Roberts Gorden CRV-B-4 Sizing and Design Considerations, Radiant Adjustment to Heat Loss, Example

SECTION 4: SIZING AND DESIGN CONSIDERATIONS

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

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® 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.

9