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Low flow rate

\015 I have hired someone to move the ducts into the attic because of rusted out floor ducts. The new a/c and heat system is a 3 ton Rudd Achiver 90 Plus. The air flow in the vents seems low to me. How do I know if this system will have enough circulation to heat my house this winter. I live in Oklahoma.\015

Answers :

While a load calculation is the approved method it is far from your question regarding air flow.

Now let me help you understand something about air flow and if I understand whats going on here.
You say the total is 2800 CFM? Well how much is up and then down stairs? The furnace may be in fact have the capacity to blow more air than is required by the AC "LOAD" Calculation. Which is why it carries little weight with me except in special circumstances like large windows and southern or western exposures unshaded or open foyers. The advantages are this If you move more air (and have higher humidity like here in Oklahoma during summer) it will help increase the air changes per hour and help eliminate cold and warm spots. Thus making the system and home more comfortable and the system run less to achieve this.
Now your previous system was installed down stairs I assume. And some how the air had to be turned towards the attic for the up stairs, right?

I have deliberately over sized the blower on many applications to keep air flow up and thus keep the air moving and changing. Making the result a comfortable home. Now your home has some serious design problems that require more - larger ducting and probably larger blower. The blower will work properly and the system will not be adverse to larger design in most cases. However they may need adjusting to cause pressure to build up and keep flow adequate in some places. Experiment with these duct outlets to see if you can reach a happy balance.

Remember this too:
The air flow before would probably move curtains along the windows because that is where the outlets were and curtains are looser at the bottom. The curtains are tighter at the top where the rods and I assume ducts are now. And the system for down stairs now is just 3 tons. BUT the air flow may still be higher than 3 tons and the ducting may be sized larger than 3 tons (my standard was 225CFM for 8" round ducts and 125CFM for 6" round ducts. If these are rectangular I would have to consult with a calculator for measurements (400 CFM per ton at 1" water column static air pressure is the industry standard to prevent freeze up and noise problems)

The Rule here in Oklahoma is 400 Sq Feet / Ton if the walls are 4" insulated, no excessive west or south windows
Now if its newer (1990 on up) and 6" walls insulated and no excessive south or west windows, yes 600 sq ft /ton would be acceptable.

AND I add that with my experience with load calculations and my rule of thumb the measurements come out too close to make a difference given if all the facts are known up front.

NOW here is what I believe you might be seeing. First make sure your talking about Cubic Feet per Minute and not feet of throw. These are 2 different measurements and the feet of throw has less to do with load and more to do with air change and static air in a room.

I don't know about Don's but this sounds like he is a reputable fella.
He is on the same page as I am.

Lets go back to your original definition of trouble.
What is it specifically that makes you think you have a problem? Too long a run time?
Well what was the previous system doing? Remember this you had 4 tons before doing both up and down stairs. Typically the down stairs cools first because:
1 cold air is heavier
2 most of the time spent with the AC on is while you are awake.
3 the thermostat is usually down stairs.
4 The night time some people (may be you too??) turn the thermostat a little warmer and sleep fine with a ceiling fan or air mover on in a 1 thermostat zoned home.
5 the system being new and the duct work being installed in a 2 story application ? Is this correct? If this is so all bets are changed because the structure will have places that restrict application of even air flow and capacity. In other words you cant put the ducts in the center of the room over head in the entire down stairs but maybe some of them are not accessible and have no attic over them, right?.

Does the air handler or furnace have a variable speed blower?
Is the air flow better the closer you get to the air handler? It should be just a little if the duct pipe is the same size and the longer run is under 100 equivalent feet total, meaning you add on for ells and wyes and such.

Now I hope I haven't gotten off on a tangent but Ive reread your problem description and it lead me to write the lengthy above. If this helps you understand better what may be happening I'm glad (and if you would please let me know by posting back). If however Ive miss your problem please restate to me what it was that you needed to know.
The only real, exact way is to do an "Heat Load" on the house. Too many tech use the "Rule of Thumb" method. Usually 1 ton for every 600 square feet. This does not count windows, the way the house is hit by the sun, and so on. A unit should supply air in a gentle fashion so you do not have drafty areas. I'd look into the heat load calculation. Takes all the guess work out of what is required.
\015\012Good Luck.

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