|
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."
|
|
References:
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
|
