How to determine what grade air filter is appropriate

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.

You'll have to find a unit that can match the required intake air flow of the furnace.

Estimating required air flow.

To estimate the required amount of air the furnace needs, you'll have to know what type of furnace you have, and how many BTUs the furnace is rated for. Once you have this information, you'll divide the furnaces rated BTU value by 10,000.

100,000 BTU / 10,000 = 10

Next you'll multiply that value by the CFM/10,000 BTU value for your type of furnace.

• Natural Draft Furnace = 100 CFM/10,000 BTU.
• Induced Draft Furnace = 130 CFM/10,000 BTU.
• Condensing Furnace = 150 CFM/10,000 BTU.

130 * 10 = 1300 CFM (Cubit feet per minute)

So if you had a 100,000 BTU induced draft furnace, you'd need a filter with at least an output of 1300 CFM.

As the filter gets dirty, air flow will naturally be restricted. Because of this, you'll want to choose a filter system with a higher CFM rating than you need. In our example above we wouldn't want to get a unit that is rated at 1300 CFM, we'd want one rated somewhere closer to 1400 or 1500 CFM.

Measuring Air Flow

You can determine the actual air flow using a handheld Anemometer, like this one

Start by measuring the area of the duct (Length * Width). Next measure the air speed using the anemometer (ft/min). Finally multiply the area of the duct by the air speed.

(20" * 25" = 500 sqin.) / 144) = 3.47 sqft.
3.47 sqft. * 375 ft/min = 1301.25 CFM

To determine if the air filter is restricting air flow, calculate the CFM both with and without the filter connected.

Watching for signs of reduced flow.

If you notice any of the following problems after installation, you likely have restricted the air flow too much.

• Furnace has trouble producing a flame, or flame is small.
• Furnace constantly overheats.
• Furnace struggles to keep up with heating demand.
• Output air flow is weak and/or cold.