Open fresh air return in attic
We are in process of building a home in North Texas. As part of the construction, we have encapsulated the attic with open cell spray foam. In essence, the attic is part of the home's conditioned air space.
My HVAC contractor installed, with our blessings, a system whereby the fresh air return entails about three to four feet of duct and then is open into the attic. This is lieu of having fresh air returns using duck work to the air handler. The HVAC contractor feels the air in the attic needs to be conditioned as well as the home (1 story home).
Prior to starting construction, I engaged an independent energy inspector primarily for the spray foam insulation. As part of his services, he also inspects the installation of the HVAC system. When he saw the open fresh air return on the HVAC, he went ballistic stating that it violated code to use the attic as part of the fresh air return.
My HVAC contractor provided me several names of other builders that use the open fresh air return in the attic with great success and counters that these have passed inspection. He, however, indicated that I wanted enclosed return; he would modify the system without charge. Please note that I have a green tag from the city building inspector.
I wanted to submit this question for discussion. I ask that you not debate the enclosed attic system since it is a given.
I would appreciate if anyone else has used the concept of using the attic as part of the return without corresponding ducting.
Thank you for your comments.
Why would you aircondition your attic if you are not living in it? The attic has a great amount of surface area and unnecessary heat gains resulting in additional losses. Is the attic warmer then the house? How "Sealed" is the attic? Have they done a door blower test to prove that the attic is air tight?
If you turn off the AC, how fast does the temperature climb and how much in the attic vs the house? Every degree adds to a loss in space you don't occupy.
On a paranoid note, I wonder what kind of emissions the foam will have being directly sucked into your indoor air at a high rate? What ever emissions there may be, they probably taper off fairly quickly, but still, how many 20 year studies are there for the health effects of the foam used in your home when being used as part of the airflow path directly effecting indoor air quality?
That's what I think... The pros will comment on facts.
a sealed attic can work great
but I would not use it as a big plenum. I would duct the return
I would also have a dedicated fresh air intake directly from outside
you are supposed to condition the space, but some thing as simple as a 40 cfm supply and return from there is fine.
People are used to doing work in stripmalls with flat roofs. They call the space below the roof and above the ceiling a return air plenum. You have the samething, but if you are not well sealed to the top of the wall plates, you have the entire perimter of the home to be a big potential leak , meaning you could get a lot of uncontrolled outside air moving in when you have the main system drawing all its return from the attic
I tested my own sealed attic for 6 months beofre I finally conditioned it with a small amount of air. I added the air as I was storing some valuables up there- just a little safeguard to keep things extra dry.
The main opposition from the building inspector to doing what the builder wants is because it is a new thing as far as residential goes. I would vote to duct the return for the reasons I gave.
Last edited by Carnak; 07-17-2008 at 11:11 PM.
Help me understand
I understand the concept of using the space above the ceiling as a return air plenum. It's done all the time in commercial buildings, when a dropped ceiling is used. Are return "grates" placed in rooms to allow the air to be circulated? Your contractor wanted the attic space conditioned...does that mean supply air is also going into the space? And why would you want to have to heat or cool the attic air as it enters your furnace. Seems counter-productive to me.
Conditioning the attic space is a waste of money.
Unless your using the attic for more then storage.
I hope your HVAC contractor didn't install a bigger A/C to condition unused space.
Here is my opnion. Close off the attic. It is completely senseless to make the attic a part of the HVAC system if it is not occupied. And in most places it is NOT up to code.
Wow, Just some quick remarks (I'll have to think on this one)..
What is the heating source? electric/heat pump or gas
IMO, your attic is now a return plenum, so is it fire rated ?
Will the plumbing and electrical in the attic have to be fire rated?
How much and how long will the spray foam offgas ?
Dust,humidity and odor issues ?
Building pressures ?
Storage in the attic nulled?
What city are you dealing with?
Not trying to be negative,but I have done houses in N.Tex with the
sprayfoam, with great success.. But I did everything possible to seal the
living area from the attic...
I think we have a confusion of terminology here. A "fresh air intake" is different from a "return air intake". Fresh air intake inducts fresh air from the outdoors. A return air intake is where air returning from the conditioned areas is inducted into the air handler or furnace.
So, I'll assume you mean your sealed attic has a return air provision. And reading your post further, I think you are trying to use the entire attic as a return air plenum, with openings in the house below to allow air to return to the unit. If so, this is what is known as an "open return" system.
I'll be frank with you...I'm not fond of open returns, period. I think they're a shortcut around a proper return air system. In humid climates, open returns cause lots of problems in commercial buildings if the area being used for the return space is not absolutely airtight. In construction, that is very hard to accomplish.
I would need to be assured the sealed attic is indeed airtight before I'd sign off on it being an open return system. Any air leak to the outside will draw moisture through construction materials during hot, humid weather, and any fireplace chases, thermal bypass areas, etc. may do the same thing if they also are not sealed with respect to the attic and house.
less heat coming through the ceiling its not a 140F suana up there any more
Originally Posted by beenthere
air handler and duct work up there are not losing 10% capacity while being radiated. Air handler will last longer.
technically get a bigger conditioned space for no extra energy penalty.
40 cfm keeps it a couple degrees warmer than the space below, I have no shortage of data to back this up, i built a four pleax like this
Last edited by Carnak; 07-17-2008 at 11:27 PM.
40 CFM is not nearly the same as using it for the full return of the system.
40 CFM is more of a tempering, not conditioning.(perhaps a perspective thing)
what is your heat source??
put a 80% furnace up there and the attic return plenum becomes a big no no.
Originally Posted by beenthere
My own attic i could have ran as a return plenum, its tight.
I just looked through a couple test runs, before I conditioned it, with a very small amount of air, it would average about 81F and 50% RH up there, and that makes it into ASHRAE's comfort zone.
The attic temperatures were always the hottest after the sun set mainly because I have some surface mounted lights mounted on the ceilings and it traps the heat from the lights.
So I was keeping my place about 78 F, the thermostat is 5 feet above the floor. I would imagine the air up below my ceiling is 79 degrees or so, so I had a 2F differential through the sheet rock ceiling. Not a lot of heat transfer driven to the space below.
I added a very small amount of air 40 CFM tops, and it now averages 79F and RH about 43%
I have seen buildings retrofitted with the foam and the attic is a plenum and I never noticed any problems. No inspector has seen this either.
To me. An 80° attic would be tempered.
I keep my place cold.
Doesn't the attic add about 4000 to 6000 BTU to the load, depending on roof pitch, orientation etc. On say a 2000 sq ft single story. If you try to bring it down to what the nomal occupied temp of the house is. That temp being say 72°F?