The 'Cold Water Sandwich': Why Your Tankless Heater Runs Cold

Tankless water heaters are famous for providing "endless hot water." So why, when you turn the shower off for a minute to lather up and turn it back on, do you get hit with a freezing blast of cold water before it gets hot again? In the plumbing world, this frustrating phenomenon is known as the Cold Water Sandwich.

Why Does It Happen?

Unlike a traditional tank that has hot water sitting ready at all times, a tankless unit heats water exactly when you ask for it. Here is the sequence of a Cold Water Sandwich:

1. You are taking a hot shower. The pipes are full of hot water.
2. You turn the shower off. The tankless unit senses the water stopped flowing and immediately turns its gas burners off to save energy.
3. A minute later, you turn the shower back on. The unit senses the flow and fires the burners back up. However, it takes a few seconds for the burners to ignite and heat the new water.
4. During those few seconds of ignition time, a slug of unheated, cold water slips past the heat exchanger and into your pipes.

When that water reaches your showerhead, you get the hot water that was already in the pipe, followed by the slug of cold water (the "meat" of the sandwich), followed by the new hot water.

How Do We Fix It?

While this was a common quirk of early tankless models, modern plumbing technology has completely solved it. If you are experiencing this, the experts at Pipedex Plumbing & Drains have two primary solutions:

• Built-In Buffer Tanks: If you are upgrading to a new system, we install premium condensing units (like Navien) that feature a small, internal "buffer tank." This tank holds a tiny amount of hot water ready to deploy during those few seconds of ignition time, completely eliminating the cold slug.
• Recirculation Pumps: For existing units, we can install a dedicated hot water recirculation pump. This keeps hot water continuously moving through your pipes, ensuring the temperature remains perfectly consistent from the second you turn the handle.

Stop settling for freezing blasts of water. Contact the tankless experts at Pipedex Plumbing & Drains at 864-900-0300 to upgrade your home's comfort today.