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Introduction
The two most common myths around
radiant cooling is that it will cause condensation and you
will have cold floors - that's it...go to any online chat site
and ask about radiant cooling and watch what happens.
The reason why
these
radiant cooling myths get
spread has to do with
misunderstanding four principles:
-
the true definition of heating, ventilation and air
conditioning
(HVAC)
-
the difference between
latent and sensible loads and,
-
the concept of a "lean" mixture (grains of moisture per
pound of dry air)
-
how
comfort cooling works.
First we need to remind
ourselves that HVAC is not exclusively about conditioning the building,
rather its primarily about
conditioning you and I which includes both conditioning
the air and our bodies.
What does this mean?
It means you have to learn to
separate comfort quality from air quality which is hard
thing to do if you've spent your whole life thinking they
are the same or believing you can effectively do both with
one HVAC system. Look at it this way...have you ever
considered the body has independent system to deal with
thermal comfort (skin to brain) and respiration (lungs to
brain)...and have you ever
considered there are
two separate standards one for
thermal comfort (ASHRAE 55) and one for air quality (ASHRAE
62)? The problem with
most builders and HVAC people is they try to do two separate
tasks with one system which is contrary to what the body
does and contrary to what is reflected by standards.
So when do we feel cooling
comfort? Well
when our body loses more heat than it can generate we
experience a cool sensation. If the heat loss from our body
is such that it goes from cooling comfort to cold it causes
a reduction in skin temperature and
goosebumps...if
the body can't shed its heat fast enough it goes from
heating comfort to hot which leads to
a rise in skin temperature and
perspiration.
If the body continues to loose excessive heat we can go into
hypothermia...if we can't get
rid of excessive heat we go into hyperthermia - both
scenarios can lead to death - which is really hard to
recover from...
To cool our body just enough
to be comfortable we can lower the mass temperature in our
home a few degrees cooler than our
skin temperature which
creates the same experience you get when you walk into an
underground garage on a hot summer day or into the frozen
food section at the super market. Why do we feel cool in
the underground garage or freezer section? It's because the
temperature of the concrete or glass in relation to
our skin
is lower so our body releases its
heat via radiation to the
cooler surface - thus the reason we feel cool. The best
part of it all, is a body at rest,
transfers over 50% of its heat
via radiation so radiant cooling makes perfect sense. In
fact the transfer of our body heat to the cooler surface via
radiation is called "sensible" heat transfer. So now you
know that radiant (sensible) cooling works because you have
experienced it a thousand times before just like the cave
dwellers of years past…you just never thought about it.
Where most uneducated voices
speak up and against radiant cooling has to do with
condensation. You see they don't know about
sensible vs.
latent cooling. Sensible can be thought of as dry heat or
heat without moisture. The solar heat inside a
greenhouse is a great example of sensible heat. In
 a
building, sensible cooling is the absorption of the radiant
energy from the sun and other radiant heat sources such as your
body, lights, motors, compressors, stoves, ovens, dryers,
computers and home theatre systems. All of these
loads which raise the mass temperature and
mean radiant temperature of the home can be
removed by absorbing the excess heat with a cool surface.
Think of a cooled surface as 'sponge' for sensible heat
What shouldn't be absorbed by a
cool surface is the heat being held in the moisture content
of the air. This heat is called the "latent" load.
Latent means dormant, hidden or concealed. Latent heat shows
up every time you sit outside on a hot humid day with a cool
drink in your hand...water actually forms on the glass
surface. Where did the water come from? It
 was
actually hidden in the air and the amount is based on an
every day term that you are familiar with called relative
humidity. The visible presence of moisture or condensation
occurred because the ‘humid’ air (latent heat) met a chilled
surface (sensible). Can
you guess where some of the heat in the moisture went to?
That's right...it went into the glass via conduction.
It's one of the reasons why you need to keep putting ice
into the glass to keep the drink cool. The rest of the heat
went back into vapor or dripped all over your shirt or
shorts.
So what does this mean? It means
a radiant cooling surface is used to absorb heat it can
thermally feel in the absence of moisture.
This is very important to
understand because in some
climate regions of North America the
relative humidity in the summer is very low almost desert
like. In other parts of the country the humidity is very
high. In other parts it changes based on how much rain
falls in the region and for how long.
Regardless whether there is a radiant cooling
system present or not, the
latent load must always be controlled for thermal and
respiratory comfort, to
prevent moisture damage in buildings, to enable dimensional
stability in hydroscopic materials and to disable the
environmental support of pathogens and allergens.
So the key to confidently cool with
radiant is to first control the moisture in the air (lean)
through dehumidification for the reasons stated above and then control the surface
temperature of the floor (dew point) which is easy
to do.
go to part II -
click here., Controlling the Relative
Humidity
go to part III -
click
here,
Heating and Cooling Comfort
Systems
go to part IV - click here, Radiant Cooling Educational Movie Clips
go to part V - click here, Featured Project, Manitoba Hydro Office Building
People who visited these radiant cooling pages have also visited our Radiant Design Guide,
Psychrometrics - Made Easy and
Introduction to Indoor Environmental Quality.
Suggested reading:
Leach, M., Lobato, C.,
Hirsch, A., Pless, S., Torcellini, P., Technical Support
Document: Strategies for 50% Energy Savings in Large Office
Buildings, National Renewable Energy Laboratory, Technical
Report, NREL/TP-550-49213 , September 2010, <http://www.nrel.gov/docs/fy10osti/49213.pdf>
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 |
