When to change filter in air cleaner?


Help Support UKworkshop.co.uk:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.


Established Member
UKW Supporter
27 Feb 2023
Reaction score
Probably a silly question but I've got a Record Power AC400 air cleaner which has had intermittent use in my single garage workshop for about a year now. The filter is starting to go a bit beige and I'm considering replacing it.

How do you know when is the correct time to change the filter?
They are supposedly cleanable, blowing them out with compressed air or washing - brief instructions in the manual, if I remember correctly. If you've already done that but think they need replacing, then they probably do...
I think it's subjective, when mine looks bad I unclip the outer and inner filter, take them down the garden and give it a good brush/bash out over the compost heap. Has it 3 years, similar use to yours, and have just bought a replacement ready for next time.
I've got a few different types of similar, and as mentioned either blow them out or brush outdoors or vacuum them clean, haven't had to replace any.
Having some dust buildup in the filter isn't necessarily a bad thing. Big bag extractors build up a dust "cake" on the inside and that helps trap more fine dust.
The problem is when the filter becomes too loaded, the airflow slows down noticeably and you are not getting as many air changes / hour in your shop.
Just take note of how long the filter takes to clean the workshop air with a fresh filter and once it's taking noticeably longer to do that with a dirty filter, time to change or clean it.

Professional kit measures the pressure diference across the filter - often with nothing more than a simple u tube of water - because the difference increases as the filter loads up. It would be easy to add this as an upgrade
> Professional kit measures the pressure diference across the filter - often with nothing more than a simple u tube of water - because the difference increases as the filter loads up. It would be easy to add this as an upgrade

That sounds like a great idea. I can 3D print or buy a cheap manometer. My only question is how to position/install it. Should I drill a hole into the side of the air filter inlet and poke one end of the tubing through it to measure the pressure there?

I'm thinking something like this would do the trick:

A good tip for these type of filters is to place a section of spray booth media ( the fine one like white fluff not the coarse fibreglass one ) known as secondary filter media or similar in front of the filter. I also use it in my Microclean filter.
I use a double thick layer as the width of the roll folds conveniently in half to about the right size, clip it in the filter holder tab top and bottom.
This significantly improves the life of the inner filter and is quick and easy to vacuum off once you notice it is getting dirty.
You can wash the inner cloth one but the one in the box frame needs replacing eventually.

Put a piece of normal paper over the clean filter with the motor running (doesnt need to cover all of it). Switch off the power and note the time it takes for the piece of paper to fall off the filter. Write this time on the side of the unit. Every now and then repeat. When its falling off in half the time (or whatever time you think is appropriate) thats the time to clean the filter.
Very unscientific but gives a good gauge of how blocked your filter is.


> I'm thinking something like this would do the trick:

View attachment 170002
You could use a big easy to read gauge like that or even make a smaller one that would attach to the side of your filter if it is accessible to read.
I'm not sure how many inches water gauge the fan will suck. Just try it with a piece of tube and a bend to see before making something up.
Personally I'd simply leave one end of the pipe open to ambient and the other needs to poke into the fan body behind the filter and on the suction side of the fan. Use a plastic bulkhead connector or just drill a hole that gives a snug fit to the tube you can get and seal it with silicone, bluetack, whatever.
Why not simply set a regular schedule/date to remove filter and clean it? I use a soft 1inch paint brush to clear between the vanes on my air purifier filters, after first gently tapping sed filter rim (it's plastic) with the brush. Tapping first looses the dust and then continually rotating the filter whilst tapping gets rid of much of it. Then use the brush to gently remove any remaining dust and repeat tapp/brush ritual umtil clear?

My filters have an out layer of vertcial sections of course which is where thebulk of dust gathers...

Just remember to stand up wind of the dust - for obvious reasons...

Clean the filter say once a month - cerrainly no less than say three monthly intervals if in heavy use.

They'll last an age before requiring renewal - at least 6 month for my in house filters according the manufacturer. They're not designed for heavy workshop use; but I know they will work quite well there as well - even if requring the above cleaning of the outer layer more frequently.
> Why not simply set a regular schedule/date to remove filter and clean it?

This is probably the simplest solution, but not really optimal as I go through phases where I'm creating a lot of dust and then months when I'm only making the odd cut here and there. I'd prefer to just look at a gauge to tell me that the fan isn't sucking like it used to.

I've got as far as printing the manometer and using 6mm OD PVC tube to fit it. The problem is that when putting one end of the tube right next to the fan outlet i.e. to capture the air pressure from the air filter's fan, I get exactly zero movement of water in the tube.

I've tried mounting the manometer at an angle, and using oil instead of water. No movement, even with the air filter on maximum suction. Not sure if I'm missing something obvious. Maybe a wider diameter tube?

I think the place you need the sensing inlet end is through the side of the fan case between the back of the filter and the suction side of the fan. That will read a negative pressure used to draw air into the filter.
You can then test it by placing a piece of kitchen roll across the front of the filter to simulate it becoming blocked. The suction should increase.
You'll need to drill a 6mm hole in the case, poke in the tube and seal it there with bluetack or chewing gum :)
I tried this and unfortunately the water inside doesn't move at all between having a new filter and covering the air inlet entirely.

New air filter:


Inlet completely covered with a piece of plywood


Any other ideas or should I scrap this as more trouble than it's worth?
You've stumped me there.
Clearly your gauge can see a little suction but not much.
I downloaded the ebm papst catalogue for backard curved impeller fans - a type of fan that might be inside a filter like this, and are the style inside some of the microclene units.
6 to 8" dia fans, power around 130 to 160 Watts, they are specified to pull upto about 2.5 to 3.5 inches water gauge, far more than you are seeing
You have given it a good try, might be time to resort to a simple replacement schedule :-(