does my air purifier not work in my living room?

    The most common reason is that it is too small for the room, giving fewer than two or three air changes an hour. Other causes are open doors and windows, a unit pushed against a wall or behind a sofa, a clogged pre-filter, or a filter due for replacement. Check the PM2.5 reading in different parts of the room.

    How many air changes per hour should an air purifier give?

    For Indian homes, about three air changes an hour is a reasonable minimum for living rooms, and four to five is better during smoggy months or for allergy sufferers. Bedrooms are smaller, so most purifiers can reach five or more there. Below two air changes an hour, outdoor air leaking in can outpace the purifier.

    Is it better to buy two small air purifiers or one big one?

    For large, L-shaped or zoned spaces, two smaller units placed apart often clean more evenly than one big unit, and you can move one into a bedroom at night. For a square, open room with one seating area, one large unit is simpler. Either way, make sure the combined CADR meets what the room needs.

    Does ceiling height affect air purifier size?

    Yes. CADR needed is floor area multiplied by ceiling height multiplied by air changes per hour, so a higher ceiling means more air to clean. A 3 m ceiling needs 20 percent more CADR than a 2.5 m ceiling for the same floor area. Double-height living rooms or rooms open to a staircase need noticeably more.

    Boon Airvana delivers 380 m³/h through dual-suction filters; for halls beyond about 484 sq ft, two placed apart do the job. ₹14,900 at the time of writing.

    See Boon Airvana →