How a tank less water heater works?

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Water heaters aren’t rockets. They don’t have a whole branch of physics designated to knowing exactly how they work.  They’re simple beings. And they work on simple mechanisms even if the details seem complicated.

Rinnai V65IN Tankless Water Heater, Large, V65iN-Natural Gas/6.5 GPM

Types of tankless water heaters

There are two general types

Both of these work on fairly simple principles with the major difference being how they heat water. Gas heaters heat water by combustion (or flames) while electric water heaters use heating elements. Combustion is fairly simple. We observe it quite often. If anything’s burning, it can be said to be in the process of combustion. It’s that easy. Heating elements, however, use a different principle. Not very complicated one but certainly different. Rather than giving you the straight textbook definition, let’s use a simpler example to explain.

Irons are a very common household item probably present in almost all areas of the world. How do they get so hot just by using electricity? This is where it gets a little scientific. The principle is called Joule’s law and it basically states that heat is emitted from any element or material that’s carrying current.

The amount of heat generated depends on two factors

  1. The larger the current, the larger the emission of heat there is. This is why an iron consumes so much electricity. To generate lots of heat.
  2. The second is the resistance of the body through which the current is being made to flow. An object that presents more of a hurdle to electrons that are trying to pass through (or simply has ‘higher resistance’) consumes more of their energy. This energy is then converted to heat.

So when a conducting body (a body that can have current passing through it) has a lot of current trying to tear its way through is placed inside or close to a body of water, the heat that gets emitted heats up your water.

Rinnai V65IN Tankless Water Heater, Large, V65iN-Natural Gas/6.5 GPM

Steps involved in water heating by tankless water heater

  • Flux Sensor senses the flow of water
  • Ignition
    • Combustion (in gas heaters) or
    • Starting flow of current through the heating element (in electric heaters)
  • Exchange of heat
  • Calibration
  • Maintenance of required settings

Stiebel Eltron Tankless Water Heater - Tempra 24 Plus - Electric, On Demand Hot Water, Eco, White


Step 1: Sensing of water influx

This step is just the heater getting to know that there’s water on its way to be heated. The sensor can be mechanical, as simple as a flap going down due to the flow of water against it or as advanced as lasers. It depends on what model of heater or company you prefer.

Step 2: Ignition

This step is the heater getting its feet planted for work.

  1. Combustion

The process of combustion, like you may have already read, can be defined as anything burning. To burn means the reaction of oxygen with any other material or the material that is burning. Technically, you cannot ‘burn’ anything unless you have oxygen. The reaction of oxygen with the fuel in your vehicle or with glucose in your body are both examples of material ‘burning’.

In the case of gas-based tank less water heaters in homes, it’s either natural gas or propane that is being burned. The heater mixes both oxygen and methane in one place and releases a discharge or uses a pilot light to start the burning process. Once started, the heater usually does not need any current or pilot light to carry on the burning process. It is a self-sustaining process.

  1. Current discharge in the heating element

In electric water heaters, we don’t need to concern ourselves with combustion. All that’s happening here is the flow of current. One factor that is key in heating elements is the material. If a material (like silver or gold) lets current flow too easily, the amount of heat generated is too insufficient for heating flowing water. But if a material presents a hurdle to the current, the current will have to spend more energy to try passing through it. And if the current is having to spend energy, you’ll be getting heat. Think of it as a truck on a road. If the road is bumpy, the truck will have to spend more fuel trying to keep going on going while if the road is smooth and bump-free, getting across it will be much easier and economical when it comes to fuel.

The most commonly used material for heating elements is nichrome. An alloy or mixture of nickel and chromium. What makes it such a good choice as a heating element is the fact that it doesn’t let the current just pass on by (which wouldn’t generate enough heat) but it doesn’t resist current so much so that it doesn’t flow at all.

Remember, if the current flowing is smaller, a lesser amount of heat is produced and if the resistance gets too high, it will stop the flow of current entirely.

  • The other key factor in heat production inside heating elements is the geometry or the shape of the heating element.

A heating element shaped like a circle will not heat water up as quickly as a long bar-shaped heating element. The answer to why that’s so is surface area.

You can think of it as a ball of clay. The flatter you make it, the more area there is that you can touch directly.

In the case of water, it results in more areas than it can take heat from directly.

Step 3: Heat exchange

Producing loads of heat will not do you any good unless you have a set-up that can utilize it efficiently. This is the job of the heat exchanger. Remember resistance? Well, the more resistance a material has, the more difficult it is for current to flow. Similarly, the more resistance a material has, the more difficult it is for heat to flow. This being due to the fact that the flow of heat is actually because of electrons making their way through the material.

So to conduct heat efficiently, there needs to be a material that can transfer heat from one side to the other as quickly as possible. The commonly used material is stainless steel which is actually an alloy of iron and chromium. Stainless steel is used because it’s a very sturdy material even in paper-thin forms. It also is rust-resistant due to the presence of chrome and is more stable at relatively higher temperatures. One thing that designers have to keep in mind when making electric heating elements is the danger of electrocution. Water is a considerably good conductor and having water that is to come in direct contact with the skin be even remotely side by side with electricity can spell disaster. So to prevent conduction, heating elements sometimes have a coating of an insulating ceramic on them. It doesn’t break down at higher temperatures and makes a very good barrier between the electrons on one side of the heating element and water on the other side.

Another solution can be to use highly stable, non-combustible liquids or oils between heating elements and the water inside pipes.

Step 4: Calibration

Hard part over with, the electronics come into play to maintain a steady flow of water at the desired temperature. When you open a tap or fixture, the outlet can only give you a limited volume of water per minute (or flow rate). Keeping the volume of water outflow consistent while maintaining a steady temperature requires calibrations. Some water heaters make these necessary calibrations themselves while others put the responsibility on the user.

Step 5: Maintenance of required settings

Let’s say, for example, a shower was already running when another tap was opened in the kitchen. As the volume of water naturally increases, the flame in the combustion container or current flowing in the heating element also has to grow more and more intense. If it grows too intense, the water may get scolding hot and may cause serious burns or injury to the users. And if it doesn’t increase, the amount of heat being produced will be too insufficient to maintain a steady temperature. All of these things have to be kept in check in order for a water heater to provide a steady flow of water with a consistent temperature.





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Growing up with a keen interest in the ever-evolving tech landscape, Andrea hails from the tech-savvy city of San Francisco. She pursued her passion at the University of California, earning a degree in Computer Science. Andrea's academic journey laid the foundation for her comprehensive understanding of technology and its impact on our daily lives.

Andrea's professional journey has been marked by hands-on experience with a variety of tech devices, from laptops to cutting-edge gadgets. Her practical expertise has made her adept at translating complex tech topics into digestible, informative content for tech enthusiasts of all levels.