Online educational resource on achieving indoor environmental quality with radiant based HVAC systems
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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 contributing to thermal discomfort. The two authoritative documents addressing thermal comfort are ANSI/ASHRAE Standard 55 Thermal Environmental Conditions for Human Occupancy and ISO 7730 Ergonomics of the thermal environment -- Analytical determination and interpretation of thermal comfort using calculation of the PMV and PPD indices and local thermal comfort criteria.

Among the combined influence from 10 major factors contributing to thermal comfort is humidity. To be clear, Olesen et al have stated, "...the recommended humidity limits have caused a lot of discussion during each revision of Standard 55. The influence of humidity on preferred ambient temperature within the comfort range is relatively small... In EN ISO 7730, a humidity range of 30 – 70 % RH is recommended, but mainly for indoor air quality reasons." Others such as the Commonwealth of Australia , Bureau of Meteorology have noted, "...people moving from a less humid to more humid environment will immediately notice the effect of the greater humidity. In many sub-tropical regions of Australia the humidity is usually quite low, but occasionally can become quite high, again reducing comfort to those people not acclimatised." These statements illustrate the differences between steady state conditions and transient conditions. and thus statements on humidity and its effect on thermal comfort always requires context.

There are several approaches to communicating thermal comfort and likely the most popular is the graphical method as shown in Figure 1. You can see there is no lower limit for humidity as it relates to thermal sensations and the upper limit is fixed at 0.012 lb of H2O/lb of air. This means using the graphical method for thermal comfort the humidity can vary anywhere from appx. 80% RH at 68°F(20°C) to 60% RH at 80°F(27°C) down to 0% RH at
71°F(22°C) to 0% RH at 83°F(28°C). But as we have already noted in parts one through five that high and low relative humidity's are not recommended and so we say in our design practice the preferred range is between 30% RH to 60% RH.

There is an exhaustive supply of research addressing this topic and readers are encouraged to seek out these documents for detailed study. For our purposes here, it is enough to say once again moisture must be controlled in habitable spaces which ultimately enables the successful use of radiant cooling. 

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

Figure 1 Graphical illustration of the thermal comfort zone.



1. Berglund, L.G., 1998. "Comfort and Humidity." ASHRAE Journal, V.40(8).
2. Fang L, Clausen G, Fanger PO. Impact of temperature and humidity on the perception of indoor air quality. Indoor Air 1998;8:80–90.
3.Seppänen, O., et al. Performance Criteria of Buildings For Health and Comfort. ISIAQ-CIB Task Group TG 42, CIB number 292, 2004

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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