Return Air Duct

[mcp6453]

The return air duct on my upstairs heat pump (the same one with the thermostat issue also discussed here) sucks air through the filter at such a high velocity that the filter is not of much value. (It also pulls the bathroom door up against it, if I'm not careful.) The present grill is 20x25x1. I cannot find a much larger one. Is there any reason that I could not or should not add a second 20x25x1 above it? There is room on the wall, and the AC guy says that the duct work is not that hard to do.

[DerbyDad03]

re: "the AC guy says that the duct work is not that hard to do."

What did the AC guy say about the benefits of adding another grill?

[EXT]

I don't understand how the air moving through the filter quickly means that the filter is of little value unless it is ripping the filter media -- or are you using those crummy fiberglass filters that don't do anything except stop the big lumps of dust bunnies.

If air is moving that fast, possibly the fan is set too fast. Also add a real filter such as a quality pleated type.

[dpb]

Sounds like the air return wasn't properly sized for the furnace.

If this is a new installation I'd expect the installer to fix it.

I'd still want an explanation of the air balance etc., before I just added that big an additional opening in the same location.

It's possible the new opening could cut the velocity down, but the size of the return ducting is still going to be limiting if it isn't larger than the opening as far as actual flow.

The physical process of making the entry isn't too bad but a neat job requires some tools for sheet metal work most homeowners don't have. As for the particular situation, depends also on what the path of the return is--if it's vertical in the wall then you could make an additional opening. You say there's room in the wall but is the duct behind that location or does it make an L and end at the current opening?

Upshot is, I think you could _perhaps_ minimize a symptom but don't know it really would solve a more fundamental problem...

--

[DerbyDad03]

re: The physical process of making the entry isn't too bad but a neat job requires some tools for sheet metal work most homeowners don't have.

Unless the return ducts are nothing more than the stud/joist space like they are in my house.

Many of the returns on the first floor of my house are simply openings at the base of the wall above a joist space in the basement. The joist space is sealed with a flat piece of sheet metal nailed to the bottom of the joist. If I wanted to douple the size of a return opening, I could just cut higher up the wall and cover it with another grate.

[ransley]

Air Bear is a good filter that seals well, April Air dont seal to the case. Isnt ductwork sized for the cfm of the blower, a sizing chart should not be hard to find to see if it was done right.

[Steve Barker]

Sounds like to me you need to slow the blower down. But the speed of the air does not affect the filtering capability.

s

[Tony Hwang]

Hi, Some filters are air flow restrictive for better filtering. Tried different kind of filter?

[mcp6453]

The higher efficiency filters make too much noise because the air flow is so high.

[BobK207]

How about some more info? return air duct size? Home size?

[ransley]

Thats bs, I have 4", its no quieter without the filter I have

[mcp6453]

You want photos of a grill? By the way, there is no ducting between the grill and the unit. The air handler is located immediately behind the grill in a closet.

[BobK207]

No....I don't need photos of the grill.

No ducting, AH immediately behind the grill.

Ask your HVAC if a 20 x 25 grill is large enough for the AH's cfm at current fan speed.

Is the grill in the bathroom?

cheers Bob

[Tony Hwang]

Hi, Typically blower runs at medium speed in heating mode and high speed in cooling mode for obvious reason. Wonder if installer had proper calculation done for sizing the ducts.

[DerbyDad03]

ide quoted text -

re: But the speed of the air does not affect the filtering capability.

Is this really true? Here's why I ask...

Let's say I have a dust particle of 5 microns and a filter that blocks anything above 4 microns at whatever speed they use to determine MERV ratings.

Is it possible that at some X times the standard MERV air speed the 5 micron particle could be either mis-shapen enough (or even broken up) to be forced through the 4 micron hole?

[mcp6453]

The grill is in the hallway outside the bathroom, between the bedroom and the bathroom. There is no ducting per se behind the grill. The air handler is directly behind the grill.

Several HVAC guys have worked on this unit. None has said that they suspect a sizing problem. That doesn't mean there isn't one.

To others, I don't want to be argumentative, but if you try to push a piece of straw through a 2x4, the straw will break, and the 2x4 is unaffected. If that piece of straw is forced into the 2x4 by a tornado, the high wind velocity will cause that piece of straw to embed in the

2x4. Using that example, I don't see how the effectiveness of a filter is not affected by the velocity of the air (plus particulate matter) passing through it.

For the person who said that better filters are not noisier, if you're in my neighborhood, stop by with a 20x25x1 pleated filter, and I'll let you educate yourself.

[ransley]

Maybe a 1" is noisier, the 4" units are designed well and filter much more on 1st pass which is probably what you want. I dont think most 1" filters do a good job on 1st pass percentage dirt removal. With a 4" filter 1 yr replacement is normal they have at alot of media and are designed well.

[Zyp]

Not on a heat pump - read the original posting Tony.

BTW: any air speed above 400 fpm on the grille will cause enough noise to be heard. It is likely that, with a 20 x 25 grille face, making enough noise to be heard is a 5 ton unit trying to draw 2000 cfm .... adding a 2nd return duct and grille at a 2nd location will reduce the noise [air speed] at the 20 x 25 grille and be less noticeable. Without respect to the supply ducting, simply cutting the air speed by adding a 2nd duct will reduce the noise.

[Zyp]

Ahh, you're in a condominium... built to "cheap as we can get standards."