System short on return?

[545GAlady]

I've been always wondering if our first floor system is short on return or not. Our service company never mentioned anything about the returns. I already know that the furnace and heat pump are over sized. There are a total of 17 supplies and only 7 returns. The returns are all 8 inch. The furnace is 90,000 BTU and the heat pump is 3.5 ton, which both are an overkill for 2200 sq. ft. in Atlanta, GA. If we really are short on return, it's impossible to add any more because expensive dry wall work will have to be done since the basement is finished and the ductwork is above the basement ceiling. Do you think the system is short on return? I apologize if I didn't include enough information. Thank you in advance for any input.

[George2]

If the ductwork is covered, how do you know they are (all) 8"? The important size to know is the duct that "feeds" or leads to the 8" RA runs.

Lets say you have 4-8" runs being feed by a 12" main line (from your earlier posts I remember it's all round flex), then the 12" is less volume than the 4-8" runs.

Many people count the registers or return grilles and ask if it's "enough". It all starts with the main line. If that's not large enough, the number of registers or grilles mean nothing.

[George2]

You asked if you had enough return air. If the main return duct that feeds the 8" runs is large enough, I still like to see more return. Your blower is a 4 ton (1,600cfm) and what is there, if done to perfection, is just large enough for 3.5 ton (1,400cfm) at a higher static or resistance to the airflow.

Does it work...probably....efficiently......no.

[545GAlady]

Thanks! It's really hard to tell what the size really is. I might post some pics soon, just in case it helps. This means you get to see the crappy ductwork!

[Rodney28334]

Adding to what George said, I would like to see more than 7 - 8" flex returns for a 3 1/2 Ton, especially with a furnace (to save heat exchanger). The main issue is how the flex is tied into the return duct. He said in a previous post you said it was all flex, but there is some metal ducting somewhere, be it a square/rectangular plenum, or a branch duct setup. I am going to Over simplify this. I like to have a larger central return and a few smaller ones for big bedrooms, bonus rooms, ect. But if I was going to run just 1 large return for a 3 1/2 Ton. 1- 18" metal pipe has a area of 254 sqin, a 8" round pipe has area of 50 sqin. 7x50=350 sqin. I dont know what model you have but lets assume it has a 20"x20" return opening. 20x20=400 sqin. We are ignoring friction loss for this scenario but with it being flex and several small returns in real life it cant be overlooked. Assuming the duct is adaquite size the situation may not be as bad as you think. Is this new construction or a changeout/remodel. You can have the company come out and check the duct pressure or if you do not trust them have another company do it. Also if this home is older and not had any/many renovations it may not be grossly oversided depending on other factors, and the efficiency of furnance.

[George2]

Again.........I've seen it before. I can't forget it and I can't believe contractors have stooped so low.

Contractor have found that they can sell about anything and then let the homeowners deal with the poor workmanship.

[George2]

Adding to what George said, I would like to see more than 7 - 8" flex returns for a 3 1/2 Ton, especially with a furnace (to save heat exchanger). The main issue is how the flex is tied into the return duct. He said in a previous post you said it was all flex, but there is some metal ducting somewhere, be it a square/rectangular plenum, or a branch duct setup. I am going to Over simplify this. I like to have a larger central return and a few smaller ones for big bedrooms, bonus rooms, ect. But if I was going to run just 1 large return for a 3 1/2 Ton. 1- 18" metal pipe has a area of 254 sqin, a 8" round pipe has area of 50 sqin. 7x50=350 sqin. I dont know what model you have but lets assume it has a 20"x20" return opening. 20x20=400 sqin. We are ignoring friction loss for this scenario but with it being flex and several small returns in real life it cant be overlooked. Assuming the duct is adaquite size the situation may not be as bad as you think. Is this new construction or a changeout/remodel. You can have the company come out and check the duct pressure or if you do not trust them have another company do it. Also if this home is older and not had any/many renovations it may not be grossly oversided depending on other factors, and the efficiency of furnance.

Wait until you see the pictures she'll send. It's flex ALL the way. Unbelievable. A metal box on the furnace and then flex (size?) off of that box with the round 8" flex tapped into (?) that round flex. Thank God it's covered by the sheetrocK! I can only imagine how it's hung.

The quality of work in our area has declined in the 35 years I've been doing HVAC but not even close to the level you'll see in GA.

[545GAlady]

Here's the pics: All of this looks horrible. Soon might be the time to call a really good contractor, but I can't find any "really good" ones around here. There are several good ones though.

[545GAlady]

The house is 10 years old and the first and second floor systems were installed before we moved in 10 years ago. We have never had a failure with the furnaces or old A/C's. We had our upstairs A/C replaced because of a huge freon leak in the A coil and we wanted the first floor A/C upgraded.

[545GAlady]

Here's the pics: All of this looks horrible. Soon might be the time to call a really good contractor, but I can't find any "really good" ones around here. There are several great ones though.

I can't tell if the ducts in the picture are 8 inch or 10 inch (I think it might be 10 inch, but still not sure). Are you able to tell? I can post some better images if you need me to.

[George2]

I can't tell if the ducts in the picture are 8 inch or 10 inch. Are you able to tell? I can post some better images if you need me to.

Are there only the two flex ducts coming off the box? Normally the size is stamped on the flex. If not, just measure it. If it measures 12" (OD) on the outside, then it's 10" ID.

The lower flex starts with a kink which reduces the size right off the bat. What size is the A/C on that system?

Was it 3.5 tons? That's 1,400 cfm minimum of airflow needed. I always design for the blower plus I add at least 200 cfm extra to be on the safe side.

[545GAlady]

Are there only the two flex ducts coming off the box? Normally the size is stamped on the flex. If not, just measure it. If it measures 12" (OD) on the outside, then it's 10" ID.

The lower flex starts with a kink which reduces the size right off the bat. What size is the A/C on that system?

Was it 3.5 tons? That's 1,400 cfm minimum of airflow needed. I always design for the blower plus I add at least 200 cfm extra to be on the safe side.

No there's actually 3 ducts coming off the box. One of them is hiding right behind the one you see on top (The one behind looks to be the same size as the one in front). I'll measure them eventually.

Yes, the A/C is 3.5 tons and it's a heat pump (installed back in late April 2012 and the unit is a Trane XR15). I'm enjoying having a dual fuel system!

Thanks for the reply.

[545GAlady]

Did I ever mention that the air rushing through the returns are sort of noisy?