Online educational resource on achieving indoor environmental quality with radiant based HVAC systems
Not for profit educational resource on indoor environmental quality.
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Fundamentals of indoor environmental quality / thermal comfort and air quality solutions using radiant based HVAC


One of the myths perpetuated with radiant cooling systems is condensation risk on the cooling surface. This would be a real risk if there was no moisture control. Without moisture control numerous risk factors develop such as deterioration of materials. According to The Image Permanence Institute (IPI), a university-based non-profit research laboratory devoted to preservation research,  the Preservation Index (PI) represents the overall rate of chemical decay in organic materials based on a constant temperature and RH. A higher number indicates a slower the rate of chemical decay. The Image Permanence Institute developed the Time-Weighted Preservation Index, or TWPI, to illustrate chemical decay as the environment changes over time.

Clearly you can see that at a nominal temperature of 73F, a no risk conditions exists for  organic materials between 25% RH and 35% RH for a dew point condition between 45F and 59F. Now this range of space conditions is well below what a radiant cooling system operates at with a very large safety factor - in fact it is so large that the system could easily operate up to 60% RH albeit in the presence of a progressively greater risk to the natural aging of preserved materials. If the space doesn't hold materials to be preserved then 73F @ 60% RH space conditions with a cooling panel above 61F is perfectly acceptable. For other operating conditions visit the The Image Permanence Institute.

The American Museum of Natural History regarding preservation and temperature and relative humidity (RH) says this;

"Different types of collections have substantially different relative humidity requirements and so it is hard to give specific set-points.  Specimens with metal components may benefit from RH levels that are as low as possible.  Organic artifacts require more moderate RH levels to prevent desiccation or embrittlement.  Most specimens benefit from RH levels that are moderate and stable to prevent physical damage that can be caused by wide climatic shifts.  Generally, recommendations for museum environments are given as to 50% while attempting to minimize dramatic swings to between 40-60%, even if broad seasonal trends are hard to avoid."

Again, if one is not prepared to control moisture regardless of the HVAC system then cooling should not be done. Full stop. If moisture control is going to be implemented then it also enables the use of radiant cooling systems. There is no logic in stating, "don't use radiant cooling because of moisture concerns" then proceed to design an air based HVAC system which provides moisture control. Do you now see why this a Huh? moment?

This content is a key component from our course, "Integrated HVAC Engineering: Mastering Comfort, Health, and Efficiency."


See also:

Radiant Cooling - Part I, Fundamentals
Radiant Cooling Systems: Calculation Example
Tres Bien for Large Scale Radiant Cooling
Radiant Cooling for Sceptics: How to do radiant cooling in high humidity geographies
Radiant based HVAC systems - bibliography / resources
Radiant Cooling Systems: Condensation Concerns Part 1 of 6, Preservation of Materials
Radiant Cooling Systems: Condensation Concerns Part 2 of 6, Microbial
Radiant Cooling Systems: Condensation Concerns Part 3 of 6, Hydrolysis
Radiant Cooling Systems: Condensation Concerns Part 4 of 6, Dimensional Stability of Hygroscopic Materials
Radiant Cooling Systems: Condensation Concerns Part 5 of 6, Respiratory Discomfort
Radiant Cooling Systems: Condensation Concerns Part 6 of 6, Thermal Comfort



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