Tankless water heaters save energy, but not water, they waste water.
While it’s true that tankless water heaters save energy and can provide an endless supply of hot water, it’s also true that tankless units take longer to get the hot water to the fixtures. Since a tankless heater needs to heat the water before it can deliver hot water to you, you have to wait longer.
The only way to get the hot water of course is to run the faucet. So while you run the faucet and wait, you are dumping gallon after gallon of pure clean water down the drain. Water that required energy for pumping and treating it. Pumping and treating the water uses energy which in turn means green house gasses being released into the atmosphere.
The heater can be pictured as a long piece of pipe coiled up and placed over a fire. As the water flows through the pipe it gets hotter. In order to reach full temperature it has to stay in the fire for a certain period of time. It will take longer if the water starts out colder. If you speed up the flow of water it will get cooler, unless you add more fire. With the old storage heaters the tank full of hot water is ready to go the moment the faucet is opened so you get your hot water faster.
With tank type heaters what has been done in the past to both speed up the delivery of hot water and eliminate running the water down the drain while you wait is to create a loop in the hot side of the plumbing so you can pump the hot water around the loop, keeping the pipe full of hot water. That way when you turn on the faucet you get instant hot water every time whenever you want it. It works great with a tank type heater except for the fact that it uses a whole lot of energy keeping the piping warm.
But you can’t do that with the tankless unit because tankless heaters turn on when they detect water flowing through them. Usually it takes 1/2 gallon per minute of flow or more to activate most tankless heaters. Most circulating systems don’t produce that much flow. If they did, then the water heater would be on all of the time. Using a standard circulating system with a tankless heater will void the warranty in almost every case anyway.
There is another type of hot water delivery system called an on demand system that will work just fine with tankless water heaters. The way it works is a small pump is installed under the sink furthest from the water heater. When you want hot water you press a button and the on demand system pumps the water at high speed from the water heater to the fixture. The system has a temperature sensor monitoring the temperature of the water, and when it senses an increase in temperature it shuts off the pump. This prevents hot water from entering the cold water lines.
Since the pump only runs for a few seconds at a time it uses very little energy. Typically it uses less than $1.00 per year in electricity. It doesn’t use any more heat energy than a standard plumbing system. It saves 100% of the water normally run down the drain waiting for the hot to arrive.
On-Demand systems are suitable for use with tankless water heaters since the pump only runs for a short time, and will not void the warranty of the water heater. Not all of these systems have strong enough pumps to run tankless water heaters though, so you should check with the manufacturer before you make the purchase.
Metlund makes a demand system they call a D’mand system. They have several models, and I think the largest model has a strong enough pump to use with a tankless heater. Metlund’s d’mand pumps range from 1/40 horsepower for the smallest to 1/8 horsepower for the biggest. Chilipepper makes an on-demand system, and they only make one model, the CP6000. The CP6000 has the most powerful pump of any system on the market, (approximately 1/3 horsepower) and will work with any tankless water heater.
Another reason for using a more powerful pump is that the more power the pump has the faster the hot water will arrive at the fixture.
By combining your energy efficient tankless water heater with a hot water on demand system you get the best of both worlds… you are energy efficient and you conserve water. It’s a great feeling to know that you are doing your part to protect the environment and conserve our natural resources every time you use hot water.