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View Full Version : How much return grill for a 3.5 ton air handler

navyjoe
01-05-2009, 01:02 PM
I have a Trane XL16i. The outside unit is a 3 ton with a inside air handler of 3.5 tons model #4TEE3F40B1000AA. How much return grill area should this system have? I currently have 423 sq. in.

Also, the manual on that air handler is missing. Can you get that online from Trane?

jwiehagen76
01-05-2009, 01:51 PM
recommended free area of return grill(sq in) =cfm of return air/2.0 cfm/sq in

navyjoe
01-05-2009, 02:40 PM
thanks, how do I find out what the CFM of my unit is?

motoguy128
01-05-2009, 03:00 PM
Typically a 3 Ton A/C should require approx 1200CFM (400CFM/Ton). I know Carrier installation manual for their air handlers are easy to find online. I've found it a little harder for Trane Units.

motoguy128
01-05-2009, 03:01 PM
You might also try e-mailing Trane.

jeff520
01-05-2009, 03:04 PM
Recommended airflow is typically 400 cfm per ton of AC. A 3 ton AC would therefore need 1200 cfm airflow. With only 423 sq in of return your air velocity through the grille would be just over 400 ft/min, a good number as far as noise is concerned.

Air velocity of more than 500 ft/min will be too noisy for a residential system.

As important as air velocity is for noise, also consider the pressure drop across the grille and filter. Too high a pressure drop will cause problems maintaining recommended airflow and also stress the air moving fan motor.

Kevin O'Neill
01-05-2009, 05:35 PM
thanks, how do I find out what the CFM of my unit is?

navyjoe
01-05-2009, 06:19 PM

Jeff520 you feel that 428 sq ft is OK if I understood you correctly, However I noticed that jwiehagen76 was suggesting about 600 sq in based on the formula of cfm/2 if I understood his notes correctly.

Right now I have 3 returns. 2 are in the mater bedroom/bath and total 195 sq in. This area consist of a little less than 500 sq feet of living area.

The 3rd return is in the hall outside of the bedroom. It is 228 sq inches for the remaining 1500 sq feet of living area and ocasionally whistles.

PS I tried posting this 30 minutes ago and something went wrong. I apoligize if it double posts

Swampfox
01-05-2009, 06:32 PM
You need 600 sq/in minimum

that is not a 3 1/2 ton air handler, it is a variable speed model, compatible with multiple capacities

you must have some strange size return grills to get those odd numbers

mbarson
01-05-2009, 06:39 PM
There should not be any returns in the bathroom!

jeff520
01-05-2009, 07:03 PM
Navyjoe, Not sure where the 428 sq ft number came from. I said that your 423 sq in would give an airflow velocity of just over 400 ft/min.

I found jwiehagen's formula a little hard to interpret ("recommended free area of return grill(sq in) =cfm of return air/2.0 cfm/sq in") because of the cfm/sq in at the end. If you interpret it as return grillle size in sq in should be half the value of the cfm of return air number, then he is aiming for a return air velocity of 288 ft/min. A better number than the 400 ft/min that I calculated you have now but not needed to prevent excessive noise.

Ductwork is not really as simple as just measuring the size of your three return grilles, a lot depends on the size, length and turns in the ductwork between the grille and the furnace.

The simplest way to put a value on what you have is to measure the pressure difference between the return air entering the fan and the supply air leaving the fan. If the difference is too high, then the fan is not working within its design parameters and you need to figure out where the pressure is being generated.

A high negative pressure at the input to the fan could be a dirty filter, excessive duct length, obstructions, duct too small, grilles too restrictive, etc.

Too high a pressure at the fan output could be dirty evap coil or any of the duct problems mentioned.

To check your system have a professional determine the actual airflow through the air handler and the static pressure across the air handler. 400 cfm per ton is a typical number for airflow, but this value may need to change depending on your situation. It would be lower if you wanted more humidity removal or higher if you live in a very hot but dry climate.

Static pressure drop usually has an upper limit of around 1 inch of water pressure, and this number is set by the manufacturer of the air handler. Within reason, less is better, and if your number comes out to less than .5 then you are in good shape.

jimj
01-05-2009, 07:20 PM
Also, the manual on that air handler is missing. Can you get that online from Trane?

Here ya go.

navyjoe
01-05-2009, 09:13 PM
You need 600 sq/in minimum

that is not a 3 1/2 ton air handler, it is a variable speed model, compatible with multiple capacities

you must have some strange size return grills to get those odd numbers

You are right on the grill sizes. 2 of the grills in the bedroom are actually 8.5x 11.5 (Opening size) floor registers hence the 195 sq. in.

The other register in the hall is actually a 12x20 filter box with no filter (filter is at the air handler) and I just measured the grill on it which is 12x19 or 228 sq inches.

navyjoe
01-05-2009, 09:14 PM
Here ya go.

Fantastic! thanks!

navyjoe
01-05-2009, 09:46 PM
Navyjoe, Not sure where the 428 sq ft number came from. I said that your 423 sq in would give an airflow velocity of just over 400 ft/min.

I found jwiehagen's formula a little hard to interpret ("recommended free area of return grill(sq in) =cfm of return air/2.0 cfm/sq in") because of the cfm/sq in at the end. If you interpret it as return grillle size in sq in should be half the value of the cfm of return air number, then he is aiming for a return air velocity of 288 ft/min. A better number than the 400 ft/min that I calculated you have now but not needed to prevent excessive noise.

Ductwork is not really as simple as just measuring the size of your three return grilles, a lot depends on the size, length and turns in the ductwork between the grille and the furnace.

The simplest way to put a value on what you have is to measure the pressure difference between the return air entering the fan and the supply air leaving the fan. If the difference is too high, then the fan is not working within its design parameters and you need to figure out where the pressure is being generated.

A high negative pressure at the input to the fan could be a dirty filter, excessive duct length, obstructions, duct too small, grilles too restrictive, etc.

Too high a pressure at the fan output could be dirty evap coil or any of the duct problems mentioned.

To check your system have a professional determine the actual airflow through the air handler and the static pressure across the air handler. 400 cfm per ton is a typical number for airflow, but this value may need to change depending on your situation. It would be lower if you wanted more humidity removal or higher if you live in a very hot but dry climate.

Static pressure drop usually has an upper limit of around 1 inch of water pressure, and this number is set by the manufacturer of the air handler. Within reason, less is better, and if your number comes out to less than .5 then you are in good shape.

My bad, 423 was the correct number. Technically I suspect I should have said the register was 240 sq. in. since it was designed for a 12x20 filter. It still seams too small for 1500 square feet.

As for having a professional do it. I wish I had one doing it. I am building a house and this is the "pro" the contractor used. I started out requesting a XL16i 2 ton unit for my 1st floor of 1100 square feet (very well insulated, very energy efficent house) and was going to use mini-split units on the 2nd floor bedrooms of 800 square feet.

The contractor and the HVAC talked me into going with a zoned system instead. Now I have a very noisy system. I sleep with ear plugs.the fan always runs on high. The air often whistles going through that 12x20 register (Its just outside the bedroom door). The AC contractor has switched the bypass fron one return duct to the other and is now planning on trying a third attempt using a larger bypass straight from the supply plenium direct to the return plenium. The AC contractor is standing firm that the 12x20 is adequate in size and won't change it. Unfortunately when I close the bedroom door at night the air swoushes under the door trying to get back to those bedroom returns and the bedroom supply vents really turn on! It also causes the bedroom the be 3 degees hotter than the res of the house. To add insult to injury I requested the the honeywell programable thermostat, he responded that he would use the Trane thermostat then he installed the Trane bottom of the line XR402 because he said Trane told him that was the only one that could be used for zoing (the XR803 would work wouldn't it?). To add insult to injury the supply air usually runs abot 6 to 8 degees warmer than the room air on stage 1. One chilly house. One time I did get it 20 degress warmer by it was brief and used stage 2 with Aux heat.

Pardon my raving, but this has been going on a month.

navyjoe
01-05-2009, 10:04 PM
Jeff, just to further clarify. The return ducts are only about 10 feet long. The unit is in the basement directly under the bedroom.

hvac248
01-05-2009, 10:55 PM
Note the area is the Free area of the grill not the grill size,larger grill size may be in order.
Check with the manufacture for the free area of the grill.

lentz
01-06-2009, 07:23 PM
The return grills are to small. Need to check the size of the return ducts also. Ask the contractor to check his sizing manual and do it right. You know if it making this much noise something is wrong. Keep that filter clean.