Demand dispatch refers to a collection of distributed control techniques to obtain grid services from flexible loads. A carefully designed control architecture can enable a collection of loads to behave in aggregate as a large virtual storage device. Grid-level ancillary services can be provided with minimal communication, while guaranteeing quality of service to the consumer. This work expands on prior work in several directions: • A natural notion of energy capacity is proposed for the special case of thermostatically controlled loads (TCLs). It is shown that this quantity is closely approximated by thermal energy capacity, which is a component of the "leaky battery model" introduced in prior work. • Simulation experiments in a distributed control setting show that these energy limits, and accompanying power capacity limits, are reliable indicators of online capacity, even for a heterogeneous population of loads. • A feedforward/feedback control scheme is proposed for a large collection of heterogeneous loads. At the local level, control loops are used to create cooperative responses from each load in a given class of homogeneous loads. This simplifies control of the aggregate based on two pieces of information: aggregate power consumption from each class of loads and the state of charge surrogate that is a part of the leaky battery model. This information is required at a slow timescale (say, 5 minute sampling). • The paper concludes with economic implications. In particular, given that dispatchable loads are a form of virtual storage and not virtual generation, it is not surprising that the use of real time or time-of-use pricing has been problematic.