# Need help with cold air return vent



## rustic_philosopher (9 mo ago)

Thanks in advance for the help - trying to make an educated decision about something im not educated on.
So my wife and I built a new home (1.5 years ago) in far NE Texas and we have two central air / heating units (electric). System is Lennox and one unit is 3 ton and the other is 2 ton I think. Each unit has a 4" 20x25 filter that we change occasionally.

We have a lot of dirt and dust brought into the house on account of the new yard which has a lot of sand that hasn't filled in with grass yet and our three Great Danes. Also we have 3 orchards in the middle of the rest of the pine plantation so there is no shortage of pollen in the air. It feels like we change the 4" filters all the time just due to dirt and what feels like "bigger" stuff.

I was thinking about replacing the cold air returns (30x16 and 30x12) to be the vents with a door that you could put a 1" thick filter in to "pre filter" before the return air got to the main expensive 4" filters. My thinking was A) the cheaper low merv initial filter would catch a lot of the bigger stuff floating and leave the main filters in better shape while helping keep the air cleaner for the wife and B) It would be more cost effective to replace the cheaper 1" low merv filters often vs replacing the 4" filters all the time.

So my question is around increasing load on the system as I know too much air restrictions can shorten the lifespan of the system and Texas is already a beating on a HVAC. Is this a good idea and if so should I use a lower merv value 4" filter if I am going to be adding 1" cold air return filters?

So instead of say a single 4" MERV 13 filter would I get more benefits from say a 1" MERV 1-4 pre filter along with a 4" MERV 8-10 filter in our case and more importantly would that be bad for the HVAC system in terms of load?


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## channellxbob (Sep 27, 2020)

Usually, your system was designed for a certain filter pressure drop, adding another filter will increase that pressure drop and reduce air flow. A lot of people think a furnace can be a house air filter, and unless you design for it, that's just not the case. You would be better off adding some sort of air purifying system in your house than trying to monkey with your system. Generally speaking, and as I said before, unless your system is designed for a high effiency filter, the filter in you furnace has basically only one function, and that is to protect your evaporator and blower from debris, and clogging up air flow. I would talk to an HVAC pro in your area and get some idea of what they recommend.


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