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View Full Version : Why would lack of a filter stop A/C?


BigT
08-05-2010, 09:42 AM
Alt title: Cold air again, but why?

This may be my silliest question yet, but it does fit GQ.

A few weeks ago, my dad removed the dirty filter on the climate control unit. You know, that big filter in front of the vent that is used to get the air into the system. It was so dirty he was afraid the air blowing on it was actually releasing more dust and allergens into the air (or something like that).

Last night, he finally replaced it--finally being able to afford getting a new one. And, this morning the air conditioner that everyone assumed was broken started working again.

I understand the basics of how air conditioners work. A liquid is compressed until it gets really hot, then the ambient air cools it down, and then liquid is decompressed back to normal, and is thus cooler, and a fan blows air on top of it, and blows the now cold air through the floor vents.

Why would having or not having a filter affect that part of how the air conditioner works?

Turek
08-05-2010, 09:55 AM
WAG: Is there something that senses that a filter is present, and disables the unit it it's not there? Kind of like the switch on the dryer that won't let it start if the door is open.

Caffeine.addict
08-05-2010, 09:55 AM
One reason I can think of is that maybe there is a switch that needs to be pressed for the A/C to be running. On mine, if the door isn't closed properly, the unit won't run.

chorpler
08-05-2010, 10:01 AM
Yeah, mine has a nearly-invisible switch that won't let the A/C unit run unless the filter is in place. Might want to check around for that. It caused me no end of grief until I discovered it.

BobArrgh
08-05-2010, 10:10 AM
Besides blowing dust and allergens through the house, there is another reason you need to clean/replace your AC filters regularly, and that is having decent airflow through the system. If your air filter is extremely dirty, it can block the air from getting through the system, which in turn can cause the evaporation pipes to freeze.

That used to happen to us on a regular basis until I got into the habit of changing our filters every 3 months. FWIW, we don't use any fancy filters, so the filters only cost a few bucks apiece.

bordelond
08-05-2010, 10:27 AM
BigT, does your OP say that ...

a) without the filter, the A/C ran (you could still hear the fan), but did not cool

or

b) without the filter, the A/C did not run at all (no fan noise)

?




Last night, he finally replaced it--finally being able to afford getting a new one ...
:confused:

BobArrgh is right -- filters are about the cheapest thing to replace on an A/C unit. Even the fanciest Rolls-Royce of filters might cost $20. Good-enough filters are $3 to $10. Was there more going on than just a clogged filter?

Dag Otto
08-05-2010, 11:52 AM
I would doubt a filter switch would be in place on a residential unit, but there might be one. If so, when filters get dirty enough they can actually be blown out of the filter frame by the force of the fan. I've seen this on commercial air handlers operating at higher pressures than a residential unit operates at, but it's possible.

More likely is that the dirty air filter blocked too much air from passing over the evaporator and the evaporator iced up. At that point you are pretty much getting no cooling until the ice is melted and the filter replaced.

Chefguy
08-05-2010, 11:59 AM
In all of the years that I responded to "AC not cooling" trouble calls, the cause in 99.9% of the caes was dirt, either in the form of dust bunnies, bird nests, mouse nests, pollen or dust. As mentioned, once air flow is restricted, effectiveness drops, and icing can occur in the case of bad clogging. If the compressor overheats, it will likely shut down on thermal overload.

Snnipe 70E
08-05-2010, 09:56 PM
Dirty air filter means low air flow. Low air means lots of problems.

BigT
08-05-2010, 11:13 PM
No, they just really don't have a lot of money. They're practically hand to mouth. Food, then bills come first. Heck, the only reason they can afford what they can is that I pay them almost my entire check for rent/utilities/food. And I've never seen a filter that fits our system for less than 30 dollars.

And yes, the fan was running, and air came out the vents, but no cooling. And, like I said, removing the filter altogether did not fix the problem--only finally putting a new one in. And there does not appear to be a switch.

I could understand a blocked filter causing problems. I could understand junk in the air. I could even understand it being iced up. What I can't understand is why suddenly filtering the air, and doing nothing else, would suddenly cause it to work again.

I will admit that it has also been a bit cooler outside today, as well. But the air coming from the vents is definitely cooler than the outside air.

raindog
08-08-2010, 06:08 PM
Alt title: Cold air again, but why?

This may be my silliest question yet, but it does fit GQ.

A few weeks ago, my dad removed the dirty filter on the climate control unit. You know, that big filter in front of the vent that is used to get the air into the system. It was so dirty he was afraid the air blowing on it was actually releasing more dust and allergens into the air (or something like that).

Last night, he finally replaced it--finally being able to afford getting a new one. And, this morning the air conditioner that everyone assumed was broken started working again.

I understand the basics of how air conditioners work. A liquid is compressed until it gets really hot, then the ambient air cools it down, and then liquid is decompressed back to normal, and is thus cooler, and a fan blows air on top of it, and blows the now cold air through the floor vents.

Why would having or not having a filter affect that part of how the air conditioner works?

The A/C resuming operation when the filter was re-installed is a coincidence. A little strange, but a coincidence. (as was the A/C not working when the filter was out.)

Dag Otto
08-08-2010, 06:50 PM
The A/C resuming operation when the filter was re-installed is a coincidence. A little strange, but a coincidence. (as was the A/C not working when the filter was out.)

If the air conditioner was off long enough during or after the filter change (like overnight), an iced up evaporator could have thawed during this time.

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