Brigadoon - An Occasional Corner on the Internet
Houseboat Hot Water System
Designed by Mark Little
 
The houseboat My Lady used to have a storage Hot Water System (HWS) that used a gas pilot light to ignite the gas when required. This unit failed after more than 20 years of service on the houseboat.

When the unit failed, the current environmental rules in South Australia required that a new HWS has an energy rating of at least six (6) stars. This requirement effectively rules out a HWS with a pilot flame. The most energy efficient units for this sort of application is an instant heat HWS that is computer controlled and uses an electronic spark to ignite the gas when required.

The only disadvantage of this system is that it needs the HWS computer system to be connected to mains power to operate. Unfortunately, the HWS is not used for about 23 hours a day, and it is a waste of energy to power the HWS continually. Assuming that the HWS uses about 12W of power when inactive and the inverter has a standing current of about 0.5A, the HWS wastes about 1.5A. Over the period of a day, this waste amounts to about 35 ampere-hours (AH). Given that the domestic battery has a capacity of 250AH, this standby power wastes about 14% of the battery's charge ever day! Not significant in the sunny summer with its long days, but more significant in the winter months with its long periods of overcast weather.

While active during the day, the HWS uses about 3AH, or just over 0.5% of the battery's charge which is nearly comparable with the battery's rate of self-discharge (the loss of charge that occurs when a battery is just sitting there with no load). This project seeks to investigate how save the energy wasted during the time the HWS is idle, by turning off the inverter when the HWS is not being used.
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