At the present time the most widely accepted procedure for sizing storage-type water heating systems is based on work of Werden & Spielvogel (1, 2). Their work was sponsored by Edison Electric Institute and ASHRAE and the results are also incorporated in the ASHRAE Guide & Data book.

However, this and other works published so far deal only with a class of water heating system in which energy is available whenever the controlling thermostat calls for energy. It is becoming increasingly common practice to have additional controls on the energy availability, depending on the conditions of energy supply. For example, when electricity is the energy source, these restrictions are in the form of peak load stabilizers, time clocks, utility pilot wire control, etc. Thus, there is a need for design data on water heating systems in which energy is not always available.

This report deals with such systems as applied to large apartment buildings with 150 suites or more. A design method of off-peak water heating systems is described and is applicable to all buildings irrespective of energy source. However, the application details and the results are presented only for all-electric apartment buildings where there is a need for such information.