Heat Drying; an Old Concept Article written by Ken Horvath

A modest 2000 cfm heat drying system can exchange the air volume in a 2500sf building with 9’ ceilings once every 11 ¼ minutes. Air exchange is an integral part of most heat drying systems and air exchange can keep up with the fast evaporation rates heat drying produces.

A typical heat drying system sits outside of the structure, takes outside air, heats it up and blows it into the structure. Air-movers are placed and distribute the heated air like a blender mixing your favorite drink. Air is then sucked or blown from inside to outside the structure. Many systems now come with digital thermostats that are inside the structure. This allows you to set any temperature (ex: 70F, 90F, 135F) you like and the system cycles to maintain the desired temperature.

Heat drying structures is similar in concept to how a clothes dryer works. Fresh air is brought into the dryer, heated up, circulated and then vented out of the dryer and out of the house through a hose. This process continues until the clothes are dry.  Does our clothes dryer care about relative humidity? Not really. Can the clothes dryer dry clothes when it is raining? Not only is the answer yes, but with seemingly little effect to the drying time. In one hour a clothes dryer can dry our clothes. We water damage professionals are proud when we dry a house in 3 days and just fine if it takes 4 or 5 days. I shake my head every time I think of this. Are we not drying professionals?

Let’s say the air is 80F and 100% RH. If you put a pan of water on the stove will any of the water evaporate as you are heating it up? Yes it does. Basic theory is heat up the water and it will evaporate. Heat energy also penetrates building materials and causes evaporation beyond the surface of the material. This additional heat (energy) means you can typically dry a structure in 1-3 days without demolition.

I have measured the temperature of the hose on the back of my clothes dryer at the wall and it was 126F. It doesn’t seem to damage anything I’m aware of. Does that mean 126F is an ok temperature to dry with? I live in Arizona. 125F in a structure is nothing to us. In the summer I take thermal image readings on exterior walls over 140F at 9am in the morning. 150F plus in the attic, 160F plus in the car, heck, in June we can blow hot dry air from outside into the structure with an air-mover and dry if we want. OOPS, can’t make any money that way. Retract statement.

Pull out your psychrometric calculator. If you take 80F air at 75% RH, then heat the air to 125F, now you have 125F air at 19% RH (approx.). Not only are these great conditions to dry the structure but you would take days, if at all possible, to duplicate conditions with typical dehumidifiers.

Knowing when you are dry is as important as knowing when you are not.  Conditions in the structure change quicker when heat drying. Over drying can be a problem in some scenarios so monitor diligently, twice a day would be best. No one likes to grossly over dry their clothes. The same thought should apply to drying structures.

Article written by Ken Horvath