What is a flow water heater, how it works and how to use it

It is common for everyone to have the expectation of having hot water running through the pipes in their apartment. However, there are times when, due to various reasons, there is a lack of water. This is especially true for dachas and country houses, where centralized hot water supply is not available. In order to have a continuous supply of hot water in these situations, an autonomous water heater is necessary. There are many different types of devices available, each with their own unique features. One of the most promising options is a flow water heater. So, let’s explore how to select the right flow water heater for your needs.

Operation of the flow water heater

Before delving into the details of how a standard flow water heater operates, it’s helpful to understand its predecessor – a storage water heater. The concept is simple and straightforward. Water is poured into a large tank where it is heated to the desired temperature and used as needed.

However, there are significant drawbacks to this type of system. The amount of hot water available at any given time is limited by the capacity of the storage tank. Once the stored water is consumed, one must wait for the next batch to heat up. This inconvenience can be avoided by having a larger storage tank, but that presents challenges in terms of placement.

Furthermore, the continuous consumption of electricity is necessary in order to maintain the desired temperature in the tank.

These disadvantages are eliminated with a flow water heater.

The principle of operation of any flow water heater

Like its name implies, this device is designed to heat water as it flows through. When the water is not being used, the heater remains inactive.

A flow heater consists of a reservoir where the water is heated. However, unlike a storage tank, this container is compact and can be easily installed in any convenient location.

The incoming water flows into the reservoir, where powerful heating elements are situated. The power of the heating elements typically depends on the speed of the water flow and ensures a constant supply of water heated to temperatures between 40-60 °C at the outlet. All the components of the heater are contained within a single housing, allowing it to be built into wall cabinets or mounted on the wall.

A flow water heater is installed on the wall below the sink.

The diagram illustrates the internal structure of a flow water heater.

A schematic illustration of the inner composition of an electric heat flowing heating device

Types of flow type water heaters

Heating devices can be categorized according to their methods as follows:

• Electric devices;
• Gas devices;
• Liquid (diesel) devices;
• Solid fuel devices (wood, coal).

Liquid and solid fuel heaters are not very common.

Devices that use gas heating are often used in houses where gas columns are still installed. One of the main advantages of gas heaters is their low maintenance cost, as gas prices are generally low. However, gas heaters also have some significant drawbacks:

• When it comes to safety, gas appliances are inferior to electric ones.
• Proper ventilation is necessary to remove gas combustion byproducts.
• A gas water heater can only be switched on when the water pressure exceeds 1.5 liters per minute.
• Gas equipment requires regular inspections by a specialist.

Electric heaters are the most commonly used type of heaters. They offer safety features and multiple adjustable heating modes. Additionally, they are widely available as almost all residential properties have access to electricity.

With that being said, the following discussion will exclusively focus on the various options and features of electric heaters.

Typical characteristics of electric flow heaters

• Power – ranging from 3 to 20 kW. However, powerful appliances necessitate a three-phase electrical system with a voltage of 380 V. In residences with outdated electrical wiring, it may pose a challenge to utilize heaters with even a medium power rating (4-6 kW). This might necessitate the installation of a dedicated power line.
• The overall dimensions of flow devices do not exceed 400 mm. The approximate measurements are 350 x 200 x 100.
• The water heating temperature ranges from 30-45 °C. This refers to the change in temperature at the outlet compared to the inlet, not the actual temperature of the water itself. It should be noted that the inlet temperature of cold water can vary depending on the season.
• The capacity of medium-sized flow heaters is 2-6 liters per minute of heated water.

There are two groups of flow water heaters, classified according to their method of connection to the water main.

Unpressurized water heaters

The heating systems in this category are not directly connected to the main water supply. They are specifically designed to provide hot water to a single point of use. The non-pressurized device receives water through an existing faucet or tap. This type of heater is not exposed to the high water pressure of the main supply and operates at atmospheric pressure. At the outlet, the pressureless heater is equipped with its own rotating spout or shower hose, or a combination of both.

There are various options available for these devices:

• An individual heater housing, installed adjacent to the sink or shower faucet.

A unique feature of this water heater is that it combines a faucet and a shower hose at the outlet, providing pressureless hot water.

• One option is to have a heating nozzle attached to the faucet, allowing for hot water whenever needed. However, the downside to this option is that the nozzle is quite large and may not fit on low taps.
• Another option is to have a faucet with a heater already built into its body. This eliminates the need for a separate heating nozzle and provides hot water directly from the faucet.

A faucet equipped with an integrated electric heater

Typically, non-pressure models have a lower power range (3-7 kW), which is sufficient to supply hot water to a single outlet. It is also possible to install a non-pressure duct connection to two separate faucets or taps, as illustrated in the diagram.

Schematic diagram of connecting a non-pressurized flow heater to two points

However, utilizing a non-pressurized device in accordance with this diagram may not provide sufficient heating when both faucets are in use simultaneously – there may not be enough power. However, in the case of alternating usage, the diagram is perfectly viable.

In order to operate the heater, it must be connected to the power grid. The low power consumption does not require special connection schemes with protective devices. Simply plugging it into a standard home wiring socket is sufficient. Modern models of non-pressurized heaters come equipped with protective valves that shut off the heating when the water pressure drops.

The ease of connecting non-pressurized water heaters and their low energy consumption make them suitable for temporary use during periods when hot water is not available or in temporary residences. They are particularly useful for summer cottages, where one or two connection points, such as the kitchen sink and the outdoor shower, can benefit from these devices. However, if you require a continuous supply of heated water all year round, it is advisable to opt for the pressurized version.

These types of heaters are capable of providing hot water for multiple outlets, making them ideal for apartments or larger houses. In essence, they function as miniature heating systems, often referred to as system water heaters.

To connect the pressure heater, it must be integrated into the main water supply line. In houses with central water supply, this means tapping into the cold water riser, while country houses with their own water supply would connect after the pumping station.

Prior to reaching the pressure heater, all water must pass through filters to ensure purity. Once heated, the water is then distributed to all points of consumption. The heating system is activated only when water is being used, as indicated by a sensor that detects water flow. It typically takes 1-2 minutes for the system to reach the desired heating temperature.

If the pressure system is installed in the house and there is already a hot water supply, it is important to have a backup plan in case it goes out. The connection scheme should be designed to allow for a quick switch from one source of hot water to another.

An illustration of the flow-through pressure system, designed to switch the source of hot water supply from the central system to an internal one.

To ensure simultaneous heating of water for multiple points of consumption, it is essential to have heating elements with the appropriate power. In a single-phase electrical network operating at 220 V, pressure heads with a capacity of up to 12 kW are available. However, for more powerful systems (up to 25 kW), a three-phase voltage of 380 V is required. Therefore, the installation and connection of such devices should be performed by professionals.

Even when connecting a high-power appliance to a single-phase network, it is crucial to provide protection against overloads. It is highly recommended to use a protective disconnection device (RCD) and an additional automatic circuit breaker that monitors the phase line.

A diagram illustrating how to connect a flow heater to a single-phase electrical network is shown below:

Along with using the appropriate wiring, it is crucial to have proper protection in place to handle the high current drawn by the powerful flow heater. Typically, a separate power supply line needs to be installed to operate the flow system. In houses where electric stoves are already present, it may be possible to utilize their power supply line as a last resort. However, it is important to note that the electric stove and the water heater should be operated independently in this scenario.

Control of flow water heaters

In order for heating devices to function properly, it is necessary to adjust their operation modes:

• Ability to choose the desired heating temperature.
• Switching between different modes of operation, such as water pressure and heating speed.
• Integration of additional convenience and protection functions.

There are two methods of controlling flow heaters – hydraulic and electronic.

Hydraulic control

Hydraulic water head control enables mechanical switching of heating modes using a step switch. The output temperature is regulated by the mechanical stem which adjusts the water head. In this type of control, the heating system is always turned on at maximum output regardless of the heating mode.

However, this control method has its drawbacks. One of them is its inability to accurately set the temperature and maintain the desired mode consistently. The efficiency of the hydraulic switch relies on the pressure levels in the mains. If the pressure is low, there is a possibility that the heater may not turn on at all. This type of control is commonly found in affordable models and is suitable for non-pressure water heaters.

Utilizing electronic signal control from multiple temperature and pressure sensors, this system enables precise adjustment of the outlet water temperature to within one degree. It remains unaffected by fluctuations in the main line pressure. A microprocessor oversees sensor control and heating mode maintenance. Some models even feature LCD screens that display the heater’s parameters.

An electronic control flow water heater

Criteria for choosing a flow water heater

The capacity is the main parameter to consider when choosing a water heater. To determine the minimum capacity required for a tankless water heater, you need to consider your specific needs – how much water you will need and at what temperature.

Here is an approximate water consumption guide:

Purpose	Desired Water Temperature	Water Flow Rate, l/min
Hand Washing	35	2-4
Dishwashing	50-55	3-5
Shower	40-45	4-8

You can calculate the required power using the following formula:

To provide a comfortable shower with a desired flow of 4 liters per minute at an inlet water temperature of 10° C (which can be even lower in winter), a power of 10 kW is required.

In case of simultaneous water supply to multiple locations, the power required for each point should be summed up for pressure heaters. Unpressurized models, on the other hand, are designed to serve only one point, so the calculation needs to be done separately for each individual point.

Once the required power has been determined, it is important to consider other parameters such as the type of power supply, availability of protective devices, and convenience of control. Moreover, special attention must be given to the power supply parameters of the chosen options, including whether they are compatible with the existing wiring or if a separate line needs to be installed for the water heater.

Like any other household appliance, flow-type water heaters have their own advantages and disadvantages.

Among the undeniable advantages are:

• Capability to provide hot water both in situations of temporary absence and in a continuous mode.
• Occupying minimal space, as they do not require large storage tanks.
• Electricity consumption is limited to the time when water is being used. They do not need constant heating in the storage tank. No water consumption means no electricity consumption.
• No restrictions on the amount of water used – water is heated to the necessary temperature throughout the entire usage period.
• Depending on the operating conditions, there is a choice to select a different type of heater – either a pressure or a non-pressure one.
• Flow devices are so compact that they can be installed anywhere in the house, including inside wall cabinets.

A kitchen cabinet can be equipped with a convenient flowing water heater.

However, there are also some disadvantages:

• Flow-through units of small and medium capacity have limitations in heating water above 45-50 ° C. This temperature range is usually sufficient for regular household use, but if you need higher temperatures, you will need a more powerful unit.
• Maintaining the temperature of flowing water requires a significant amount of electricity. In cases of high consumption, it may be more cost-effective to install a large storage heater, if there is enough space for it.
• High power consumption also puts additional demands on the electrical wiring. In some cases, a separate power supply or even a three-phase power supply may be required.

If you’re not looking for particularly high-powered models that run on three-phase power, two of the top brands in the market for flow system water heaters are Stiebel Eltron and AEG, both of which are German companies. These models are known for their impeccable German quality and wide range of protection and control functions. However, they do come with a corresponding price tag.

In the budget segment of the market, popular options for water heaters are Electrolux and Atmor.

The Stiebel Eltron DHC-E 12 is a relatively expensive model. This pressure water heater has a capacity of 10 kW and dimensions of 360x200x104. It is equipped with electronic control, temperature regulation, and protection against overheating. The approximate price is 31,000 rubles.

Stiebel Eltron DHC-E 12

The Stiebel Eltron IS 45E is a non-pressurized flow meter that has a capacity of 4.5 kW. It comes complete with a shower head and is priced at a maximum of 10,000 rubles.

The Stiebel Eltron IS 45E is a non-pressurized model.

The AEG RMC 8 E is an affordable 8 kW water heater with electronic control and can be purchased for approximately 15,000 RUB.

AEG RMC 8 E is a flow heater that operates based on pressure. It is well-known for its effectiveness in providing hot water.

In Russia, Atmor water heaters have gained popularity due to their affordable prices. Most models, including the Atmor In-Line 7 with a power of 7 kW, can be purchased for less than 5,000 rubles.