View Full Version : filter at the return grille
I posted this at Residential HVAC. This group seems different. Iíd like to read your thoughts on my little practice.
My first house was a new construction. I started modifying the regular air filter and placing in behind my return grilles to keep my duct work clean. Although that is the only new construction I bought, I have been modifying the air filters from Home Depot for my return grilles for more than 10 years. Recently Iíve seen many duct cleaning ads and would like to think placing an air filter at the return grille is a good idea, especially for the new constructions.
As long as you don't have a filter at the unit you will be ok! Just sounds like a lot of work for not much return (LOL return ) to me!!
11-17-2005, 11:07 PM
Let's see - if ZERO LEAKS in return ducts AND new construction AND all duct was cleaned when you made your modification, AND the filter area was not decreased by the modofication, then all should be fine. But there are a bunch of 'ifs' there.
First, I've never seen ducts that didn't leak, not even my own. There are pictures on the wall of shame of what some people have found in new ducts (beer cans, cigarette packages, food wrappers, and tons of drywall dust) - you might want to filter that stuff out. And most return grills block a lot of the filter area, even if they started out the same size as the filter. Hmm, this might not be a good idea after all. But it's your system.
Truthfully, the biggest concern I would have is the filter area loss. Many systems I see are grossly undersized on the filter area so that could be a real problem. The rest of them shouldn't happen, but usually do.
drk, Very true. Not much return.
cxagent, I am sure you are right about duct leaking. Just like duct dusting is mostly psychological, it is just a thing I do to make myself feel better. I do keep the filter at the unit. The filter I crafted is flat. They are an addition, not to replace the one at the unit.
Two filters is likely too restrictive to maintain required air flow for the equipment,have a Pro test it, or go back to one filter.
My home has 5 return grills of 3 different sizes. I hope a thin filter at the return isn't too restrictive, since the air flow at each return is a fraction of the whole. There maybe enough margin in my system to handle my thin filter.
I am even considering the idea of crafting a very simple return grill allow a thin filter. An option of having a filter at the return shouldnít be a bad thing. Even if there isnít any margin at all in the required air flow for the equipment, an option is better than no option. It may not be very innovative, as long as duct cleaning exists, there should be a place for prevention - keeping it clean.
12-03-2005, 10:25 PM
I'm a fan of low efficiency filtration (MERV 4) at the filter grilles combined with high efficiency such as Aprilaire MERV 10 at the equipment.
Keeps the return ducts clean, as well as the equipment and reduces overall small particulates.
It can work fine as long as the filter grilles are oversized compared to standard practice. A good rule of thumb is 1.5 to 2 gross square feet of filter grille per ton-400 cfm. This keeps the velocity below 300 feet per minute and pressure drops to a minimum. So a 3 ton system would need 4.5 to 6 square feet. A 24 x 30 central grille of 5 square feet works well, you'll almost never see one this large.
In general, the bigger problem is that most 4 and 5 ton systems need 2 aprillair filters to avoid excessive pressure drop.
Good filtration without causing excessive pressure drop is a complex topic.
12-04-2005, 08:09 AM
Just a tip...if you'll seal your blower access door with some duct tape you'll make a huge difference in leakage on all units, may be a bit unsightly, I tear the tape down the middle and seal 3 sides.
Look inside any blower door that is 3+ years in service and you can see the evidence on the metal from the leak. All manufacturers have this problem. There are gaps in the corners of the door where the metal isn't close at all.
Some brands have less than 2% air leakage in their air handlers.
12-17-2005, 05:53 AM
Thanks Dash...that is my point.
2%@cfm/ton x runtime = too much.
My reasoning is the equipment is drawing the 2% from probably the last place you want the leakage from; Attic,
laundry room, exterior closet ....
If I have to give up 2% it will be on the supply side.
(I love Florida)
12-17-2005, 04:09 PM
Supply duct leaks are usually no better ... if you end up with a negative pressure in the house as the building infiltration increases to make up for the lost supply air. That air usually comes from somewhere nasty like the attic. Here's an excellent quote from the Florida Solar Energy Center on the issue:
ďAnother emerging phenomenon from buildings research in cooling dominated climates shows that duct system supply air leakage can lead to negative pressures within the house interior when the air handler is operating. This, in turn, can result in hot air from the attic being drawn down to the conditioned space through interior wall headers, recessed cans or other bypasses from the attic to the interior. This phenomenon is commonly encountered in slab on grade homes in the Sunbelt states in the U.S. where the dominant leakage plane to the exterior is through the ceiling.Ē
Here's another thought to follow what Brendan is saying in regard to negative pressures. If the return runs through the space in which the appliance is located, when the filter gets dirty the leaks become more important. I had one home where the occupants were not feeling well towards the end of the month but feeling better at the start of the next month. They changed their filter regularly. This was a central return system with a filter grille and the furnace/water heater were located in a closet with louver doors. As the filter got dirty, the closet went into a negative pressure due to the (not obvious) return duct leakage and back-drafted the water heater which was making 2000+ ppm CO and then delivered the CO to the house. My recommendation would be to seal the return ducts very well if you are going to have a remote filter.
12-23-2005, 12:40 PM
"not feeling well towards the end of the month but feeling better at the start of the next month."
instead of a filtration problem,
was it a house full of menstruating ladies?
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