COVID 1 9: Guidelines for Air Conditioning and Ventilation

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Salient features:

This article focuses on:

  • COVID-19 Transmission Routes
  • Effect of Environmental Conditions
  • Relative humidity
  • Temperature
  • Residential Applications
  • Room air conditioners
  • Evaporative coolers
  • Fans
  • Commercial and Industrial Facilities
  • Categories of indoor units
  • Operating guidelines for all categories
  • Recommendations for category i) , ii) and iii) indoor units
  • Recommendations for category iv), v) and vi) indoor units
  • Additional recommendations for industrial facilities
  • Healthcare Facilities
  • Converting general patient rooms or icus into covid-19 patient areas – considerations pertaining to hvac systems
  • Treatment of exhaust air from covid-19 patient areas
  • Setting up make-shift isolation enclosures
  • Quarantine areas
  • Portable room air cleaners
  • Operation and maintenance
  • Residential applications
  • Commercial applications
  • Guidance For Service Technicians
  • Additional precautions at hospitals & laboratories
  • How to disinfect tools and tackles

Detailed summary

 COVID-19 Transmission Routes

  • The size of a coronavirus particle is in the range of 80-160 nanometers. It is transferred via infected microscopic airborne particles and contaminated aerosol droplets which are released coughing & sneezing and to a lesser extent, even by talking and breathing
  • Large cough droplets remain active on surfaces and objects for hours and even 2-3 days. People can get infected by touching those contaminated surfaces or objects and then touching their eyes, nose or mouth.
  • If people are standing within 1-2 meter of an infected person, they can be infected by breathing-in droplets sneezed or coughed out or exhaled by them.
  • Small particles (less than 5 microns) released during cough stay airborne for hours and can be transported over long distances.
  • The infectious particles ranging from 0.65 to 3.3μm in the cough-generated aerosols are immediately respirable.
  • Apart from cough generated aerosols, the particulates suspended in the air also represents a substrate for viruses and consequently their transmission through this path.
  • In the indoor environment, one of the sources of dust is atmospheric dust (PM2.5 and PM10) coming in through fresh air intakes. The other prominent source is the dust generated by humans and processes. Reduction of indoor dust levels is a step towards mitigation of this source of COVID-19 transmission.
  • The faecal transmission route for SARS-CoV-2 infections is implicitly recognized by WHO. Use of exhaust fans toilet air should not come into the other occupied areas. It is recommended to flush the toilets with closed lids in order to minimize the release of droplets and droplet residues from plumes in the air.
  • Another path for transmission is when virus-laden aerosols are deposited on apparel and on the floor and then get re-suspended into the air due to the movement of people. That is why sanitizing the floors frequently and limiting the movement of people is helpful in curtailing this route of transmission.
  • In healthcare facilities sanitization of apparel is recommended as per protocol.

Effect of Environmental Conditions   

1.Relative Humidity

  • Relative humidity is found to affect the infectivity of virus through the respiratory route.
  • The normal human body cannot fight off foreign particles or invaders when we’re in a dry environment.
  • The infectivity of the bacteria increases with low humidity.
  • Relative humidity of at least 40% is considered the threshold.
  • 80% relative humidity and above tend to neutralize the COVID-19 virus.
  • Too much humidity leads to higher levels of dust mites and fungi, two of the worst culprits for indoor allergy sufferers. Mold and fungi are known to exacerbate respiratory conditions such as asthma.
  • The relative humidity level of 40%~70% is considered to be the most suitable environment for humans & decreases problems from pathogens.


  • Comfort temperature is generally considered between 24 – 30 °C after accounting for air velocity, relative humidity and clothing.
  • High temperature and high humidity significantly reduce the transmission of influenza.
  • Viral culture methods of low temperatures (7–8 °C) were optimal for airborne influenza survival with virus survival decreasing progressively at moderate Temperatures (20.5–24 °C) and further decreases at higher (greater than 30 °C) temperatures.
  • SARS-CoV-2 has been found highly stable on surfaces for 14 days at 4 °C; one day at 37 °C and 30 minutes at 56 °C were needed to inactivate the virus.

Residential Applications

1.Room Air Conditioners

  • Room Air conditioners control temperature and reduce humidity.
  • Recirculation of cool air by Room Air conditioners, must be accompanied by outdoor air intake through slightly open windows and exhaust by natural exfiltration.
  • Fresh Air intake through a fan filter unit will prevent outdoor dust entry (containing high levels of PM 10 and PM 2.5 particles) and exhaust through the kitchen and toilet exhaust fans kept operational.
  • Room Temperature should be set up between 24°C and 30°C.
  • Relative humidity should be maintained between 40% and 70%.
  • In dry climates, Relative Humidity should not fall below 40% .
  • If it falls below 40%, water evaporating from a pan kept in the room will increase humidity

2.Evaporative Coolers

  • Provide effective cooling in hot and dry climates.
  • They reduce temperature by evaporating water.
  • Evaporative cooler tanks must be kept clean and disinfected and the water drained and refilled frequently.
  • Windows must be kept open to release humid air.
  • Portable evaporative coolers that do not draw outdoor air are not recommended, since their cooling reduces with humidity rising inside the space.
  • Evaporative coolers must draw air from outside to ensure good ventilation.


  • Fans provide body comfort by convection and evaporation due to increased air motion.
  • Fans should be operated with windows kept partly open.
  • If an exhaust fan is located at a nearby location then it must be kept running to exhaust air for better ventilation.

Commercial and Industrial Facilities

  • Commercial Establishments and Industrial Facilities have multiple occupancy as well as transient visitors. It is this aspect that necessitates precaution in operating their Air conditioning Systems.

1.Categories Of Indoor Units :

  1. i) Multiple Cassette Units: Ceiling mounted units that can each cool up to 50 sqm and can be controlled individually or as a group.
  2. ii) Multiple Hi Wall Units: Used due to ease of installation and low Cost .

iii) Tower Units: For larger spaces, where most occupants are not stationary thus allowing for higher drafts.

  1. iv) Ducted Units : A mini central Air conditioning system that is easy to operate.
  2. v) Fan Coil Units : Installed in guest rooms, individual office spaces or patient wards.
  3. vi) Air Handling Units: Can provide better ventilation, filtration and Coil disinfecting.

2.Operating Guidelines For All Categories

  1. A) Air Filters must be kept clean as given in the Section – Operation & Maintenance.
  2. B) Provide adequate Ventilation ( Fresh Air and Exhaust ).
  3. C) Inspect and clean the indoor unit Coils, as given in the Section – Operation & Maintenance.
  4. D) Set Room Temperature between 24°C and 30°C. Maintain relative humidity between 40% and 70%. (In humid Climates set temperature closer to 24°C for de – humidification and in Dry Climates closer to or at 30°C Use Fans to increase air movement)
  5. E) Heat Recovery Wheel (HRW) : It is advisable to keep this wheel in off mode to reduce cross contamination. Upon restarting, the wheel must first be sanitized.
  6. F) Toilet and Kitchen Exhaust Fans must be kept in operating mode.

3.Recommendations For Category  I) , II) and III)  Indoor Units :

  • If fresh air is not provided, it is advisable to introduce a fresh air duct attached to a central inline fan filter unit and distribute the fresh air by grilles into the space or near the indoor units.
  • For Cassette Units the fresh air duct should be connected to the available port of the Cassette Unit.
  • In case fresh air cannot be provided through a fan it is recommended to actively use operable windows.
  • A minimum fresh air volume of 3 cum/hour per person and 3.75 cum per hour per sqm (5 cfm per person and 0.6 cfm per sq ft ) is recommended.
  • A separate Treated Fresh Air DX Unit should be provided in the case of a multiple unit installation. This will reduce the impact of reducing available cooling capacity by supplying non treated fresh air.
  1. Recommendations For Category IV), V) And VI) Indoor Units :
  • Fresh air should be provided by an inlet duct and fan.
  • It is advisable to provide a MERV 13 or higher filter fitted on the Air Handling Unit.
  • If a filter of higher filtering capability is retrofitted into an existing system, it should be ensured that the fan and motor capacities are adequate to handle the higher pressure drop.
  • A minimum fresh air volume of 3 cum/hour per person and 3.75 cum per hour per sqm (5 cfm per person and 0.6 cfm per sq ft) must be provided .
  • In buildings without mechanical ventilation systems it is recommended to actively use operable windows.
  • Add a TFA (treated fresh air) unit if recommended Fresh Air intake impacts cooling performance.
  • UVGI (Ultraviolet germicidal irradiation) should be installed for larger Ducted Units and AHUs to keep Coils continuously clean and disinfected.
  1. Additional Recommendations For Industrial Facilities
  • Minimum air changes of around 10-15 ACHP is advised for good ventilation.
  • The mechanical exhaust air should be 70% to 80% of fresh air quantity to maintain necessary positive pressure in the space.
  • In cases of evaporative cooling / air washers it is advisable to disinfect the water using UVGI or Ionization or chemical dosing.
  • Run the system in fan only mode for 30-60 minutes every day to dry the cooling pads. Then run only the pumps for water circulation without fans in operation for 30 minutes, to wash out any bacterial growth. Finally flush the water from the tanks and re-start the system with fresh water.
  • In case of re-circulating system, it is recommended to limit return air circulation. The return air system could be converted to an exhaust system.

Healthcare Facilities

1.Converting General Patient Rooms Or ICUs Into Covid-19 Patient Areas – Considerations Pertaining To HVAC Systems

  • COVID-19 positive patients and patients with COVID-19 related symptoms should be accommodated in designated “Airborne Infection Isolation Rooms” in hospitals to control spread of the disease.
  • In case of insufficiency of rooms healthcare facilities would need to convert their existing patient rooms or ICUs into COVID-19 patient rooms or COVID-19 ICUs.
  • Virus laden airborne particles should not leak out of the rooms occupied by COVID-19 patients and also to maintain the concentration of virus laden particles inside the COVID-19 patient room at a minimum.
  • To convert an existing patient room or ICU into a COVID-19 patient area, it is first necessary to convert the room into a non-recirculatory system (100% once through system).
  • On an emergency basis, this is achieved by blanking (blocking) off the return air vents in the COVID-19 patient room.
  • It is important to make sure that the AHU will have provision to receive adequate outdoor air supply.
  • The outdoor air source for the AHU shall not be from within the building and all care shall be taken to avoid intake of outdoor contaminants.
  • An independent exhaust blower should be provided to extract the room air and exhaust out into the atmosphere.
  • The exhaust air quantity shall be greater than the supply air quantity such that a negative pressure of minimum 2.5Pa (preferably >5 Pa) is achieved in the room .
  • The supply air quantity shall be such that it will provide a minimum of 12 air changes per hour.
  1. Treatment Of Exhaust Air From Covid-19 Patient Areas
  • Treatment of exhaust air can be done by HEPA filtration.
  • These HEPA filters should be a minimum of H13 (EN1822-1) filter class or equivalent.
  • When not possible, treatment of exhaust air by Chemical disinfection is acceptable.
  • When both the methods are not viable, the exhaust air shall be let off into the atmosphere through an upward plume at a height of 3 m above the tallest point of the building, thereby lowering the viral load concentrations to insignificant levels by dilution.
  • When HEPA filters are used to treat the exhaust air, it is preferable to install them at the primary point of air extraction in the room and the exhaust blower shall be at the discharge end of the exhaust duct.
  • Chemical disinfection of the exhaust air from COVID-19 patient room can be done by bubbling the exhaust air through a “Diffused air aerator tank” holding a 1% sodium hypochlorite solution.
  • The concentration should be checked on a regular basis and dosing undertaken based on need.
  • The aeration tank shall be placed in an unpopulated outdoor area and not inside enclosed space.
  • PPE should be used while handling the hypochlorite solution and direct contact with skin and eyes should be avoided.
  • The other two options available for exhaust air treatment being UV irradiation and heating.
  • An exposure time of 45min at a temperature of 75 °C resulted in complete inactivation of SARS-CoV.
  • An UVC (254 nm wavelength) irradiation with an exposure time of 15 minutes at irradiation intensity of 4016 µW/Cm2 resulted in complete inactivation of SARS-CoV.
  • The treatment of exhaust air by HEPA filtration is the preferred method and the other suggested methods may be adopted due to non-availability of HEPA systems.
  1. Setting Up Make-shift Isolation Enclosures
  • A make-shift patient isolation enclosure could be a temporary makeshift cubicle or tent constructed out of a skeleton structure (of plastic or metal) and plastic sheet or canvas covering.
  • The tent should be covered on all sides excepting the front, where PVC strip curtains can be provided.
  • Arrangements for light and a fan inside the tent can be provided for the comfort of the patient.
  • The tent shall be provided with an exhaust blower to extract the air inside the enclosure and exhaust out into the atmosphere after suitable treatment.
  • The exhaust blower shall be so sized that a negative pressure of >2.5Pa is maintained inside the enclosure.
  • In case of air-conditioned patient rooms where exhaust blowers cannot be fixed, or in case of patient rooms served by unitary air conditioners, the Isolation enclosure can be installed inside the patient room with HEPA filtered exhaust air.
  • The exhaust air, after HEPA filtration can be discharged inside the room. This provides a localized negative pressure zone inside the room and helps control infection spread and offers protection to the healthcare workers.
  1. Quarantine Areas
  • Quarantine refers to separation of individuals who are not yet ill, but have been exposed to COVID-19 and therefore have a potential to become ill.
  • Quarantine can happen at home or in quarantine centre.
  • In a quarantine situation, the key is to maintain physical distancing and avoidance of contact .
  • A quarantine centre should be well ventilated and preferably be maintained at a negative or neutral differential pressure.
  • When mechanical ventilation is resorted to, it shall be a once through system (non- recirculatory system) that provides a “clean to dirty” (towards the patient and away to the exhaust) air flow pattern.

Portable Room Air Cleaners

  • Portable air cleaner will only clean the air that passes through the cleaner and help reduce the indoor concentration of contaminants through the action of dilution.
  • With any active filtration technology, it is important to ensure the single pass efficiency and the possibility of any harmful byproducts that may be released due the active nature of the cleaning.
  • Some of the technologies used are Ionization, Bi-polar ionization, PCO, ESP, ozone generators etc.
  • UVGI is proven to be useful in inactivating bio-aerosols. .
  • Passive technologies include HEPA filtration that can remove particles down to 0.1 micrometers or even smaller particles.
  • An H13 certified or equivalent HEPA filter should be able to remove these viruses from the air that passes through the cleaner.
  • Ideally, one must design air cleaners with about 3-4 air changes per hour. The higher the air changes, the better the efficiency of cleaning.
  • One must be careful when reading the CADR numbers on most commercial products as they will give either the highest number (based on highest fan speed) which will be very noisy or will give the free-flow air handling capacity of the fan motor (i.e. without filters installed), rather than the actual airflow rate (with all filters installed)

Operation & Maintenance

  1. Residential applications :
  • The room should be well ventilated during the period of non-use of AC with fans running.
  • The frequency of service should be increased.
  • It is advisable to clean the indoor unit filters by the end user as per the instruction given in the user manual by the manufacturer.
  • In case there is need to call a technician, it is advisable to call the company authorized technician.
  • Careful evaluation should be done by the customer and the service technician on the nature of breakdown and decide on the repairs location site / workshop.
  • In-case of a major breakdown, it is advisable to request for a replacement machine to have minimum contact time with the technician.
  1. Commercial Applications :

The steps recommended for the start-up of air conditioning system:

  • The user or the owner should get the area sanitized.
  • Fresh air and exhaust system adequacy should be studied as per the guidelines and user should be informed to modify the system if found inadequate.
  • Ppreventive maintenance on all the units should be carried as per manufacturer’s guidelines. This should include disinfecting and cleaning of :
  1. Filters, grilles, diffusers & internal surfaces : it is recommended to use 5% Cresol solution (containing 50% Cresol and 50% Liquid soap solution). Mix 1 liter of this solution in 9 liters of water. The surface shall be sprayed with this solution, left for 10 minutes and then washed / wiped clean with water / cloth.
  2. Condensate drain pan : Disinfecting / treatment of condensate drain pan is suggested using UV treatment or 1% sodium hypochlorite dosing. This will apply only if the HVAC equipment is working on a re-circulatory mode.
  3. Coils : Follow standard recommendations of coil cleaning and then sanitize.
  • In case the area has ducted air distribution, it is advisable to clean the ducts by an appropriate method that may include santization.
  • The following process is recommended at start-up :
  1. All the doors and windows of the space should be opened.
  2. Ensure that all cleaning protocols are complete
  3. Run the fresh air system at the maximum intake of air setting.
  4. Start and run the exhaust systems if available.
  5. Start the air conditioning system in fan mode only, without filters and run it for minimum of two to four hours with doors open and exhaust system operational.
  6. Install the clean & sanitized filters
  7. Start the AC in normal mode and run for two hours with doors open and then close the doors and windows.
  • The fresh air and ventilation system should be kept on throughout the off cycle and on the weekend and holidays in air circulation mode

Guidance For Service Technicians


  • Company identity card, authorized letter (during lock-down) should be carried .
  • Public transport should be avoided, use personnel vehicle as far as possible.
  • Vehicle should be disinfected before and after the visit on daily basis.
  • The driver if accompanying, should stay inside the vehicle, except for essential health breaks & emergency.
  • Home quarantine stickers/ stamp at the customer’s location should be checked.Do not enter in case of any such stamp/sticker, and re-schedule the service.
  • Social distancing norm should be maintained and a face mask should be worn. Avoid personal contact, including handshakes with co-workers /customers
  • Carry your own water bottle and food as required.
  • Hands should be washed often for at least 20 seconds using soap. Always carry alcohol-based sanitizer and use whenever hand washing is not possible
  • Prescribed PPE ( Personal protection Equipment ) should be used.
  • Carry a sanitary bag of adequate size, to keep the replaced item and dispose safely.
  • Proper disposal methods for used PPE’s should be followed.
  • Monitor your health closely. If you develop symptoms like fever, cough, sore throat, tiredness or shortness of breath, immediately inform the office and take necessary action as advised by the government. Do not continue to work under these circumstances.
  • Alcohol-based sanitizer, before working with electrical sources should be avoided.


  • Unknown persons should never be boarded in the vehicle.
  • Never touch the common items provided at the customer place such as newspaper, common towel, etc.
  • Never handle other person’s belongings or share food and water from others.
  • Never use another person’s crash helmet or PPE
  • Do not hand over one’s mobile phone to another person/colleague.
  • Do not share the pen especially for service report signature from customer, instead ask the customer to use their own pen.
  • Never enter premises which are marked as isolated or quarantined.

Additional Precautions At Hospitals & Laboratories


  • The tools and tackles should be sanitized before and after every visit including the bag.
  • Compulsorily wear Surgical nose mask, before entering inside the Hospital, & Laboratory sites. Change the mask every six hours or as soon as it becomes wet.
  • All technicians should wear a clean, long-sleeved gown or full body suit, N95 Respirator, gloves, face cover if they need to enter into COVID patient’s isolation ward or COVID-19 lab. After exit, dispose the used PPE’s into the designated in “RED” color bin.
  • A disposable bag of adequate size should be carried to keep the removed/replaced item and disinfect them at suitable location in the hospital premises.
  • Barricade the area, while rendering the service to avoid trespassing. For rendering the service outside the conditioned zones such as for chillers, outdoor units, Fresh Air AHUs, cooling towers ensure that the authorities have cleaned and disinfected before the work is started.
  • Complete record of the time spent in the premises should be maintained.


  • Come in close contact with the patients and visitors in the hospital site.
  • Touch bio-hazardous components such as surgical knife, needles and lab equipment’s, used cotton and clothes etc.
  • Use handrails, touch objects such as lift buttons, door handles and fixtures with your bare hands.

How To Disinfect Tools & Tackles:

  • Surfaces should be cleaned with soap water and hand towel removing any contaminants, dust, or debris.
  • A surface-appropriate disinfectant should be applied after that . The quickest and easiest way to do this is with the following:
  • Disinfecting wipes
  • Disinfectant spray
  • Isopropyl alcohol
  • Hydrogen peroxide

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