It provides the most accurate method of energy measurement, but at a cost much higher than the other methods. Volume metering involves the measurement of only the water flow very much the same as in municipal water system operations. The equipment to accomplish this consists only of a water meter suitable for use in hot water. Because the customer's cost is determined only by the volume of water used, and the energy content of a given volume is directly related to the temperature difference, it is in his best interest to design and operate his system in such a way as to achieve a high temperature difference between the supply and return. This is also important to the district system operator since the capacity of any system is related to temperature difference.
Flat rate is the least sophisticated of the methods for customer billing.
In most systems that use a flat rate, there is some mechanical device installed to limit flow to the customer or regulate temperature. One of the primary advantages of flat rate is simpler marketing. There is no question in the customer's mind as to the savings, meter accuracy of impact of his current system efficiency. This approach works well with existing, small, simple customers for which there is a history of previous heating energy use. In states where district heating is considered a regulated utility, the Public Utility Commission may have specific requirements for customer metering.