If the pressure switch contacts cannot be closed, the draft inducer sends a pressure switch error to the integrated furnace control. If the pressure switch contacts are still in the open position, the draft inducer will increase speed again until the contacts close or the maximum allowed time for proof of pressure switch closure expires. If the contacts are ever sensed to be open, the draft inducer increases in speed, then checks the contacts again. The variable-speed draft inducer continuously monitors pressure switch contacts during normal heating operation. The learning routine is repeated periodically to ensure the draft inducer is operating at the lowest and quietest possible speed. The learning routine for high-fire operation takes place in a similar sequence, noted Storm. “When the furnace is first started after installation, the learning routine will slowly reduce the speed of the draft inducer while it learns the optimum combustion efficiency and operating speed for this furnace under the current installed conditions, which includes vent pipe size, vent length, line voltage, gas pressure, and climatic conditions,” said Storm. Tim Storm, furnace product leader for Trane and American Standard, Tyler, TX, notes that in the company’s high-end furnaces, the microprocessor-based variable-speed draft inducer also includes an adaptive learning routine, which allows the draft inducer motor to learn the optimum speed and efficiency for both low- and high-fire operation.
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