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The Inner Workings of a Tankless, Creating Continuous Hot Water

June 23, 2022
tankless water heater how it works

It’s quite a mystery. How does such a small water heater produce a continuous supply of hot water? From the outside, our tankless water heater looks like a simple gray box. However, there is a lot going on in there and we’re showing you the textbook version of how a Rheem® tankless water heater works. It won’t be a mystery anymore. We’ll walk you through how condensing and non-condensing gas tankless units function.

How a Rheem Condensing Water Heater Works

Condensing tankless water heaters are much more efficient than non-condensing water heaters because they have two heat exchangers. Water passes through a secondary heat exchanger first which preheats the water before it flows through the primary heat exchanger. This process pulls heat from the exhaust causing a vapor to form which then cools the exhaust to a much lower temperature allowing for lower cost materials such as PVC to be used for venting.

condensing tankless water heater

1. A hot water faucet is opened, which starts the water flow.

2. Cold water entering the unit passes over the flow sensor, which activates the water heater (.4gpm). The water then flows into the secondary heat exchanger, followed by the primary.

3. The gas valve opens and sends gas to the burners.

4. Ignitors in the unit spark, and gas is burned to heat the water.

5. Heat from the flame is transferred to the water in the primary heat exchanger. Any remaining heat is transferred to water in the secondary heat exchanger. Combustion gases are used to heat the water in the secondary heat exchanger before exiting the water heater. Piping in the heat exchanger goes back and forth where the water is heated continuously before exiting. Because so much heat is pulled from the combustion gases, condensation occurs which is acidic. This acidic condensate passes through the built-in condensate neutralizer so it can be safely sent down the drain.

6. Hot water exits the water heater and travels to the open faucet. Once the faucet is closed, the flow sensor stops detecting the flow and the water heater goes to a ready state. Our exclusive HotStart Programming™ feature will minimize cold water bursts if the faucet is opened again within five minutes.*

How a Rheem Non-Condensing Water Heater Works

Non-condensing tankless water heaters have one heat exchanger that produces exhaust gases at a much higher temperature as they leave the unit. In the concentric air inlet and exhaust outlet, fresh air goes into the unit from the outside ring, while exhaust gas is released from the inside ring.

non condensing tankless water heater

1. A hot water faucet is opened, which starts the water flow.

2. Cold water entering the unit passes over the flow sensor, which activates the water heater (.4gpm). The water then flows into the heat exchanger.

3. The gas valve opens and sends gas to the burners.

4. Ignitors in the unit spark, and gas is burned to heat the water.

5. Heat from the flame is transferred to the water in the heat exchanger. Piping in the heat exchanger goes back and forth where the water is heated continuously before exiting.

6. Hot water exits the water heater and travels to the open faucet. Once the faucet is closed, the flow sensor stops detecting the flow and the water heater goes to a ready state. Our exclusive HotStart Programming™ feature will minimize cold water bursts if the faucet is opened again within five minutes.*

tankless continuous hot water

More Tankless Information

We have some great resources that will help you understand tankless water heaters better. They will help you learn about how they may impact your everyday life. Find out more about the benefits, things to consider and all the details. Explore the Rheem tankless water heaters available.

*Based on a comparison of Rheem tankless models having the Hot Start Programming against consumer reviews of leading competitor models.

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