Temperature Systems Inc.

DIRTY CEILINGS

By Dave Jones
Well, hey there everybody!  I’m a little bit late in writing this month’s exciting newsletter.  As I sit here, it’s the first day of SPRING!  Woo Hoo!  So, what goes along with spring?  Why, spring cleaning of course!  “So, Dave……What does that have to do with HVAC?” you’re thinking. Everybody is probably thinking were going to talk about duct cleaning.  But…. Nooooo.  The subject this month is actually a lot simpler: dirty ceilings.

You know you can always tell an HVAC guy – as soon as he walks into a store or restaurant, he starts looking up at the ceiling.  So, since we are ‘ceiling nerds’, we’ve all seen it: ceiling diffusers that are dirt-stained, and the dirt stains usually extend to the surrounding ceiling surface.  It’s ugly.  I don’t know about you, but I really hate to see it on a restaurant ceiling.

The first reaction from a non-hvac guy is “Wow, the filters need to be changed,” if they even notice at all.  Hopefully, those of us involved in HVAC business know that dirty or poorly functioning filters are not the only, or even the most common cause.  ASHRAE tests have shown that “smudging,” as we call the “deposition of dirt particles on the air outlet and surrounding surfaces,” is more likely to be generated from room activity rather than from dirty supply air.  The dirt gets stirred-up and suspended in the room air.  Then, it can be entrained (drawn) into the discharge of the ceiling diffuser. Additionally, ceiling diffusers with tumbling air patterns that contact the ceiling, similar to the Hart & Cooley #RENPS  

Perforated supply diffuser, or a linear slot diffuser like a Carrier #35BD, are more likely to generate smudging on the ceiling.  

Now, a register that blows the air angled away from the ceiling, like a Hart & Cooley #682 register, or a Shoemaker #HVD, will usually have a lot less staining of the ceiling.  

 

 

Dirt & dust can come from just about any source:  fuzz off of clothes, carpet fibers, durt off of peoples shoes, etc.  The amount of aerosol crud varies with the geography, season, weather, room furnishings, room construction, and activity.  Dust, carpet fibers, tobacco smoke, greasy fumes, lint, and pollen are common sources of ceiling stain crud.  The smallest particle sizes are the worst offenders.  Heavy foot traffic through a room will stir up the fine particles and keep them in suspended in the air. Most of the time, the crud particles are so small, you don’t even see them. Cooking, printing, and paper dust and are some more contributing activities to smudging.

One interesting example occurred in a new local food store near my home.  Dust from the coffee grinder produced an obvious brown blossom on the ceiling around the diffuser that was located above the grinder with nothing appearing on the more remote outlets, even though they were on the same air system.

How can we control (notice I didn’t say “eliminate”) smudging in susceptible areas?  A combination of keeping the air filters clean, frequent mopping and vacuuming of floors, and room air cleaners can all help.  But these ideas may not always be practical.  A careful selection and mounting of air diffusers will minimize dirt smudging, but recognize that we are addressing the symptoms, not the problem.
 

 

A ceiling diffuser with a beveled or stepdown margin like a Hart & Cooley #SRS, or a Shoemaker #100,  will help deflect the air in a slightly downward angle, keeping the air from contact with the ceiling and, hopefully, reduce the amount of dirty crud on the ceiling.
 

Another potential solution is to use a ‘panel’ style diffuser, similar to a Shoemaker #700DVD, or a Hart & Cooley #ART.  This style of diffuser still has some crud build-up.  But, the crud is easier to clean off since it’s on the metal panel and not on the rough ceiling surface.

 

 

 

 

 

What we are trying to do is prevent the entrained room air that suspends the dirt particles from reaching the ceiling surface where the forces of electrostatics, vapor pressure, direct impingement, and temperature difference cause the dirt to stick.  The (relatively) high velocity of the supply air stream creates a localized lower pressure that the room air-suspended dirt will want to flow toward.  As my wife likes to say, “Nature abhors a vacuum” (which she learned from me).  That’s another way of stating that air will flow from a region of high pressure to one at a lower pressure.  Keeping this supply jet off the ceiling surface can help reduce staining.