Goedjn wrote:
Hepa filters put a huge strain on a blower as the pores are so small,
never do this one without an experts advice on capacity.


If the filter is, say, 3" thick and had many, many pleats then the extra
area can compensate for the extra resistance of the HEPA filter medium.

To me, that's the way to go. Rather than a high maintenance electrostatic
filter which, typically, requires 6" of space in the direction of flow, get
a pleated HEPA filter that fits in the same space.




The issue with HEPA filters that the poster complained about is that in
order to get the 99.97% entrapment of 0.3 micron particles and larger
takes large amount of fan power. Furnace fans cannot do this by
themselves. To move 1500 CFM (typical flow number for 4 ton AC units)
through a typical HEPA filter requires almost 1HP of fan power. Air
handler fans are woefully inadequate to this task as a typical air
handler fan only has a tiny budget left for filter resistance.i.e. MERV7
OVERLOADS THE FAN!!!


Shouldn't you be able counteract this by increasing the area of the
filter?

To some degree, yes. That is why pleated filters get so much attention,
as they increase the surface area of the filter dramatically. Have you
noticed that the 1" UltraAllergen filters from 3M have DRAMATICALLY
increased the number of pleats?

You can increase the size of the filter say by going from 20x20 to
20x25, more filter area. Change filter media to get higher levels of
filtration (MERV goes up), or change nothing and the load on the air
handler fan drops.

You can increase the number of pleats in the filter, again more filter
area. Change filter media to get higher levels of filtration (MERV
increases), or do nothing and the load on the fan drops.

You can increase the total depth of the pleats, going from 1"to 4",again
lots more filter area. Change filter media to get higher levels of
filtration (MERV increases), or do nothing and load on the fan drops.

With each of these steps, we increase the total filter area. If the
media stays constant, say MERV7, then the total dust holding capacity of
the filter increases, we don't have to change it as often. But if the
filter media changes, and we get a higher MERV value, the life of the
filter drops, as it traps more dust faster.

To get HEPA level filtration at the flow rates needed to handle your
WHOLE house, i.e. 1200CFM or 2000CFM, the depth of the filter increases
to 12 inches or more, filter size increases to 24x24 or more, filter
media area increases to 185sq ft or more, and the type of media
changes. In most big HEPA filters, the air flows thru 12 inches of
filter media.

In our household MERV 7 pleated media, the filter depth for airflow is
under 0.1 inch (thickness of the material in the pleat).

To push air thru 120 times thicker media, thru media that has higher
density, requires lots of fan power.

Goto www.airguard.com and look at the cut sheets (drawings) of the HEPA
filters they make. This style is quite common for the 600CFM and larger
HEPA filters. Hospitals and laboratories use these by the truck load.
No, they don't cost $20, its more like $220 and up.