Filter too small?

[John Kogel]

An HVAC guy was checking out a 5 ton heat pump system.

He told my client a 16 X 25 filter in the return air duct was undersized for 5 ton.

Does this sound reasonable?

[Marc]

If there's more than 3 tons on a 16X25, I write it up. Truth be told, only a D calc knows for sure but such a calc costs as much as an HI in my area so I use a figure of one square foot of cross sectional area per ton installed. By that yardstick, 16X25 comes out to 2.78 tons max. Everything in my area uses a single air return duct. Rarely do I ever see a distributed return in residential.

That 'one SF per ton' figure originally came from the back side of an old cardboard duct calculator I have but it's reinforced by my own professional experience as an AC serviceman. Solved lots of 'iced evaporator coil' issues by using that figure.

Marc

[John Kogel]

One square foot per ton

1SFPT

Thanks, Marc. Hope you survive the tornadoes so y'all can brace up for the hurricane season!

[mjr6550]

I don't recall standards for this, but on larger size furnaces at least some manufacturers require that return air plenums enter from both sides of the furnace.

ACCA recommends a maximum velocity of 300 FPM at filter grills (not exactly the same thing). That works out to 833 CFM for a 16x25 filter. That is about 2 tons. That velocity may be low due to noise concerns or because the grille restricts airflow. So 3 tons seems about the right limit for a 16x25 filter.

However, I don't recall ever calling this a defect.

[Marc]

I don't recall standards for this, but on larger size furnaces at least some manufacturers require that return air plenums enter from both sides of the furnace.

ACCA recommends a maximum velocity of 300 FPM at filter grills (not exactly the same thing). That works out to 833 CFM for a 16x25 filter. That is about 2 tons. That velocity may be low due to noise concerns or because the grille restricts airflow. So 3 tons seems about the right limit for a 16x25 filter.

However, I don't recall ever calling this a defect.

It's more of a concern that the constriction represented by the air return filter and return duct will be great enough to bring evaporator temps to 32 or under. At that point, long run cycles might make enough ice to survive the off-cycle and culminate over several consecutive run cycles in a total loss of air flow through the coil. As for heat cycles, inadequate air flow could result in the failure of hot refrigerant to condense before reaching the expansion valve in the outdoor section. It's the flash of liquid refrigerant to a gaseous state that causes it's temperature to drop so it can pick up more heat from outside the house.

Wind noise from grilles starts becoming a nuisance at about 600 FPM. 300 FPM sounds like a good safe design value.

Marc

[MechAcc]

Filters should maintain around 300-350 fpm air flow to prevent dirt from unloading. One article I read stated one way to determine if filter or duct work is to divide the required cfm for the outdoor unit ( 400 cfm x tonnage) by the square footage. In this case 400 cfm/ton x 5 / (( 16 x 25) / 144 sq inches per square foot) or 2000 / 2.78 = 719.4 fpm. Too high of air flow through filter which will cause dirt unloading of the filter. Sq ft = cfm / fpm or 2000 / 350 = 5.7 sq ft of filter area would be required. To get this much filtration the unit would require dual returns. Returns on both sides or a return on one side and the bottom should be installed to keep a low face velocity.

[Marc]

Filters should maintain around 300-350 fpm air flow to prevent dirt from unloading. One article I read stated one way to determine if filter or duct work is to divide the required cfm for the outdoor unit ( 400 cfm x tonnage) by the square footage. In this case 400 cfm/ton x 5 / (( 16 x 25) / 144 sq inches per square foot) or 2000 / 2.78 = 719.4 fpm. Too high of air flow through filter which will cause dirt unloading of the filter. Sq ft = cfm / fpm or 2000 / 350 = 5.7 sq ft of filter area would be required. To get this much filtration the unit would require dual returns. Returns on both sides or a return on one side and the bottom should be installed to keep a low face velocity.

First you say you need a minimum velocity to prevent dirt unloading then later you refer to a high velocity as causing dirt unloading.

My suggestion; reread your post and write it a little better.

I can't make head or tail out of what you're saying.

Marc