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Up in Smoke

By Joseph W. Lstiburek, Ph.D., P.Eng., Fellow ASHRAE

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©2019 This excerpt taken from the article of the same name which appeared in ASHRAE Journal, vol. 61, no. 11, November 2019.

About the Author
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in Westford, Mass. Visit www.buildingscience.com.

So, what do candles and joints (aka “doobies”) have in common with baseboard electric resistance heating and heat pumps? Read on Macduff.

Over the years I have been in countless buildings that experienced discoloration of carpeting due to particulate accumulation (Photo 1). I have also observed particulate deposits on window blinds, inside of kitchen cabinets and on appliance surfaces (microwave ovens and refrigerators). Additionally, I have seen lots of “ghosting” of framing members on the interior gypsum board surfaces of exterior walls. Yes, of course, more with steel studs…but also with wood studs. Yup, wood.

It used to be smokers that caused all the problems…. we solved that by banning smoking in buildings and throwing them outside to face Mother Nature’s furies.

We had a relapse with candles, especially the scented/aromatic ones. Ah, ambience and decoration. Who would have thought that those candles could cause so much damage and generate so much soot? A wonderful “bench top” experiment by an old school engineer shows the amount of soot from a 30-minute candle burn.

The issue kind of went away for a while and now is back—with heat pumps. Huh? Time to set things up a bit so sit back and read on.

The problems of particulate accumulation and deposition all share similar characteristics. The only big differences are the sources of the particles.

The problems are related to four deposition mechanisms: filtration, electrostatic attraction, thermal deposition due to Brownian motion and impaction. These deposition mechanisms cause tiny microscopic airborne particles to:

  • “Filter out” in carpets;
  • “Plate out” on charged surfaces due to electrostatic attraction;
  • “Plate out” on cold surfaces due to “Brownian motion”; and
  • “Plate out” on surfaces due to impaction.

All of these mechanisms require really tiny particles— particles that can become aerosolized or lofted into the air and remain airborne for many minutes or hours. Big particles are not related to the problems observed. Big particles fall out of the air too fast. Their movement and distribution are dominated by gravity rather than airflow, electrostatics or molecular movement such as Brownian motion. The particle sources for the types of really tiny particles that can cause the problems observed are many:

  • Tobacco smoke, cannabis smoke;
  • Candle soot;
  • Fireplace smoke;
  • Soot from standing pilot lights on gas fireplaces;
  • Carpet fiber pyrolysis on hot heat exchange surfaces such as electric resistance baseboard heaters and resistance heating elements in heat pumps;
  • Blown insulation pyrolysis on hot heat exchange surfaces such as electric resistance baseboard heaters and resistance heating elements in heat pumps; and
  • Aerosolized clay, concrete cutting particulates and gypsum sanding.

Aside from really tiny dirt, concrete cutting and gypsum sanding most of these sources have something in common: something has to get burned, cooked, fried or pyrolyzed in order to create the size of particles required. There are a few other potential sources of particles that could provide them in the size range necessary.

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