Return Air Sizing

[crisscross66]

Hi

Presently collecting bids for a 3.5 ton 13 seer system. Present return is 20x20 with a 16" duct going into the plenum. (Old system was 3 ton, based on Load calcs, bidders are telling me I need 3.5 ton)

One bid includes sizing the return grill to 20x25, but leaving the 16" return duct in place (they will get back to me if it should be 18").

Do I really need to increase the return to 20x25?

Seems strange that 4 other bidders did not mention this.

Thanks
Chris

[jwiehagen76]

Did new contractors do a load calc? If increasing the size of the a/c duct sizing could be an issue as well as return grille sizing.

[thatguy]

It depends on how much free area the grill has. some of the more decorative grills reduce airflow by over half where as a simple and most basic r/a grill may be only 15%. A grill will commonly be upsized in order to maintain consistent velocities and minimize noise. it looks to me like a 20 by 20 would pass enough air that the furnace will potentially require. considering lack of info I am only using very basic rules of thumb though. The 16" may be an issue with the higher cfm.

[sktn77a]

The 20x20 grill is 400sq in while the 16" duct is only 200sq in. Unless you use a VERY restrictive filter, I wouldn't change the grill size unless they want to increase the duct size (which might not be a bad idea with a 3.5 ton A/C).

[motoguy128]

Wow, I've never heard of a load calc increasing the size of a unit. But I suppose it happens.

The duct calc I use says 16" is too small and 18" is marginal for 1400CFM (3.5 Tons). 20" is ideal. Defnitely go with the larger grill. Is this a filter grill, or is the filter located at the air handler? It's best to use a 3-5" media filter located on the air handler itself. Much less restriction and a little logner filter change intervals than a 1" filter.

[crisscross66]

Sorry little more info - contractor performed the load calcs and indicated that a little more than 3 tons for my upstairs unit. He indicated 3.5 tons.

Looking at a Trane XR13 with vs furnace XV80. At this point, he indicated that the 16" should be fine but still recommending that I upsize the return air grill in the ceiling to 20x25. I don't remember but he used his calculator and said I am not "pulling" 3.5 tons of air through the 20x20 grill.

This contractor recommends I leave the filter (pleated) in the ceiling grill, whereas the other bidders are all advising that I place a 4" filter in my attic and no longer use the filter in the return grill.

Chris

[beenthere]

If its a 20 x 20 return filter grille, its too small.
And so is the 16" flex return duct.

[crisscross66]

My other unit is 3 ton and also has 20x20 with 16" duct. The load calcs say that 3 ton is fine for this unit. The contractor suggested i upsize this grill and leave the duct at 16"

What should I be looking at doing here?

Chris

[motoguy128]

My other unit is 3 ton and also has 20x20 with 16" duct. The load calcs say that 3 ton is fine for this unit. The contractor suggested i upsize this grill and leave the duct at 16"

What should I be looking at doing here?

Chris

16" is a little small for 3 Tons too. 18" is better. 20"x20" is also a little too small. Better to go to 20"x25".

[crisscross66]

Ok,

The contractor that recommending the upsize used the formula 1sqft per ton to size the grill

The other contractor measured the static pressure (-0.20) and indicated it was acceptable for 16"duct and 20x20 feeding a 3ton. He indicated that a 16" can supply 1300CFM provided no restrictions in the routing of the duct. I have short run (10-12').

I do want to upsize the grill and duct, but looks like I need to convince the contractors for the duct increase. Don't know enough about static pressures.

Chris

[beenthere]

Is that a filter grille, or just a grille.
Where did he measure the static at.
The air handler?

[crisscross66]

Hi

Its the filter grill for the return in the ceiling which holds a 20x20x1 filter. I want to move to a filter in the attic and use a 4" filter (a few of the contractors recommended this).

The contractor measured the static pressure by turning on the blower and putting a probe inside the return plenum.

Chris

[beenthere]

If he measured with a clean filter in the grille. Then the 4" will add about .1 to .15" additional static to the return.