Living with a Hot Water Demand System

Hot water demand systems are green plumbing products. They save tremendous amounts of water and they are a convenience for the homeowner or user of hot water.

Hot water demand systems work with any type of water heater, storage, tankless, solar, and even combination solar and tankless systems. Any type of water heater as long as the cold water inlet to the water heater is hooked to the same cold water supply as the cold water faucets at the fixtures.

However, they are quite dependent in their operation on the plumbing layout of your home. The hot water plumbing, cold water plumbing, and pump location can affect the demand system performance, and even cause some unusual behavior from the demand system.

Hot water demand systems consist of a small pump placed at the sink furthest from the water heater and connected to the hot and cold water supply lines. When you want hot water you “demand” it by pushing the start button. The pump speeds hot water from the water heater to the fixture, returning the cooled off hot water left in the hot water pipes from the last usage back to the water heater inlet through the cold water pipes.

When hot water reaches the pump an internal temperature sensor shuts the pump off to prevent filling the cold water lines with hot water. Now when you turn on the faucet you get nearly instant hot water and you didn’t run any water down the drain. The hot water was also delivered to you more quickly than by just running the faucet.

The demand systems shut the pumps off when they see a temperature rise, not at some absolute temperature. The reason for this is that it takes time for the heat to pass through the sensor housing and bring the temperature sensor itself up to the shut off temperature. By the time the sensor detects the full temperature of the water in the pump you will have filled the cold water line with hot water.

However, a sudden increase in temperature means hot water is just reaching the pump. Most hot water demand systems such as Metlund, Armstrong, and Taco use a 3 degree temperature increase to shut off the pump. The Chilipepper hot water demand system however has an adjustable temperature shut off. There is a knob on the front that you can use to adjust the shut off temperature increase between 3 degrees and 12 degrees.

One reason for an adjustable shut off temperature range is that sometimes due to the plumbing layout the demand system may shut off too soon. For instance, if the hot water pipe comes out of the water heater in the garage, goes through an interior wall, then passes through the center of an exterior wall and on to the bathroom. In the summer the exterior wall can heat up much warmer than the interior wall spaces and end up heating the hot water pipe due to the ambient temperature within the wall space.

The hot water demand system see’s a sudden increase in temperature from the warmed pipe and shuts off too soon. An adjustable off temperature can compensate for warm spaces that the pipes may pass through.

Typically, but not always, when you use hot water at the fixture you end up using mixed hot and cold water. This purges any warm water that the pump put into the cold water line before it shut off.

However, if the usage was entirely hot water with no cold water usage then there may remain some tepid water in the cold water line near the pump.

The hot water that remains in the hot water piping after a hot water usage event begins cooling immediately and cools at around 1-2 degrees per minute depending on the temperature of the hot water in the pipe, the amount of pipe insulation if any, and the ambient temperature.

If the hot water is normally at 140 degrees, which means that after a half hour it could still be at over 100 degrees. Since the pump won’t run unless the water temperature is less than 96 degrees, you won’t be able to use the system until it finishes cooling below 96 degrees.

Even so, you aren’t starting out with a pipe full of cold water so you should still get your hot water faster than normal, running less water down the drain than you would have if you didn’t have a demand system at all.

Because demand systems turn off once hot water reaches the pump, and the water begins cooling immediately, using the system with a timer can be problematic. You would want to use the hot water fairly soon after the timer activates the pump otherwise you may have plenty of warm water but still have to wait for hot.

Motion sensing can also be used to operate the pump, but keep in mind that with a motion sensor to activate the system you will activate the system more often than necessary. There potentially many times when someone may go into the bathroom and not need or want hot water.

Living with a hot water demand system is better than living without one!