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Cabin A/C air filter replacement a necessity or a nice to have?

oldtimer 11, where are you finding them for $7?

I don’t find them that cheap, but rockauto has the for a decent price.

Why should I change my air filter?

  1. A dirty air filter is the #1 reason for HVAC system failure. A dirty filter restricts the air flow into your HVAC systems air handler. This restricted air flow places additional strain on the air handler fan motor and could, over time, burn out the motor and cause your system to overheat and ultimately fail. Filter replacement is a small price to pay to extend to life of one of the biggest financial investments in your home.

A dirty filter can be the major factor behind a long list of A/C dysfunctions. Checking and changing the A/C filter is key to preventing or resolving many of these issues. Here are some of the reasons why:

A dirty filter causes the blower motor, one of your system’s most expensive components, to strain due to restricted airflow. An overheated motor may have reduced service life and require premature replacement.
As components like the blower fan and compressor run longer to offset restricted airflow, more electricity is consumed and utility bills increase. Costs will decline when the filter is replaced.

I don’t know about your car; but the cabin filters in mine are one of the cheapest, easiest owner service items you can have. You can even warm up the car and change it in the wintertime and be toasty warm. Almost every auto parts auto store carries them for nearly all cars. It’s the air you breath and even cleaning an old one and putting it back, is not as good as putting a new one in once a year; or just after you inspect it. Make it a Holiday stocking stuffer for your wife’s car and she for you. Or, use it to impress your mate that you’re spending hours on end changing the" filters". Don’t tell her it’s a five minute job.

I would also like to know where oldtimer finds cabin air filters for $7.00!

The last time I bought one (at AutoZone), it was $13.99.
Since I am due for a new cabin air filter, it should be interesting to see whether the price has gone up in the past year.


I’ve been in the Industrial Ventilation/Filtration design profession for thirty years. And fan laws dictate that the volume of air moved by the fan will dictate the type of fan that is to be used and the horsepower of the motor.

I’ve attched fan curves that show that the less air the fan has to pump the less horsepower is required from the motor. Once the fan is no longer pumping any air the horsepower drops to zero. So the fan motor isn’t doing any work, so it can’t damage the motor.

The exact opposite can happen tho. If the motor is undersized for the fan used and the volume air the fan has to pump this can damage the motor. This is called over-horsepowering the motor.


Of course that’s true. The curves are based on the only work being moving air. But when the fan blades have to do work by keeping the blades moving against a back-vacuum, from a clogged filter, there is an additional source of work–not normal and not in your tables.
Try this experiment: Jam a screwdriver into some fan blades, then turn the fan on and leave it.
No work is being done if the blades can’t turn. See what happens to the fan motor.

melott, you are fighting a loosing battle here. Spinning a fan in a vacuum is not the same as sticking a blade in the vanes. Tester is simply right on this. Sometimes you have to admit your wrong, I’ve had to do that a few times here myself. (more often than I like to admit)

@Tester wrote: “Once the fan is no longer pumping any air the horsepower drops to zero.”

That doesn’t take into account free running losses in the motor and “windage” loss in the squirrel cage fan.
An electric motor, especially a brushed DC motor, will still consume some power and generate some heat with no load.

Another thought…if the air flow is severely blocked by a plugged filter, the electric motor might not get enough cooling and fail.

Advance auto parts- order online to pick up in store with a 30% discount code you can find online by Googling Advance Auto Parts discount code.

@insightful, that was a key point I forgot to make in my earlier post.
Fan motors often depend on the air stream for cooling.
Car blower motors sometimes have a small duct to carry away the heated air coming out of the motor separately, so it doesn’t detract from the AC cooling.

Not only is there a cooling issue, but if there is a blocked filter the fan creates a partial vacuum on the intake side. This means that the blades have to do work against this vacuum. If there is an air flow, the vacuum does not exist. Thus the engineer’s tables of flow versus horsepower are meaningless. Bernoulli’s Law.

There is no work done against a vacuum except for the internal friction of the motor. That is why the fan speeds up, just like when an outboard prop “catches air”. That plus the lack of cooling light @insightful said is why it’s bad for a motor.

The blower motor in a cars HVAC system does not rely on airflow from the fan for cooling. The motor is separate from the fan.

But even though it does no harm to the fan motor, the filter should be replaced periodically to keep debris out of the evaporator core and or heater core.

But right now, the cost of those filters is not justified. As more and more manufacturers start getting into making them, their cost will eventually come down to a reasonable level. Anything over $1.50 is overpriced.

Besides mechanical friction, there is resistance in the windings and the carbon brushes, and eddy currents in the stator plates, and all that can heat up a DC motor even if it isn’t under any load. It’s true that in the ideal case, a motor under no load draws no power and therefore wouldn’t heat up, but most motors are not ideal.

I agree, GeorgeSanJose. But, with no air flow, the blades must move against air pressure differential, which means there is work done. Normal air pressure upstream, partial vacuum on the filter side.

Doesn’t a lot of this discussion depend on whether the filter is before or after the blower motor?

@melott - you misunderstand the meaning of ‘work’. Work requires something to be pushed, pulled, or moved. A fan spinning a vacuum does no work, as does a fan that moves no air. That’s why the fan speeds up, the motor has much less resistance to spinning the fan blades when they do no work.