Pre-1994 residential and pre-1997 commercial flush toilets in the United States typically used of water per flush (gpf or lpf). The United States Congress passed the Energy Policy Act of 1992, which mandated that from 1994 common flush toilets use only . In response to the Act, manufacturers produced "low-flow" toilets, which many consumers did not like because they often required more than one flush to remove solids. People unhappy with the reduced performance of the low-flow toilets resorted to driving across the border to Canada or Mexico, or buying salvaged toilets from older buildings. Manufacturers responded to consumers' complaints by improving the toilets. The improved products are generally identified as "high efficiency toilets" or HETs. HETs possess an effective flush volume of or less. They may be "single-flush" or "dual-flush". A dual-flush toilet permits selection between different amounts of water for solid or liquid waste. Some HETs are "pressure-assisted" (or "power-assisted" or "pump-assisted" or "vacuum-assisted").