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What is a heat loop at a water heater?
Thursday, December 20, 2018
It is an old-fashioned, but effective, technique of adding a loop in the pipe above the hot and cold water connections to a water heater to avoid the flow of hot water out of the tank by convection. Hot water rises and cold water falls, as everybody knows. That can start a small circulation of water, called thermosiphoning, and the subsequent energy loss during standby periods. The loop can be upward or downward, and effectively traps the heat.
But a heat loop is just one type of heat trap, and it has been displaced by mechanical traps that are installed in the water heater tank at the pipe connection by most manufacturers today. Heat trap nipples can also be bought and retrofitted. The heat trap mechanism can be a ball, flap, or a flexible disc like the one shown below that is included in Bradford White water heaters.
A heat trap is required by the International Energy Conservation Code (IECC) for all tank-type water heaters, except ones that are part of a solar water heater system, because solar systems depend on thermosiphoning to operate.
By the way, although heat loops are fine at the hot and cold water connections to a water heater, a heat loop is unnecessary and actually forbidden by the building code at the Temperature and Pressure Relief (TPR) valve. The water heater shown at the top of this page has one loop too many (the center one), which will allow stagnant water to collect behind the loop after the TPR valve opens if the tank overheats.
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