# Blocking Furnace Air Return

| Posted in Mechanicals on

My wife wants to move a bookcase right over the floor register for our furnace air return.  I figure I can build a little stand for it to give air a clear path underneath.  So the question is, how tall does this stand need to be so a to have negligible effect on air flow? Since it’s a return rather than a supply, perhaps it doesn’t take much? The footprint of the bookcase is roughly the same dimensions of the floor register.

## Join the leading community of building science experts

### Replies

1. | | #1

I’m not an expert here, but the vertical cross sectional area of the opening created by the stand will need to be at or larger than the return air vent to not change the static pressure seen by your air handled. So if the vent is 2’x3’ (6 sq feet), you need 6 sq feet of surface area. Assuming the sides of your stand are solid (blocked), and your bookshelf is against the wall (so the back is blocked as well) your stand would need to be 2 feet tall…. You might want to consider a different location for the bookshelf :)

Of course, if the stand is open on all sides, you can reduce the height. But the total vertical area needs to add up to the horizontal area of the vent.

1. | | #3

Thanks, that seems like an obvious rule in retrospect. So now I have to decide if the opening against the wall is big enough to count it, and if not, how big of a price is paid for shrinking the opening.

2. Expert Member
| | #2

Is the return that will be blocked the ONLY return? If it's not, you might only minimally affect your system. If it IS the ONLY return, then it's more important to try not to restrict the airflow.

Most returns aren't super high velocity, so it's not super critical how they are sized, within reason. As Tom mentioned, if you can keep roughly the same open area as the original register, you should be fine. Just don't do something like make several square feet of opening out of a narrow 1/2" slit or something like that -- you get turbulent flow effects with very small openenings that result in them having more restriction than an equivalent area of a single, larger opening would have. If all your openings a few inches or more in their smallest dimension, you shouldn't have any issues.

Bill

1. | | #4

Yeah, unfortunately it is the only return. Although maybe I can make the case that having the bookcase there will actually result in the register being kept clearer than if it weren't there. Before we rearranged the room that register pretty consistently had pillows on top of it. And that gives me an idea for some experimentation--block part of the register and see if how much more the furnace runs as compared to a night at the same temperature unblocked. I can also see if the the fan draws more current, but I think for this furnace that won't change.

1. Expert Member
| | #5

If you block a lot of the return, you probably won't see a difference in runtime -- but the heat exchanger in the furnace will run hotter due to the reduced air flow, and you'll have issues with the blower motor too. I would avoid blocking the register as much as possible, and try to arrange things with the bookcase so that you maintain the same total open aperature area as the register with whatever vent opening you are left with.

Bill

• |
• |
• |
• |