Flow-Through Electric Water Heater For The Shower: Selection Guidelines

To solve the problem of constant hot water supply in the bathroom, it is easier to install a flow-through electric water heater for the shower. It has significant advantages over the storage device, which we will consider in detail in this article. And we will also pay attention to the disadvantages, the main one is the need for a special approach when providing electricity.

Also in this material, we will give recommendations on choosing a suitable water heater, including the calculation of sufficient power. We will identify the best manufacturers of electric models, focus on installation and maintenance. For a trouble-free operation, it is necessary not only to calculate the parameters correctly but also to install them in a professional way.

Article contents:

  • Features of an electric water heater
    • The internal structure of the appliance
    • The advantages of flow-through models
    • The problem of electric power supply
  • What to look for when choosing a water heater?
    • Calculation of the optimal heater power
    • Popular series from well-known manufacturers
  • Nouches for connection and operation
    • How to connect the water heater correctly?
    • Use and maintenance recommendations
  • Conclusions and useful videos on the subject

Features of an electric water heater

The choice of a shower water heater between a flow and storage type device is not always straightforward. Each has its pros and cons, and the decision in most cases depends on the capacity of the electrical system.

Interior structure of the appliance

In bathrooms, non-pressurized water heaters are installed for a single split point. They are designed for a single consumer, have 1-2 heating elements, and rarely exceed 8 kW. And the water pressure sufficient to operate such devices should be from 0.03 to 0.6 MPa.

The device is turned on when the flow sensor is triggered. The overheating sensor detects the water temperature and switches the heating elements off when they are outside the permissible range.

The indicator signals to the user the operation of the heating elements. If it is on but the water heating does not occur, it means the heating element breakage - the most frequent problem with flow-through water heaters.

Flow-through electric water heater for the shower: selection guidelines

The structure of the Thermex flow-through water heater does not differ from similar devices of other companies. However, it is better to use original parts when repairing

Flow-through electric water heater for the shower: selection guidelines

All water heaters have a blue water pipe connection and a red one for the shower head or tap

The water flow and overheating sensors do not allow the flow of the heated water in the opposite direction. Therefore, for the safe operation of the device it is necessary not to confuse the connection points of the cold water inlet and the heated water outlet.

Also for many devices a limit on the maximum temperature of the incoming water is prescribed. As a rule, it is 30-35 degrees. If this value is exceeded, the overheat sensor will definitely be triggered and the water heater will shut down. Therefore, the device should not be connected to a hot water supply riser when, in the opinion of the consumer, the water there is not hot enough.

The advantages of flow-through models

The first advantage of a flow-through device is the much smaller size. Placing it in the shower room and installing it yourself does not cause problems, whereas finding a place for the massive tank of the storage water heater and its installation creates noticeable difficulties.

The second advantage is the much lower cost of a domestic flow heater designed for a single point of consumption. Depending on the equipment and brand, the price range of such devices is 1700-8000 rubles, while the price of a simple storage water heater with a tank volume of 30 liters starts from 5000 rubles.

Service and repair of storage devices must be done more often, and it costs more. In addition, their own installation and connection can not carry out not every consumer, which incurs additional costs for the services of a specialist.

When using a storage water heater it is necessary to consider the number of people when calculating the volume of the tank. However, in the case of the arrival of guests, it may not be enough. Flow-through counterpart does not have such a disadvantage.

More criteria for comparing a flow heater and a storage water heater are discussed here.

Flow-through electric water heater for the shower: selection guidelines

Comparing the parameters of a six-kilowatt flow heater and a fifty-liter storage unit. Weights are for empty units without water weight

The problem of electricity supply

The only significant obstacle to the decision in favour of the flow heater is the peak load on the electricity grid. It is several times greater than a storage unit.

The purchase of a flow heater greatly increases the electrical cable requirements. The combined capacity of common bathroom appliances such as the washing machine (with a heating element of 1.5 - 3.0 kW), the towel rail (0.4 - 0.6 kW) and the light line (0.1 - 0.25 kW) rarely exceed 4 kW.

To provide this voltage, a wire with a copper core section of 1.5 or 2.5 mm2 that is often supplied to such rooms is sufficient.

Flow-through electric water heater for the shower: selection guidelines

The flow water heater is becoming increasingly popular as an independent of the water supply as a way to generate energy for heating water. That's why when you renovate a bathroom you have to make good wiring right away

But having a flow heater increases the maximum power consumption of the circuit section to 6-10 kW and then a cable with a section of 4 or 6 mm2 is already required. This means that before installing the device it is often necessary to replace the wiring, and it is better to separate it into a separate branch up to the distribution (internal) switchboard.

The second problem after the wiring can be the load on the electrical socket. These are labeled with the allowable voltage and amperage.

From this information it is possible to calculate the maximum power of the appliance to be connected without causing damage to the socket:

P = I * U


  • P the equipment power (Watt);
  • I the current (Amps);
  • U the line voltage (Volt).

Mains outlets for 220 V standard voltage have a current rating of 5, 10 and 16 Amps. Consequently, devices with a maximum consumption of 1100, 2200 and 3520 watts, respectively, can be connected to them. If a higher power heater is to be used, power sockets must be installed.

They have the following standard parameters:

  • 25 amps (up to 5.5 kW);
  • 32 amps (up to 7.0 kW);
  • 63 amps (up to 13.8 kW);
  • 125 amps (up to 27.5 kW).

In case of difficulty in mounting the power socket, it is possible to make a connection to the terminal block of the supply cable. However, this procedure should only be carried out by a professional electrician, as unskilled work may overheat the connection and cause a short circuit. This should not be allowed in a room with high humidity.

Flow-through electric water heater for the shower: selection guidelines

Seal socket and plug are often included with power-intensive equipment. They can also be purchased at almost any store that sells electrics

If the existing electrical supply of the bathroom allows for alternating use of power-consuming devices, then you can settle on this option. To exclude by forgetfulness their simultaneous switching on it is enough to use one socket for two appliances.

The last problem can be the maximum connected load of the apartment or house when connecting to the common energy infrastructure. For horticulture and private houses with old power lines it can be as little as 4-6 kW.

Then you can only use a water heater of the flow-through variety if almost all other appliances are turned off. But even with the standard 15 kW allowed power it is necessary to make a peak load calculation.

What to look for when choosing a water heater?

First of all, you should choose a flow water heater with regard to the sufficiency of its power, and then pay attention to the brand. When buying a device of a widespread series of a well-known manufacturer, you can be sure of its quick repair in case of a breakdown. This is facilitated by the availability of spare parts and experience in solving typical problems.

Calculating the optimum appliance power

The relationship between electric water heater power, water flow rate and water flow temperature increase is calculated as follows:

P = 0.073 * Q * (T1 - T2)


  • P - power device (kW);
  • Q - water flow rate (l/m);
  • T1 - flow temperature at the outlet (°С);
  • T2 - flow temperature at the outlet (°С).

The leaving water temperature can therefore be calculated as follows:

T1 = T2 + 13.7 * P / Q

The temperature of the water coming from the own well does not vary much during the year. Its value can be measured and is usually 8-12 degrees. The temperature value of centralized cold water supply is more strongly dependent on the time of year and ranges from 5 degrees in winter to 15 degrees in summer.

The maximum flow of water can be measured experimentally. To do this, simply measure the time in seconds it takes to fill a liter bottle and then divide the number 60 by the recorded value.

Flow-through electric water heater for the shower: selection guidelines

The water flow rate in the shower depends not only on pressure, but also on the flow capacity of the watering can. That is why it is better to use the water flow through it when experimenting

You can calculate the value of the flow temperature at the outlet.

It can be adjusted as follows:

  • When the pressure is reduced with the tap, the water temperature increases;
  • when the water heater power is reduced, it decreases.

For most people the comfortable range of the shower is from 35 to 42 degrees. To be able to flexibly regulate the flow, it is better to use a water heater with several output modes.

It is also important in case of centralized water supply, since the water pressure may be reduced for a long time as a result of repairs on the mains. In this case, the unit operating at maximum capacity will produce a flow hotter than required.

Flow-through electric water heater for the shower: selection guidelines

The main cause of pressure drop in the water mains is maintenance works that may last a long time. Therefore, water heaters with reduced capacity modes are in demand on the market

For example, consider a standard apartment with a centralized water supply. The cold water temperature is 5 degrees in winter and 15 degrees in summer. Maximal stream pressure is a bit more than 4 l/m. Desirable head for showering - not less than 3 l/m. A comfortable temperature range is from 35 to 40 degrees.

P in winter = 0.073 * 3 * (35 - 5) = 6.6 kW.

P in summer = 0.073 * 4 * (37 - 15) = 6.4 kW.

So a water heater with a total heat exchanger capacity of about 6.5 kW must be purchased. If there is a periodic drop in pressure in the water mains, you need a device with the function of switching off one of the heating elements.

Popular series of well-known manufacturers

Almost all well-known manufacturers of flow-through electric water heaters offer the same model range of devices in terms of characteristics.

This is due to the standard plumbing pressure and approximately the same water flow parameters that contribute to a comfortable shower.

Flow-through electric water heater for the shower: selection guidelines

Electric water heaters are sold in many stores. In addition to the technical parameters and price, you should pay attention to the warranty and the location of the service center

Smartfix 2.0series.

  • 3.5 (1.5 + 2.0) kW;
  • 5.5 (2.2 + 3.3) kW;
  • 6.5 (3.0 + 3.5) kW from the Swedish manufacturer Electrolux is represented by three units equipped with two heating elements with the following maximum power

The presence of \"T\" at the end of the appliance name means the tap and \"S\" the shower head. They are connected by a standard 1/2\" plug, so existing nozzles can be used. They are assembled in China and Russia.

Series Stream by Italian manufacturer Thermex is represented by three devices with maximum power limits of 3.5; 5.0 and 7.0 kW. The water heaters also have two heating elements each. Russian assembly, lower price range.

Series Whell of the Swedish manufacturer of water heating equipment Timberk is also represented by three models with maximum power 3,5; 5,5 and 6,5 kW. There is a restriction on the minimum allowable temperature of water flow at the inlet (16 degrees), although, according to reviews, the device works well at lower values. Chinese production.

Series H-IWR of the Korean manufacturer Hyundai is represented by three models with maximum power parameters 3,5; 5,5 and 6,5 kW. Buyers note the smell of plastic, which disappears after some time (Chinese production).

The above common series of water heaters have a bottom supply and are designed for use in bathrooms. All devices of the same model range have the same design and construction, differing only in the capacity of the heating elements.

The rating of the best models of flow-through water heaters, composed by popularity among buyers and positive reviews,  we led in the following article.

Nuances of connection and operation

For trouble-free operation of the flow-through water heater you will need to study its operation principle and know the main technical characteristics of the device. There are also nuances of connection and maintenance that will avoid problems during its operation.

How to connect a water heater correctly?

There are water heaters with bottom, top and side water inlets. The bottom version is usually used in bathrooms. It is necessary to decide on the best way of connection before purchasing the unit, as it is forbidden to install it in any other position.

Cold water connection is made in two ways:

  • to the shower connector of the tap-shower switch;
  • direct to the cold water mains.

The first method leaves the option of using a standard water tap to obtain cold water bypassing the water heater.

In a user-designed plumbing system, the shut-off valve cannot be used after the running water heater. Otherwise, it will cause it to overheat, even if the thermo-sensor shutdown function is present. This rule is directly indicated in the instructions of many devices.

Flow-through electric water heater for the shower: selection guidelines

A typical example is the connection of a flow heater to a cold water system. The shower switch and tap are positioned before the water heater and after it, the flow is free

According to the electrical safety requirements for hygienic rooms GOST R 50571.7.701-2013, outlets for 220 Volt in bathrooms must be installed in zone three (further in plan than 60 cm from the shower, bath or basin tap) and conform to protection class IPX4 or higher.

According to protection class IPX1, an ordinary socket can be installed within 3 metres of the water supply points.

Despite the fact that the instantaneous water heater is designed for humid environments and has a good water seal, the undesirability of a direct jet of hot water must be considered when choosing the installation site. So there is a heating of the body, which will adversely affect the tightness of joints and the functionality of the control panel. In addition, direct water exposure reduces the fixing strength of the device to the wall.

The fixing to the wall is done using standard methods with screws. The weight of the device is 1 to 3 kg, so no reinforcement measures are required. Some owners do without fixing the device, but this should not be done, as there is a risk of the device falling and depressurizing during operation, which is dangerous for people in the room.

Would you like to install the flow heater yourself? We recommend that you read the step-by-step installation instructions.

Flow-through electric water heater for the shower: selection guidelines

If it is not possible to install the water heater outside the bath or shower stall, then it should be placed as high as possible. This will reduce the amount of water that falls on it

Use and maintenance instructions

Besides wiping the surface of the water heater with a soap solution, periodic maintenance consists in changing the filter installed on the cold water inlet.

The following steps are taken:

  • disconnect the unit from the power supply;
  • close water tap;
  • unscrew the hose;
  • remove and wash the strainer;
  • reassemble the unit in reverse order.

For problem free operation it is important to set the appliance correctly.

The water temperature setting should be made in the following order:

  1. Open the tap and wait until water flows from the water heater.
  2. Turn on the water heater. After 10 seconds the temperature reaches a constant value.
  3. If the temperature is not suitable for showering then change the temperature by adjusting the water heater operating modes.

Wait 10-15 seconds after each change of the water flow or the heating elements power to establish constant temperature values.

For detailed instructions on the use and maintenance of the water heater we refer to this article.

Summary and useful video on the subject

Wall fixing, connection and start up of the Electrolux Smartfix water heater:

Example of incorrect use of the water heater. The device is not fixed, the tap is installed after the hot water outlet:

The calculation of the necessary capacity of the flow water heater, its fixing and connection to the water supply can be done independently. The greatest difficulties are caused by the safe supply of electricity to the device. Choosing in favor of a reputable manufacturer will ensure a quick solution to problems if the device breaks.

Want to ensure the year-round availability of warm water in the shower by mounting a flow-through water heater model, but friends convince you of the ineffectiveness of such a solution? Ask your questions under this article - real flow water heater owners will share their experiences with you and their own opinions on the feasibility of its installation.

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