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Old 12-01-2019, 11:15 AM   #31
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Re: AC filter question


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On some industrial engines the air filter has a pop out indicator when the filter needs to be replaced. I suppose this is due to pressure differential. Is this type of indicator available for residential HVAC applications?
I don't think so, but there are several thermostats that have "filter reminder" feature that alerts to the need.
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Old 12-01-2019, 02:05 PM   #32
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Re: AC filter question


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On some industrial engines the air filter has a pop out indicator when the filter needs to be replaced. I suppose this is due to pressure differential. Is this type of indicator available for residential HVAC applications?
You can install a manometer to measure pressure drop across the filter and change the filter when it gets too high.

some high end systems like the carrier infinity can monitor static pressure and flash a filter change reminder on the proprietary stat when extra restriction is detected.

a lot of stats have a reminder and it's based on fan run time. days of run time has to be set for the application.
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Old 12-01-2019, 02:49 PM   #33
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Re: AC filter question


I suspect a large percent of indicators, including the pop out I mentioned, would go unattended unless there was a irritating audible sound similar to the smoke detector. On the industrial engines we never looked because the filters were changed, at 30 day intervals regardless, when the oil was changed.


For residential without an audible it would be similar to tire pressure on the transportation. Not concerned until the tire is flat.
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Old 12-01-2019, 03:01 PM   #34
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Re: AC filter question


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You can install a manometer to measure pressure drop across the filter and change the filter when it gets too high.
I have a 10' roll of manometers that I got from Lowe's. 1/4" x 10' clear plastic tubing.

I think the knot adds character and flair to it!
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Old 12-01-2019, 05:36 PM   #35
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Re: AC filter question


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I have a 10' roll of manometers that I got from Lowe's. 1/4" x 10' clear plastic tubing.

I think the knot adds character and flair to it!
That's what I use to check the pressure on Ole Faithful's gas valve and engine crank case vacuum but mine isn't connected to a board so I can use it as a siphon hose to drip water irrigate specific garden plants and suck water from a toilet bowl. Spit, Spit.
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Old 12-02-2019, 01:13 AM   #36
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Re: AC filter question


that style "manometer" is dangerous when used to check furnace gas pressure - forget to add liquid or spill it when gas is on and lots of raw gas gets dumped into the house.
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Old 12-04-2019, 11:44 AM   #37
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Re: AC filter question


The type of filter you should use also depends on the air intake size. In example, the proper sizing formula for a 20"x20" (400 sq in) R/A opening is (400 x2cfm= 800cfm). At 400 cfm per ton, a 20x20 intake will properly handle 2 tons.


Most return air intakes in my area are undersized, thus I often recommend the fiberglass throw-aways.


The problems with restricted airflow are serious. In addition to the problems mentioned, restricted airflow can shorten the life of the compressor, since it reduces airflow across the evaporator and can send liquid back to the compressor, washing oil of the bearings.


The only accurate way to measure airflow in terms of SP and cfms is using TESP with a manometer. Once you have TESP, it can be checked it against the blower performance charts to obtain expected cfms for the model furnace installed at the measured TESP. The digital dual manometers work well for taking TESP. The pressure drop across the filter should also be measured. You'd be surprised at how much a 1" pleated filter can restrict.


Another good method was posted here:


https://www.diychatroom.com/f17/ac-f...2/#post5994825



In short, make sure your return is properly sized as per the above formula. If it fall short, you should use fiberglass, as the pleated filters (even the merv 8) will become more restrictive as it loads up with dust, reducing airflow to a greater degree.


It would be worth calling a pro for this. Making sure you have proper airflow reduces service calls and extends equipment life.
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Old 12-04-2019, 02:18 PM   #38
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Re: AC filter question


The advantage of pleated filters is that they provide a great deal more surface area for filtration than a flat filter. That is why 100% of the air filters used for car engines are pleated ones. Important to understand that filters are designed to allow for adequate airflow when dirt accummulates and so need to be loaded before they will work at their rating. Air filters are less effective at filtering when new as with fuel filters.


I use a combination of pleated filter cartridges and an electrostatic filter on my furnace but also have a custom 4" thick filter that I replace annually in the return duct space. https://www.airfilterusa.com sells any type of filter in custom sizes to fit the return for the furnace and this additional filter helps a great deal to keep dust from inside the house from getting into the ducting.


Frankly it is silly to think that a filter is going to reduce the efficiency of a furnace blower over the course of a year. The biggest improvement is with a variable speed motor and being able to have the furnace recirculate the air more efficiently. This is especially true with air conditioning where with an oversized AC the house will be less comfortable as the shorter cycle times for the AC (which is cycling on and off based solely on the temperature at the thermostat) will result in far less moisture removal from the living space.
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Old 12-04-2019, 02:27 PM   #39
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Re: AC filter question


I'm not an AC expert, although I am a Mechanical Engineer. I focused on the structural aspects of the field. That being said, it's my opinion that the filter keeps the heat exchanger from getting clogged up with (what's the technical term?) CRAP. I agree that if you stress the system by using filters that are too restrictive, you'll cause damage. But those cheapy fiber filters let too much stuff through before they clog enough to stop it. A heat exchanger clogged up with dust, dust bunnies, and stuff, is going to cause problems.
I use the cheapest pleated filters I can find. Lowe's has a 3-pack of filters for about 8 bucks. I think this provides the protection necessary, without interfering significantly with the air flow.
Just my 1 cent.
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Old 12-04-2019, 03:50 PM   #40
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Re: AC filter question


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The advantage of pleated filters is that they provide a great deal more surface area for filtration than a flat filter. That is why 100% of the air filters used for car engines are pleated ones. Important to understand that filters are designed to allow for adequate airflow when dirt accummulates and so need to be loaded before they will work at their rating. Air filters are less effective at filtering when new as with fuel filters.


I use a combination of pleated filter cartridges and an electrostatic filter on my furnace but also have a custom 4" thick filter that I replace annually in the return duct space. https://www.airfilterusa.com sells any type of filter in custom sizes to fit the return for the furnace and this additional filter helps a great deal to keep dust from inside the house from getting into the ducting.

Frankly it is silly to think that a filter is going to reduce the efficiency of a furnace blower over the course of a year. The biggest improvement is with a variable speed motor and being able to have the furnace recirculate the air more efficiently. This is especially true with air conditioning where with an oversized AC the house will be less comfortable as the shorter cycle times for the AC (which is cycling on and off based solely on the temperature at the thermostat) will result in far less moisture removal from the living space.

1" filters don't provide enough additional surface area to overcome the restriction. Again, on a properly sized intake, merv 8 is not a problem, assuming they are maintained.


In addition to the other high SP issues mentioned, high static pressure is one of the leading causes of ECM motor failure. Given the cost of these motors, it's definitely worthwhile to make sure your static pressure is within or nearly within spec.


Lower CFMs will better remove humidity, but it's best to not go under 350 cfms/ton. To do so can cause many unpleasant issues...sweating ductwork and grills, organic growth in the supply duct system, shortened compressor life, etc. Setting 350 cfm blower speeds are usually reserved for hot & humid climates.

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Old 12-06-2019, 10:05 AM   #41
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Re: AC filter question


The fiberglass filters will stop the airborn "chunks" of debris. They let a LOT of things pass through. You can probably read a paper while looking through a fiberglass filter. If you want to get rid of pollen, dust, plant spores, pet dander, mold spores, etc., then use a pleated filter.

We always use a pleated filter and don't have a restriction until it is dirty. If your air filter is at the end of a return air duct, then it sounds like the return air grille was sized too small.

The purpose of the filter is too keep dirt, etc. out of the cooling coil, so air can pass through it easily.

Fiberglass filters will let a lot of dirt get up into the coil, which will then restrict air flow and cause poor operation of your system. Then, a service man will have to remove the coil, clean it and reinstall it.
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Old 12-06-2019, 10:37 AM   #42
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Re: AC filter question


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Fiberglass filters will let a lot of dirt get up into the coil, which will then restrict air flow and cause poor operation of your system. Then, a service man will have to remove the coil, clean it and reinstall it.
Quite so pogo. I recently replaced my leaking A-coil with a new cased-coil from Goodman. When it arrived, I was disappointed to see that it didn't provide a removable panel to allow removal and cleaning the coil. I can just see me (me being the serviceman) with a die grinder cutting my way into it someday. I decided to use a MERV 8 filter to make that day further into the future so my heirs can worry about it. If the fan motor suffers as a result and fails earlier than expected, I can easily replace that compared to reworking the coil housing.
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Old 12-08-2019, 11:28 PM   #43
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Re: AC filter question


Pleated filters are all good and those are not restrictive until they are dirty and all clogged. So, replacing them as soon as they get dirty will be enough. But there are more options too.
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