[bldg-sim] Overhead Radiant Heaters

[Steven Gates]

Interesting discussion; the reasons given for radiant in a high-bay
environment pertaining to stratification/infiltration make a lot of sense.

I have a related question:  How is radiant heat typically controlled in
either a high-bay environment or a residence?  The controls I have seen in
radiant systems appear to be on/off thermostats (I haven't seen many
systems).  I have heard the argument that the temperature setpoint can be
dropped a few degrees, but don't understand how that actually works with an
on/off controller.  

For example, assume that the space is comfortable at 65F when the radiant
heat is on.  Also assume that the temperature in the current hour is 66F.
Given the temperature, is the radiant heater off?  If so, then there is no
radiant heat effect and the space might be uncomfortable; causing the
occupants to raise the setpoint.  The net effect is that, over time, a
radiant system could wind up having the same setpoint as a convective
system.

A more sophisticated way of controlling the space would be to implement a
modulating control, but I do not know whether this is commonly done.  This
type of system would need to start adding heat at the same setpoint as a
convective system, but not in sufficient quantity to maintain the setpoint.
More heat would be added as the space temperature continued to droop, until
equilibrium is achieved around 65F.

Is anyone aware of field studies comparing the actual setpoints used with
radiant vs. convective heating in similar environments, where the occupants
are "uneducated" regarding radiant heating's "benefits"?

Steve Gates


-----Original Message-----
From: bldg-sim at gard.com [mailto:bldg-sim at gard.com] On Behalf Of Jon Maxwell
Sent: Friday, October 28, 2005 7:51 AM
To: bldg-sim at gard.com
Subject: [bldg-sim] Overhead Radiant Heaters

I think you're on the right track.  I don't have a paper to offer, just an
opinion that I have loosely correlated with field observations.  Reduce the
temperature for two reasons:  (1) People are comfortable at a lower
temperature with radiant than convection heating.  So reduce your modeled
inside setpoint a couple of degrees on account of that.  (2) Radiant
delivers major stratification-related savings as well.  With a 40- to 50-ft
ceiling and forced air heat, you might have to maintain 90F air at ceiling
in order to have 70F air at the 5-ft high thermostat.  That means the
average inside temperature is 80F even if the setpoint is 70F.  Radiant
won't have that problem.  So that would be another 10F reduction on the
average inside temperature.

Third, by eliminating the aforementioned stratification you will reduce the
stack effect-driven infiltration.  This can have a measurable effect on the
heating load if your building is not pressurized with a lot of OA.

Fourth, check your candidate equipment rated efficiencies.  Depending on
venting and burner types a radiant heater can be more efficient than a
convection unit.

Jon Maxwell
Aspen Systems

----- Original Message ----- 
From: "Mike Tillou" <miket at etcgrp.com>
To: <bldg-sim at gard.com>
Sent: Friday, October 28, 2005 10:04 AM
Subject: [bldg-sim] Overhead Radiant Heaters


Hi all,

I am currently modeling an overhead radaiant heating system in a large
industrial building.  The application is similar to a Public Works
garage, large overhead clearence (50' center height), insulated metal
building, space doesn't have to be heated but they need heating for
employees at the ground level.

I plan on using DOE2.2 because I don't have the budget for using a more
appropriate modeling tool.

Does anyone have good ideas about how to approach this using a DOE 2.2
model?

My initial thoughts have focused on reducing the space temperature
setpoint but how much is appropriate 2 degrees , 5 degrees or 10 degrees
?   Are there any papers or articles that discuss air temp vs mean
radiant temp of the surrounding mass while using overhead radiant
heaters?

Many thanks in advance for any help/guidance that can be offered.

Mike


Michael Tillou, PE
etc Group, Inc.
PO Box 7, Williamstown, MA 01267
ph. (413) 458-9870 fx. (413) 458-9875
www.etcgrp.com





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