[bldg-sim] Radiant Heating/Cooling

[Peter Simmonds]

I have attached my replies in the text.

-----Original Message-----
From: stvgates at pacbell.net [ mailto:stvgates at pacbell.net]
Sent: Tuesday, April 13, 2004 8:49 AM
To: bldg-sim at gard.com
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 is not quite true. The radiant component either heating or cooling can balance the mean radinat tempertaure exchange in a space. Onec the MRT has been balanced, incresed for heating or lowered for cooling then the space temperature can be adjusted to an optimum temperature as long as the occupant comfort remains within PMV=/- 0.5. Typical values for heating would be about 68F and for cooling about 78F.

This suggests
that one can lower the thermostat setpoint. yes 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.  Not if the solar gain to the space remains.

So are the people comfortable
when the space is 66F This would be calculated, my gut feeling would be that 66F would be pushing the minmum PMV of - 0.5., or do they raise the thermostat? most propbably but to what tmperature, most humans would reset to say 70F and this would be a slight overkill, us engineers would be happy with 68F. The point here is the very gery area we now find ourselves in. The new ASHRAE standard 55 reverts back to the PMV +/- 0.5 identical with ISO 7730 which then states comfort criteria for a space as opposed to dry bulb temperatures being used to dictate space conditions and sometimes hopfully comfort.

2.  Granted, infiltration heat losses can be lowered by reducing the air
temperature. Infiltration will not be reduced by lowering the air tempertaure however the effect of possible infiltration would be reduced. The heat losses would be reduced if lower temperatures were used. But the radiant heat source is also warming the room surfaces,
including the exterior wall surfaces and window surfaces.  Good point, however, most programs and engineers are incapable of calculating this. My experince has been that the inside surface temperature is calculated using the component u value, the temparture differential across the component and then using the heat transfer coefficient of the inside surface to calculate the inside surface temperature. There is another longer explanation which is basically the diffrenece between  the  responese method of calulation against forward finite element. 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. correct, but what are we heating the space or the losses. In simple terms if a space were calculated to maintain say 70F at outside conditions of say 20F ( not for californians), then the dalta t = 90F and the heat loss from the space is calculated. Now if the inside space temperature is reduced to say 68F then the delta t= 88F and the calculated heat losses would be slightly less. The discussion being that the heat losses to the exterior would be slightly higher if the surface temperatues were higher, I would agree, but the heat gain to the space or heat loss (assuming the laws of thermodynamics havent changed this week) would be slightly less as the relationship between space temperature and inside surface temperature would be decreased. 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
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>
>
> Chris Jones, P.Eng.
> 14 Oneida Avenue
> Toronto, ON M5J2E3
> Tel. 416 203-7465
> Fax. 416 946-1005
>
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