Health, Reviews, Solutions

How to cheaply convert your air-con into an air purifier

Beating the Haze!

By now, the world has once again heard about Singapore & mistaken us as some China state. Hint to these people, Google ASEAN or South-East Asia. Some people are still hazy about who we are (pun intended.)

But I digress. There are many ways for people to beat the haze but the best is to stay indoors with the air-con on and all doors & windows closed as tightly as possible. If you have put off installing an air-con in the house, please consider doing so now, as the haze will be with us for many more weeks. Look at my post here on how to choose an air-con suitable to your house in Singapore.

Offices are usually the best as the air-cons are maintained by professionals regularly. The air-cons at home, may not be maintained as regularly so do clean them today if you haven’t done so in the last 3 months. I just cleaned mine today & there’re a lot of grey ash on it.

Many people are also turning to air purifiers to help clean the air in their home or office and while many of the good ones are effective, they also tend to be noisy ( >60dB on high setting), not to mention they cost many thousands of dollars.

Let’s take a look at some that I’ve come across.

Air Purifier

Air purifiers work by pushing & pulling air through multiple membrane filters. Due to the resistive nature of membrane, the fan motor must be very powerful in order to move enough air to be effective. Hence, you have 2 types of noise, mechanical noise from the motor & wind noise. The situation is worst in air purifiers with HEPA filters because those are so restrictive to air flow, hence some makers like Rainbow & Delphin use water as the main filter + a “L-Lamella” separator cone to reject fine dust.

Click to expand
I bought this German made Delphin Vacuum system 10 years ago & it’s still working. However, I rarely use it for air purifying because the wind noise is annoying.

Rainbow still include a HEPA filter but this results in the motor having to work very hard & it’s really loud when vacuuming. When purifying air, it’s not as annoying but still noisy. The air flow also doesn’t seem as high as the Delphin.

Rainbow Vacuum with HEPA filter.

The advantage of water-based system are they’re less restrictive to air flow and thus saves energy + can recycle the air faster. Even if they can’t filter the same amount of dust in 1 cycle, the amount of air they can process in the same amount of time makes up for their lower efficiency. This is especially true for the Delphin system which doesn’t have any membrane filter at all!

Water is also able to dissolve much of the harmful chemicals & odour in the air that membrane filters are not able to remove. This is why many air-purifiers also include a charcoal (or activated carbon) filter which adds to the cost of purchase and maintenance.

OK fine, I just want to know the cheapest & fastest way to purify my air!

Why I’m glad you asked. If you’ve skip the above explanation, that’s fine. I recommend using the air-con as your MAIN air purifier for 2 simple reasons. It’s the most efficient and fastest mover of air in the house, and it’s also very quiet (typically 25dB on Auto).

BUT, obviously the filters in your air-con are not actually very good. Some air-cons have UV and Plasma technology. UV kills virus & bacteria so it only sterilize the air, not filter dust.

Cold Plasma technology is basically air-ionization, which introduce an electrical charge to dust particles so they can stick to another charged surface. However, this introduce a small amount of ozone which is unhealthy with prolonged inhalation. The charged dust particle must still stick to another opposite charged surface before it’s removed from the air so its efficiency is not consistent in every environment.

SO, the best solution is to introduce another filter behind your air-con filter.

I recommend the 3M Filtrete Room Air Cleaning Filter with Carbon.

I bought the blue box Filtrete at a DIY shop for less than $30.
I bought the blue box Filtrete at a DIY shop for less than $30.

It’s an electrically-charged membrane filter with carbon so it’s able to filter particle down to 0.3 micron + the carbon will help to absorb harmful chemicals & odour. It’s not as efficient as a HEPA filter but because the air-con is pushing so much more air through the filter per minute, the result is still very good with noticeably cleaner & neutral smelling air in 10 minutes.

The black mesh is the carbon & the membrane filter is behind it. Just stick it or place it behind the normal air-con filter, then reinstall your air-con filter.
The black mesh is the carbon & the membrane filter is behind it. Just cut it to size & stick it or place it behind the normal air-con filter, then reinstall your air-con filter.

It’s recommended to cover up to 70% of the air-con filter to allow good air-flow so the air-con cooling power is not compromised. Don’t worry, after 1 month, the membrane will turn a icky grey to let you know the filter IS working. The filter is rated to work for 2 months but with this haze, I recommend changing it sooner, when the filter turns completely grey.

I install the 3M Filtrete behind the normal air-con filter to prolong its life-span.

Each box contains 2 pieces of 60 x 30cm filter, which is enough to last an air-con blower unit 18 months if you change it every 2 months.

For less than $30 per air-con unit per year, I’d say that’s VERY cheap!

Lastly, for you people who are lazy, you can install the 3M filter OVER the air-con filter so you don’t even need to wash the filter in future. Obviously, the 3M filter will get dirty faster but since the haze wouldn’t last forever, it’s completely fine. 😉


26 thoughts on “How to cheaply convert your air-con into an air purifier

  1. Ingenious. Air purifiers are not that expensive though – why not just get one for under 100


    1. There’re many types of air purifier. Depending on the technology used, the cost & effectiveness of the air-purifier can vary widely. Which kind of air purifier are you talking about?


    1. Thanks for the video. A small air purifier takes a very long time to purify the air. If the door is opened & closed often, outside air will pollute indoor air & a small purifier will still be ineffective.

      In the video, it took 20minutes to purify a 30m³ room. Imagine you have many people walking in & out of the room. An air-con is a lot more powerful so it can still be effective.

      Anyway I wish to point out that PM2.5 means a particle size 2.5micron or 2.5μm aerodynamic diameter. The 3M Filtrete is effective down to 0.3micron which is much smaller than PM2.5.

      In fact, it can filter most bacteria & some viruses.


      1. Just a small point to note, the 3m filter may remove up to 0.3 microns but do remember to factor in the efficiency of its ability at being able to remove the particulate matter. Given the high air flow the air conditioner has and the nature of using electrostatically charged filter, it’s unlikely that it will have an extremely high level of efficiency. As a benchmark, True HEPA filters are rated to 99.97% efficiency for 0.3microns. While this is the idea case, true HEPA filters and their machines don’t come cheap and tend to be very noisy. I think at this point, it is really about finding the right balance for your needs. Having tested this filter, I’ve found it to be effective at removal of odors, however after trying out a HEPA filter based air purifier, I found the air quality to be better than the filters.

        Another point to note as well is that for others reading this, please heed the Writers advice to only cover up to only 70% of the air conditioner. Besides a reduction in cooling performance, by covering more than 70%, its likely to reduce the lifespan of your Air conditioner fan coil motor in the long run, because the motor needs to work harder due to the reduction in air flow.


  2. Yes, Air purifier has great technology in it as it can purify your in-house air and you can get best results.


  3. Just a heads-up to readers, seems like the 3M Filtrete is running low on stock. The guy at the DIY shop advised me to buy and stock up, meaning buy 2 boxes when it becomes available.

    I will head to NTUC Fairprice Extra to see if they still have it.


  4. I have one concern over air purifier using water – it is presumably blowing out humid air. Won’t that extra humidity encourage moulds to grow?


    1. Not at all, the “L-Lamella” separator cone rejects water and dust equally well. So much so that you wouldn’t even smell anything even if the water is absolutely dirty. If dirty (evaporated) water is ejected by the Delphin or Rainbow vacs, you should be able to smell a very unpleasant odour. In my experience, it’s quite odourless.

      Unless you’re talking about those air purifier which IS DESIGNED to also emit cold steam as a humidifier.


  5. Even though the AC is blowing more air through the filter, are you sure that makes it more effective than an air purifier??

    After all, an air purifier continuously the same air through the filter again and again.

    An AC on the other hand is just blowing in outside air which passes through a less effective filter ONE time.

    Have you even tested with a particle meter?


    1. No, I don’t have a particle meter. This is based on the documentation from 3M. However, residential air-con blowers also cycle the air in the room. Obviously, I’m not talking about central aircon that don’t use casette/ceiling blowers.

      Where did u get the idea that air comes from the outside?


  6. Eric, thanks for a simple and very effective solution.
    Since you are into aircon, would you know where I can buy cut-to-fit foam aircon filter that you slide-into the filter frame that used in central aircon? Thanks.


  7. Hi can i know which side should be the front whereby the air flow in. The mash woolen side or the a bit plastic side?


  8. Air conditioners have become a prominent part in people’s life. In the heated days imagining a day without the air conditioner can be horrifying. Therefore knowing a competent air conditional service provider is absolutely necessary.


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