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HEPA filters - replace or reuse?

Hi all,

We’ve published a fair bit of content recently on air purifiers and bushfire smoke. One of the the key points of advice is that for best performance, an air purifier needs a HEPA filter.

HEPA filters generally need to be replaced - their design usually means they can’t easily or effectively be washed or vacuumed and reused. They aren’t usually cheap, so this cost can add up over time.

But along with the current shortage of air purifier stock in stores, the replacement HEPA filters are apparently also hard to find at the moment, at least for some brands. (I guess the filters have been worked hard in all the smoke…) So the temptation to reuse rather than replace must be strong.

My questions:

  1. Has anyone tried cleaning a HEPA filter (whether from an air purifier, vacuum cleaner or other appliance) and reusing it? What was the result?
  2. Do you replace the HEPA filter on schedule, as dictated by the machine or its instruction manual, or do you just keep using it until you notice the performance has dropped?

Thanks!

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My current CPAP machine has a replaceable HEPA filter, it also has a normal fibre filter on the outside which is replaced when greyish in colour. It must be replaced when the warning appears. Washing one to try to see if it worked resulted in a warning within days or reusing it. Not worth the risk in my opinion. The outside filter we have at times washed and reused without too much hassle but the HEPA filter lasted less time when we did so.

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Chris Barnes asked:

I tried giving my face mask P3 filters a shake-out and quick wash, but it had no effect. Being paper-like material, they could not be wet properly without risk of disintegrating, so I think they are strictly one use only, until they become too restrictive to breathe through.

My Breville air purifier has a warning light for when the filter needs replacing, which is apparently after 4320 hours (180 X 24 hours), rather than when it is clogged up with particulates. They say the filter should be cleaned at least monthly for best performance.

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@gordon According to the Breville instructions, the 4320 hours is based on operating hours - so it will last longer if used less often, though as you say, it is simply counting time rather than actually measuring how clogged the filter is. Out of interest, do you run it 24/7 or only as needed?

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Yes, 180 days of continuous operation, but that takes no account of how polluted the air is, and I can’t imagine mine will still be ok after even 3000 hours with the amount of smoke there has been lately.

I only use it when needed, which has sometimes been for a couple of days continuously, but with the reduced density of smoke in recent days, only from about 6pm (when it arrives from fires to my SE) until morning.

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You cannot “clean” a HEPA filter and ensure that it will still remove 99.97% of particulate matter.
Any cleaning process is destructive to the sensitive filter media and can also lead to distortion of seals, causing bypass.
You may also find that cleaning a HEPA filter will cause your appliance to overheat/work.

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It is worth checking ones user manual to see if the manufacturer recommends cleaning or replacement of filters. Some filters are not designed for being cleaned and doing such may reduce their effectiveness to either filter particles or say VOCs.

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May I ask which machine? I’m not happy with the filtering system on my Philips Dreamstation and went back to using my old Remstar machine which had very effective filters, but neither are HEPA as far as I know.

In answer to the original question, I tried washing the HEPA filter on my old Bissell cat and dog vac because you cant buy new ones in Oz anymore. Waste of time, did not work. Ended up buying some generic ones from ebay which came from China. They don’t seem to be as good as the original but what can you do…

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You can get Hypoallergenic Ultra Fine filters for the Dreamstation (https://www.cpapaustralia.com.au/philips-dreamstation-disposable-ultra-fine-filter-6-pack/) + a pollen filter (https://www.cpapaustralia.com.au/philips-dreamstation-reusable-pollen-filter/) that might improve your experience. They also have an in line Bacterial filter available that helps a bit with those who suffer allergies (https://www.thecpapclinic.com.au/shop/item/philips-respironics-in-line-bacterial-filter).

Mine is the ResMed AirSense 10 with both the standard fibrous filter and what they term the Hypoallergenic Filter but others call HEPA (https://www.amazon.com/CPAP-HEPA-Filters-Packs-Machines-Medihealer/dp/B07QG1ZVND)

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Yeah I already have those and the pollen filter and a bag of bacterial filters… none of them are HEPA and the bacterial filters are effective but annoying… why not build them to be able to replace the filter inside, instead of having to get whole new filters. They arent lasting long at the moment and none of them are cheap :(. Oh well, thanks for trying to help :slight_smile:

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The AirSense has been very good with those filters in place. It gives a warning when the fine filters need replacing (perhaps air flow related). They last about 2 weeks or so a filter at the moment, before around 4 weeks a time. I don’t think there are many that reduce the particulates to very low levels but the ResMed devices are able to have ResMed filters that replace the standard filters and effectively capture very fine particles. That may be an useful alternative, they in the one filter incorporate a large particle filter and a fine particle filter (I have used these before but in very dusty/smoky conditions you replace them frequently so can get expensive):

https://www.resmed.com/epn/en/healthcare-professional/products/accessories/hypoallergenic-air-filters.html

Why can’t they build them to incorporate finer filtering? Most likely they would need very large filters to allow unimpeded airflow to the machine. As the motors and fan are not very powerful the extra strain of pulling air through a small sized filter will wear the motor out much sooner than would normally be the case. Some filters I have seen in the past have had a tacky substance that “grabs” fine particles but of course this adds to the expense of the filters, they are not reusable, and may need replacing often to be at their best effectiveness.

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Probably why my old machine seems to be filtered better. Those old filters are about 6x the size of the new ones, they last longer and filter better.

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The design wisdom for filtration.
To filter to a very high standard and remove very fine particulates.

Filtration is most efficient when done progressively in stages, EG with several layers of graded media. Coarse particle filters, followed by medium particle filters, then fine and ultra fine. The coarser layer filters can often be washed or cleaned with ultrasonic cleaners.

For dusts there are a number of different technologies used for industrial scale systems. For the home, the use of physical media to trap small dust particles remain the simplest. The beta ratios for physical media determine performance. Efficient filter media is not a simple flat surface. Layering in depth provides high performance, and collects greater quantities of particulates. It may seem counter intuitive, but what is trapped remains trapped and cannot be readily removed or cleaned. Filter media is not a flat surface. It is a ridged fibre like matrix like a thick bale of straw.

Coarser particulates that remain on the outside of filter media can often be removed, but not the fine particulates collected and embedded deep within the media.

PS.
Some of us may be familiar with oiled foam outer covers for engine air filters or precleaners on 4WD air intake filters. Eg Donaldson, with typically two internal paper air filter cartridges. One outer coarse, one inner final/fine.

Effectively cleaning the air in a home is no different. It needs to be done in stages. Typical home air purifiers rely on one or two layers only, limiting capacity or particle size performance. Perhaps fine for everyday use, but pushed with our current bushfire fallout.

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I have a Miele Complete C3 Comfort Total Care vacuum cleaner with HEPA. I used to replace the filter every few months, but at $50 a pop, I thought I’d try attempting to clean it at least once with my air compressor. Being careful not to damage it with too high a pressure, I was able to blowout quite an impressive amount of dust - blowing from the underside of the filter back out through the topside, using a back and forth sweeping motion. When I couldn’t see any dust (about 1-2 minutes), I gave the top and bottom a brief tangential blast to help clear any remaining dust from the two surfaces.

Did it work? How would I know? Graholl points out that there will be finer particles trapped and inaccessible to this approach. However, I am now purchasing new filters only half as often, and haven’t noticed any difference in the vac’s operation. I did the same with the foam-type initial coarse filter. However, they are so cheap I now replace them instead.

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I use a HEPA filter on my Ryobi 30L wet/dry vacuum cleaner. I blow it out occasionally and it seems ok but I have no way of measuring particle size before and after. I have taken to using a disposable paper bag from Powerfit, which really annoys me, but I get a lot more vacuuming before emptying. I use an orange cloth prefilter which came with the machine. There is always fine dust on it when I open the machine to change bags. The HEPA filter is stopping the finer dust I hope.

I always work with a respirator.

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