The effect of adding more
The effect of adding more filters in tandem does not improve the filtration significantly. If the filter 1 captures 90% of certain particles, only 10% will get through and reach the filter2. Filter 2, being the same as filter 1, will capture 90% of the particles that reaches it, which was only 10% of the original number of particles: that is 0.9% of the original pollution. Ergo, the improvement you will get from putting two filters in tandem is below 1% of filtration.

This is a great site. I’m an
This is a great site. I’m an engineer so I particularly appreciate the detailed technical info. Now, here is my question. As the MERVE rating increases, is it a function of the density of the filter? If so, can I gang them for better filter performance? So for example, assuming I have the space for ganging filters, which I do, if I gang an 8 MERVE and an 8 MERVE do I get the basic equivalency of a 16 MERVE for filtration efficacy and airflow? I am not concerned about using more electricity due to some reduced air flow. I want higher performance of the combined filter combination without having to search out MERVE 16 filters, which are not all that easy to find in reasonable thicknesses! Thanks! Ed

Robert, you will find the
Robert, you will find the model number on many furnaces and boilers by lifting the door off. It will be on on either side, below the gas burners or on the face plate. On some furnaces it may also be found on the outside of the furnace. Your air conditioning tag, if not faded away, can be almost anywhere on the unit and is sometimes hidden down by where the copper refrigerant lines connect to the unit. As an alternative they may be listed on a service ticket if you have one. Use about 5 characters of the numbers a space and PDF “58STA PDF”. If you play around with that and don’t find anything chances are it’s time to replace your equipment. To tell how old the equipment is google the manufacturer’s serial number nomenclature, “Rheem Serial Number Nomenclature”. If you can’t do that there are 2 hacks. Try to find the 4 digit date code on components “3602” would be the 36th week of 2002. Components get replaced so you have to find at least 2 from the same year and that’s still not a guarantee. The other hack will only get you close. Look for a year after ANSI at the bottom of the furnace tag. This is NOT necessarily the year the furnace was made this means the furnace meets the ANSI Standards for that year “ANSI – 2001”. Those standards change every few years so that just gets you close. Good Luck

Layman homeowner here – I
Layman homeowner here – I note higher MERV rated filters create duct rumble, but only with some filters. Clearly, not all filters are the same.

Via this site and the trails it led me down, I’ve learned two MERV 11 filters can have the same filtering capacity, but one might allow more air to transfer. From that information, I did a bit of digging and one very well known company provided a demonstration of their MERV 13 filter up against others. The flow capacity was, VISUALLY, significantly improved, but you paid nearly three times as much for their high end filter.

Sadly, most of us homeowner types don’t want to or don’t have the time to learn the basics about different filters, let alone the means of monitoring and testing them.

I wish more companies pushed manometers and Magnehelic gauges being installed, as well as simplistic info brochures explaining why jumping to a MERV 13 from a 50 micron filter can be problematic, or letting a filter double in depth due to dust build up (saw it, as a handyman – the furnace still worked (kind of).

Sadly, even so called reputable companies can be less in-the-customer’s-best-interest. When I moved into our current house, I noted one of the flex ducts was crushed in half, because their “expert installers” didn’t cut the basement paneling sufficient to allow the duct to pass.

They, also, put the return in the floor, in a traffic area, something no one could convince this layman was a good plan.

I have been in the HVAC
I have been in the HVAC Industry going on 40 years. In the real world we don’t see a huge failure of ECM motors. My own furnace is 12 years old, has a 4″ media filter and runs 24/7/365. A simple indication that an ECM Motor cannot overcome the static pressure of the system is called “hunting” Because the ECM responds to a torque sensor rather than static pressure, it will start surging up and down in RPM if it can’t maintain the selected CFM. We see horrible duct systems and yet the ECM motors overcome the resistance almost always. Having said that, a poorly executed duct system will probably have a negative effect on motor longevity whether PSC or ECM.