I have a question regarding the use of filters in forced air heat systems.
I have a new home with forced air heat. There is a filter at the furnace in the return air plenum and also a filter at each of four air returns in the house.
I need to replace the filters which are the cheap .75 jobs. I’m going to get a better grade filter.
My question is this. Should I be “double” filtering, that is to say, replace all of the filters, or should I simply filter at the furnace only and omit the filters at the various returns. My concern is that there may be a possibility of reduced air flow if I replace all filters with the improved grade.
Thanks for your help. (I posted this on the heating board as well, but didn’t know how much play that board gets) Thanks again!
Replies
Mako, fist, the "cheapies" or "see through" are only for keeping large items from fouling the equipment, like stray cats. Remove the furnace filter, close the door and aluminum tape the door seams. Be sure to check for any duct leaks and air handler seam leaks. Then buy good (merv 6-8 or higher)pleated filters for each return grille. Be sure there are return air filter grilles at each return. Also, look behind the back edge of the grilles. You will likely find a gap between wall and duct, or grille frame and duct etc. Tape the seam so that there are no gaps from grille housing to the duct interior.Use aluminum tape, not cloth duct tape.
The final step is to put a strip of adhesive backed window weather stripping along the inside of the grille where the cardboard filter frame meets the grille metal. This allows a good seal and forces all air through the filter. It also may stop some whistles. Close the grille, and replace filters in 60-90 days. You will greatly improve the air quality and keep the ducts clean, as well as the fan vanes in the blower.
While in the taping mood, pull down some supply grilles or lift from floor, and look at air tightness between sheet rock or floor and the duct work. Seal the gaps. Typically, I find systems the leak pulling and pushing air, and some systems loose 30 % of the conditioned air.
Hope this helps, hate to see a new clean system get fouled from the start. Paul
Paul, I wonder if it isn't better to have the filter at the blower compartment, instead of at the duct runs.
If we knew the ducts could be well sealed, perhaps, but they usually aren't and many areas of the country still pan the space between studs and joists for returns and I hate to think of the junk that could get into the return air between the filters and the blower compartment.
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Bob, panning returns isn't the best practice, but is ok if the sheet metal, and rock or other seams were properly caulked. If not well sealed, than all kinds of imbalances can happen. Every leak between floors, wire runs etc become air flows. For those systems, efficiency is somewhat a bust, and I can't quite equate it to fine home building practice, but it is real world too often. That said, even a leaky fitting of wall to stud seems to be tight enough to cause most of the air to flow through the return grilles.
If someone suspects a leaky return system, then there is an easy test.
They can put in return grille filters, and a streamer of clean kleenex tissue at one of the supply grilles. If the streamer of tissue is clean after 6 months, then very little dust is bypassing the filter system.
Another way is to look for dust trails around the duct work and seal where possible.
As a last resort, lower merv filters at the returns and one in the unit will at least filter even the up stream leakers. I hate to see people buy filters that I can read a newspaper through, if you catch my drift. The cleaner the system the better it functions, and that goes for lungs as well.
I know you see the horrors that are out there, and I try to point people in the do-it-right up front mode. Always appreciate your help. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
>>If the streamer of tissue is clean after 6 months, then very little dust is bypassing the filter system.
And if it's not, just think about how the condenser is looking, and the heat exchanger and blower vans, after that 6 months and what we've done to efficiencies of the system.
Maybe I'm just being human and having trouble letting go that which I am used to. I'm gonna have to ruminate on your suggestion!
>>I hate to see people buy filters that I can read a newspaper through, if you catch my drift. The cleaner the system the better it functions, and that goes for lungs as well.
I'm with you there. Whenever I run across one (that isn't too dirty) I hold it up to my client, shine my light on whatever is behind it from the client's perspective, and suggest they consider how much dust it stops and how much it's going to let through.
_______________________
Tool Donations Sought
I'm matching tool donors to a church mission to Haiti - we're shipping a bus converted to a medical facility in (now it looks like) April and can fill it with clothes, tools and all sorts of stuff needed in that poorest of all countries. A few hand tools or power tools can provide a livelihood for an otherwise destitute family. Please email me if you have tools to donate.
Thanks to Jeff and David and Jim and Rich and Steven and Mark and Jason and Shep and Jen and Mike and Joe and Bill and Ken for their offers!
Several donations have arrived! Thanks and God bless!
Bob, if in doubt about the over all design and tightness of the system, then Boss Hog's suggestion should give peace of mind, even though the cheapies don't trap much.
For others following this; The high merv pleated filters are designed to allow proper air flow, due to the much larger surface provided by the pleats. Direct swap out is ok. I like the returns filtered because they provide even more surface area. Our up system had a 20x20 filter in the air handler. The ceiling return was 14x24, and the low return was 14x20. By filtering only at the returns, I have 616 sq in vs 400 at the unit, or over 50% more surface area not counting the pleated addition. I unsealed the air handler last fall and checked the interior after 5 years of dual season use. The evaporator fins were clean and bright, and the case interior was clean. There was no scum buildup in the condensate drain slot. I have seasonal allergies, so maintaining a clean, tight system is both energy cheaper and health wise more comfortable.Energy Consultant and author of Practical Energy Cost Reduction for the Home
I've never heard of filters in return registers. Are you saying that there are some designs that properly have filters in return registers? It certainly seems silly to have multiple filters all over a house when all the return air can be filtered by a single filter in the return duct at the furnace.
Wayne, go to the big box stores or any HVAC supplier and ask for a return air filter grille. Here is why they are better. 1. They trap the dust before the dust coats the duct work 2. Multiple return grilles provide greater filter surface area than just the filter at the air handler. Greater surface area means less air velocity at each filter which means greater dust/pollen capture rate since some dust not pulled through the filter. I am talking about clean air for your family to breathe. 3. Have you ever seen an air tight filter door on an air handler? Most I have seen let a lot of air leak into the handler. I seal them and use the return grilles. 4. The low air restrictive spun glass filters are equipment protectors only, and don't do much of that job well. With the evaporator coil and sometimes other internal parts getting wet in the air handler, a layer of dirt forms a muddy crust, both on the coil and the fan vanes. Air flow efficiency goes down. 5.If you have multiple returns, you can install high efficient dust trappers down to very small particles, without excessive air flow restriction. 6. Last but not least, filters in reachable return grilles get changed more timely than the filter in a crawl space or screaming hot attic.
That's my take on it. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
But I understand that two much filter area can cause problems.
When I belonged to a local woodworking guild a speaker talked about home made air clearners for the shop.
He did a lot of research on different types of filters and said that they are designed to work with a certail air velocity. That a lot of filter media is based on the particals bounceing around and inpinging on a barrier. That will to low a velocity (to much area) that dust particles can slide through.
Bill, not sure I follow the guy's reasoning on too low. Certainly not enough air flow will not carry dust to the filter, but once there, the filter should hold the dust, and the higher the air flow/velocity, the more dirt will be pulled through the filter.
In any event, I have doubled filtration by adding second return air filter grilles, and the effects have been happy house wives with less dust, better air flow from cleaner systems(after cleaning the air handler/fan vanes), and less trash air being pulled in around the AH filter door.Bottom line, better air,lower bills and often air flows to rooms that used to starve for air. I take a total system approach where I can; clean it, seal every seam, boot, and grille frame to duct, keep it well filtered, and never accept the phrase "we don't mind a 10-15% air loss" from the HVAC guy. He is not paying your bills. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
Now this was for an air filter system for home workshops and to be effective they require a fair amount of air flow and I think that he was trying to get down to .5 micron and they used 3 stages of filtering.
So I am not sure how applicable this is to "normal" HVAC filtering.
I don't remember the details, but the way that filters work and the way that they are tested it based on an average of what will get through them. It is not a absolute value that nothing will go through bigger than that number and 100% smaller than that will go through.
One of the reason is that the particals are irregular shapped.
As I said I don't remember the details behind this, but I remember that it made sense at the time.
But look at this example. Try screening some crushed rock with say a 1/2" screen. If you drop gravel on it one at a time some thing that is save 3/8x3/8x3/4" might bounce around until it alings and goes through.
But dump a whole buckent of that size and lots of it will end up sideways and the other rocks end up on top of it so that it can't bounce around and go through.
Bill, appreciate your thoughts. I have seen so many houses with poor air filtration that the owner could easily correct, that I get too serious sometimes. Kind of like when Bob Walker finds a house with high CO practices.
For my conversion from garage to work shop days, I bunge cord a 20x20 pleated filter to the back of each of my 20" box fans, and let them blow on low.
The filters really trap a lot of dust. I don't have a fancy tool vac system, since 95% of time it's just a garage. PaulEnergy Consultant and author of Practical Energy Cost Reduction for the Home
I like the paper filters, but they seem to restrict the airflow on my system too much.
I've also never seen filters at each air return. But why not take advantage of them?
My suggestion would be to use the pleated paper ones at each air return, and a cheap "see through" filter right at the furnace. That would give you the best overall filtration, but you'd keep the filter at the furnace to protect the fan from anything that accidentally found it's way in there.
I'm not 40-something. I'm $39.95, plus shipping and handling.
Thanks guys!