Traditionally, if you had a hot water circulating system, it meant that you had a hot water pipe that ran in a big loop from the outlet of the water heater to each fixture one after the other, and then back to the inlet of the water heater through a small pump. By continuously pumping water through the loop you have instant hot water at every fixture.

This is very convenient. Hotels and motels have hot water circulating systems; otherwise it could take hours to get hot water with such long pipes as they have.

However, instant hot water is also very expensive. Not only do you have to pay for the energy to run the pump continuously for long periods of time or even full time, but you also have to pay for the heat energy being radiated into the environment from that big loop of hot water piping. The cost of heating the water is considerably more than the cost to run the pump.

There are a number of manufacturers producing hot water circulating systems that utilize the cold water line as the hot water return line.

Some of these pumping systems use small pumps that pump water very slowly, and are temperature controlled. The pump turns on when the water temperature drops below a set point, and then shuts off when the water temperature in the pipe reaches another set point. This keeps the water your entire piping system full of luke-warm water. The water is warmer near the water heater and colder the further you get from the water heater, but it is tepid through out the piping system.

Most folks don’t really want the cold water pipe full of warm or tepid water. You don’t get “Instant Hot Water" as the manufacturers claim. I think they should be honest and call them “Instant Tepid Water Systems". The Lang Auto-Circ and The Grundfos Comfort System are two such systems, and the Hot Water Lobster is another. The Hot Water Lobster doesn’t have a pump, but relies on the fact that hot water rises, and the Lobster valve has to be substantially higher than the water heater for the system to work.

Unfortunately these “tepid water" systems consume a lot of energy since you are heating that big loop of piping and return line and its surroundings. The cost of the energy will far outweigh any monetary savings from water conservation.

Another class of systems that use the cold water line as the return line is the “Hot Water Demand Systems". These systems only pump the water to the fixture when hot water is “demanded" by the user. The pumping costs are small; typically demand systems use less than $2.00 per year in electricity costs. This is because they run for such a brief time, typically less than 1 minute per use.

When the user turns on the pump it runs until hot water reaches the fixture and then the pump automatically shuts off. The cold water line does not end up full of warm water.

With the demand system the energy consumed is no more than if the user just ran the tap as normal. Hot water is not being circulated. And since the hot water is not being circulated it does not affect the life of your tankless water heater or the warranty.

The water is pumped more quickly than if you ran the faucet full throttle. You save time, water, energy, and money!

There are at least three manufacturers of demand systems, Metlund, RedyTemp, and Chilipepper. The Chilipepper will run any tankless water heater. Some models of the Metlund system will run tankless water heaters, and some won’t, so be sure to inquire before you make a purchase. The RedyTemp will not work with a tankless water heater.

Choose the type of system that best suits your needs and enjoy the added convenience of quick hot water while being kind to the planet.