Condensation is not the Dark Side of Chilled Water and Radiant
Copyright © 2016, Robert Bean,
R.E.T., P.L.(Eng.) All world rights reserved. Originally for
HPAC Canada Magazine.
Ok in this sequel of defending the universe of
chilled water systems we need to take a deep breath…ala Obi-Wan
Kenobi, "must - use - The Force." Let’s start off by
stating the obvious, there is no shortage of articles describing
the risks of using chilled water and radiant
cooling system and how these systems cause condensation. It’s
like those with a hidden agenda from a Star Wars
plot handed out scripts to industry authors instructing them to
repeat in a James Earl Jones voice, “Luke, be wary of the
radiant cooling systems – it shall condense and rain upon the
galaxy”. To this I whisper in my famously subdued Yoda voice – WTF (i.e., Where’s the Facts)?!!
For cynics of chilled water and radiant cooling
system here is a serving of universal logic; 100% of
all condensation problems in buildings conditioned exclusively
with refrigerated air, did not have a chilled water radiant
cooling system to blame. That would be all, as in
every one of them with moisture problems could not be tied back
to a chilled water system. Stay with me on this...
I’m going to get to the DNA of the problem
shortly but let me first ask a rhetorical question; why do we
not hear those Darth Vader voices from the “refrigerated air
only” camp draw our attention to an equivalent amount of
cynicism to the well-known moisture problems on ducts,
insulation, grills, registers, steel decking, and drywall and
T-bar ceilings? Standing as judge and jury over chilled water
system while failing to give equal time to the plethora of
microbial and building material problems associated with
refrigerated air based systems seems to be a common
characteristic amongst the Rebel Forces. So follow along, it
gets better. You see only bad designers and bad installers can
be held accountable for sweating refrigerated air based systems
because good designers and good installers would never let
moisture become a problem. Why you ask? Well good designers and
installers spend hours evaluating moisture loads and assembling
building and HVAC components properly so sweating doesn’t occur.
But apparently with chilled water and radiant cooling the good
logic of moisture management gets tossed to the far corners of
the galaxy because (wait for it) evidently only bad designers
and bad installers are permitted to work on these systems.
That’s dark side thinkin’ don’t ya think?
It’s time for the brain washing to stop - stop
holding radiant cooling and chilled water systems to some
unreasonable double standard and talk about the real problem
which is moisture. Moisture is the root of all that is good and
bad in the universe. It is an equal opportunity offender and
much to the anguish of the Rebel Forces, moisture holds no
discrimination for HVAC system types. Just for effect, consider
statistically one is far more likely to find moisture problems
in refrigerated air systems than chilled water radiant systems
simply due to the installation ratio of air over water so put
your light sabres away and let’s get real about the real issue.
Ok…deep breath (again)…must return to a state of peace…Namaste.
Here are six reasons why condensation with
chilled water and radiant cooling panels is a straw man argument
so read carefully; regardless of HVAC system type, moisture
must be managed for biological concerns, hydrolysis, dimensional
stability of hygroscopic materials, and preservation of
materials, respiratory comfort, and thermal comfort. Aside
from discussions around pipe insulation and building tightness,
if you control for these six things then condensation becomes a
moot point. It also begs to ask, if designers and installers of
all HVAC system types are not focusing on these far more
important elements, what exactly are their systems based on?
Let’s look at each of these conditions:
Without moisture control numerous biological risk
develop which support the growth of bacteria, viruses, fungi
(moulds/molds) and mites. According to a ASHRAE Transaction,
“Human Exposure to Humidity in Occupied Buildings”[i]
and the ASHRAE Handbooks, humidity at less than 30% or more than
60% can introduce higher multiple microbial risk factors. As
noted in the Environmental Protection Agency (EPA) Indoor Air
Plus program, "You have to control humidity to below 60% RH."
You’ll also find support from the medical community on this
range. Stephanie H. Taylor, M.D. states, "The movement and
infectivity of bacterial, viral, and fungal organisms vary with
the RH of the air…" This is supported by Dr. R.L. Dimmick from
the Naval Biological laboratory (NBL), Univ. CA, Berkeley who
said, “Moisture content may, indeed, be the most important
environmental factor influencing the survival of airborne
microbes.” Taylor goes on to say, “Maintaining the relative
humidity of hospital indoor air between 40% and 60% can
significantly decrease healthcare associated infections.”[ii]
In addition to ASHRAE, the EPA, and NBL this position is
supported by numerous authoritative organization including the
Canadian Centre for Occupational Health & Safety, Health Canada
and ACCA, The Indoor Environment & Energy Efficiency
Figure 1. Controlling moisture for biologicals.
Hydrolysis is a water based reaction that is used
to break down certain chemicals. Studies by Dr. Richard L. Corsi,
University of Texas (Austin), show paint emissions (specifically
HC-O-O) are affected by rising relative humidity.[iii]
Matthews et al, noted that changing the indoor conditions from
68°F (20°C) and 30% relative humidity (RH) to 79°F (26°C) and
60% RH would result in two to fourfold increases in formaldehyde
concentration for the same air change rate. Hodgson et al,
stated in their study on the topic, "This suggests that indoor
humidity has a substantial impact on formaldehyde emission rates
and concentrations." [iv]
Figure 2. Controlling moisture for hydrolysis.
Stability of Hygroscopic Materials
When hygroscopic materials such as wood are
operated in an uncontrolled environment their moisture content
can fluctuate. Such changes lead to dimensional instability due
to shrinking and swelling. Both Canada Mortgage and Housing
Corporation’s Wood Frame Envelopes Best Practice Guide
Products Laboratory’s (U.S. Department of Agriculture/Forest
Services) Wood Handbook[vi]
provide for the ideal "in service" wood moisture content as
between 6% and 14%. This ideal in-service range corresponds to
relative humidity between 40%+/-10% and 60%+/-10% at
temperatures typical for space heating and cooling.
Figure 3a. Controlling moisture for dimensional
Figure 3b. Controlling moisture for dimensional
According to the Image Permanence Institute (IPI),
a university-based non-profit research laboratory devoted to
preservation research, at approximately 73F dry bulb, “no risk”
conditions exists for chemical decay of organic materials
between 25% RH and 35% RH for a dew point condition between 45F
and 59F.[vii] The
American Museum of Natural History regarding preservation and
temperature and relative humidity says this; "Different types of
collections have substantially different relative humidity
requirements…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."[viii]
Figure 4. Controlling moisture for preservation of artifacts.
5. Respiratory Comfort
Research showing the effects of high and low
humidity on respiration discomfort supports the humidity ranges
above. In one study the least amount of people dissatisfied (PD)
at 10%, corresponds to space conditions of 20% to 60% relative
humidity for a temperature range between 68°F(20°C) and
78°F(26°C). Increases in discomfort were observed during
increases in relative humidity at a given air temperature. For
example at 72°F(22°C) there is a 10% increase in people
dissatisfied going from approximately 40% RH to 65% RH; and an
additional 10% dissatisfaction going from 65% RH to 80% RH.[ix],
Figure 5. Controlling moisture for respiratory
Two authoritative documents addressing humidity
and thermal comfort are ANSI/ASHRAE Standard 55
[xi] and ISO 7730.[xii]
At the humidity conditions defined in items 1 through 5, indoor
climate engineers will meet the requirements of both the ASHRAE
and ISO Standards.
Figure 6. Controlling moisture for thermal
So now that you have 60% RH as recommended
maximum in your intellect do you know what the sea level dew
point is at say a 75Fdb? It is 60F. Do you know what the lowest
you should go for say a radiant floor cooling system should be?
It is 66F. That would be a 6F safety margin far more than
good engineering practice. Condensation on the
floor, ya right – give me a break! For those developing
additional arguments in your heads see the bibliography and
Alright, if you’ve made it this far and if you
have any of Princess Leia’s DNA in you, then you should be
concluding there is zero logic in stating unequivocally, "don't
use chilled water and radiant cooling systems because of
moisture concerns". It’s a silly statement. You must provide
moisture control regardless of the HVAC type for the six very
good reasons outlined above.[xiii]
With chilled water systems the control of space moisture is done
dedicated outdoor air system
(DOAS). Noted DOAS expert Stanley
A. Mumma, Ph.D., P.E., Fellow ASHRAE, Professor Emeritus of
Architectural Engineering, Penn State University states, "The
DOAS approach effectively eliminates biological contaminants and
inadequate ventilation. It also avoids building-wide
distribution of indoor chemical contaminants". [xiv]
For those of you whose brains are heading in the
direction of, “yes but then you need a hybrid with two system,
one for ventilation and one for cooling”. This is actually a
good thing as indoor air quality and energy specialists will
attest that dedicated ventilation with radiant cooling provide
superior control and efficiency over dual duty air only systems.
In commercial systems Rumsey and others have demonstrated these
hybrid systems can also be installed for less cost.[xv]
So hybrid systems can enable better air quality, better comfort,
better efficiency and for some projects - a lower capital cost.
Controlling moisture is a fundamental principle.
It enables the use of chilled water and radiant cooling systems
without going off the deep end on condensation concerns. If
mechanical designers, builders and HVAC installers don’t focus
on this fact, then they shouldn’t be doing cooling systems.
So Luke, condensation is not the dark side of
chilled water and radiant cooling systems. Moisture is…take care
of the moisture and you take care of the dark side.
End of story.
Sterling E M, Arundel A, Sterling T D., 1985 CH-85-13 No
1, ASHRAE Transactions, 1985, Vol 91, Pt 1. 11p.
[iii] Corsi, R.L., 2013. Relative humidity and
paint emissions (HC-O-O). Building Energy & Reactivity
Complex Interactions. Simple Solutions, IAQ 2013 –
Environmental Health in Low Energy Buildings –
Vancouver, BC, Canada October 17th, 2013
[ix] Toftum, J., Jørgensen, A.S., Fanger, P.O. 1998.
Upper limits of air humidity for preventing warm
respiratory discomfort, Energy and Buildings, Volume 28,
Issue 1, August 1998
[x] Fang L, Clausen G, Fanger PO. Impact of
temperature and humidity on the perception of indoor air
quality. Indoor Air 1998; 8:80–90.
[xi] ANSI/ASHRAE Standard 55 Thermal Environmental
Conditions for Human Occupancy
[xii] 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.
[xiii] Setting aside discussions on duct and pipe
[xv] Sastry, G., Rumsey, P. 2014. VAV-vs-
Radiant-Side-By-Side-Comparison, ASHRAE Journal, vol.
56, no. 5, May 2014. <
Bean, R. Part 2 Together Forever HPAC
Magazine, March, 2012. <www.hpacmag.com/news/together-forever/1001534096/?&er=NA>
Bean, R. 2013. Très Bien for large scale
HPAC Magazine Canada, Sept. 2012.
Bean, R. 2013. Radiant Cooling for
Sceptics: How to do radiant cooling in high humidity
Bean, R. 2014. Why And How To Do Radiant
HPAC Magazine Canada, Feb. 2014. <http://www.hpacmag.com/news/why-and-how-to-do-radiant-cooling/1002912708/?type=Print%20Archives>