# Inline whole house filters and their effective filtration performance

I would like to build a fairly aggressive whole house water filter system.

However, the 1 micron filters that I see are low flow and are for point-of-use (under the counter for a drinking spigot) and are not appropriate for the flow an entire house needs.

I do, however, see many 5 micron filters that have sufficient flow.

If I cascade two or three 5 micron filters together, do they have an effective filtration that is actually smaller than 5 micron ?

Further, do two filters with the same flow rate maintain that same flow rate regardless of how many filters are in line ? I realize there might be a non-zero reduction in flow just because of the additional couplings, but I am hopeful that two or three filters, in line, with the same flow rate, maintains roughly that same flow rate.

Am I correct that I can get an effective 2 or 3 micron system with, for instance, three 5-micron filters in a row ?

Thank you.

• You can increase the flow by using multiple fine filters in parallel, or a larger capacity filter. Oct 25, 2018 at 17:37

No. If a filter allows a particle size to pass, more filters of the same porosity will pass the same particles down the line. As the first filter clogs, it may stop finer particles, but it will also impact the flow rate due to being clogged.

Multiple filters in line are typically either arranged coarser to finer, or are filtering for different things (i.e. physical and then chemical filtration.)

You can get more flow by using finer filters in parallel, or you can use a pressure tank after fine filters to provide a certain amount of filtered water at a high flow rate, before you hit the flow rate of the filter as the pressure tank empties.

Many high-volume water uses do not require superfine filtration - most notably in the average house, toilet flush water is an absurd thing to spend filtration effort/cost on.

• Consumer filter micron ratings are averages, so a 5 micron filter sometimes lets through 6 and 7 micron particles and sometimes blocks 3 and 4 micron particles. But if I chain two together, my odds of stopping each particle double ... and so my average micron rating should go down - possibly below the rating of each individual filter. Right ? That is why I think two identical filters inline have a finer filtration than just one ... Oct 29, 2018 at 6:20

Old question, but I think it still merits a response (especially since a newer comment implies the poster was not fully convinced by a prior answer).

The short is no, more filters will not change the design of a filter (i.e. multiple 5 micron filters still cannot efficiently capture 1 micron particles) and depending on your setup, more filters can impact your flow rate, but for just 2-3 filters, if installed correctly, it should be negligible.

1. If I cascade two or three 5 micron filters together, do they have an effective filtration that is actually smaller than 5 micron? - No. I am a professional who works on ultra-fine water purification systems for pharmaceutical manufacturing, so believe me when I say that more of the same type of filter does not typically nor remarkably improve water purity. A 5 seat SUV seats 5 people. You could jam another person in the trunk and have someone sit in a lap, but you increase the passenger number and decrease safety. Increasing more 5 micron filters in line does not change the size of the pores in the filters. It can, plausibly, reduce maintenance costs in the long run, if you do it correctly (i.e. cycling filters and only replacing 1 at a time). However, this would also increase your risk of decreasing flow rate. Perfect segway!

2. Do two filters with the same flow rate maintain that same flow rate regardless of how many filters are in line? - It depends on the current flow rate and the installation. If the flow rate is exceedingly high, you would not likely see an appreciable decrease just from 1-2 additional filters to the 1 that is in line. However, if your flow rate is, lets say 4 GPM, you could theoretically see a decrease of .4 GPM or greater (10%). If you have a pressurized water tank down line from the filters, then this becomes a moot point so long as the tank volume is sufficient for your daily demand. As for the filter installation, in my line of work we have a term for a parameter of installation that we try to minimize, "dwell or void volume". This term means the amount of additional water you have in your system between the well and the tank (if applicable) or tap. To much dwell volume will equate to more back pressure and therefore a negative impact on the directional pressure (or flow rate) of your system. I would recommend keeping the connecting lines short to reduce this impact. If you added 20 5 micron filters, and your flow rate is 4 gpm, you will likely experience an empirical change in flow rate and possibly water pressure (if you do not have a down line water tank).

I hope this is helpful for the poster or anyone else that finds this.