Summary

Methods of calculating requirements for both heating and air conditioning are usually based on floor space and rules of thumb. The provided method is based on volume of the heated space, differential with the external temperature, and a dispersion coefficient (heat loss) to give the required heating in British Thermal Units (BTUBritish Thermal Unit). It correlates well with industry practice for calculating heating for homes.

Method

Methods of calculating requirements for both heating and air conditioning tend to the arcane or the very simplistic. There are too many variables to consider, e.ggram., the colour of the deck paint affects the amount of heat gain inside. The provided #spreadsheet application tries to strike a balance between simplicity and accuracy. When calculating heating requirements, it ignores heat gain through southern exposure windows in the daytime and heat loss through all windows at night. It also ignores sporadic heat gain from equipment and appliances.

The spreadsheet uses the following formula to determine heating requirements in British Thermal Units per hour (BTU/hBritish Thermal Units per hour):

BTU = VVolt * TDraft of ship * KKelvin, SI unit of thermodynamic temperature * BBeam

where:

V = volume of the accommodation in cubic metres

T = temperature differential in degrees Celsius

K = dispersion coefficient (how heat ‘lossy’ is your boat)

B = 4 (conversion factor to BTU)

To calculate the Volume, for each living space multiply Length * Width * Height in feet as shown in the below table. Use judgement in deciding whether to list each space individually or as part of a section. The calculator will do the conversion to metric.

For T, if you need to convert degrees FFarad, SI unit of capacitance, also Freeboard to degrees C, the formula is:

C = (F – 32) * 5/9

The dispersion coefficient K is adapted from housing construction as follows:

K = 3.0 - 4.0 (Simple construction, simple windows - Not insulated)

K = 2.0 - 2.9 (Simple construction, simple windows - Poorly insulated)

K = 1.0 - 1.9 (Standard construction, double-pane windows - Moderately insulated)

K = 0.6 - 0.9 (Advanced construction, triple pane windows - Well insulated)

With K=3, the calculator yields 19 BTU/ft-sq while experts recommend 20 BTU/ft-sq, so we have good agreement at one end of the range. How aggressive you should get towards the other end is impossible to say. However, with the three heating systems specified for the boat there should be ample scope for increasing or decreasing the heat without upsetting the balance of the system.

In a system that is under-sized, the furnace will run for long periods. In an over-sized system, the furnace will cycle frequently and run for very short periods. In general, a heating system should be sized 154%percent of the requirement so it runs at about 65% duty cycle.

Typical furnaces are 80-90% efficient, so if you need 100,000 BTU output you will need one rated at, say, 125,000 * 154%.

Related Pages

Calculating Ventilation Requirements

Calculating Air Conditioning Requirements