[bldg-sim] Radiant Heating/Cooling

[Laouadi, Aziz]

I'd like to shed some light on the energy saving claim by lowering the
thermostat temperature.
Despite the extensive research conducted sofar on radiant systems, there is
no a definitive answer to this claim.  There has not been an attempt to
break down the energy savings reported in the many research papers: savings
due to thermostat setpoint, transport of energy from the heat source to the
indoor space, efficiency of the heat source, etc.

Lowering the air temperature does not mean lower heat loss/gain to/from the
outside environment.  If one examines the heat balance of the building, the
heat loss/gain is by convection (between indoor air and inside surfaces of
the building envelope) and radiation among inside surfaces.  The radiation
component for radiant systems is more significant than in war-air systems
due to higher inside surface temperatures.  The radiation component may
contribute to warm the inside surfaces of the exterior envelope and
therefore more heat loss to the outside.  There exists a threshold
temperature of the radiant surface (the heated surface) that results in the
same amount of heat loss to the outside, including the ground, as the
warm-air system.  The radiant system is energy saving if the temperature of
the radiant surface is below this threshold temp., otherwise the radiant
system is energy consuming.  Obviously, the threshold temperature depends on
the boundary conditions imposed on the building, the building physical
dimensions and thermal properties of the building construction materials.
It is not known whether or not this threshold temp is within the comfortable
temperature range, or is design-specific so that one can not generalize
savings from one building of x square foot to another building of y square
foot.  Further research in needed.
 
My Second argument is that one can never maintain a constant air temperature
so that the heat loss is constant, especially for residential buildings (I
am excluding confined spaces where the exterior environment conditions
remain quite constant, or ceiling cooling systems where the ceiling does not
directly receive solar radiation) for a simple reason:  Control of the
radiant system is not imposed on the air temperature (contrary to warm-air
systems); the control is imposed on the supply water temperature or on the
heat source (e.g. boiler).  For radiant systems, the air temp. fluctuates
according to the imposed boundary conditions (outdoor temperature, ground
temperature, wind speed, solar radiation, etc.) and water supply
temperature.  In this regard, using steady state condition to design radiant
systems is merely a conservative approach that avoids the worst-case
scenario, and does not guarantee indoor thermal comfort.  Let's say, the
design conditions coincide with nighttime conditions.  Therefore, the indoor
conditions during daytime hours may not be comfortable due to a higher air
temperature (because of the usually higher outdoor temperature and solar
radiation).  Adjusting the thermostat temperature to lower values may not
guarantee thermal comfort too since the response time of the radiant system
is quite slow (in the order or hour compared to in the order of minutes for
war-air systems). This slow response is primarily due to the effect of the
heat storage in the water mass confined in the tubing volume, tubing mass
and slab construction layers.  The other scenario is when the design
conditions coincide with daytime hour, then the system may not deliver a
sufficient amount of energy to maintain the indoor air temperature to an
acceptable level. 

To wrap-up my discussion, an integrated approach combined a dynamic behavior
of radiant system as subject to different controls will bring more insight
on the performance of the system and its energy saving.



Dr. Abdelaziz (Aziz) Laouadi
Research Officer
Indoor Environment Research Program
Institute for Research in Construction
National Research Council of Canada
1200 Montreal Road, Building M-24
Ottawa, Ontario, Canada, K1A 0R6
Tel.:  (613) 990 6868;	Fax:  (613) 954 3733
Email: Aziz.Laouadi at nrc-cnrc.gc.ca 
Web: http://irc.nrc-cnrc.gc.ca/ie/light/skyvision/


-----Original Message-----
From: postman at gard.com [mailto:postman at gard.com]On Behalf Of
stvgates at pacbell.net
Sent: Thursday, April 15, 2004 12:09 AM
To: bldg-sim at gard.com
Subject: [bldg-sim] Radiant Heating/Cooling


I'd like to thank everyone who has contributed information to this
discussion.  It has been very informative for me.  I would like to revisit
one concept to give my thoughts and maybe get some more feedback, and then
I'll give it a rest.

One of the main claims to energy efficiency of a radiant system is the idea
that the thermostat can be set lower than for a forced-air system.  Yet
several people have disagreed with this claim, or said that the reduction is
typically only a couple of degrees.  Some people have also said that the
"two position" thermostat scenario that I described is not correct; that the
actual dynamics of a radiant system are different than what I assumed.

The thermostats I have observed in radiant systems are all simple
two-position thermostats with about 2F hysteresis; they are either "on" or
"off".  Some of these thermostats do not even have a labeled temperature
setpoint, they simply have a band with one end labeled 'colder' and the
other 'warmer'.  So let's look at the following scenario:

1.  The thermostat is set to 67F.  It turns on at 66F and off at 68F.
2.  The weather is mild, the house is floating in the low '70s, and the
thermostat is off.
3.  The weather starts to cool down, and the house temperature starts to
drop.
4.  The house gradually cools to 67F, and reaches equilibrium at that
temperature.  The thermostat never kicks on, because the temperature would
need to drop another degree.
5.  The occupants perceive the house to be cold, and raise the thermostat
setting.  They remember that the original setting felt too cold, so they
never set it back to 67F again.
6.  The thermostat is now set at the same setting as it would be for a
forced-air furnace, and all energy savings that could accrue by having it
lower are never realized.
7.  If they did set the thermostat lower once the weather got colder and the
radiant system was cycling and warming the surfaces, they would run into the
same problem once the weather warmed up for a period.  As soon as the
thermostat climbs above 68F and stays there for a while, the system stays
off, the surfaces cool, and once the space drops to 67F the occupants are
uncomfortable.

Theory predicts that people should be comfortable with a lower thermostat
setting, but field studies find otherwise.  Perhaps the above scenario is
why.

All of the thermostats I have ever seen have a cover that shields the sensor
from radiation; the sensor sees mainly the air temperature.  Are there any
thermostats specifically designed for radiant systems that sense mean
radiant temperature?  If so, how common are they?

----- Original Message ----- 
From: "Rick Strand" <rkstrand at uiuc.edu>
To: <bldg-sim at gard.com>
Sent: Tuesday, April 13, 2004 5:17 PM
Subject: [bldg-sim] Radiant Heating/Cooling


The discussion started by Dr. Laouadi has raised several issues related to
radiant heating and cooling systems, and I will attempt to add some of my
own perspective on these concerns raised by the various email
postings.  For those of you who are interested in modeling radiant heating
and cooling systems, a model has been available in EnergyPlus for some time
already and papers have been published regarding this model and its
predecessor model which was part of a research version of the BLAST
program.  Some of these papers are available on the web from the EnergyPlus
web site (www.energyplus.gov) and future papers describing some of the
recent enhancements are in the works.  Follow the link to Documentation and
then to Research Papers and Articles.  As Dr. Laouadi points out, an
integrated radiant model (such as the one found in EnergyPlus) is essential
to answering many of the questions posed here.

In answering Question 1 from Steve, Dr. Laouadi pointed out in his email
from earlier today that in many cases homeowners are not taking advantage
of the potential for lowering their thermostat settings.  That is not
surprising as most homeowners are accustomed to the air systems.  Steve
noted: "But, if the thermostat is lowered to 65F, and the ambient
temperature is 66F, then the radiant heat is off, and the mean radiant
temperature drops."  This is not exactly how the physics of all radiant
systems work.  Systems with any thermal mass will react more slowly than an
"instantaneous" air system.  The occupants will not notice a sudden drop in
temperature because of the mass of the system.  In addition, radiant
systems are not typically "on-off" as forced air systems.  Will there be
greater variation in temperature or comfort?  This depends on
sophistication of the controls and the construction details.  Clearly, more
thought has to be put into the design of a radiant system.  However, a
poorly designed system, whether radiant or conventional forced air, will
not make for a happy building owner.

Question 2 from Steve asked whether the reduced infiltration loss or the
increased surface heat loss will be the determining factor in whether a
radiant system will be more energy efficient.  Or, in a more broader sense,
are radiant systems inherently more efficient than forced air systems.  The
answer is: it depends.  In an ASHRAE paper, I showed that for a particular
case while the average unheated surface temperature did increase slightly
and thus increase the heat loss through those surfaces that the
infiltration losses were higher.  As a result, the radiant system was more
efficient than the conventional forced air system.  But this is by no means
a definitive answer.  It will depend on infiltration rates, exterior
envelope construction, and other details.  The question simply cannot be
answered by a single study--which is why it is important to have simulation
models that can help answer the question for a specific case.  There are
many single cases published in the literature that make great savings
claims for radiant systems.  They may be true for a particular building,
but those numbers are not valid for *all* buildings.

As for the delivery losses mentioned in Question 3, without appropriate
insulation radiant systems are known to have problems with heat loss.  Yet
one also has to concede that fans consume quite a bit of energy.  Which
factor dominates?  Again, it depends of various characteristics of the
design.

As far as setback issues are concerned, again it will depend on whether the
system is a high mass system (like one embedded in concrete) or a low mass
ceiling panel system.  It is pretty clear that there is much less
opportunity to effectively use simple setback controls with a high mass
radiant system, but with more sophisticated controls on the system, one
might be able to address these issues.

Finally, I would invite all of those who are interested in radiant heating
and cooling to become active in ASHRAE TC6.5.  It would provide a further
forum to discuss these issues and influence future research in this area.

Rick Strand, Ph.D.
Assistant Professor
University of Illinois at Urbana-Champaign

At 03:05 PM 4/13/2004, Alec Stevens wrote:
>Steve
>My 2c:
>One aspect that should save energy on a radiant hydronic system is that
your
>supply and return water temps are lower year round than in a typical
>convective system.  If you consider a condensing hydronic boiler, you
should
>always be in condensing mode with radiant heat, but your HW reset schedule
>for other forms of hydronic heat would mean you would not be condensing
>except when OA temps are relatively high (Above 40F?).  Therefore, boiler
>efficiency should be better with radiant for most of the heating season.
>
>You raise some good points about the other issues.  I've only heard the
>lower space temperature setpoint argument when it comes to spaces like
>manufacturing areas, ice rinks, airplane hangars, etc.  Don't think it
would
>apply so much to residential, especially if you are going to cycle zone
>valves on and off.  Modulating zone valves?
>
>
>
>Alec
>
>----- Original Message -----
>From: <stvgates at pacbell.net>
>To: <bldg-sim at gard.com>
>Sent: Tuesday, April 13, 2004 11:48 AM
>Subject: [bldg-sim] Radiant Heating/Cooling
>
>
> > I also have some questions on radiant heating that I hope someone can
> > answer:
> >
> > 1.  It is well understood that, for a given comfort level, a higher mean
> > radiant temperature allows for a lower ambient temperature.  This
suggests
> > that one can lower the thermostat setpoint.  But, if the thermostat is
> > lowered to 65F, and the ambient temperature is 66F, then the radiant
heat
>is
> > off, and the mean radiant temperature drops.  So are the people
>comfortable
> > when the space is 66F, or do they raise the thermostat?
> >
> > 2.  Granted, infiltration heat losses can be lowered by reducing the air
> > temperature.  But the radiant heat source is also warming the room
>surfaces,
> > including the exterior wall surfaces and window surfaces.  If those
>surface
> > temperatures are then higher than they would be with a convective
heating
> > system, their conduction losses are now greater, even though the air
> > temperature is lower.  Also, infiltration/exfiltration losses are
>typically
> > through cracks.  If the cracks in the interior surfaces are warmer from
> > radiant heating, then the crack warms the exfiltrating air, and space
> > temperature is not a valid criterion for calculating exfiltration loss.
>So
> > does a radiant heating system REALLY save any energy?
> >
> > 3.  If the radiant elements are imbedded in the ceiling, which is
common,
> > the interior ceiling temperature can now be in excess of 90F, which
> > increases the conduction losses to the attic.  So if this loss is
counted
>as
> > a delivery loss, the efficiency of the system drops compared to the
> > theoretical.  The same argument applies to radiant elements in floors.
So
> > is a radiant system REALLY any more efficient than a convective system
in
> > terms of delivered energy?
> >
> > 4.  With a setback thermostat and a convective heating system, I can
turn
> > off the heat at night, but my home will be comfortable in less than a
1/2
> > hour the next morning.  But most radiant heating systems have a slow
> > response time.  Do people turn them off/down at night, or do they run
them
> > continuously?
> >
> > ----- Original Message -----
> > From: "Jon Maxwell" <jmaxwell at aspensys.com>
> > To: <bldg-sim at gard.com>
> > Sent: Monday, April 12, 2004 8:46 PM
> > Subject: [bldg-sim] Radiant Heating/Cooling
> >
> >
> > > I have modeled the savings for radiant systems for unvented low
>intensity
> > > gas fired radiant tube heating systems in high bay warehouses and
> > > manufacturing facilities in particular by:
> > >
> > >     1. Reducing the setpoint dry bulb temperature a few degrees
because
> > > human comfort with radiant heating is reached at a lower ambient than
>with
> > > convection heating systems.  I am certain that comfort research
supports
> > > this.
> > >     2. Reducing the setpoint temperature a few more degrees because
>there
> > is
> > > less floor-to-ceiling temperature stratification. With the desired
> > > temperature at belly button level, radiant systems will have a lower
> > average
> > > temperature floor-to-ceiling than unit heaters overall.
> > >     3. Reducing the amount of infiltration, due to reduced stack
effect,
> > due
> > > to reduced temperature stratification
> > >     4. Increasing the heating system combustion efficiency slightly
due
>to
> > > having no intermediate media such as air or water between the
combustion
> > air
> > > and the space to be heated and lack of venting.
> > >
> > > While I cannot cite studies to validate the adjustments or quantify
them
> > > generally (though I have rules of thumb based on ceiling height), such
>an
> > > approach has predicted savings roughly in the right ballpark, close
>enough
> > > to make a do/don't do decision at least.
> > >
> > > Would love to be able to cite rigorous research that proves or
disproves
> > my
> > > approach.
> > >
> > > Jonathan B. Maxwell, PE
> > > Senior Engineer
> > > Aspen Systems Corporation
> > > 710 Park Place
> > > College Station, TX 77840
> > > (979) 764-6779 wk
> > > (979) 764-7810 fax
> > > (979) 575-1281 mobile
> > > jmaxwell at aspensys.com
> > > www.OPUSPOWER.com
> > > www.aspensys.com
> > >
> > > ----- Original Message -----
> > > From: "Chris Jones" <cj at cr-jay.ca>
> > > To: <BLDG-SIM at gard.com>
> > > Sent: Monday, April 12, 2004 6:25 AM
> > > Subject: [BLDG-SIM] Radiant Heating/Cooling
> > >
> > >
> > > In your research with radiant heating cooling savings, have you found
>any
> > > energy "savings" that can be attributed directly to the use of the
>radiant
> > > system vs other systems (air supply in particular).  For example, I
have
> > > seen some papers that note that the heating setpoint can be relaxed
>while
> > > still maintaining thermal comfort with a radiant system.
> > >
> > >
> > > At 10:47 07/04/2004, you wrote:
> > > >Dear All,
> > > >
> > > >For those interested in the simulation of radiant heating/cooling
> > systems,
> > > >IRC has developed a semi-analytical model for integration in energy
> > > >simulation software that use the one-dimensional numerical modeling
to
> > > >calculate the heat transfer within the building construction
>assemblies.
> > > >
> > > >The model combines the one-dimensional model of the energy simulation
> > > >software with a two-dimensional analytical model.  The advantage of
>this
> > > >model over the one-dimensional one is that it accurately predict the
> > > contact
> > > >surface temperature of the circuit-tubing and the adjacent medium,
> > required
> > > >to compute the boiler/chiller power, and the minimum and maximum
> > > >ceiling/floor temperatures, required for local moisture condensation
> > > >(ceiling cooling systems), thermal discomfort (heating floor systems)
>and
> > > >controls.  The model predictions for slab-on-grade heating systems
> > compared
> > > >very well with the results from a full two-dimensional numerical
model.
> > > >
> > > >The model was implemented in the Canadian software HOT3000 and the UK
> > > >software ESP-r as a plant component. The implementation of this model
>in
> > > the
> > > >ESP-r program offers additional flexibilities to the radiant system
> > > designer
> > > >community, mainly:
> > > >·       Designers can use any control algorithm possible in ESP-r
with
> > the
> > > >new plant component (e.g.., use the flux or temperature control, and
> > > compare
> > > >their performance).
> > > >·       Designers can specify any number of radiant surfaces of the
> > > building
> > > >fed by the same or different heat source.
> > > >·       Designers can size realistic radiant systems, and get
realistic
> > > >energy consumption (from the source side) and cost.
> > > >
> > > >
> > > >A copy may be downloaded from:
> > > >Laouadi, A. "Development of a radiant heating and cooling model for
> > > building
> > > >energy simulation software," Building and Environment, 39, (4),
April,
> > pp.
> > > >421-431, Apr, 2004
> > > >(NRCC-46099)
> > > ><http://irc.nrc-cnrc.gc.ca/fulltext/nrcc46099/>
> > > >
> > > >
> > > >Thanks
> > > >
> > > >Dr. Abdelaziz (Aziz) Laouadi
> > > >Research Officer
> > > >Indoor Environment Research Program
> > > >Institute for Research in Construction
> > > >National Research Council of Canada
> > > >1200 Montreal Road, Building M-24
> > > >Ottawa, Ontario, Canada, K1A 0R6
> > > >Tel.:  (613) 990 6868;  Fax:  (613) 954 3733
> > > >Email: Aziz.Laouadi at nrc-cnrc.gc.ca
> > > >Web: http://irc.nrc-cnrc.gc.ca/ie/light/skyvision/
> > > >
> > > >
> > > >You received this e-mail because you are subscribed
> > > >to the BLDG-SIM at GARD.COM mailing list.  To unsubscribe
> > > >from this mailing list send a blank message to
> > > >BLDG-SIM-UNSUBSCRIBE at GARD.COM
> > >
> > >
> > > Chris Jones, P.Eng.
> > > 14 Oneida Avenue
> > > Toronto, ON M5J2E3
> > > Tel. 416 203-7465
> > > Fax. 416 946-1005

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