How to find an appropriate replacement air filter for the furnace

Dirty filters not only make the HVAC equipment not last as long, but the low air flow makes them less energy efficient. A dirty furnace does not conduct heat into the air as well and the dirt sticks to the fan blades making it not last as long or spin as easily.

There are two main types of air filters:

The 1" thick ones that look like this:

And the 6" thick filters look like this:

_{(source: filterscanada.ca)}

Generally the 1" filters are good for a month or two. The 6" thick ones are good for 6 months.

There are also 2", 4", and 5" filters available but generally not at the local hardware store. Some folks who have 6" filters use a 2" + 4" filter and replace the 2" one every other month and the 4" once a year. This seems like a hassle to me, but it might save you $50 a year if you have 6" filters.

You are quite correct to note that pressure drop across an element within a dynamic fluid system is related to volumetric flow…this has certain implications.

To return to your original question, “Can I sense filter replacement need based on pressure drop?” the short answer is, not really. This is for the reason you state, that such a differential pressure value is dependent upon the volumetric flow, and if the flow is indeterminate or unknown, the value of the pressure differential does not reveal anything quantitatively as to the state of the filter element.

By way of example, a brand new filter element could indicate need of replacement if the fan speed were increased, resulting in a pressure differential increase to whatever level deemed to constitute the service limit of the filter.

The volumetric ‘nameplate’ capacity of the fan or blower purchased cannot be assumed to prevail within the target system, as the impeller speed and volumetric flow will depend upon many variables, not least those of the filters employed.

In commercial and industrial scale systems incorporating multiple elements, pressure differential measurements prevailing across individual elements may be monitored in addition to volumetric flow, which is controlled at specified process levels, as well motor energy required to sustain process levels, and these parameters and other cost factors integrated in calculations to arrive at maintenance schedules yielding least cost operation.

In a domestic DIY situation, the first and second filters could be changed alternately to determine which one, or whether both require replacement to restore desired system performance.

‘Filter capacity’ has been reached when the filter(s) in whatever combination do no longer yield the desired performance (basically CFM/min) which can be quantitatively determined. I hope this explanation will be of assistance.