You can’t suck your way out of a distribution problem

Here’s an excerpt of an email I wrote in support of adding large central returns in a finished basement, directly next to the unit, with transfer grilles at the top of the basement stairs. The overwhelming majority of HVAC technicians reject this as something that can never work.

One of the amazing things about trying this is that the effect can be tested before doing the work, by temporarily removing the blower cabinet door and taping the door switch shut. (Danger: this can cause serious safety problems by making combustion appliances spill, so it requires a continuous return path. Don’t try this if you’re not an HVAC professional!)

The point of why I went there was to test if the hypothesis was right. In this case, it appears to be right.

There are basically two parts to the problem:

1. Airflow is restricted, specifically on the return side. This reduces the total quantity of airflow. That statement is totally uncontroversial, and there are multiple ways to test it. You can check the total external static pressure of the duct system, estimate what a reasonable static pressure would be after the duct changes, and check both values against the blower chart for the unit. PSC fan motors in particular have a pretty sharp airflow curve, so reducing static pressure increases airflow significantly.
2. Delivery temperatures tend to improve, especially in summer, through better mixing. We’re (at least initially) moving the coldest air through the system.

Lots of technicians believe they can fix distribution problems by sucking air out of individual rooms. This rests on some false assumptions and leads to confusing outcomes. In general, you can’t suck your way out of distribution problems. In some cases you can increase total system airflow by adding return, much as I proposed on this job, but where the return is located doesn’t tend to matter much.

I think about it with Occam’s razor: the simplest answer tends to be the correct one. Why are rooms hot on the second floor in summer? Because not enough cold air is being supplied to them. At what point does a room being too hot stop meaning insufficient supply and start meaning insufficient local return? And how do so-called “hot air returns” know to pull the hot air out of a room and not the cold? When we pull air from a room, what temperature is the air that replaces it? Doesn’t the supply air displace the room air anyway, just by being supplied? (Yes, or the room would explode.)

I design the return side of a system to do two things: support the system airflow effectively, and ideally keep the pressure difference between rooms under 1 pascal. Local room returns are really good at reducing pressurization problems, which is what they should be used for.

If you still have skeptics, test it under hot conditions. Go to the house at 2pm on a really hot day and record the temperatures on all three floors. Then take the cabinet door off, leave the basement door open, and watch what happens. Ideally record airflow before and after. It might take a little while, but in most houses this will be highly effective.

Originally published on the Tay River blog. Lightly edited.