Hi, I'm looking for some advice on how a typically house air duct damper system should be set up. I have a two story house (and an unfinished basement). I have a damper for the main second floor air duct (located in the basement near the blower). I usually close this 75% in the winter, to direct hot air to the main floor, as hot air rises.

Should I also have a damper on the first floor air duct? My thought would be to close this most of the way shut in the summer to direct the colder air to the second floor. I realize cold air falls, but am not sure how this works in the real world (would be main floor cool down when most of the air flow is directed to the second floor?)

I have a lennox AC13x (I think it's 4 ton). My second floor is a good 10 degree warmer than the main floor even with the AC cracked. I have tried closing the each of the vents on the main floor at the registers, but this has not helped.

thanks,
Joe

My second floor is a good 10 degree warmer than the main floor even with the AC cracked

The problem is lack of a high return air on the second floor. The heat stacks up there not allowing the cool air a place to go. Fix the return air problem and your system will work better. You can play with the dampers all you want but until the second floor has a high working return air it will not make much of a difference as you have found out.

hvac1000, you're correct, the 2nd floor return is low, near the floor, while the vent registers are close to the ceiling. I understand your point on moving the return higher up to capture the hot air. Would I also need to move the vent registers from the ceiling to the floor? I'm trying not to spend more money than I need to in order to fix this problem.

Thanks,
Joe

First of all many home builders did not run a large enough supply duct to the second floor to start with. To determine that you should do a manual J for the BTU require calculation then a manual D for the main trunk size and then the feeders for each room.

I am not trying to make this difficult for you BUT I believe in accuracy in all things to do with HVAC.

See if your duct size is not large enough to send enough air for cooling to the second floor it will not make a difference what you do to the return air locations. If this is the case there is NO cheap cure but to replace the main trunk for the u8pstairs and any other associated takeoffs from the main trunk in the basement.

I believe you will be better served to do the calculations then after seeing what you have make a decission how you want to proceed. I can tell many stories of new and existing homes that have the same problem that you have. In some of them we replaced the main run for upstairs with a larger one. This usually requires the services of a carpenter and dry wall guy since tearing out walls or boxing in duct is usually required. In many other cases we just installed a separate A/C unit for the upstairs to keep it cool.

One other factor to consider is your attic vented well. If not this can cause a heavy heat gain in the upstairs bedrooms as the heat filters down through the ceiling and into the room. We found that a properly vented attic or the use of a properly installed power attic vent fan dropped the cooling requirements of a standard home about 6,000 to 12,000 BTUs on cooling. Just a thought.

If you feel you need the help of a ON SITE professional by all means call one. Usually the money spent for a professional on site calculation is well worth the money spent. If the cheap route is the best for you buy a small windo A/C unit and call it a day.

I have attached a chart that is used by many of the HVAC hacks for sizing A/C units. Usually there work never works. LOL

HVAC1000, thanks for the info, I will call a pro to determine if the duct size is o.k. I have seen other links that say for heating the returns and delivery vents should be low, and for cooling they should be high on the wall. My issue is more with cooling, but don't want to run into an issue this winter if I get my return moved.

Can you give me the general theory on proper placement of returns vents and supply vents for basements, first floor, and second floor?

Thanks
Joe

I agree with hvac1000 your ducts and placement of the returns are a probably a big problem. Also just to add, do you have a bypass damper from the supply to the return in the basment to alow for air flow to pass after you close one damper? Closing them too much could cause your unit to freze up.

Can you give me the general theory on proper placement of returns vents and supply vents for basements, first floor, and second floor?

Class work

(((Supply air velocity and throw distance will be the primary effector regarding air movement within a room. The return is primarily for at least two reasons: to exhaust air in the room being displaced by the incoming air, and for that exhausted air to be returned to the air handler so it can be conditioned once more (via either being heated or cooled, and filtered).

If a supply diffuser or register is placed properly and has proper amount of air emerging from it, this air will travel out from the supply and mix/entrain with the air present in the room. Ideally, by the time this air mixture reaches what is known as the "occupied zone" (from floor to six feet vertical), it has slowed to what is called terminal velocity (around 50 fpm) so it does not create a draft for the occupants of the room. In cooling it will also have warmed when it reaches terminal velocity, so the occupants won't feel chilled by the air's motion. ))



Now the best place for supply ducts/registers is along the parimater of the outside walls especially right in from of any windows. This allows a blanket of cool or warm air to hit the window area first. Windows are usually where the heat or cool gain is the most in alsmost any room.

Return airs are a bit different. The ideal placement is in the center of the room since this will cause the air that has been discharged to motivate to the center so all areas of the room stay about the same temperature. This is usually impossible to accomplish so wall areas at floor or ceiling are used instead.

The exact design placement is usually determined by the construction restrictions every floor plan has so each install has to be looked at on a separate basis.

I have both style systems in my home. High returns in one section and low in another. The same goes for the discharge air placements. Each area was designed for a different purpose.

There used to be a system on the market that was just about perfect. It was the old Coleman duct in a duct system. The supply air was actually in the inside of the return air duct. A special register grill was used to prevent cross contamination between the supply and return. That system was GREAT but it was very expensive to install and the material costs were also quite high. Sorry I cannot be more accurate than that but a ON SITE survey from qualified contractor will surely help you to make up your mind as to which way you want to go.

Thanks for the quick reply guys.

Joshdta, I have two supply vent lines coming out of the furance (blower), which are basically on top of my vertical standing furnace. There is a damper on the supply vent going to the second floor, which I close 3/4 of the way in the winter to force more air to the first floor. I do NOT have a damper on the first floor supply vent. Although it sounds like my return placement on the second floor (near the floor) is my main problem, it seems like it would help if I had a damper on the first floor supply vent to force more cold air to the second floor.

Hope that answers your question, I'm not exactly sure what a 'bypass' damper is.

Thanks,
Joe

HVAC1000, good info, it seems like my supply vents on the 2nd floor are located in the correct places (high on the wall, and near the windows). My returns are in the center of the room/ on the interior walls, but unfortunately they are low to the floor.

Based on you comments, it seems like returns can be low or high just depending on the house design. My first floor heats/ cools great with returns 3/4th of the way up on the walls, and the supplies on the floor. However, with returns near the floor on the second story of the house, clearly they are not sucking in the hot air which is rising toward the ceiling.

I plan to get a pro into to give some suggestions next week. High 90's in Kansas right now, pretty brutual.

To backup on your original response, my attic has a ton of blown in insulation, with 5 or 6 vents on on the roof. Those vents do not have a fan or them, and it's certainly very hot in my attic.

Joe

Post back with the pro's suggestions once he visits.

I talked to the pro a few times. Basically, they have to do an full house "analysis" to determine if I have the correct duct sizing etc. This would take several hours (estimate of $500 bucks). I simply can not afford that and the costs to move returns/ install dampers.

So, I am having them move my 2nd floor returns from the floor to the ceiling, as well as install a damper at the furnace so I can direct cold air to the 2nd floor in the summer.
If this stuff does not solve the problem, I will go down the full analysis method.

Also, based on my conversation with this guy, he said air returns are simply air being sucked between two joists and drywall. In other words, air returns do not use metal ducts until they rearch the furnace. Is this correct? It seems like I could move the returns myself if it is simply cutting a hole in drywall at a higher location and moving the register.

Thanks,
Joe

he said air returns are simply air being sucked between two joists and drywall

In most homes this is true and they pan the floor joist in the basement over to the furnace.