Section 4: Sizing And Design Considerations; Radiant Adjustment To Heat Loss; Radiant Height Adjustment Factor - Roberts Gorden CoRayVac CRV-B-2 Design Manual

SECTION 4: SIZING AND DESIGN CONSIDERATIONS

The building heat loss must be calculated in accor-
dance to accepted energy load calculation methods.
ASHRAE (American Society of Heating, Refrigeration
and Air-Conditioning Engineers) offers in-depth infor-
mation that is useful in calculating energy loads. The
CRV-Series system input is determined in concert with
the required radiant adjustment to heat loss and height
adjustment factors.

4.1 Radiant Adjustment to Heat Loss

The practice of applying an adjustment factor to heat
loss calculations for radiant heating systems is well
known within the radiant heating industry, having been
used by manufacturers for over 25 years. A number of
studies have been conducted to identify the values of
the adjustment factor in the range of 0.8 to 0.85
depending on efficiency (higher efficiency uses lower
factor). This adjustment can be more thoroughly
understood when considering the following radiant
effect issues:
• Infrared energy heats objects, not the air.
• Lower ambient temperatures reduce the amount
of air infiltration.
• Less air stratification with radiant heat.
• Lower ambient air temperatures reduce the trans-
mission heat loss through walls and roof.
• Elevated floor temperature provides a thermal
reserve capacity.
• Increased mean radiant temperature allows occu-
pants to perceive thermal comfort at the reduced
air temperature.
Each of these issues impacts favorably on the reduc-
tion of the installed capacity of the radiant heating sys-
tem. This fact, together with the realization that the
standard ASHRAE heat loss calculation methods
(particularly the transmission heat loss coefficients)
have been developed specifically for conventional hot
air systems, demonstrates the need for the heat loss
adjustment factor.
• In general, a .80 adjustment factor should be
used for CRV-Series systems.

4.2 Radiant Height Adjustment Factor

As discussed above, the installed input capacity of
radiant heating systems is typically reduced as com-
pared to the calculated heat loss due to the radiant
effects associated with a properly designed radiant
SECTION 4: S
heating system. The ability of a radiant system to pro-
vide the advantages of these radiant effects rests
largely with the ability of this system to establish a
reserve heat capacity in the floor. Without this reserve
capacity, radiant comfort cannot be achieved. (The
exception is station heating/spot heating applications
where sufficiently high levels of direct radiation are
received from the heater.) The height adjustment fac-
tor is a means to insure adequate floor level radiant
intensity to "charge" the floor heat reservoir.
Proportionately larger wall surfaces also remove
energy from the floor to a larger degree, decreasing
the heat reservoir.
The increased input capacity recommended by a
height adjustment factor is not extraneous as com-
pared to the heat loss calculation. Rather, it is a real-
ization that in order to maintain radiant comfort
conditions (and the economic benefits), a minimum
radiant level must be maintained at the floor.
It is recommended that an adjustment to the heat loss
of 1% per foot (3% per meter) for mounting heights
above 20' (6 m), be added up to 60' (18 m). Above this
height, additional correction overstates the BTU
requirement as determined by the heat loss.
EXAMPLE 1:
Given a building with a calculated heat loss of
350,000 (Btu/h), what is the installed capacity
required of a CORAYVAC
30' (9 m)?
CORAYVAC ® Installed Capacity = Heat Loss x
Radiant Adjustment x Height Adjustment
For CORAYVAC systems, a .80 radiant adjust-
®
ment factor is used.
The height adjustment is 1% per foot over 20'
(3% per meter over 6 meters), or 1.10.
∴CORAYVAC
Installed Capacity = 350,000
®
(Btu/h) x .80 x 1.10 = 308,000 (Btu/h)
A 12% reduction in installed capacity vs. a
conventional heating system.
D C
IZING AND ESIGN ONSIDERATIONS
system mounted at
®
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