This system is arguably, the most popular installed system build for the last 20 years. And, most of them also have a 16x25x1 pleated filter. A true recipe for call-backs, failed equipment and lost revenue. 

How many of you know the CFM requirements for an 80K btu furnace? This is relatively easy to figure out using the CFM/BTU formula: Output BTU divided by 1.08x delta T. The delta T target we use should fall in the middle of the temp rise range of the furnace, which is around 45*F for most manufactures. Now, we take 45 and multiply by 1.08 which equals 48.6. We then divide that into the OUTPUT btu's of the furnace. A 80,000 btu furnace at 96% eff will have an OUTPUT btu rating of approximately 76,500. So, we take 76,500 divide it by 48.6 and we get 1574cfm. We can use a higher delt T target if we want and get a lower cfm requirement, but that will come at a price, limit trips, lockouts, short cycling, failed heat exchangers ect.

So we have a system that needs around 1575 cfm for heat, lets look at the cooling side. 3 tons of ac using the nominal 400 cfm per ton requirement (ideal for our market), needs approximately 1200cfm total. 

Two systems that are installed together that have airflow requirements 400cfm apart. But wait, it gets worse.

Lets have a look at a 3ton evap. Below is a snip from the spec sheet of a 17" wide 3ton coil. Notice the pressure drop (restriction) of that coil at 1200cfm vs. 1600cfm. If this coil is on a 80k btu furnace, the pressure drop alone is almost at the MAX RATED DESIGN TESP (total external static pressure) of the furnace. We still have ducting, registers and filters to add to this yet.

But wait! There's more. Now lets add the most popular size and style of filter on the market today, the 16x25x1 merv 8. 

This chart straight from 3M shows the pressure drop of their filter at the recommended residential face velocity (yellow) and commercial face velocity (red). Basically what we see here is that this filter is really only appropriate for 2 tons of AC and approximately 50k btu at 95% furnace. Also, notice the resistance if we use this furnace at a higher cfm demand. A filter at .46 and an evap at .45 gives us a total external static pressure of .91"wc. Well above cabinet specs and pushing the limits of the blower motor itself (assuming we are still on an 80k furnace).

The real problem starts at the beginning with design and load calculations. Determine if the application really needs a 80k btu furnace, is the AC system too small for the application? Have a load calculation done to verify this information, Just because an 80k came out, doesn't mean an 80k needs to go back in. Evaluate the duct system and see if it supports the CFM requirements of both pieces of equipment.  Know and understand the equipment, part and pieces you are installing and how they will impact the performance of entire system. Spec sheets are wonderful tools.

Remember, its your duty as a trade professional to know this information better than anyone not in your trade. Understanding information like this and how to apply it, can be helpful for the books of your business as well. Potentially higher profit margins, added revenue with system upgrades and not to mention, reduced call backs.