The answer to the question of how long it takes for a water heater to drain lies in the plumbing and the volume of water to be depleted from the tank. In order to drain from the tank's lowest point, a valve must be opened higher in the system to allow air to replace the escaping water. This effectively nullifies your water system's pressure and the tank drains by the action of gravity alone.
When an object is released from a height it falls through the atmosphere at a rate of 33 feet per second, per second. Water flowing is also affected by gravity although not falling. The downward force of water, or flow rate, is affected by how close to vertical the water drops, the outlet pipe's inside diameter, the type of material used in making the piping, and the pressure head.
According to analysis done at the Ministry of Agriculture and Land in British Columbia, "1 foot of elevation drop = 0.433 psi of pressure head," or, "2.31 feet of elevation drop = 1 psi of pressure head." While the average homeowner will never need to know these statistics, it is helpful to know that the pressure head is the difference in elevation between the source and the outlet.
The history of standardizing pipe sizes has evolved along with the materials used to make piping. All pipe has an inside diameter (ID)-the actual space available for the flow of gas or liquids. The outside diameter (OD) is established by the thickness of the material used. Inside a pipe physical friction naturally slows the flow rate. There are standard formulas for determining this effect (pipe friction loss tables) based on the type of material type and ID. The American Society for Testing Materials and its international counterpart have established standards for the nominal size of pipe. Under older standards, a one-inch pipe was not one inch in diameter outside or inside. You will be concerned with the inside diameter of the outlet from your water tank.
Hydraulic Theory and Airlocks
Inside a section of pipe the gravity-induced flow of liquid at one point is equal to the flow of liquid at any other point. Its flow rate is affected by the angle of the pipe in relation to vertical and it may be computed accurately according to hydraulic theory. The flow may be interrupted, or stopped completely, if an airlock forms within the pipe. Airlocks can occur when the liquid is static (standing still) or dynamic (moving). There are ways to overcome an airlock in your system if necessary.
Factors Affecting Time of Flow
Your water heater's size (the amount of water) and any system obstructions affect how long it will take to empty your tank. Often a water heater is drained for repair. One reason for repair may be sediment built up at the bottom of the tank where the drain valve is located. If this valve is clogged with debris the size of the outlet will be greatly reduced. A 40-gallon tank would logically drain faster than an 80-gallon tank with all other variables the same. Another consideration is the amount of water in your system above the heater. Second story plumbing will need to drain down through your house and out your heater's drain valve. As with any plumbing project, patience is important-you cannot speed up gravity.