MERV level on home AC filters in South Louisiana

For standard 1-inch thick return grill filters MERV 4 changed monthly - anything significantly higher can vastly increase total external static pressure (TESP, resistance to airflow) and overtime reduce the life span of the HVAC.

The system has to work harder, blower motors, etc to overcome resistance to airflow with a restrictive filter. With a PSC blower motor air flow will be reduced with higher TESP, with EMC blower motor air flow may not be reduced as it will ramp up to try to overcome higher TESP but it will reduce the life span of a very expensive motor.

For thicker filters, 2 inch to 5 inch thick, then higher MERV 8 to 12-15 can be acceptable with minimal increase in TESP. One of my 2 return air grills can accommodate a 2 inch thick filter, so I use MERV 8 in that one.

The purpose of the filter is to protect the HVAC equipment not to improve IAQ for the occupants. If IAQ is a priority then I’d consider other options to deal with that - this is assuming your HVAC can only accommodate a 1-inch thick filter.

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The purpose of the filter is to protect the HVAC equipment not to improve IAQ for the occupants.

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This is what I've always been told. Use a standalone HEPA filter if you wanna improve air quality in certain spaces. Your HVAC will hate you if you stick a MERV 16 in there just because you think it helps your allergies.

Piggybacking on this: my hvac guy recommended I change the furnace filter a while back, so I swapped it with a MERV 11 furnace filter. I’ve also always run MERV 1 return air filters. Haven’t had any issues with my system and it’s 10+ years old now, but is that overkill? Should I ditch the furnace filter and run a MERV 5 or something in the return air or vice versa?

if you are doing 1" filters, go cheap/fiberglass.

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The system has to work harder, blower motors, etc to overcome resistance to airflow with a restrictive filter.

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Well, not as much work harder, just you won't get the effective air flow and run the risk of icing the evaporator. Most systems have the blower after the filter so a dirty or restrictive high-MERV filter will actually have the blower motor drawing less amps (air flow is a load for any fan). My upstairs blower with a brand new FPR 7 filter draws about ~511 W, when the filter is due to be changed it will draw around 490 W.

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so a dirty or restrictive high-MERV filter will actually have the blower motor drawing less amps (

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For old school PSC blowers, that is true. For modern ECM motors, restrictive filters will result in much higher blower motor power consumption to the point of causing the motor to burnout and die early. Undersized ductwork with ECM blowers also has the same issue.