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When the thermostat is closed it blocks water flow. With a conventional water pump there is a small bypass hose that allow limited water to circulate. With the electric pump you must block off this bypass. This means with the thermostat closed the pump will build up pressure in the block and increase the load on the pump. The holes in the thermostat allow small amounts of water to flow, this bleeds off the pressure and allows some water to circulate prior to the thermostat opening. Smaller amounts of water flowing allows the block to reach temp quickly, then the thermostat opens allowing full flow.