This letter refers to Howard McKew's "Tomorrow's Engineer" column in the September 2006 issue ofEngineered Systems, titled "Cleanliness During Construction" (page 98).

The nearest thing to a standard about the effect of allowing construction dust into air conditioning equipment may be SMACNA's "Duct Cleanliness For New Construction Guidelines," although it does not address this issue directly.

There are four very important things to remember:
  • No practical commercial HVAC air filter is 100% effective:
Glancing at the ASHRAE Handbook - Systems and Equipment, we see that even the best filters that are normally used in commercial HVAC equipment have an arrestance of less than 100%. This means that the remainder goes through the filter to the evaporator.
  • Portland cement and plaster of Paris do not harden by drying.
Concrete does not harden by drying. If it did, builders in hot climates would not have to keep concrete from drying out before it sets.

Portland cement is a compound made up primarily of calcium, aluminum, and silica. In its dehydrated form (fresh from the kiln), it has very little strength. However, when water and Portland cement are mixed, the result is a solid hydrate. Water molecules bind to the cement, and allow the formation of internal binding structures. The water becomes part of the concrete.

Plaster of Paris works very much like Portland cement, though less water is involved.
  • The evaporators of air conditioning equipment are often wet due to condensation.
If the surface temperature of an air conditioner evaporator is below the dewpoint of the room air, then water will condense onto the evaporator. In humid climates, this is a wonderful thing.

If active Portland cement dust or plaster of Paris dust touches the condensed water on the evaporator, you get concrete or plaster in your evaporator fins. This is not wonderful at all.
  • Concrete and plaster are not good heat conductors.
Compared to aluminum fins and copper tubes, concrete and plaster are not good conductors of heat. Lots of little dabs of concrete or plaster in an evaporator causes (a) a reduction of airflow, and (b) reduction of heat transfer. The net result is an immediate reduction of performance and possible long-term damage to the compressor.

The worst case of this that I have ever seen was due to "stealth concrete." The units in question were worth tens of thousands of dollars, so the original installers carefully prevented any construction dust from entering them.

The stealth arose because of a process called "tile re-pointing." After a few years, the grout lines in some tile floors get too dirty to clean. The building owner had a tile contractor re-point the tile, which means grinding out the old grout with electric grinders and then putting in new grout. As a favor to the contractor, the building owner left the air conditioners running.

This was repeated every few years.

The edge of a grinding wheel running in cement grout is hotter than the original cement-dehydrating kiln. This means that some of the dust blowing off the grinder wheels had been dehydrated to active Portland cement again. The air conditioners drew in this active dust, some of which went through the filters and contacted the wet evaporator. By the time I arrived, the evaporators looked like poorly plastered walls and the compressors had died of floodback.

In summary, if any Portland cement or plaster of Paris dust (among others) might be present in the air, it is important ahead of time to:
  • Turn off the blower for any cooling coil that might condense water.
  • Tape plastic or paper over the return grilles.

James Hogan
Product Development Engineer
Dectron, Inc.
Roswell, GA