air filter inside the return

[net_tech]

Several years ago I had my HVAC system replaced and a contractor decided to make a custom (diagonal) opening in the return, right before it enters the system cabinet, so I can slide in air filters instead of changing them inside the unit.

Is there a problem with air filter sitting inside the return or this is a common practice?

Thanks

[karsthuntr]

common practice, the diagonal cut is to give more surface area for the filter.

[Diceman]

Be glad, thats how it should be instead of taking the furnace panel off.
Used to be like all the time, back in the day.

[net_tech]

thanks for your quick responses

[midhvac]

If it's only one diagonal filter rather than a V configuration, you should have someone check the static pressure drop (resistance to airflow) across the filter. That diagonal scenario used by the old-timers LOOKS like it would work, but is actually extremely resistive to airflow. The bottom line is that the cross-sectional area of the duct still determines the filter resistance. I actually constructed some of these myself, and learned it the hard way on my own system.
Let's say for example that you have a 12" high x 25" deep duct, and you put a 20" x 25" diagonal filter rack in it. You still have the SAME AIR VELOCITY moving through that 12" x 25" duct as you would have with a 12" x 25" vertical filter installed. And the velocity is what determines the pressure drop.
That setup is fine as long as your system can get by with a filter that's the same dimensions as the return. But typically, this is not the case, because the filter needs a lot more cross-sectional area than the return.
The only advantage to the diagonal filter is that it obviously has a higher dust holding capacity. But that's a moot point if it's WAY too restrictive when you put a brand new one in it, when it's undersized.

[net_tech]

will post a picture so you guys can criticize. It’s hard to explain using word