# Heat Transfer

The equations are designed for quick and easy application in the field. They are intended for the technician who needs to determine if a system or a component is performing in the desired range of operation. Some equations are approximations that provide a practical degree of accuracy. Some norms are also provided that indicate what can be expected from a typical installation.

Topics covered include:

## ESTIMATING INPUT OF A GAS-FIRED BOILER

To estimate the input in Btuh of a gas-fired boiler:**cubic feet of gas per revolution**(cu. ft./rev), indicated by the timing dial on the gas meter. This is called the

**meter factor**.

**seconds per revolution**of the timing dial.

CFH = (cu. ft./rev x 3,600)/Seconds per revolution

**heat content**(calorific value) of gas is approximately 1,000 Btu/cu. ft.

Btuh = CFH x 1,000 Btu/cu. ft.

MBH = (cu. ft./rev x 3.6 x 1,000 Btu/cu. ft.)/Seconds per revolution

## ESTIMATING INPUT OF A NO. 2 OIL-FIRED BOILER

To estimate the input of a No. 2 oil-fired boiler:gph = gal. x 3,600/Seconds timed

**Example:** If 2 gallons requires 30 seconds:

gph = 2 gal. x 3,600/30

gph = 240

Btuh = gph x 140,000 Btu/gal.

## ESTIMATING OUTPUT OF A GAS-FIRED OR OIL-FIRED BOILER

To estimate the output in Btuh of a gas-fired or oil-fired boiler, assume an efficiency of 80 percent, or determine efficiency from flue gas analysis. Estimate the boiler output with this equation:
Output (Btuh) = Input (Btuh) x Efficiency

## CALCULATING HEAT EXCHANGE AT ANY POINT IN HYDRONIC SYSTEM

Determine heat exchange in Btuh at any point in a hydronic system with this equation:Btuh = gpm x âˆ†T x 500

The âˆ†T is the difference between the entering and leaving water temperature.

## CALCULATING TONS OF REFRIGERATION FOR CHILLER OR COOLING COIL

To calculate the tons of refrigeration used by a chiller or cooling coil, first calculate the Btuh:Btuh = gpm x âˆ†T x 500

Since one ton of refrigeration equals 12,000 Btuh, use this equation to calculate the tons of refrigeration used:

Tons of refrigeration = Btuh/12,000

## COUNTERFLOW PIPING FOR COOLING COIL

Fluid (chilled water or refrigerant) must enter the cooling coil on the side of the coil that is downstream of the airflow and leave on the upstream side. This pattern is called**counterflow**. If a coil is piped in the other direction (parallel pattern), the desired leaving air temperature will not be obtained.

## TYPICAL AIR PRESSURE DROP ACROSS A COOLING COIL

Typical air pressure drop across a clean dry coil is 0.5 inches to 0.7 inches wg.## TYPICAL âˆ†T IN COILS AT MAXIMUM LOAD

Typical chilled water âˆ†T in cooling coils:Typical hot water âˆ†T in heating coils:

## ESTIMATING SENSIBLE HEAT FLOW THROUGH HEATING OR COOLING COIL

For standard air, estimate the sensible heat flow through a coil with this equation:Btuh = 1.1 x cfm x âˆ†T

**Note:** 1.08 is more exact for this equation, but 1.1 is often used for estimating.

The âˆ†T is the difference between the entering water temperature and the leaving water temperature.

## COEFFICIENT OF PERFORMANCE (COP)

COP (coefficient of performance) is a term used to indicate the efficiency of refrigeration equipment. The higher the COP, the more efficient the equipment is. Effective cooling depends on the temperature as well as the moisture content of air.COP = Effective cooling out/Work in

*Excerpted and reprinted from* HVAC Technician's Handbook *by Leo A. Meyer, one of the books in the Indoor Environment Technician's Library series published by LAMA Books. For over 30 years, Meyer has been writing and publishing training materials for the HVAC industry. His books cover a wide range of topics, including heating and cooling, indoor air quality, sheet metal work, electricity basics, safety, and others. For more information, visit www.lamabooks.com.*

**Publication date:** 02/06/2006

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