New construction in Houston area with conditioned attic space. The attached picture shows the close proximity of the return air duct to one of the supply ducts in the master bedroom (spacing is about 24 inches and they are both somewhat tucked into a corner). Both my client and I think they are too close, but the builder is resisting.
I’m looking for an authoritative source / citation that addresses minimum required separation and what the minimum is. I suspect that it’s in the ACCA Manual D, but I don’t have it. Any help is appreciated.
From what I can see, the louvers on the diffuser are aimed away from the return grille. In which case, the supply air should be blown away from the return. In fact, blowing air away from the return could enhance the air flow toward the return.
In general, though, all diffusers have throws that are vary with supply air flow. The throw is the distance the air will flow before reaching a terminal velocity of 50 fpm. In designing supply air systems, the goal is not to have the return air opening toward the outer edge of the throw. That means that the supply air diffuser is usually placed as far away from the return as possible. That being said, one manufacturer used to make a supply air diffuser that had the return in the middle of the diffuser. The theory was that the movement of the supply air helped pull air toward the return air opening. I believe it didn’t succeed because of the logistics of installing the diffusers.
I will also add that no matter how return air diffusers are designed, they usually do not work as designed. The only ones that have a chance of working right are ducted ones. Even then, most of these systems are not balanced, and return air systems need to be balanced just as supply air systems do.
Chuck, are you sure that return air grill is ducted to the AHU? It might just be a transfer or jumper duct to a nearby hallway to help equalize pressure and allow a path for air to get back to a hall and eventually a return when the door is closed to that room. If so, then I wouldn’t think the location shown would be an issue. If it is ducted to the AHU then take a look at this document.
I guess one thing that could be done to test the affect that the proximity of the supply to the return may be to place a temp probe in the suspect return duct and another in a non-suspect return duct with the system operating and compare the temperatures to see if the suspect return air temperature is being significantly influenced by the supply (i.e., when the heat is running is the return air temperature inside this duct higher than other return ducts). YES - I know that this goes beyond the normal scope of a home inspection.
Thought I would share another “interesting” duct location from this house. Note the location of the fresh air intake duct in this picture (yes they used an exhaust duct and placed silicone caulk in the louver pivots to hold the louvers open). The intake is placed directly above the hot discharge of the condensing unit (remember this is southeast Texas).
I have to wonder if some people do not understand how ceiling diffusers work. If they are designed properly, the air blows out along the ceiling. To get an idea of the air flow pattern for a single throw diffuser, look at Figure 14 in this article: http://www.hdgrant.com/hpacart.pdf. The louver direction dictates which direction the air flows. If you look closely at the photo, you can see that the louvers are aimed away from the return grille. If you look at Figure 14 in the article, you can see that if the diffuser is working correctly, that is designed with the proper air flow for the diffuser, then the return air grille is in an optimal location in the room.
If you want to verify what is going on in the room, use a smoke tube to visualize the air flow. It is probably one of the most under-rated diagnostic tools out there.