Many people find it annoying that they have to wait for their hot water to reach the fixture. If you water heater is a substantial distance from your fixture, you could have a very long wait.
There are a number of factors involved with how long it will take for the hot water to reach your sink, but there are really four main variables. The first is how long the pipe is between the heater and the fixture, the second is the diameter of the pipe, and the third is the flow rate of the fixture itself. The last variable is how much heat the cold pipes pull out of the water on the way to your fixture.
Let's begin with the piping layout. When a home is built there are no plumbing blue prints to follow, so the plumbers just connect up the piping however the individual doing the job feels like it. Whatever is easiest often is the only criteria. I've encountered tract homes that have identical floor plans yet the plumbing is connected up substantially different.
If your heater is 30 feet from the fixture as the crow flies you probably have at least 40 feet of pipe. The simplest plumbing layout would have a pipe connecting from your water heater outlet, running either down to the crawl space or up to the attic, or even under the slab if you have slab floors.
Now add the 30 feet of pipe to the fixture, and the pipe running either up to the fixture or down to the fixture. So you now have at least 40 feet of pipe. However, seldom is the pipe run diagonally, usually following along beams or through walls etc, making right angle turns here and there. The pipe could be 50 feet or longer by the time it reaches your fixture.
The amount of water flowing through the pipe and the pipe diameter determine the speed at which the water flows. The flow rate of the water is most likely determined by the flow rate of the faucet or fixture.
At only 40 psi a ½" diameter copper pipe 100 feet long would have a flow rate of over 6 gallons per minute with a velocity through the pipe of over 10 feet per second. At that rate your hot water would arrive at your sink in about 5 seconds! Large diameter pipes would have even higher flow rates.
The Federal Energy Policy Act of 1992 required all faucet and shower fixtures made the USA to have a flow rate of no more than 2.2 GPM at 60 PSI. Since the pipe itself will allow a much greater flow rate, your water velocity in the pipe will be limited by the flow rate of the fixture. The larger the pipe is, the lower the velocity will be with a fixed flow.
At 2.2 gallons per minute, the velocity of the water in ½" diameter copper pipe is about 3 feet per second. If you are at a kitchen sink it will take about 16 seconds to get the hot water, assuming the flow rate of your fixture is 2.2 gallons per minute, if the cold pipe wasn't sucking any heat out of the water.
Normally bathroom sink fixtures are restricted to 1 gallon per minute or less, and many shower heads are 1 gallon per minute. At one gallon per minute the time to travel 50 feet would be 32 seconds.
Now we come to the part where the cold pipes are pulling the heat out of the water. There are some variables involved here, with the important ones being the ambient temperature of the piping material, what the pipe is made out of, and once again, the flow rate of the water.
Colder pipes absorb more heat, heavier pipe material hold more heat and so pulls more heat from the water, and the slower the water travels, the more heat gets removed. Insulating the hot water piping helps in that it keeps the pipes from getting as cold and reduces the ongoing heat loss from the pipes to the air, resulting in a higher temperature during usage.
The time it takes to get hot water at the fixture can easily double when the heat loss to the pipes is taken into account.
If you have a tankless water heater, then you are even worse off, since the whole water heater is like a long cold pipe. In order to get full temperature hot water out of the outlet it has to pass all the way through the heater from the inlet to the outlet. Instead of starting with hot water your starting with cold water and a cold heater, which takes time to heat. The owners of tankless water heaters need to add 10 or 15 seconds to the wait.
The fastest hot water delivery would be provided by using small diameter pipe, insulating the pipe, using plastic pipe instead of metal, and keeping the length of the pipe to a minimum.
The good news is that there are several products available that help you get your hot water faster without changing your plumbing system. Not only do you get your hot water faster, you don't run any water down the drain while you wait.
There are basically two approaches to the problem. One approach is to circulate warm water through the piping so that the pipe doesn't suck heat out of the water while it's on its way. This provides a substantial improvement in delivery time. These products consist of a small pump that mounts under the sink furthest from the water heater, which is temperature controlled, turning off and on as needed to keep the water in the piping system at between 85 and 95 degrees F.
The water is circulated from the water heater through the hot water pipes to the pump, and then on into the cold water line and back to the water heater inlet.
The down side is that the cold water pipes no longer have cold water...it's more like luke-warm, which some people don't mind, and some don't care for it. Another problem is that it uses a lot more energy since the water heater has to work harder to keep the water in the pipes at above ambient temperature.
The warm-water systems will not work with tankless water heaters.
The second approach is to again mount a small pump at the sink furthest from the water heater, but instead of keeping the pipes full of warm water the pump is only activated when hot water is wanted. When activated these pumps pump the water rapidly to the fixture at higher flow rates than the fixtures could provide. When the hot water reaches the pump, it shuts off.
Since the water is flowing at a higher than normal flow rate it arrives more quickly, and since it is traveling at a higher velocity, the pipe absorbs less heat from the water. Again, no water gets run down the drain. These systems are called "demand hot water systems" since they only operate on demand, i.e. when the user pushes a button that starts the pump.
Demand type hot water systems will work with tankless water heaters as long as they produce enough flow. Some pumps do and others don't, so check with the manufacturer of the demand system to make sure it will work with your model of tankless heater.
The warm-water pumping systems are manufactured by Laing, Grundfos, RedyTemp, and others. The demand type systems are manufactured by Chilipepper Sales, Metlund, Taco, and others. Suggested retail prices range from about 0 to about 0.
Installing one of these systems not only provides the convenience of fast hot water, but can also save thousands of gallons of water per year. A typical family of four can save over 12,000 gallons of water a year.
Along with the water savings comes a reduction in green house gas emissions since energy is used to pump and treat the water in most residential water systems. And don't forget about the reduction in sewage, the same energy reduction applies to it.
