Question: I have recently moved into a house and the furnace uses an electrostatic air filter filter and I am unfamiliar with it.  We have 2 filters one is in the furnace and 1 is leaning up against the wall…the slot is big enough for both filters, do I put them both in or would that restrict the airflow…is the sole purpose of having 2 to alternate when cleaning the other.  Thank you in advance for helping me with this question….thank you also for the info on cleaning it this website is very helpful.

It is fairly common for people to have two electrostatic filters. While one is drying after being cleaned the other is used and they are alternated this way. You are also correct about using both of these filters in a heating or air conditioning system. Installing both filters at the same time would definitely cause problems with air flow.

Restricting air flow can cause many different problems:

Restricting air flow in an air conditioner

• Evaporator coils icing up
• Poor air circulation in the conditioned space
• Increased operating costs due to longer run times
• A decrease in equipment efficiency

Restricting air flow in a furnace

• Furnaces cycling on high limit switches
• Cracked heat exchangers
• Increased fuel consumption due to longer run times
• A decrease in efficiency

The above are only some of the potential problems that can result from restricting air flow.

Question: 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 20×20x1
Grill #2 is 20×24x1

The under furnace tray is 24×24x1

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 filters from the return air grills.

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.