... Or a flowerbed, or a new form of organic fertilizer, or a piece of absorbent ground matting...
That, is what they took off of my heat exchanger. Or rather, that is one of two 1-2" thick mats they took off the bottom of my heat exchanger, and two thinner mats they took off the top.
My air conditioning system (as most do, though combo units are more common around here) uses an outside compressor/condenser and an inside heat exchanger and air handler unit.
Two years ago, the compressor/condenser was replaced, and theoretically the air handler was serviced (it should have been replaced, but wasn't for reasons explained later). Then last year we paid someone to come in, clean the ducts, condenser coils, and air handler unit.
Only as is now apparent, neither of those times did the heat exchanger actually get cleaned, as it was supposed to be.
We had some freezeup problems two years ago, but they went away after we replaced the compressor/condenser, which died a screeching horrible death the first weekend of September.
Then last year we had some freezeup problems again, which is when we hired the cleaners in. When that not only didn't help but made the problem worse, we had a different guy come out and look at the system again; and he said the coolant pipes into the heat exchanger were fitted to the new compressor/condenser improperly (which they were. Thats one of the messiest pipe joins I'd ever seen, and at some point it still needs to be fixed).
We both noted the airflow was less than it should be, even after we let it thaw for two days; but there was still a fair bit, and after all we had just had things cleaned, theres no way that the air heat exchanger could be clogged up right?
The diagnoses was they would have to rip out the bad hookup and redo it at the cost of about $1500.
So we just lived with the inadequate AC and freezeups every few days for the rest of the summer and fall.
I had developed the very strong suspicion that the previous two services had skipped cleaning the air handler. As summer and fall wore on last year the airflow kept getting worse, as did the freezups; and I was SURE there were icedams building up in there that were completely unjustified if we had a properly functioning heat exchanger. I even partially disassembled the heat exchanger and ductwork to confirm it, the thing hadn't been opened up in years.
Eventually the freezups built up so bad, that the AC unit actually made it's own little flood. There was a block of ice the size of the entire header duct (a 2 foot square box) built up above the heat exchanger. I actually had to pull off part of the ducting, and unclog the drains by hand, then get a shop vac and a heat gun in there to melt the block of nasty ice.
That said, I didn't want to tear my whole heat exchanger assembly down, and then try to clean and rebuild it myself; or at the time bother paying someone else to do so, because we weren't even sure we'd still be here come this spring, or if we were whether we'd be replacing the whole AC system top to toe (which is what should have been done in 2007 instead of keeping the 25 year old air handler, but the home warranty company would only pay for the compressor/condenser).
When we figured out that we would be here for at least another year but wouldn't be replacing the AC system, it was time to get the system maintained again; but properly this time.
You can see the result. Not only had the previous two "cleanings" not done the job; they most likely made it worse, pushing dust, hair, and debris into the heat exchanger in the process.
Now remember, that is the matting that built up, even though we've been using a special 3 layer filter made specifically for dusty environments and pet hair; and changing it every 60 days (or whenever it is clogged up by visual inspection) as recommended by the manufacturer.
The fact is, Arizona has dust issues, we've got two big dogs and a cat, and we've got a 60 year old house thats leaky, dusty, and drafty; with an open attic, and an open underhouse plenum air intake. Even with a good filter, that level of crud can accumulate just through the air that gets into the system around the filter.
Given that, our new AC guys (who were great by the way. Mazon Air Conditioning and Heating out of Scottsdale), advised us that rather than use the expensive pet hair filter changed every 2 months, we should just use the thinnest and most free flowing cardboard and fiberglass cartridge type filter we could; and just change it every 2-4 weeks depending on the level of clogging by visual inspection.
That said, it took YEARS for this to build up. If you have your heat exchanger cleaned every year or two, this shouldn't ever happen to you.
It's a miracle that we got any heat or AC at all... or that it didn't catch on fire... or for that matter that it didn't give us diseases. It's entirely possible that the respiratory problems I've been having were aggravated by this thing.