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