[bldg-sim] Radiant Heating/Cooling

Rick Strand rkstrand at uiuc.edu
Tue Apr 13 17:17:10 PDT 2004


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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