I am currently living in China and want to improve the indoor air-quality. We have a Hitachi R410A HVAC. All the room in the house are controlled separately. Just to make it clear up front, I am not a technical guy and have no idea about air conditioning or HVAC (just learned what a HVAC is recently while doing some research into my issue).

The air is 'fresh' from outside. Since the Pm2.5 count is very high outside, I would like to filter the air that is blown in. I was thinking of a very straightforward solution, but I am not sure if it is possible. I want to buy some HEPA filters and cut them to the right size and fix these before the A/C outlets.

Will this work? Or does someone expect the pressure drop will be too high and create malfunctions or breakdown of the HVAC?

Thanks for the replies! As requested, I added some pictures of the unit that is placed outside (pic 1 and 2), example of the vents where the air from the room is sucked in an goes to outside (pic 3) and some of the vents were the air is blown in to the room (pic 4). Each room has its own set of vents. The HVAC is central and each room can be controlled separately with a controller on the wall.

HVAC unit outside the balcony

Type of HVAC

air out vents living room

air in vents on top living room

Also attached a link to the unit that we have (Hitachi multi set free, R410A, FSNQ) link to product Technical data brochure In the brochure I nowhere find any mentioneing of HEPA filters, so I guess these are not present.

Any more suggestions, thoughts about if it would be possible the attach a HEPA filter (need to think how without causing to much leakage) without causing serious HVAC malfunctioning would be appreciated.

  • 1
    Hitachi R410A seems to be only the compressor, which is only a part of an air conditioning system. Do you have a central air conditioning system, or a single independent unit per room? Also, most air conditioning systems don't bring air in from outside, they simply circulate and condition the indoor air.
    – Tester101
    Commented Nov 8, 2013 at 16:12
  • Some images of your equipment could help clear up some of the points that @Tester101 raises, and help get you a better answer. I kinda suspect this may be a mini-split system, which wouldn't have ducts, much less outside air intake.
    – mac
    Commented Nov 8, 2013 at 17:12
  • @Tester101: I assume "R410A" is the refrigerant used, not the compressor model. It's probably stamped on the HVAC unit somewhere. We really have no useful information about the equipment.
    – Hank
    Commented Nov 9, 2013 at 4:39
  • Hi all, I updated my question with more details that were requested. Thanks for taking the time trying helping me out.
    – berend
    Commented Nov 9, 2013 at 5:18
  • I'm not familiar with this unit, but it appears that the filters may actually be at the outlet vents. If you remove the cover of the vents that blow air into the space, are there filters there?
    – Tester101
    Commented Nov 9, 2013 at 14:01

2 Answers 2


After reviewing your pictures and the promotional brochure that you posted, I am fairly confident that your system does not take in "fresh" air from outside. The outside air is used only to cool the compressor/condenser unit outside, there is no duct work bringing air from outside to inside. The inside and outside units are connected only by refrigerant piping.

Your indoor units draw air in (likely through the grate on the vertical part of the tray ceiling), condition the air (either heat or cool), and then blow the conditioned air out through the register in the lower part of the ceiling.

annotated photo from OP

Above the ceiling, your unit likely looks something like this:


There is likely an air filter behind the intake grate.

filter location

The filter should be accessible either by removing the intake grate, or by going in from the bottom, perhaps through the square access panel in your ceiling that is shown in your picture.

filter service

From my experience with mini-split systems, I would imagine that this filter is a washable/reusable type, rather than a disposable type. This may limit your ability to find high-efficiency aftermarket filters.

Again, remember however that your system does not appear to take in air from the outside, so your concerns about bringing in environmental pollution may be unfounded.

  • Dear mac, thanks a lot for your clear explanation and the research you have done! You have found more than I was able to. Highly appreciated. Yesterday my wife was finally called back back technical support from Hitachi. They said the same, the air is not coming from the outside, oly circulates inside. SO as long as we keep the air clean inside (separate air purifiers) we should be ok ! Furthermore they mentioned that the current filter is only stopping large particulates, and not the PM10 and lower stuff that I want to 'catch'. Thanks again !!
    – berend
    Commented Nov 21, 2013 at 14:49

Adding a filter onto an HVAC unit that's not designed for one will almost certainly cause problems... definitely with efficiency, possibly mechanical problems as well. If your model does not have a built-in filter you would probably be better served by getting a standalone air filter.

However, many units do have at least a basic filter built-in, and you may be able to clean it or replace it. Typically they are on the input side of the unit, to remove any particles or debris before the air passes through the unit. If you have or can find a user's manual, check there for instructions. Or it may be obvious once you start looking. In the USA they are often large square filters that come in standard sizes and slide into the unit, but on a smaller model like a window unit it may be a non-standard size. I do not know anything about HVAC equipment in China.

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