Hot Water - How Fast?
When you turn on the hot water faucet, how fast do you expect hot water to arrive? Better yet, how long does your customer expect to wait?
The hot water supply has been impacted by the change in lifestyle, the increase in the size of homes, and the increase in the number of plumbing fixtures. In the good ole days, someone was always home running the plumbing fixtures on a periodic basis. The hot water always seemed to be there after a few seconds.
Today, we disappear from our homes for hours at a time. When we return, we turn on the hot water at the most remote location and wait, and wait, and wait, for the hot water to arrive.
When we build larger homes, or even condominiums, with more plumbing fixtures, the size of the hot water piping increases. So does the length of piping. After being away for a period of time, the hot water in the piping system is cold. You must first bleed this cold water to get to the hot water. Besides being annoying, it is wasting a considerable amount of precious water.
The plumbing engineers used to have a rule of thumb that you could wait up to a minute for hot water to arrive at a faucet. But time it. You will never realize how long a minute is until you are sitting there waiting for hot water. Even the engineers have reconsidered this time factor.
There are options available to speed up the delivery of hot water. Some choose to install more water heaters or local instantaneous water heaters, and locate them throughout the building close to the faucets using hot water. While an option, it is not always practical.
The other option is to speed the delivery of hot water or provide some means of maintaining the hot water temperature in the piping. The available piping system for speeding the delivery of hot water is a PEX manifold piping system. If you run separate 3/8-inch hot water tubing to each fixture, it takes less time to bleed the water out of the tubing. A 60-ft. run of 3/8-inch tubing is bled in a matter of seconds. If the water is hot in the manifold, you almost have instant hot water. If you have to bleed the manifold, it is still less water to bleed than a long run of 3/4-inch or 1-inch tubing.
Temperature maintenance. A temperature maintenance system is a fancy term for installing heat tracing tape on the hot water piping. The modern heat tracing tapes are far superior to the olden days. These heat tapes are self-limiting, meaning that they reduce the energy use when the hot water reaches the set temperature.
The problem with temperature maintenance is that it is expensive, uses a lot of electricity, and is limiting in its use. Most UL listed heat tracing tapes are supposed to be visible throughout their length after being installed. That means you cannot bury the pipe in a wall or ceiling cavity.
Recirculating system. The old reliable standby is a hot water recirculating system. At the end of the run of hot water piping, a smaller pipe is connected and run back to the water heater. A small, and let me repeat that, a small, or very small, pump is installed to circulate the hot water.
Of course, when you install a recirculating system, it increases the amount of energy used since the hot water must constantly be maintained. The plumbing codes require the hot water mains and returns to be insulated when a recirculating system is installed. This is to reduce the heat loss in the piping system, saving on the amount of energy required.
There are some who claim that you can install a hot water recirculating system by gravity, without the need for a pump. I am not one of those believers. As far as I am concerned, gravity return systems don't work. I grew up in a house with a gravity recirculating system. It never worked.
When designing a system, the water in the return piping should be flowing 1 to 2 feet per second at the fastest. That is in comparison to 5 to 8 feet per second in the water supply piping. Hence, the need for a very small pump. All the pump needs to do is move the water. It is not pressurizing the system nor is it moving huge volumes of water.
Think Small
My favorite story is about a recirculating system designed by an engineer. The engineer calculated the size of pump and specified a 1/24-horsepower pump. When the plumbing contractor received the specifications, he called the engineer an idiot for specifying an undersized pump and increased the pump to a 1/12-horsepower pump. The supply house realized that the contractor probably got it wrong and changed it to a 1/4-horsepower pump. Finally, the sales representative for the pump company realized that the supply house probably undersized the pump and shipped a 1/2-horsepower pump, which was installed. They couldn't figure out why the copper tubing was pitted to shreds in less than a year. Of course, the hot water was screaming through the piping at about 15 feet per second. But they had hot water instantly!Did I mention that the pump needs to be very small? I consider a 1/16-horsepower pump big for a recirculating system in a residence or condo.
The other nice thing about a small pump is that you can run it all day, 24/7. There is no need for a timer cycle. Consider it similar to having a 60-watt light bulb burning all the time. Not a lot of energy.
The problem with installing a time cycle is that people use water at different intervals. You may have a shift worker that gets up at different times each week.
There are demand pumps that circulate the water when there is a call for hot water, however, you still can have a lag time waiting for the hot water. The purpose of the recirculating system is to avoid the lag time.
A better approach is a pump that operates based on the temperature in the piping system. When the temperature drops, the pump turns on.
There is a product on the market that doesn't have any recirculating piping. The system connects through a pump on the furthest fixture and returns the water to the cold water piping system. The concept is that rather than waste the hot water that became cold, move it into the cold water piping and use it there. When the hot water arrives, the pump shuts down.
I am not a big fan of this type of system. Many plumbing codes prohibit such a system because the hot is directly connected to the cold water. My concern is that I want my cold water to be cold, not lukewarm. However, in many parts of the country people are used to lukewarm cold water.
I did see a system at ISH that I fell in love with for recirculating. They oversized the hot water main supply pipe and ran 3/8-inch PEX tubing on the inside of the pipe. At the end of the run, the PEX tubing fit into a special fitting that would allow the water in the pipe to circulate back into the inside PEX tubing. At the end of the run, the PEX would connect to a very, very, small pump and recirculate the water to the water heater.
The beauty of this system is that it reduces the need for insulation on the recirculating pipe, and it makes it easier to install. Plus, all of the piping for the supply and recirculating system is together.
I have been told that this system will be introduced into the United States in the near future.
I guess I can't emphasize enough that when you install a recirculating hot water piping system, the pump should be very small. Don't worry; it will still be very effective, with hot water in a matter of seconds.
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