I have a whole house electric AC with gas heat in my home, which is approximately 1800 sq ft. There are 2 return air grills with filter in the house and a filter tray at the bottom of my furnace in the return air system.
The sizes are as follows:
Grill #1 is 20x20x1
Grill #2 is 20x24x1
The under furnace tray is 24x24x1
What filter combination should I run? Filter in all 3? And what type. I’m looking for the best airflow (low pressure drop) with the most filtration.
If you are looking for the best combination of air flow and filtration I would recommend a pleated filter with a relatively high MERV rating 10 – 12 and change it monthly. Although filters with higher MERV ratings have more pressure drop than air filters with lower MERV ratings, they will provide the best filtration. Change the filter monthly to prevent the air flow from being effected greatly.
Stacking filters is not a good idea because combined they may cause excessive pressure drop and can be far worse than having one pleated air filter with a high MERV rating. I would remove the filter from the filter tray and rely on the return air grill filters. Combined, the return air filters have a bigger surface area than the one at the air handler and will restrict air flow less.
If you are interested in upgrading your system, you can install an electronic air cleaner and install a pleated air filter as a pre-filter. Electronic air cleaners have almost no effect on air flow. Remove the foam pre-filter that comes with the electronic air cleaner and use a pleated filter instead. This combination will provide the best filtration and the least pressure drop.