Importance of AC filters

I can’t stress the importance of keeping the AC filters changed often.
I inspect all evaporator coils. I see dirty coils all the time!
Sellers may change the filter out to make it APPEAR that they have been on top of it but the proof is in the pudding.

Here is an image of a coil that is covered in dust. The seller put a brand new filter in the slot. Does this look like he has been changing his filter?

This can cause the unit to freeze over since there isn’t sufficient hot air to transfer to the refrigerant. All that dust can also clog up your condensate lines.

I was called to this home because the AC was dripping water into the interior.
The lines were clear, there were no blockages in the line yet water was dripping into the ceiling still.
When I looked inside, there was a build up of debris in the condensate tray. This debris was creating a dam which was pushing the condensate over the edge of the tray and into the interior of the home.
The landlord tried to clean the coils and stick new filters in but he didn’t know that the tray was blocked from all the previous neglect.

There is no need to spend big bucks on fancy filters. Honestly a simple cheap filter changed every 30 days is good. Be sure to get the right fit. Don’t just buy what was in there when you moved in. There should not be any gaps around the sides of the filter.

Keep your filters cleaned. Get your system checked out and serviced. Take care of your AC. Its working hard to take care of you!

Anyone in Central Texas looking for an inspection or to network feel free to contact me.
My website is
My number to schedule and inspection is 254-669-1497.
Please only call to schedule an inspection.

If you’re interested in networking you can reach me at


I see it quite often.


Just as important is the outdoor condenser fins. They are always plugged full of debris in my area.


Welcome to the InterNACHI forum community, Larry Palmer.
Damm…what a way to start. Great post, Larry. Thanks!

Personally, I often wondered, how many inspectors look inside the plenum of a forced air system when they have a chance?
When ever I can, I take several images in the plenum and attached ductwork including images of current hygrometer readings. I am looking for,(1) condensate stains and (2) bulk water.
I also do measurement readings. Temp/RH - Temperature and Relative humidity inside the plenum. Moving upstream I compare readings at floor supply registers and overall average atmospheric Temp/RH in the residence/building I am inspecting. From the basement to the top floor and all points between. I try to keep my CO and Hygrometer monitoring continually. CO detector sent to alarm above 009 PPM. Normal CO levels in air are10-29 ppm. Chronic problems, headaches or nausea, 30-35 ppm.

Even mini-split evaporators filters require cleaning. Almost all my clients and the venders did not realize it. Simple filter maintenance changes comfort, energy efficiency, reducing energy loss and reduces the chance of condensate, bulk water leaks at the cabinet.

Dirty filter problems…

  1. Overheating: When a HVAC system filter is clogged or dirty, it may/can cause overheating.
    In some, I think most modern day mini-split models, a failsafe is built into the system. When the unit filters are dirty, the unit only circulates air, there is no conditioning. This is to prevent catastrophic failure caused from overheating. Clean filters monthly.
  2. Increased Energy Usage:
  3. Unclean Air:
  4. Comfortable:

Thanks again.

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Changing filters requires ‘pride of homeownership’. When they hand you a home with zero down and 3% 30 year loan, there is no pride or care. Too busy with TikTok and FakeBook, sorry.


There’s truth to this.
It’s nice to be able to have a home but for many the ease creates a lack of that respect for the investment.

How many times has your CO meter gone off? What was the issue in each case?

Evening, Bryce.
Hope this post finds you and your loved ones well.

How many times has your CO meter gone off? Twice in 6 years.
It is set to alarm at 009 PPM.

CO2… ><10 in 5 years.

Sorry. I forgot homes that have been unoccupied for longer than 30 days. It would be several more.

If there is not some previously made opening in the plenum, how do you do that without disassembling the plenum or cutting a hole in the plenum?
I rarely see an opening that I can easily reopen to see the evaporator.
I agree that there are many evaporators needing cleaning. A clue is a dirty blower.


Too many people in a room will NOT set off a CO detector.


Yeah, if people are exhaling CO, we start suspecting someone is a gas-powered terminator from the future. :stuck_out_tongue_winking_eye:


Humanoids [not just terminators from the future] do exhale minute amounts of Carbon Monoxide:

Carbon monoxide (CO) arises in the human body as the natural product of hemoprotein turnover, most of which originates from circulating hemoglobin… Under physiological conditions the rate of endogenous CO production has been estimated at ~18 μmol CO per hour. [1umol is 1ppm]

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Sorry. CO2. Thanks for the correction.

Gotta say that I find that very interesting. Live and learn…
I love this quote from the study abstract.
A trumpet model with axial diffusion is employed to fit simulated exhalation profiles to the experimental expirograms, which provides equilibrium airway and alveolar CO concentrations and the average lung diffusing capacity in addition to end-tidal concentrations.
But more interesting is the conclusion which is basically…nothing. This is just an observation, with no conclusions about any significance from it. I thought there might be something about the amount of CO being generated by 17,000 people at a hockey game or something, but nope. :stuck_out_tongue_winking_eye:

Morning, Lon.
Interesting post by Simon.
Bravo, Simon.

Hypothesizing, a scientific model could to be created in an air tight space, a room/space that does not exchange air, with atmospheric conditions mirroring exterior atmospheric condition CO PPM as a reference point. Using a stadium would not suffice. The mechanical ventilation would not produce a good model I feel.

More seriously, this looks like the kind of study that broadens our knowledge but on its own, it is not very consequential. I would not call it worthless like, oh say, spending $50k finding out that some fish can count to three.

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What was interesting, how CO PPM can increase in an unventilated home.
:thinking:Hope I am not reading to much into this.