Russell King, M.E.

Which is better for distributing heated air to a house, ceiling registers or floor registers?

This seems like an easy question. Hot air rises so blowing the air up would improve the flow. This makes sense on the surface, but let’s look deeper.

First of all, let me make it clear that if the system is properly designed, both will work just fine.  But, all things being equal, is one better than the other, even if only slightly?

Recall that the purpose of blowing heated air into a room is to maintain a constant temperature over time and an even, consistent temperature everywhere in the room.  That temperature is whatever the thermostat is set at. Let’s say that’s 70 degrees.  When the heating system stops, the room begins to cool off.  Hopefully the thermostat will sense that and turn the heating system back on. This cycling on and off can cause problems.

The air that we are blowing into the room is substantially hotter than the air in the room.  In other words, we are adding concentrated btus into a volume of air to replace the btus that the air has lost.  It’s sort of like adding red food coloring to white frosting, but the red keeps fading away and we have to keep adding more concentrated red coloring.  We want the frosting to have a very even color, no dark streaks (hot spots) and no light streaks (cold spots).  To do this we have to mix as much as we can.  Mixing is the key to even temperature distribution in a room.

The next thing to look at is the register itself.  What is the purpose of the register?  Take a typical stamped-face 2 way ceiling register and a similar floor register.  Why are there 2 directions?  To send the air to different parts of the room, of course.  Why do we want to do that? So we don’t have hot spots and cold spots.  In other words, the register is designed to distribute the air around the room, which is another way of saying to mix the air. 

Also notice that the registers are angled to direct the air away from whatever surface the register is mounted in.  Ceiling registers throw the air down and floor registers throw the air up.  Also notice that they have a horizontal direction, parallel to the ceiling or floor.  This horizontal distance the air travels before slowing down to a certain velocity is what is referred to as the “throw distance”, but there is also a significant vertical component.  Register manufacturers provide specifications for their registers, including throw distance, static pressure drop, and noise criteria, at different face velocities and flow.  Again, supply registers are intended to push the air to all parts of the room to ensure even temperature distribution.  So, hopefully you will agree, that the key factor for selecting a good register location (and type) is to promote mixing. 

Another issue that comes into play is that warmer air is less dense than colder air.  Notice the “-er” at the end of those two important words, warmer and colder.  It’s not correct to say that “hot” air rises, but of course when people say that they usually mean “hotter”.  Hotter air rises in the presence of colder air.  It’s relative.  Most people would consider 120 degree air “hot”.  I could make 120 degree air come out of a wall register and sink to the ground like fog at a Transylvania cemetery.  How?  Make the room 160 degrees first.  Not very practical, but you get the point.

How do we reduce stratification? By reducing the temperature difference (delta T) between the room air and the supply air.  How do we do that?  One way is to reduce the supply air temperature by increasing cfm.  You can do this by increasing ducts sizes and reducing restrictions.  You can also do it by increasing the speed that the air handler runs on in heating mode.  Other than that, the easiest and best way to reduce the temperature difference between two masses of air is the mix them.  The sooner the air mixes together, the less chance there will be of stratification.

So, how do we mix the air?  A giant blender in each room would be great.  That’s basically what a ceiling fan is.  Ceiling fans are awesome! Make sure it is blowing up in the winter and down in the summer.  They beat the air like a scrambled egg, virtually eliminating stratification. Unfortunately, they use electricity and home owners tend to leave them on too much. Other than ceiling fans, we can help the air mix with register placement and selection.  Mixing is helped by turbulence.  Turbulence is created by making the air do things that it doesn’t really want to do.  Blowing the air the opposite direction that it wants to go can create turbulence, like a bunch of people going out the entrance of a building while other people are trying to come in, like cars going the wrong way on a freeway.  If hotter air wants to rise, blowing the air up will only get it up to the ceiling faster, where it will stay.  Blowing hotter air down will make it go down through the colder air and then fight its way back up, by that time it has mixed and cooled off: lower delta T = less stratification.

Note that there are two types of air movement in a room that is caused by the incoming supply air.  The primary airflow is caused by the force and velocity of the air coming out of the register.  The secondary airflow takes over when the air has lost its momentum and other forces take over.  These forces are usually stratification (buoyancy pressure) or the fact that the room is being pressurized, assuming there is no return grille in the room, the air has to leave the room and is being pushed out by the air coming in behind it.

Note that higher face velocity of the air coming out of a register can improve mixing but it can also have other negative affects, such as higher static pressure drop (resistance) and noise.  It’s very important to realize that face velocity is completely different than the velocity of the air in the duct.  You can have extremely slow air in a large duct and very high face velocity if the air is coming out of a small register.  Velocity is cfm/area.  The area of the duct is usually very different than the net free area of the register.

The image below shows what happens when hotter air is blown up into a colder room.  The primary airflow sends it up toward the ceiling and there is little secondary airflow to make it go anywhere else.  This exacerbates stratification.

This next image shows what happens when hotter air is down into a colder room.  The primary airflow sends it down toward the floor and the secondary airflow causes it to want to rise back up toward the ceiling.  This promotes mixing and reduces stratification.

Based on this and with all else being equal (airflow, delta T, face velocity, etc.) registers in the ceiling are more likely to promote mixing of heated air blown into a room and the ceiling is therefore a better location for supply registers in heating mode than floor registers.