Service manual

SECTION 4: SIZING AND DESIGN CONSIDERATION
9
SECTION 4: SIZING AND DESIGN CONSIDERATION
The building heat loss must be calculated in accor-
dance to accepted energy load calculation methods.
ASHRAE (American Society of Heating, Refrigera-
tion and Air-Conditioning Engineers) offers in-depth
information that is useful in calculating energy loads.
The CRV-Series system input is determined in con-
cert 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 follow-
ing 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
system. 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
heating system. The ability of a radiant system to
provide 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 factor is a means to insure ade-
quate 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
®
system
mounted at 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.