400cfm per ton not cold enough?

17 years ago

I was wondering if anyone else has noticed this. I have a 60,000 Trane XV95 furnace and a XL15 2 ton AC. With comfort R enabled and 800cfm of air, the system has always been sluggish and even though the thermostat would read 72F with 45% humidity, the house didn't feel very cold. If I bump up the fan to 450cfm per ton the system seems to perform better and the AC actually makes the house feel cool. Does anyone else find they prefer more airflow for the AC, not just in performance but comfort? I am in New York, the house is 1250sft, the ducts seem in good shape (No obvious leaks). I had the install company check the Freon and everything was good. I plan on adjusting dampers to possibly distribute the airflow better. I guess I am looking for a good explanation as to why 450cfm works great in my house, but I should be using 400cfm per ton. I have seen Udarrell mention that using floor Supply and floor Returns may require more airflow for maximum efficiency, which I have. Thanks

Teddy

Sort by:Oldest

Comments (4)

funnycide
17 years ago
400 cfm per ton is a typical value that works well with cooling and dehumidifying. The trend has been to reduce the cfm a little to 350 or 375 cfm per ton to produce colder supply air which dehumidifies a little better. Going to 450 cfm/ton would only have an effect on the dehumidifying capability. Since you are not in the desert southwest I would not recommend going with 450.
72Â° and 45% is a very comfortable environment I can't believe you weren't happy with that. Maybe your tstat is out of calibration
paulbm
17 years ago
If your supplies and returns are both in the floor and your air flow is not too hard for the amount of openings you have, then the cool air coming out only goes up a few feet up and drops back down to the floor. This way you have hot air from 4' and up that does not get moved or cooled. When you speed up the blower and blow out the registers harder it blows the cool air up further and mixes with more of the hot air . If you can't take return from hi in walls or ceiling, then you would need ceiling fans to mix the air so you can get that hot humid out of there and cool it. You should have at least 80% return hi and 20 % . this would work better for both heating and cooling. Later paulbm
Related Professionals
Kankakee General Contractors · Youngstown General Contractors · Mission Viejo General Contractors · Colorado Springs General Contractors · Lexington Home Remodeling · Branson Home Remodeling · Waco Home Remodeling · New Bedford Home Remodeling · Newark Home Remodeling · Garden City Home Remodeling · Hopkinsville Bathroom Remodelers · Cincinnati Bathroom Remodelers · Gloucester Bathroom Remodelers · Trenton Bathroom Remodelers · Wausau Bathroom Remodelers
udarrell_2007
17 years ago
Are sure the CFM measurement is accurate?

It wasn't a gravity flow heating system converted to a duct system cooling & heating?

Preceding response on throw & air mixture is on target.

In a lot of situations, I personally like 450-CFM per ton
of cooling.

Different flow rates should be tried & go with what works best with one's duct system air handler & both coil set-ups.

Especially, the system with a piston orifice (TXV,too) should be superheat tweak charged with the CFM & within a few degrees of room temp t stat setpoint you're going to use. - udarrell

Here is a link that might be useful: Air Side of Air Conditioning
teddyc
Original Author
17 years ago
Thanks for your comments paulbm.

To answer your questions udarrell:

1. Im not sure if the cfm mesurement is accurate. It is a variable speed fan I am setting with jumpers.

2. I am not sure if it was a gravity flow. Is there anyway I can easily tell?

When the Freon levels were checked last season it was at 800cfm per ton. Once I decide on what AC setting I am going to stick with, I will have them back out to recheck everything. Thanks

Teddy