Yeah perhaps. But on my purifiers there is at least one screen sorting out a lot of debris. And then I vacuum the filters every now and then. I guess one might notice if the airflow significantly reduces. I am running a test on mine to see how long they last. You have prompted me to get a new filter and later on just test how much air flow is reduced with the old filters—thanks for the inspiration. But I think one can safely run the home air filter for much longer than the 6-12 months suggested by manufacturers.
Increasing pressure drop across a filter is probably the best proxy for whether a filter needs replacement. In the field, a new, high MERV filter will often have a measured pressure drop between .15 and .3 inches of water column. An increase of ~.5iwc is probably sufficient to indicate replacement. Where high merv filters are used, it’s not uncommon to have very fine particulate be the primary source of clogging which won’t be visible so visible dirt isn’t entirely reliable. It’s not uncommon for sophisticated control systems to automate this. It’s also fairly accessible as a diy tech.
Do you know why, apart from increased energy usage and perhaps fan wear and tear, one needs to replace filters as pressure increases? My understanding is that filter efficiency does not decrease but maybe I missed something. And I agree measuring the pressure drop is best, but just thinking for some DIY, at-home quick fix without any special tools, it might just make sense to check if the airflow is more or less the same.
My understanding is the small particles can basically cover the surface area of the fibers and block their electric field. Here’s an image from one of the linked studies showing filters that are (a) clean, (b) after one test, and (c) having absorbed 2 grams / m^2 of smoke and having its efficiency drop from 92% to 33%.
Fan energy is a mixed bag depending on motor type. Constant airflow fans have a fairly flat pressure:CFM curve and will increase energy but PSC motors will see much lower airflow and reduced energy as a consequence.
In heat/AC systems airflow reductions outside a fairly narrow range will cause shutdown/failure due to high heat/freezing coils. In filter only systems you’d probably see low airflow and premature motor burnout. In standard motor assemblies going beyond .2 iwc increase would widely be considered quite aggressive (I have a fairly high risk tolerance for crazy stuff in buildings and probably wouldn’t do this)
I guess just a last question from me: Does it seem necessary to you to replace filters every 6-12 months as manufacturers suggest? I have run filters for >2 years and cannot really discern any increased load on the fan motors, just listening for changes in motor sound. Nor can I sense any significant reduction in air flow, just feeling the air flow with my hand. I am asking as I am telling people not to replace the filters as often as manufacturers suggests—it is expensive and people might think that their purifiers do not work well after a filter is 6-12 months old.
My experience is mainly in central HVAC systems and I mostly agree that filters could have a much long service life. However, I’ve seen significantly shorter life via fine particulate via the use of tap water in humidifiers. Ime this will clog a MERV 13 filter in a few weeks. If you can afford a manometer (~$100) capable of reading increments of IWC ~.1, testing pressure is quite simple and many more people would benefit from adding this type of measurement to their skillet! I’m happy to provide further guidance!
Yeah if $100 can save you even just 3 filter replacements, that sounds like a good investment. Maybe I should do this myself. For now, I will just hope motor sound + my intuition/tacit knowledge of air purifier air flow is enough for me to realize when a filter definitely needs replacing. Thanks Jesse!
Yeah perhaps. But on my purifiers there is at least one screen sorting out a lot of debris. And then I vacuum the filters every now and then. I guess one might notice if the airflow significantly reduces. I am running a test on mine to see how long they last. You have prompted me to get a new filter and later on just test how much air flow is reduced with the old filters—thanks for the inspiration. But I think one can safely run the home air filter for much longer than the 6-12 months suggested by manufacturers.
Increasing pressure drop across a filter is probably the best proxy for whether a filter needs replacement. In the field, a new, high MERV filter will often have a measured pressure drop between .15 and .3 inches of water column. An increase of ~.5iwc is probably sufficient to indicate replacement. Where high merv filters are used, it’s not uncommon to have very fine particulate be the primary source of clogging which won’t be visible so visible dirt isn’t entirely reliable. It’s not uncommon for sophisticated control systems to automate this. It’s also fairly accessible as a diy tech.
Do you know why, apart from increased energy usage and perhaps fan wear and tear, one needs to replace filters as pressure increases? My understanding is that filter efficiency does not decrease but maybe I missed something. And I agree measuring the pressure drop is best, but just thinking for some DIY, at-home quick fix without any special tools, it might just make sense to check if the airflow is more or less the same.
Efficiency in terms of fraction of removed particles wouldn’t decrease, but because CFM will decrease efficiency in terms of CADR will too.
Efficiency can decrease too, especially when there are lots of very small particles like smoke. See this reddit thread: https://www.reddit.com/r/crboxes/comments/1fznar2/comment/lr2j404/.
My understanding is the small particles can basically cover the surface area of the fibers and block their electric field. Here’s an image from one of the linked studies showing filters that are (a) clean, (b) after one test, and (c) having absorbed 2 grams / m^2 of smoke and having its efficiency drop from 92% to 33%.
Fan energy is a mixed bag depending on motor type. Constant airflow fans have a fairly flat pressure:CFM curve and will increase energy but PSC motors will see much lower airflow and reduced energy as a consequence.
In heat/AC systems airflow reductions outside a fairly narrow range will cause shutdown/failure due to high heat/freezing coils. In filter only systems you’d probably see low airflow and premature motor burnout. In standard motor assemblies going beyond .2 iwc increase would widely be considered quite aggressive (I have a fairly high risk tolerance for crazy stuff in buildings and probably wouldn’t do this)
I guess just a last question from me: Does it seem necessary to you to replace filters every 6-12 months as manufacturers suggest? I have run filters for >2 years and cannot really discern any increased load on the fan motors, just listening for changes in motor sound. Nor can I sense any significant reduction in air flow, just feeling the air flow with my hand. I am asking as I am telling people not to replace the filters as often as manufacturers suggests—it is expensive and people might think that their purifiers do not work well after a filter is 6-12 months old.
My experience is mainly in central HVAC systems and I mostly agree that filters could have a much long service life. However, I’ve seen significantly shorter life via fine particulate via the use of tap water in humidifiers. Ime this will clog a MERV 13 filter in a few weeks. If you can afford a manometer (~$100) capable of reading increments of IWC ~.1, testing pressure is quite simple and many more people would benefit from adding this type of measurement to their skillet! I’m happy to provide further guidance!
Yeah if $100 can save you even just 3 filter replacements, that sounds like a good investment. Maybe I should do this myself. For now, I will just hope motor sound + my intuition/tacit knowledge of air purifier air flow is enough for me to realize when a filter definitely needs replacing. Thanks Jesse!