Where does the cold water go in a recirculating pump?

A recirculating pump is used in some homes to provide instant hot water at the faucets. It works by constantly circulating water between the water heater and the fixtures throughout the house. But where does the cold water go in this type of system?

How a Recirculating Pump Works

A basic recirculating pump system consists of a small pump installed near the water heater, a dedicated return line that connects back to the water heater, and a series of control valves at the fixtures. Here is a simplified diagram:

When the pump turns on, it takes the cool water sitting in the hot water line and sends it back to the water heater through the return line. The water heater reheats this water and pumps it back out into the hot water plumbing lines throughout the home. This creates a continuous loop and ensures hot water is always available.

The Cold Water Return Line

The key to understanding where the cold water goes is the return line. When the pump activates, the cool water that has lost heat while sitting in the pipes gets sucked into this line and sent back to the water heater. The return line creates a dedicated path for the cold water to travel back to the heater.

The return line is a smaller diameter pipe, usually 0.75″ or so, that runs from the pump location near the water heater to a distant fixture or back to the water heater itself. It connects to the hot water lines at strategic points to capture the cooler water.

Return Line Configuration

There are two main ways the return line can be configured:

• Direct return – The return line goes all the way back to the water heater.
• Bridge return – The return line connects the hot and cold water lines at a distant fixture.

With a direct return, the cold water has a clear path back to the water heater. With a bridge return, the cold water first crosses over to the cold water line, then flows back to the heater through the home’s main plumbing lines.

Pump Activation

The pump turns on based on a temperature sensor on the hot water line or through a timer. When activated, it draws the cooler water from the hot water plumbing through the return line and back to the water heater. The water heater rewarms this water before sending it back out.

What Happens to the Cold Water

Now that we’ve looked at how a recirculating pump system is configured, let’s trace the path the cold water takes:

1. Hot water leaves the water heater and flows through the hot water supply lines and fixtures.
2. The hot water sitting in the pipes loses heat over time.
3. The recirculating pump turns on via the timer or temperature sensor.
4. The pump draws the now cold water into the return line.
5. The cold water flows through the return line back to the water heater.
6. The cold water enters the water heater and is reheated.
7. The cycle repeats.

So the answer is – the cold water gets recirculated right back to the water heater via the dedicated return line in a recirculating pump system. This allows the heater to reheat that water so hot water is always readily available when you turn on a faucet.

Benefits of Recirculating Hot Water

Understanding where the cold water goes reveals the smart engineering behind a recirculating pump system. The key benefits this type of system provides include:

• No more waiting for hot water – The piping is always filled with hot water, so you don’t have to wait for cold water to clear when using a fixture.
• Convenience – Hot water is instantly available at any fixture, no matter how far from the water heater.
• Water savings – You don’t have to waste water running it down the drain while waiting for it to get hot.
• Energy savings – The water is reheated using less energy compared to letting fixtures run.

By constantly recirculating the cooled-off hot water back to the heater, you can enjoy maximum convenience and hot water on demand.

Considerations Before Installing

While recirculating systems provide great benefits, there are some things to keep in mind before installing one in your home:

• The pump requires electricity to operate and runs frequently.
• It may increase your energy and water bills somewhat.
• The return line must be properly sloped back to the water heater.
• A licensed plumber is recommended for proper installation.
• Timer or temperature controls are needed to avoid wasting energy.

Overall though, the conveniences often outweigh the minor drawbacks for many homeowners.

Frequently Asked Questions

Here are answers to some common questions about recirculating hot water pumps and dealing with the cold water:

Does the cold water get mixed back with the hot water in the heater?

Yes, the cold return water enters the inlet of the water heater and mixes with the hot water inside the tank or heating coils. It then gets reheated before being pumped back into the hot water plumbing lines.

What causes the hot water to lose heat?

As hot water sits in the pipes, it naturally cools down through convection and conduction, transferring heat to the air and pipe walls surrounding it. The farther the fixtures, the more heat is lost over time.

Where should the return line connect?

The best spot for the return line connection is at the fixture located the farthest distance from the water heater. This ensures all the cooled water is captured. For new construction, running the return line all the way back to the heater is ideal.

What if cold water still comes out of the hot tap?

This usually means there is still some hot water line left unconnected to the recirculation loop. Try connecting the return line at a more distant fixture to capture all the cooled water. Check that all valves are open too. Bleeding the air out of the lines can also help purge any remaining pockets of cold water.

How can I increase efficiency?

Use an on-demand style control system to activate the pump based on temperature or flow. Add insulation to the hot water lines. Make sure all fixtures are low-flow designs. Limiting the length of pipe branches can help water stay hotter too.

Conclusion

Understanding where the cold water travels is key to appreciating how a recirculating hot water system provides instant and endless hot water. The cold water loops right back to the water heater through a dedicated return line. This allows the heater to reheat and maintain a constant loop of hot water at all fixtures.

By leveraging the power of recirculation, you can enjoy maximum convenience and consistent hot water delivery throughout your home.