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Heating & Air Conditioning

Why are there two air filter slots in my HVAC system?

3 years ago

Can anyone help me solve this mystery?
When we moved in, the previous owners had been putting a better quality filter in the top slot and a cheaper one in the bottom slot. That's what we've continued to do but is that what they're meant for?

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Comments (5)

A R
Original Author
3 years ago
last modified: 3 years ago

Pic
Elmer J Fudd
3 years ago
One should be enough. Two may be too restrictive for air flow.
A R thanked Elmer J Fudd
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Patrick,I did option 1 in several commercial environments,a few residential homes and have pre-filters on my units. I use spun polyester and paper pleat merv 8 pre filters along with merv 11 finals. Those with health issues should consider using merv 13 or higher as the final. Big box store 99 cent fiberglass filters are slightly better than running without a filter. I did some testing several years ago using professional tools monitoring and evaluating air flow,refrigerant pressures,temperature drop/increase across coils and compressor amps/reaction/performance. In the field,pre filters proved to enhance performance,equipment life and operating.maintenance costs. I have written about and reccomended the arrangement a number of times on this forum. Almost without fail,the idea has been met with disagreement claiming it to detrimentally restrict air flow. When dissenters are asked about flow restriction as result of poor quality filtration,they claim annual coil cleaning solve's that. Anyone know how to spell self-serving bias? The increase in final filter life is easily better than five times that where no pre filter is used. It is beneficial to performance and cost savings to install as large a filter rack as room allow's. As for option 2,that is actually how many large homes are set up and to respond to the big scary dust mite story, common sense tell's us different. If and when anything set's up shop inside the return,it will be trapped by a high quility filter and wind up at the landfill next filter change. With that in mind it should be evident that if one chose option 2 then later went to option 1,duct cleaning will not be required. Beating the drum regarding one's own credentials is pretty common on this forum. I am retired and have no reason to participate in self promotion but for what it's worth,I didn't attend the 2 week trade school training but I do have more than a few college credits in mechanical and electrical fields. Let the nay saying begin.
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Double filtration can cause problems, restricted air flow is one of them. You have a few choices with how your system was installed. The 4 inch filter is the better filter. However, if you remove the 1 inch filter eventually that return from the inlet of the filter grill to the furnace where the 4 inch filter sits is going to get filthy over time. This where all those before and after duct cleaning pics come from. You have a few choices. Pull the 1 inch filter out and keep the 4 inch in place accept the fact that the return before the 4 inch filter will get dirty. OR buy a cheap 1 inch see thru filter to use as a pre-filter and leave the 4 inch filter in place. Replace the cheap filter every month and periodically check the 4 inch filter. OR the best of the best (depending on how your 1 inch filter grill is attached) You'll have to check to see if this could work for you. Remove the 4 inch filter at the furnace and install a 4 inch lip filter at the 1 inch filter return spot. It's more or less a 4 inch filter that can fit a 1 inch filter slot, depending how the filter grill was attached to the system. This way the whole duct system is filtered and dust build up will be minimal. Home builders and HVAC installers via a GC make all kinds of mistakes nothing along that line is without error.
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Sizing can be less of an issue when you have multiple units. Because each unit is controlled independently, provided of course the unit itself is not over sized for the area in which it serves resulting in excessive short cycling. In your case 3 ton plus 2 ton isn't necessarily equal to 5 tons of cooling... unless you drive those thermostats really low. By having two systems with total capacity of 5 tons (which, most of the time this would be oversized for a 1700 - 1800 sq ft structure) has a few advantages. 1. You have the ability to use one system only during low heat days or set the opposing unit higher. The unit set higher only runs if load conditions warrant.... meaning if it's not hot, the thermostat controlling the unit does not call... the unit can not run without a call... 2. Utility bills can be much less... because you rarely use both systems at the same time. You run the bedroom system when you sleep, the other when you're awake. If you have a home office in one the bedrooms you may need to rethink things a bit. 3. Extra capacity for when it's hot out. When it's hot you have the capacity to cool the home. (even while entertaining guests --- additional people add heat) Remember heat waves come, heat waves go... a so call 'properly sized' AC system is essentially properly sized roughly 10% of the time... especially if that system is a single speed AC system. Alot of people I come across like to sleep cool. So for that reason I doubt I would change the configuration of the space. Once you realize what you have, the choices you make are mostly budget type concerns. Essentially a set up like this runs like a 2 stage system. This kind of subject is above the pay grade of many... choose your contractor wisely. I service the Katy, Texas area.
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mike_home
3 years ago
It could be that the cabinet is set up to accept a one inch or two inch filter. You would have to remove the side tracks to insert a two inch size. That is probably was easy at the time of installation. Never use two filters. A single good quality Merv 8 filter is sufficient.

Is this cabinet sitting on basement cement floor? How does the air flow through the filter?
A R thanked mike_home
opaone
3 years ago
Agree, a single 2" filter would be best. I'd shoot for MERV 13 if your system can handle it, otherwise as high a MERV rating as it will handle. Lot's of info online about MERV ratings, PMx, and why higher MERV ratings are important for your health.
A R thanked opaone
Elmer J Fudd
3 years ago
last modified: 3 years ago
There seem to be different opinions among HVAC contractors concerning what kind of filter is best to use in a forced air system. I have two homes, in two different areas, both mild weather areas where heating and cooling are not heavily used or stressed. I have had two different but both terrific and continuously trained, familiar with current thinking HVAC contractors to work with (each put in new equipment for an existing home).

Start with the premise that for replaceable standard filters, their job is mostly to keep dust and other air floating debris out of furnace (air handler) and coil air passages. With that in mind, Contractor #1 believes that even though they're a bit restrictive with airflow, the typical hanging element accordion shaped air filters are best. I don't know what the MERV value is but it must be in the teens.

Contractor #2 thinks that because most existing systems have ducts that are too small to begin with, any restriction of airflow puts an unneeded added stress on an already stressed fan and also reduces system efficiency. He recommends the thinnest, fiberglass woven filters with single digit MERV ratings - like 7 or 8 as I recall. I bought a case and I'm not there right now so I forget the specifics.

Take your pick, flip a coin. In each location, I use what the local service provider recommends. It's their job to maintain the equipment, I do it their way, even though the two are different.
A R thanked Elmer J Fudd

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