wrong return duct size

[cowboyjw]

I have a 1973 double wide mobile home 1400 sq feet the a/c unit is a 1992 4 ton colemen the return duct size is 12 inch the unit states it should be a 14 inch do you think that is why the air flow through the registers is not vary powerful and the unit cannot keep up with the temp. 78 deg in the afternoon. humidity is 50% return air temp is 77 the temp out the registers is 59 the house cools but the a/c runs alot during the day. I had a a/c contracter out and he cleaned the coils and he told me that the return duct size is fine.

[udarrell]

Well, one would wonder what your CFM airflow is compared to what it needs to be for a 4-ton system.

I did not do much service work on moble homes but don't know if Coleman had high static pressure blowers back then, but am betting those applications needed them! Does the furnace tag say .5 ESP?

The return duct appears to be very undersized however, is there a problem regarding replacing it with a proper sized duct? I would want at least an 18" rd duct on the return with a large filter area.

That duct system may not handle more than a 2.5-ton equiped system (?), the rest of the tonnage becomes pointless overkill.

http://www.udarrell.com/proper_cfm_btuh_duct_sizing_air_conditioning_syste ms.html

[cowboyjw]

the book say's it is a .45 static pressure on high speed I could change the return to a 14 inch but I dont think I can go any larger

[dash]

Package unit or furnace split system??

Your location?

[cowboyjw]

this is a package unit and I live in south florida

thanx for the reply

[dash]

It's very common to find the flex duct ,and even the ducts and return in the home undersized,in Florida.

Correcting the problem,will increase cfms delivered,increasing capacity and reducing your operating cost.

When replacing it have a load calc. done as ,it's unlikely you need four tons.

I would have thought that a Coleman could handle more than .45 static,post the model and maybe someone can check the mfrs specs.


What size return grille,where is it located,and what type/ size filter?

How many feet of flex duct and how many 90° turns in it,supply and return?

Type and size supply duct in the home?

[cowboyjw]

the floor return grill is 12x20 with a filter size of 16x20 diagnal in the return box with the 12" flex duct about 15' long with 1 90 degree angle. the supply ducts come from the unit 1 12" flext into 2 10 inch flex into a 10x6 duct going down each side under the floor with 5 registers the

[dash]

Originally posted by cowboyjw
the floor return grill is 12x20 with a filter size of 16x20 diagnal in the return box with the 12" flex duct about 15' long with 1 90 degree angle. the supply ducts come from the unit 1 12" flext into 2 10 inch flex into a 10x6 duct going down each side under the floor with 5 registers the


You need two returns and filters ,just like the one you have,plus 14" duct on the supply (to two 12" ducts)and 14"return(to two 12" returns) as a minimum,to get all the btus you are paying for ,out of that system.