The WHID measures both the tank and house pressure. The house pressure travels through the sensing line (from the check valve) to the WHID. The WHID shuts the water supply when the water pressure in the tank drops below the cold water supply to the water heater.
Normally shut: The WHID has a diaphragm which is normally in the closed position. When a faucet is opened, the house’s hot water pressure slightly decreases. This drop in pressure travels through the sensing line to the backside of the WHID’S diaphragm, causing the diaphragm to open, and the water to flow through the tank.
Activation: A loss of 1 cup (or less) of water that leaks through the tank's inner wall will cause the tank’s pressure to drop about 20psi, causing the WHID to shut off incoming water. In detail: a piston inside the WHID allows the cold water supply to the top of the diaphragm, thus shutting off the water to the tank. Once activated, the WHID will only function again once it’s RESET (see RESET procedure).
Retains Water In The Tank: When a leak occurs, the WHID’s check valve (on the tank's discharge) does two important things:
1): Stops hot water (from the plumbing system) from flowing back into the tank.
2): Creates a vacuum, which prevents the residual water in the tank from leaking out the hole (like a finger on a straw).
Dripping Faucet: When the WHID activates, NO water flows through the tank. BUT, a small amount of cold water will flow through the sensing line to the faucet's hot water side. This also occurs when the faucet has a very small drip, thus saving energy.