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Uncle Pep Pep 01-29-2012 07:45 PM

Need Advice on Return Air Filter Grill
I am trying to fix an air intake issue for my HVAC unit and need some advice. I think I know what to do, but I would welcome more experienced opinions.

See this video to get an idea of what the situation is that I am dealing with.

There are several things I am not sure about. I will start with how I should fix the problem of air being drawn from under my house. I would imagine that's not ideal from a perspective of dirt and dust getting sucked up as well as drawing air that is much different in temperature from the air in the house. I am under the impression that I should draw all my air from inside the house.

It seems there are two ways to fix this (at least that are obvious to me). One is to install a duct from the opening in my wall all the way up to the intake on the unit. This would close the loop and fix the issue of air being drawn form the wrong place (under the house). The cheaper way would seem to be to get some wood, go under the house, and close up the hole so that air will have to draw into the vent, but still let the air just travel up the open wall without a duct. I think the second solution would be cheaper and easier, so I would like to do that unless there is some reason I shouldn't that I don't know about. That's issue #1 and I thank you for advice in advance.

Issue two is the return air filter grille that I am going to install to cover that hole. Until recently that hole in the wall was covered with a very old and dirty vent cover. I am trying to replace that return air grill with a new one that holds a filter (the old one didn't have a filter). The problem is that the old return air grill was not flush with the wall, and all the new ones I see at home depot are designed to sit in between studs but as you can see in the video, my existing opening goes across studs, and I don't really want to notch out the wood. I thought what I might do is just cut a new hole that is vertical, about 12 in X 30 in and then I will be between the studs and can use something like this:

Is a 12 X 30 inch hole big enough to let the unit draw in all the air that it needs? Also, my unit does have a filter right next to the actual HVAC unit up in the attic. It's one of these:

However, I just thought as long as I was putting in a new return air grille, I might as well get one that holds a filter just for extra protection from dust. I am allergic to dust and mold, so I want all the help I can get. Will it impede the air flow too much to have a filter on the grill and up in the attic next to the actual unit?

Thanks for any advice or thoughts on this problem!

Tator1076 01-29-2012 08:49 PM

If was you try to 100% close that off. I would go into that hall way and put a ceiling return filter grill. You need make a return box but run 12inch flex duct work to some were to the return of the unit.

bobinphx 01-30-2012 11:11 AM

a couple of things to consider.

An HVAC system needs ductwork sized and installed correctly to work correctly. The air handler that you have has model numbers etc on it. I would find the information from that so that you know what amount of air flow is required on the return as well as the supply.

with the amount of air flow in hand, I would research what each type and size of ductwork can flow. Example, 6 inch metal duct can flow 100 cfm and a 14 inch can flow 1000 cfm.

Your air handler also has a max external static pressure limit. The static pressure is the friction that the ductwork causes. Good design and common sense will keep this number low low low... a manometer will allow you to test your static pressure.

so lets say you have a standard air handler that produces 1000 cfm to the supplys. you need to design a return duct that will allow at least 1000 cfm back to the air handler.

The filter is also important. if I understand, 4 inch media filters are the best for hvac systems due to the absence of major static pressure drops. This is due to the large surface area of the fiter.

You also want to design the return ducts so that the air flowing past the filter is fairly slow. This is for filtration and noise.

if it was me and the air handler needed 1000 cfm, I would install two return grills in the ceiling of the hall. I would go 24 by 24 on each return register (12 or so inches deep) and fill them with 4 inch media filters (merv rating is your choice, but watch static pressures). I would then install 14 inch round sheet metal ducts (one each return) into a "Y" and then duct to the return of the air handler. The Y would need to be as close as possible to the air handler. I would remove the filter at the air handler, just because I dont like to get in the attic

so what would this do for you.

1. great air filtering.
2. lots of air movement without a lot of static pressure.
3. low noise from the air rushing by the return filters.
4. lower static pressure and allow you air handler to breath correctly.
5. lower your heating and cooling bills and the dust in your home!!!!

biggles 01-30-2012 02:16 PM

:wink:right off...........only return conditioned air from the heated space..thats why most stat are located within arm distance of a return when you set the temp there it is actually the reurn of the room not the discharge.that grill in the ceiling is what forget about the wall the attic air handler ducted to that ceiling grill or open to the attic with a filter.just an opening in a wall with a unit above it doesn't mean it is returning air,and when its off the cold air drops back down thru that grill filter or lets hear back.i your bringing open wall air back to a furnae your bills must be reflecting it....its much easier to heat 70F conditioned return ducted air then 55F wall air $$$..units are rated for a certain rise across that heat exchanger....

Uncle Pep Pep 01-31-2012 12:12 PM

First I will say that I am sorry to be slow to post in the thread after these great responses. I thought I was going to get emails that notified me of people posting in the thread, but that didn't happen.

First a few more details about my system. I have an Aprilaire 2200 that sits right next to the unit, which I guess from Bob's post is actually called the air handler (sorry I am new at this). Basically it's MERV 10 (@ 1200 CFM), and uses a 20x25x6 filter. It seems from what I read that this system does pretty well at cleaning the air, and I was actually planning on purchasing this: ... to upgrade the thing to be even better.

Although Bob clearly has a great solution in his post, I really don't have the money or expertise to do something like that. Some day I may just have to get this thing taken care of by a real HVAC company, but right now I must do it myself for budgetary reasons. That means I am trying to do something that will perform well, but hopefully not reconfigure the whole system or where the intake comes from. Mostly I am trying to determine if my idea for a fix is going to work OK and not cause problems that I don't know about. I know it won't represent the ideal solution.

As biggles points out, I don't want to be drawing air that's 20 degrees cooler and paying higher energy bills. That's the main thrust of why I want to fix this, along with making the air in the house cleaner so I don't have as many allergic issues. Biggles mentioned that I might even be mistaken and this place I am working on is not the place that draws the return air back into the unit. It seems like it has to be, but to make sure I am not missing something I am going to go up into the attic in a few minutes and examine the system to make sure I am not mistaken about how the air flows.

Assuming this place in the video IS the return air, I would love to know if my idea for a solution can work. Basically this is my idea:

1. Seal up the hole in the floor from under the house so no air can enter from below the floor
2. Cut a larger hole in the wall in my hallway and put in a new return air filter grill that holds a one inch thick filter. Maybe I make it 20x30 or so? I know I already have the 20x25x6 filter up in the attic next to the unit, but I thought as long as I was putting in a new return grill, why not throw a small filter on that too?

It would seem that if I followed these steps the return air will still flow through this open wall into my system, but I will have forced it to come from only inside the house via the new grill in the hallway. Can this work or am I missing something major and am somehow going to damage my system? Thanks again for the help!

Uncle Pep Pep 01-31-2012 12:54 PM

Ok I went up to the attic and this area is definitely the return air. At the top of the wall that I show in the video there is a huge box to catch the air in the attic. That is connected via one huge duct to the input side of the unit and is the only thing that feeds the Aprilaire 2200 filtration system that I mentioned in an earlier post.

biggles 01-31-2012 01:28 PM

OK so that grill in the ceiling has a duct connected to it in the attic and from there it goes to the back of the air handler as you say the "input side" or as it is know the returm plenium where the filter is within the unit or can actually be in the ceiling return with a drop down grill.forget that wall opening and seal it with a piece of sheet rock.if you don't see duct work on a grill as that is.... it's not part of your heating the dead of the winter cold air will infultrate into the hallway and guess what:wink: up into the ceiling return.just for ppeace of mind pull your stat off the subbase and put your face up to the hole the tat wires hang out of...feel a draft :eek: that goes into the back of the stat and will drive the heat to run tape over ar push some shipping packing in it so you have no influence on the mounted stat....check back later....just to double check the unit in the attic is not open to the attic but ducted from that ceiling grill...:thumbsup:

Uncle Pep Pep 01-31-2012 01:32 PM

Biggles, I really appreciate your input and your responses. However, I am very sorry to say that I have no idea what you are talking about in your last post. I would love to know if you can perhaps explain it another way.

biggles 01-31-2012 01:49 PM

patch the wall opening it is useless,and has nothing to do with the heating system except to add cold to the hallway.if that is open to the house as in the the stat location cold air cold be leaking into the back of the stat causing it to run longer then set for $$ whats the final confirmation on that ducted return to the back of the furnace..?

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