It's good to hear that the company didn't just throw in a large 4 or 5 ton unit for your house if your ducts are inadequate for air conditioning to begin with. Many times, older ductwork systems designed for heat only cannot handle the added airflow of an air conditioning system. Is there easy access to the ducts? The company may be able to increase the size of the ductwork to allow for better airflow rather than installing completely new ductwork everywhere. How about the return air grilles - how many and roughly what size? Return air can help air circulation quite a bit in many cases, but it's hard to make an accurate judgment on what's best to be done without a pro personally seeing what you have. It's important the system can move the amount of airflow needed for a/c to work efficiently and reliably. Best of luck with whatever route you take to get better airflow.

thank you ryanhughes for the reply,
I had the company come back out to check the system out and they said that they are moving the right amout of CFMs out of the duct and that the problem is that the ducts are down low and our heads are up higher. Also that because cold air is heavier and the ducts are down low on the wall that it will be difficult to actually cool down the upper area (where our heads are) but that these ducts being down low is good for the winter because heat rises. My ducts are 4" and I did ask about changing to 5 or 6" and they said that Yeah maybe you would get some extra CFM but that I still have the floor getting cool instead of the upper area. My wife says that other people have the same duct design that we have and that they have AC, not sure if they just let it run and run to keep it cool. Don't get me wrong the cool air does feel nice coming in from a hot day but I think it could be morer efficient and not have to have it run pretty much constantly all day with little breaks.
Robert

I'm not sure I fully agree with their theory. If you are already getting the correct CFM's (and with undersized ductwork you wouldn't be - depending on how many runs you have 4" may not be adequate and is typical of older systems), it doesn't make sense that they would say increasing the ductwork size would give you MORE CFM's. If you are already getting the right CFM's--any many homes aren't--then increasing the ductwork size isn't going to create more airflow than the blower is moving.

I have low registers for most rooms of the house for both heating and cooling--that's the way the home came--and have no problems maintaining 72* without running constantly. So, with that said, I think the problem lies either A. in the current supply ductwork and it not moving enough airflow (however you don't seem to imply that velocity out of the vents is a problem; there seems to be adequate airflow) or B. the return ductwork is inadequate. I would ask them about the return ductwork situation.

They can verify airflow by testing the total external static pressure (TESP). Many don't think to do this, but it will tell you if you are indeed moving the current airflow by measuring resistance. They can also run a heat load calculation to determine the correct size equipment needed.

A properly sized system will run constantly during the very hottest days to keep you comfortable, but it will maintain the temperature. Without seeing your situation personally, I cannot tell you if you have a problem or not. But if it's keeping you comfortable, and airflow is adequate, then you may not have much to worry about.

Take care.

Thank you Ryan Hughes,
I appreciate the help. I will need to get back with the company and have them give me more info on the stats that you have mentioned.
Robert

well I scheduled an appointment for the company to come back out and check things out a little more. I have 2 vents in the living room, 1 ine dinning room, 1 in each of the bedrooms, 1 in the kitchen and 1 in the bathroom. these are all 4" ducts. each room has a return with the exception of the kitchen,( I would think there would be, especially when cooking and it's hot outside), and the bathroom. in the basement there was 2 vents and 1 return. I needed more humidity control as well as temp control so they added 2 more 6" vents and 2 returns in the main trunk in the basement. my home is just under 1000 sq ft. will the 4" vent lines work or should I upgrade them to 5 or 6"? Perhaps a dumb question, but if the cfm is a low number how do I get that number up higher? 1 comment that their tech wants to keep saying is " well your vents are down low and so are your returns, so you are pulling the cool air into the returns. also he keeps saying that I don't have enough roof vents in the attic or soffit vents and this will contribute to the house not cooling off. he says that I need to insulate the walls ( I know this would be a good idea but$$$$) add more attic vents and the soffit vents. does all this keep his system from working properly? Thanks for your help and time.

Normally, we would use 6 or 7" branch ducts for a 2-ton system, depending on the number of branch runs, to keep velocities low enough.

However, you now have Eight (8) supply branch runs which means: 24000 / 8-runs= 3000-BTUH per duct.

FIGURING ROOM CFM - DUCT SIZES

Quick method: The heat gain and Btu/hr of cooling is done for each room.

At 400-CFM per/ton of cooling, 12,000-BTUH / 400-cfm = 30-BTUH for each (CFM) Cubic Foot per Minute of Airflow.

Then Select Supply duct size by CFM, velocity, & optimal Supply Air *(FR) Friction Rate.

A Room requires 3000-Btu/hr divided / by 30 equals 100-CFM, or around a 6" dia. RD metal duct. Eight 8 X's 3000-BTUH each run= 24000-BTUH.

Maybe a 4" to a small bathroom, etc.

You have to balance out the duct system to meet the BTUH heat-gain in each room.

Do not make your decisions solely on my information, work with your locals! - udarrell

Here is a link that might be useful: Duct Sizing Overview

hello and thank you UDarrell,
I apologize in advance for possibly a dumb question. When you mention keeping "velocity" low, what am I missing. I had thought that veolicty would be how fast the cold air would come out of the duct and enter the room and I guess in my wrong thinking that this would also be similar to the "cfm". I am now thinking that "velocity" would be the speed that it comes out and the "cfm" would be the volume that would come out. If this is correct, then are you saying to slow down the speed in order to keep up the volume? Thanks again for your time and patience. I am having to somewhat rely on others here for information because unfortunately I have one person saying one thing and another that just wants to gut all the ducts and just redo everything.
Robert

bump for help on this subject, please

I still need some help here please. we had 94 degree days this past weekend and there was a difference between the AC set temp and the room temp of between 7-10 degrees and the unit pretty much ran all day and night to try to keep up. as I mentioned above, I am being told that I need to have our walls insulated (about $1200), add more attic vents and also add soffit vents in order for the AC to work properly. This upsets me a little because when the companies each showed up to inspect the house none of them mentioned anything like "we can add the AC for you but your house could use..... in order for the AC to properly work". I would not be so upset if this had been mentioned in the beginning instead I feel like I have wasted money and also wasting money with the system turned on. The house is just under 1000 sq ft ant the returns and vents are all 4". How much will this additional work help in the efficientcy of the AC? I know it will help some but is this really the cure. I have had the company back out and I am getting an attitude from them while they're claimimg that "our unit is doing what it should". Perhaps so but I feel that everything should have been out in the open when they each came and spent an hr plus with me while selling their systems. Thanks in advance for any help that can be offered.

I have a 1086 square foot house with a 4 inch pipe perimeter heating system. My house was built in 1956 and the 4" duct system was designed as per the National Warm Air Heating and Air Conditioning Association (Manual 10 1956). In 1968, the NWAHACA dissolved and became part of what is now known as the Air-Conditioning & Refrigeration Institute (ARI).

My house is located in Winnipeg, Manitoba, Canada and is cooled by a 1 1/2 ton 13 SEER Ruud unit. The evaporator sits on top of a 80,000 BTU Ruud up flow furnace that's designed to produce enough CFM (1259) for a 3 ton air conditioner at .5" total static pressure drop. The main supply duct supplies 16 - 4" branch ducts.

The house is quite comfortable in both winter and summer (upgraded insulation, doors, windows and weather stripping). The AC is over-sized at 1 1/2 tons (17,400 BTU ARI rating) as manual J indicates a total heat gain of only 12,600 BTU. The only problem with the AC and furnace combination is the minimum fan setting (CFM output) is too high for the recommended 400 CFM per ton of AC. At just over 7" total static pressure (with a wet evaporator coil) my CFM per ton is 481.

The outdoor temperature today was 86 F with a humidex of 98.6 F and I was able to maintain an indoor temperature of 72 F at 42% humidity.

Some time ago I calculated my air conditioner's efficiency to be 86%. My cold air return registers are located at the bottom of the walls and my supply air grills are located 10" from the bottom of the walls so I suspect part of the drop in efficiency is due to a percentage of cold supply air flowing across the floor and into the cold air returns. I've had central air in this house for 22 years and had a replacement installed in 2007 even though my old system was operating satisfactorily. However, 22 years ago I had a problem of too much supply air flowing across the floor into the cold air return so I installed a couple of ceiling fans essentially providing some lift to draw the cold air up. This help and inspired me to seek additional lift for the supply air at the source, the grills. After a bit of investigation I settled on Imperial RG0291 10" x 6" Multi-Shutter Registers with an AK of 0.28. The adjustable shutters provide for directing air down for heating and up for cooling. This made a considerable difference and combined with the higher than recommended CFM per ton fan speed I was able to adjust the lovers to get the cool air near the ceiling where the previous registers only directed the air towards the floor.

My house is located in Winnipeg, Manitoba, Canada and is cooled by a 1 1/2 ton 13 SEER Ruud unit. The evaporator sits on top of a 80,000 BTU Ruud up flow furnace that's designed to produce enough CFM (1259) for a 3 ton air conditioner at .5" total static pressure drop. The main supply duct supplies 16 - 4" branch ducts.

The house is quite comfortable in both winter and summer (upgraded insulation, doors, windows and weather stripping). The AC is over-sized at 1 1/2 tons (17,400 BTU ARI rating) as manual J indicates a total heat gain of only 12,600 BTU. The only problem with the AC and furnace combination is the minimum fan setting (CFM output) is too high for the recommended 400 CFM per ton of AC. At just over .7" total static pressure (with a wet evaporator coil) my CFM per ton is 481.

The outdoor temperature today was 86 F with a humidex of 98.6 F and I was able to maintain an indoor temperature of 72 F at 42% humidity.

As long as it pulls the humidity down to 50% or less, the 481-CFM per-ton of cooling will just result in shorter higher sensible heat absorption & transfer outside cycles.

Also, with SA/RA near the floor it needs the extra throw.
Check the diffusers & let us know what they measure & there type, Hart& Cooley, etc.).

If the diffusers are too restrictive it will result in higher ESP.

Ignoring the diffusers, heres some data on your 1.5-Ton on that duct system.

16 4" runs : 721.5-cfm / 16-runs= 45-cfm each / .0872666-sq.ft.= 516.7-FPM Vel each * .01136= 5.87-MPH Vel., each duct.

The branch duct velocity is below 600-FPM & is OK.

- udarrell

Here is a link that might be useful: Equipment Sizing & Duct Sizing

Diffusers are - Imperial RG0291 10" x 6" Multi-Shutter Register

Return Grills - Imperial 16" X 8"

http://www.eccohtg.com/files/eccohtg/product/p775-788_-_Imperial_S8.pdf

Thanks for all the information. I amwaiting for the company to come out and get me some stats and then I will come back an get more opinions. Thanks again.