filter sizing

[wyounger]

The Patient:
Mom's house in San Bernardino, CA
Old, loose house, hot-dry climate

Four year old gas-electric HVAC. Condenser is a Goodman CLT60. Furnace is horizontal, attic, 80%, and was done at the same time, so we can assume it's a match. I haven't been up there to see exactly what it is, but I know from its behavior that it's single-stage with a PSC blower. Her base ration of electricity is spoken for just with lights, refrigerator, and the pool pump. Everything after that, like AC, is done at punitive rates (25-35 cents/kwh) in California's extremely tiered electric rate tariffs. They've had $600-$800 summer electric bills with this system, even at 13 SEER (which was not shabby when it was new a few years ago!).

I'd like to be able to tell her what she needs to have done to the system to improve its cooling efficiency- with those electric rates it's worth a lot of money, and the system is too new to really justify starting from scratch. I'm only at this house a couple days a year, though, so I largely have to work with what I remember and what she can tell me.

I've learned here that in desert applications the best efficiency will be had by maximizing airflow- humidity control is a non-issue there. For 5 tons, then, we should be hoping for 2200-2300 CFM, maybe even more. That means you either have to use a system with some extra headroom (four ton condenser with a furnace that can move 2000 CFM, say)... but this is five tons. So all you can do is do everything possible to minimize static pressure.

I know she has a single return with a single 1" pleated filter. I'm working on confirming the size of it now (must be either 20x25 or 24x30, as I remember), but it looks to me like it's a pipe dream to get beyond 2000 CFM on any single 1" filter (heck, maybe even with media). The restriction of filter media (i.e. Glasfloss Z-line) is often rated relative to FPM, though.

When we've got CFM and we want to calculate the FPM through a given filter size on a *pleated* filter, do you use the basic area of the filter (20x20, etc.) for your conversion or do you use the actual area of the media given all of the pleats?

[beenthere]

Go to airguard.com


Hers a pdf about one type of filter.

Which should tell you about your return trouble.

http://www.airguard.com/downloads/DPM7.pdf

[mark beiser]

You use the basic area of the filter. I hate the way they rate those, it would be simpler to figure out if they had charts that gave the PD of the filter at varrious CFMs like most high efficiency media filters do.

I can tell you one thing with a reasonable degree of certainty. The typical 5 ton residential system has no hope of moving anything close to 2000 CFM through just one 20x25x1 pleated filter.

There really is a lot more information about the system needed, like furnace and coil model numbers, and some measurements that need to be taken, before anyone could give you real advice on what needs to be done to improve the airflow.

Adding return air usually helps a lot, but if the supply side of the system is very restrictive, adding return won't make a huge impact on the performance of the system.
Depending on wich model of furnace and coil you have, it may not be possible for the system to move 2000 CFM, even with no ductwork or filter at all.

[wyounger]

Nice PDF, beenthere. Makes a good reference, at least when you're targeting 300 fpm.

I agree that there could be lots of other problems, but clearly one is the return; the filters need to be twinned to have any chance. Even if it's a 24x30 filter, that'd be 400 fpm, and pleated 1" units are generally 0.25-0.3" in the 400 fpm range anyway. Assuming she's got a GMS8/GHS8, we should be able to get over 2000 CFM if we can keep total ESP under 0.5" (or even 0.65" on the GHS8). They actually have pretty stout blowers in them, based on the fan curves I found on their web site.

I still need to find her a good local contractor to work with... and to confirm the identity of the furnace and coil too. At least now we can be pretty certain that the filter area needs to be doubled, at minimum. I don't doubt that it will need ductwork improvements, too, given that the ductwork was done by whoever decided to let a 5 ton desert system suck on a single 1" pleated filter. I guess we'll be on the hunt for somebody who has a manometer and knows how to use it!

[gevans]

If you have a 5 ton system installed with a single 20X25 air filter, you have a jackleg installation. You need to get a pro out there to properly size your ductwork. You can perform a very simple test to see if the ductwork will handle more airflow: just remove the air filter with the system running at high blower speed (fan on or cool). Do you have to pry the filter out of the holder due to the suction? Does the airflow on the supply side pick up substantially? If so, you will benefit from a larger filter (or a second 20X25).
Undersized filters will absolutely run up the electric bill, but so will poorly designed ducts.

[hvaclover]

Gevans,
Good idea.

Just curious,don't you use a manometer to check duct press?

Pulling off the door gives some indication of lack of return air, but it won't reveal how much.

[cem-bsee]

why not tighten the house?
even this 73y/o can use a caulking gun! -- I bet she can also --

[gevans]

hvaclover:
Yes, I use a more technical method for determining duct pressures. I'm just telling a homeowner how to make a "down and dirty" determination.
I don't use the curbside system sizing method either.

[wyounger]

It's loose in the sense of having old single-pane windows. I also have a hunch that the oldest of the exterior walls may be uninsulated. It's been added on to so many times over the years that you have a hard time telling what's from when, though.

Jackleg... probably. Hopefully I can find a real pro to straighten things out for her. I'm too far away (Georgia) to be of much help firsthand.