Every so often, I get a letter from a reader that really intrigues me. I received one such letter recently from Jimmy Mahaffey. To say the least, Jimmy was passionate in his request. He simply stated, "I NEED HELP!" Who could resist such a request?
The subject of Jimmy's letter was, "How fast should it take hot water to reach a fixture?" The next question of Jimmy's was, "How do you accurately calculate the time it takes to get hot water to a faucet?"
The length of time for hot water to reach a faucet is the subject of great debate among the plumbing engineering community. Many plumbing engineering documents have stated that a good design should have hot water to the furthest fixture in 60 seconds. Over the years, some engineers have dropped that time to 30 seconds. While the 60second time is part of the plumbing engineering community design recommendations, there is nothing in any plumbing code stipulating a minimum or maximum time to receive hot water. However, there have been attempts to add this requirement to the plumbing code.
When you are waiting for hot water to arrive at the fixture, 60 seconds seems like a long time. In many homes, the hot water takes a lot longer first thing in the morning. It is not unusual to wait up to four minutes at the furthest bathroom for hot water to arrive.
In some commercial buildings, a wait of 60 seconds may mean that water never arrives at the faucet. Take, for example, a public lavatory that discharges a metered amount of water. That metered amount is 0.25 gallons. Hence, how many fixtures are calculated as being open when trying to determine the time it takes for hot water to arrive? This is why many commercial buildings rely on hot water recirculation to reduce the time it takes to get the hot water to a faucet.
Fast Flowing
The fastest means of getting hot water to a faucet is by the installation of an instantaneous water heater in close proximity to the fixture. Likewise, the closer a water heater is to the fixtures, the faster the hot water arrives.The piping system that will deliver water fastest is a manifold system. Each fixture has a separate line from the manifold to the fixture. If the manifold line is only 3/8 inch, there is minimal water in the piping, and the only large quantity of water that has to be bled is in the hot water port of the manifold and the piping leading from the water heater to the manifold. Normally, the manifold is installed close to the water heater to reduce the amount of piping.
Of course, the slowest piping system is an oversized conventional piping arrangement. The volume of the piping has to be bled before the hot water arrives. Hence, the key is not to oversize, but to properly size a system. If the piping has runs that are too long, then a hot water circulating line is a must.
So how do we determine the length of time it takes to get hot water out of a faucet? That is relatively easy. Each faucet has a given flow rate. Don't trust the aerator rating or showerhead rating when it comes to flow rate. When a manufacturer rates an aerator or showerhead, it does it at a set pressure. Most aerators are rated at 60 psi. The flow rate decreases as the pressure drops below 60 psi. Showerheads are rated at 80 psi. Hence, most showerheads will flow less water than their rating since we tend to have pressures of less than 80 psi in our plumbing systems.
Once you have the flow rate, you simply determine the length of pipe and the volume of water in that pipe. Knowing the volume of water, you divide the volume of water by the flow rate to determine the time it will take for water to arrive. You can ignore the heat loss through the piping since this would be inconsequential for calculating the time.
The key is knowing the volume of water in the piping. First, make sure you use the correct inside diameter. A 1/2inch tubing does not have an inside diameter of 1/2 inch. Using the diameter, you can calculate the inside area of a pipe. However, the answer would be in square inches, and you need to convert this to square feet. So the answer is divided by 144. The resulting number would equate to the number of cubic feet of water in each foot of piping. Since there are 7.48 gallons in a cubic foot, you would need to multiple the value by 7.48 to determine the number of gallons per cubic foot.
If you are confused, the simple formula for volume in a foot of pipe is:
Volume (in gallons) = 0.0408 d2
If you don't want to multiply the numbers out, I provided a table at the bottom of the page that lists the water volume per foot of various pipes:
 Pipe Volume (gallons)
Material
3/8"
1/2"
3/4"
1"
Copper Type M

0.0132
0.0268
0.0454
CPVC Pipe

0.0102
0.209
0.0346
PEX Tubing
0.0052
0.0094
0.0185
0.0308
Let's assume that same house had a standard piping layout with 25 ft. of 3/4inch Type M copper tubing and 30 ft. of 1/2inch. The volume in the 1/2inch tubing would be 0.396 gallons; the volume in the 3/4inch would be 0.67 gallons for a total of 1.066 gallons. The hot water would arrive in 0.533 minutes, or 32 seconds. While the manifold is nearly instantaneous, the copper tubing system isn't far behind.
If we make the home a little larger with the copper tubing installation, there may be 15 ft. of 1inch, 50 ft. of 3/4inch, and 15 ft. of 1/2inch Type M copper tubing. The resulting volume would be 0.68 gallons in the 1inch, 1.34 gallons in the 3/4inch, and 0.198 gallons in the 1/2inch for a total volume of 2.02 gallons. This would take one minute at 2 gpm to bleed out of the piping, which is the time recommended by plumbing engineers.
The problem with this calculation is assuming a flow rate of 2 gpm. Many faucets operate at lower flow rates because of system pressure. Additionally, the piping may have longer runs of pipe. Finally, people tend to turn on both the hot and cold water at the same time, especially when using single lever faucets. If both hot and cold water are running, the time it takes to receive hot water just doubled.
If the hot water takes longer to arrive than the calculated time, check the water heater. If the dip tube disappears, the cold water short circuits along the top of the heater. Remember, if you are more than 100 ft. of piping from the water heater most plumbing codes require a recirculation line.
Jimmy, thanks for the question.