Tech recommended adding return duct and vents!

[andy]

My house is 15 year old 2-story ~3000 sqft in Texas. Two hvac units: 3 ton down, 4 ton up; both have single 16" oval duct returns. The return vents are 16 x 24 down, 20x30 up. They keep the house comfortable and I'm not having problems with them.

I just had a reputable hvac tech do routine maintenance and checkup on both units. He saw the returns and said they were both two small and would shorten the life of the units. He recommended adding a 2nd return to both units.

In the limited information I've been able to find online:
- the rule of thumb is 400 CFM/ton. That means 1200 CFM down, 1600 up.
- assuming 400 FPM return, vents are sized: 1200/400*144=432 sq in down and 1600/400*144= 576 sq in up. So, 18x24 down and 30x19 should be adequate.
- That means I'm slightly undersized (16x20) downstairs and about right (30x20) upstairs.

Is my math right? Do I need additional return duct and vents? Where can I find enough information online without buying the ACCA manual?

Thanks in advance for the help.

Andy

[Hube]

Basically,without going into a lot of detail, yes, it is slightly undersized. The 16 " oval R/A is the controlling factor, and don't mix "free area", or FPM math with grille intake size. The amount of any air gained would not warrant any amount of labor and material involved to re-size. Hube

[imeduc]

With Hube here just a different way. You say 16" on both units a 3 ton and one on the 4 ton . Well by my books they show a 16" pipe as just 900cfm so thats not big enough for 400cfm a ton on the 3 or the 4. Also you said oval now when you oval a pipe it does not have the same sq inch in it as it did when round.

Another way to look at a AC duct job is 80 sq" per ton for a real good job and down to a 60 sq" for a so so job and no lower. So on the 3 ton you have only about 201sq" and not that in oval. When you should have about 240sq" for a good job. 7 ton for 3ooo sq ft Is over kill I think and Im down in FL.

ED[8D]

My mistakes dont define me they inform me.

[andy]

Thanks for the replies, but all of this leaves me a bit confused. If I understand, you both say I'm a bit short, but not enough to justify the expense of adding a return. ED, by your math 80 sq in per ton, with the 16" I'm only 20% short on the 3 ton unit - 200 vs 240. That's not worth the hassle.

By the same math, I'm short by 50% on the 4 ton unit. I should have at least a 20" return or add a 12" return to get close to the 320 sq in you recommend for a 4 ton. 50% short seems a lot. What's it costing me? Am I doing any damage?

7 ton is a lot for 3000' but I've got an office/bonus room over the garage and it gets hot. Upgrading from 3 to 4 ton upstairs allowed me to keep it comfortable. I later discovered the underlying problem, which 2-3 people previously dismissed - inadequate insulation in the floor. Everyone thought it was inadequate ceiling insulation. It'll cost me a grand or more to fix it, so for now I'll live with running my 4 ton a bit more often.

Again, thanks for the help.

Andy

[pretzelb]

What's involved in adding a return that would drive the cost to be "too much"? I may have a similar problem but I didn't think it would cost a lot to add a return.

[imeduc]

If you dont have the return you should have on a unit its just like a car spinning its wheels and getting no where. You dont get the air over the coil you should and cool it but the main thing is to dehumidify it. Get more water out and you will feel cooler at a much higher temp.
ANDY Is the attic over that office hav e the vents in it that it should. A power roof vent fan on a tstat in the attic could help a lot

ED

My mistakes dont define me they inform me.

[Hube]

Pretzelb; Adding a return will not solve the problem,as the 16" return line is the problem. Adding to this return line will not increase its 16" size. Hube

[andy]

There is no attic over the office. It's a low vaulted ceiling with 2x8 joists and 8" insulation and then the roof. The heat is coming from under the floor (ceiling of the garage) which is 2x10 joists and 6" insulation. The 4 ton keeps me cool and dehumidified, but when the air quits moving the heat starts rising from the floor. At some point I need to spend the bucks to rip out the ceiling in the garage and properly insulate it.

quote:Originally posted by imeduc

If you dont have the return you should have on a unit its just like a car spinning its wheels and getting no where. You dont get the air over the coil you should and cool it but the main thing is to dehumidify it. Get more water out and you will feel cooler at a much higher temp.
ANDY Is the attic over that office hav e the vents in it that it should. A power roof vent fan on a tstat in the attic could help a lot

ED

My mistakes dont define me they inform me.

[imeduc]

Have you tried to let the blower run at Fan on so its on all the time your in the office, could help




My mistakes dont define me they inform me.

[andy]

I have tried to let the blower run at Fan "on". It helps for a couple of minutes, but then it's just heating the house because the ducts are running through the 150 degree attic. Nope the solution is insulation in the floor. In the meantime, I'm still thinking on whether or not I need to add a plenum and another return vent.

[Hube]

Andy, I just noticed your last post said" duct is running through a 150 degree attic",,,,, By insulating this ductwork with a good quality thermal type insulation, along with taped joints, etc, it is bound to make a tremendous difference along with more return air added. Hube

[imeduc]

With HUBE here 150o in the attic???? sounds like not vented right. A power vent fan there on a tstat would sure pay.


My mistakes dont define me they inform me.

[Hube]

Imeduc; Even if a power vent fan was installed in the attic, all it would do is take the temperature of the attic area down from the 150 to whatever the outside temperature is (plus any sun, shingle heat, etc. It still could be 100 degrees OR MORE in this attic area even with a 85 degree outside temperature. With no insulation on this duct, this amount of heat in this attic area it will soon influence the 55 degree air within the duct, and will eventually warm the air up and ruin the cooling effect. Hube