Installation of a water storage heater: top vs. bottom heating, installation and connection

Installation of a storage water heater: everything you need to know before and during installation

A storage water heater is a relatively inconspicuous but extremely important device in every household. When it works, no one notices it. When it doesn't work or was improperly installed, the problems can be quite unpleasant – from moldy walls through cracked pipes to burned electrical wiring. This article is intended for anyone who is planning to install an electric storage water heater themselves or who at least wants to understand what an electrician and plumber are doing at your home – and why.

We will take a detailed look at the differences between top and bottom heating, the correct procedure for hydraulic connection, the requirements for electrical installation, the placement and securing of the device, and the most common mistakes made during installation. If you are also interested in a comparison between storage and instantaneous water heaters, or the question of whether to choose a pressurized or non-pressurized system, I recommend reading other articles in our Knowledge Centre – for example, How to choose an electric water heater: storage vs. instantaneous or Pressurized vs. non-pressurized water heater: what is the difference and what to choose?

Storage water heater – how it works and what distinguishes it from an instantaneous heater

A storage water heater (also commonly called a "boiler" or "accumulation heater" in practice) operates on a simple principle: an electric heating element – a coil or tubular heater – is immersed in a sealed tank filled with water. A thermostat monitors the water temperature and turns on the heating when it drops below the set value (typically 60–75 °C). When the water reaches the desired temperature, the heating is turned off. The entire volume of the tank is thus always ready for use.

Unlike an instantaneous heater (for example, electric instantaneous heater MK1 3.5 kW or instantaneous heater PM 3.5 kW with hydraulic switching), a storage heater does not require a high instantaneous power – heating a 30-liter tank requires only 2 kW, which is compatible with a standard electrical installation. The exchange for hot water is quick and convenient, as long as the tank does not run out. If this happens, you have to wait for the next heating cycle, which can take 1–2 hours.

Top heating vs. bottom heating – the basic difference that will affect everything

This is the first and most important thing you need to resolve before purchasing a water heater. Storage water heaters are available in two basic configurations depending on where the heating element and hydraulic connections are located – and these two versions are intended for completely different installation methods.

Water heater with top heating (cold water supply from the top)

With top heating, the heating element is located in the upper part of the tank. Cold water enters the tank from the top as well (through an internal connecting pipe that reaches down to the bottom of the tank to prevent hot water from being displaced), and hot water also exits from the top. The electrical connection is also located at the top. This type is intended for installation under a sink or kitchen sink, where there is not enough height – the tank hangs upside down. Cold water therefore really comes from the top and hot water leaves from the bottom (relatively, since the entire head of the device is at the bottom).

An example of such a device is, for example, electric storage water heater BD 2.0 kW 30 l top – a compact device with a thermostat and pressurized operation, ideal precisely for a small bathroom or under a sink, where installation with an inverted head is standard.

Water heater with bottom heating (cold water supply from the bottom)

With bottom heating, the heating element is located in the lower part of the tank. Cold water enters from the bottom, and hot water exits from the top (where the electrical head is also located). This type is intended for the classic vertical installation with the head facing upwards – that is, usually on a wall in the bathroom or boiler room, where there is enough height. Physics works in our favor here: heated water rises, cold water sinks to the heating element – natural convection speeds up heating and increases its efficiency.

The majority of standard household tanks (50 l, 80 l, 100 l and more) are precisely those with bottom heating – they are installed vertically with the electrical head pointing upwards.

Why the choice matters – and why it can't be interchanged

This is something I see quite often in practice: a customer buys a bottom-heated water heater and then wants to hang it under a sink with the head down. The result? Hot water remains at the bottom (because the head is down), cold water enters the top part and the same cold water comes out – effectively you have no hot water or only occasionally. In addition, the heating element may be out of water and burn out. Storage water heaters are not symmetrical – each type is designed for a specific mounting position and cannot be freely rotated.

Before purchasing, always clearly determine: is the tank going to be mounted on the wall in a vertical position with the head up (bottom heating), or under the kitchen sink/basin with the head down (top heating)?

Preparation before installation: what you must ensure in advance

Installing a storage water heater is not complicated, but it needs to be well planned. Poor preparation is the cause of most problems that arise during or shortly after installation.

Wall load-bearing capacity and anchoring elements

A full water tank can be surprisingly heavy. A 30-liter tank weighs about 33–36 kg when full (depending on the construction), a 50-liter tank about 55–60 kg, and an 80-liter tank easily 85–90 kg. These are loads that cannot be supported by just any screw in the plaster. Load-bearing brackets must be anchored in the wall layers, not just in the plaster:

• Full brick / thick aerated concrete: standard Fischer or Rawlplug anchors with a diameter of at least 10 mm, length min. 80 mm. For tanks over 50 l, I recommend 12 mm.
• Hollow brick or thin-walled aerated concrete: it is necessary to use special hollow wall anchors or chemical anchors – a standard expansion anchor will pull out under load.
• Gypsum board: not sufficient on its own, you must anchor into load-bearing profiles or use a bracket with at least 4 anchoring points and chemical anchors.
• Concrete: the most reliable solution, standard expansion anchors 10 mm are sufficient.

Always check whether the manufacturer supplies the brackets or if you need to buy them separately. Some tanks come with brackets included, others do not. Brackets must be installed horizontally (use a spirit level) and at a distance specified by the manufacturer in the installation instructions.

Dimensional preparation and accessibility

Before drilling, measure the dimensions of the heater including the fittings (safety valve, pressure-reducing valve, T-piece) and check whether it fits into the intended space. The tank must be accessible from at least one side – for possible anode replacement (magnesium rod, which protects the tank from corrosion) and for inspection and maintenance of the heating element. Do not hide the tank in completely enclosed spaces or cabinets without a service opening.

Electrical installation – requirements and conditions

An electric storage water heater requires a separate and protected electrical circuit. This is not just a recommendation – it is a requirement of standards and the manufacturer. The reasons are clear: the heater operates for long periods (hours), has a relatively constant load and must not be interrupted by the operation of other appliances.

• Power 2 kW (e.g. BD 2.0 kW): a single-phase circuit 230 V / 10 A, cable CYKY-J 3×1.5 mm² is sufficient
• Power 2–3.5 kW: single-phase circuit 230 V / 16 A, cable CYKY-J 3×2.5 mm²
• Power over 3.5 kW up to 6 kW: single-phase circuit 230 V / 25–32 A, cable CYKY-J 3×4 mm² (or three-phase)

In a bathroom, protection by a residual current device (RCD) with a tripping current of 30 mA is mandatory. If the tank is connected via a plug, the socket must be located in accordance with ČSN/STN 33 2000-7-701 – this means at least 60 cm from the edge of the bathtub or shower area. In practice, it is better to connect the tank permanently (without a plug), via a terminal block or directly to the distribution board – it is safer and more resistant to moisture.

The electrical installation must be carried out or checked by a person with the appropriate qualification (§22 of the decree on electricians). The hydraulic installation can be done by yourself (provided you don't have a gas installation), but it is better to leave the electrical work to a professional.

Step-by-step procedure for hydraulic installation

A storage water heater is a pressure vessel. In a standard water supply, the pressure is 2–6 bar, and in some areas even higher. The heater must be properly protected, otherwise during heating (water volume expands) overpressure and tank failure may occur.

1. Hanging brackets and tank installation

Mark the bracket positions according to the installation instructions (spacing, height). Drill holes, insert anchors, screw on the brackets and check with a spirit level. Hang the tank. The tank must hang evenly – it must not swing or be twisted.

2. Safety (pressure relief) valve

This is an absolutely critical part of the installation. The safety valve must be installed on the cold water inlet, as close to the tank as possible (not further than on the first pipe section before the tank). It opens at overpressure – typically at 6–8 bar depending on the heater model. The valve must have:

• the correct opening pressure matching the maximum operating pressure of the heater (check the technical specifications of the device)
• an overflow pipe leading to the sewer or into a catch basin – dripping from the safety valve is normal (a little water drips after each heating – it is the expansion volume)
• no shut-off valve towards the tank must be installed after the safety valve

If you do not install the safety valve or install it incorrectly (after a shut-off valve, in the wrong direction), the tank can be damaged during the first heating. In extreme cases, the tank may burst.

3. Pressure-reducing valve (if needed)

If the pressure in the home water supply is higher than 3–4 bar (which is common in panel buildings and some apartment blocks), it is recommended to install a pressure-reducing valve before the tank. It is set to 3–4 bar and protects the tank and the entire system from excessive pressure.

4. Shut-off valve on the cold water inlet

A shut-off valve must be installed on the cold water inlet before the tank – to allow the tank to be isolated for service without disconnecting the entire system. It is installed before the safety valve (so the order from the supply is: shut-off valve → safety valve → tank).

5. Connecting hot water to the outlets

The hot water outlet from the tank is connected to the existing hot water distribution (or directly to the taps/batteries that the tank serves). Important: there must be no shut-off valve or check valve on the hot water outlet – water must be able to flow out during expansion through the safety valve.

6. Filling and air venting of the tank

Before switching on the electrical part, the tank must be completely filled with water. Proceed as follows:

1. Open the cold water supply (open the shut-off valve).
2. Open the hot water outlet valve (e.g., a tap on the faucet) – air from the tank will be pushed out by water and will exit through this opening.
3. Wait until a continuous stream of water without air bubbles starts flowing from the hot water tap. Only then is the tank fully filled.
4. Close the hot water tap.
5. Check whether water is dripping from the connections or fittings – if so, tighten or seal them.

Never switch on the electric heating element in an empty tank! A heating element that is not submerged in water will immediately overheat and burn out. This fault cannot be repaired – the element must be replaced, and customers usually discover this only when the tank stops working.

Electrical connection of the storage water heater

A storage water heater is usually a single-phase device (230 V). Electrical connection includes:

• Phase (L): brown or black wire
• Neutral (N): blue wire
• Protective conductor PE (ground): green-yellow wire – must always be connected, the tank must be grounded

The terminal block is located in the electrical head of the tank (a cover that can be unscrewed). The wires are connected according to the diagram in the installation manual. The thermostat is usually part of the tank and is manually adjustable (rotary control) – the factory setting is typically 60–65 °C. A temperature below 60 °C is not recommended for hygienic reasons (risk of Legionella bacteria growth).

Some models of storage water heaters allow connection to a night tariff circuit (HDO – mass remote control), where the electricity distributor switches the heating on during the cheaper night tariff. If you have a contract with a D45N or similar tariff, it is financially interesting – the tank heats up at night and during the day you draw hot water from the accumulated heat. For such a connection, a separate circuit controlled by an HDO signal from the distributor is required, which is installed by an electrician during inspection.

Most common installation errors with storage water heaters

Over the years of practice, I have seen truly various installations – good and catastrophic. Here are the errors that repeat most often:

• Missing or incorrectly connected safety valve: The most dangerous error. The safety valve must be of the correct class, properly oriented (arrow indicating flow direction), and have a drain hose leading to the sewer.
• Switching on before filling with water: The heating element burns out immediately. Customers do this because they want to "test" the tank before completing the plumbing work.
• Incorrect installation direction (a tank for bottom heating installed with the head down): The tank works technically, but does not heat the water efficiently and the heating element may be in an empty space.
• Shared circuit breaker with other appliances: The tank must have its own circuit breaker. If it shares a circuit breaker with a washing machine or another high-power appliance, the circuit breaker trips or the cable overheats.
• Missing shut-off valve on the inlet: When the tank needs to be repaired or replaced, you have to shut off the entire cold water supply in the apartment/house.
• Sealing with old sealing material (silicone instead of Teflon or hemp with paste): Silicone is not suitable for pressure plumbing connections. Proper sealing is Teflon tape, hemp with paste, or compression fitting.
• Thermostat set too low (below 60 °C): Risk of Legionella pneumophila growth. At temperatures of 40–50 °C, this bacterium actively multiplies. The minimum recommended operating temperature is 60 °C.

Differences in installation between pressure and non-pressure water heaters

In most cases, a storage water heater operates under pressure – it is directly connected to the water supply, the water in it is under pressure and supplies all the outlets at once. Non-pressure storage water heaters exist, but are less common (in practice, non-pressure are mostly small-volume devices up to a few liters, e.g. 5–10 l, directly above the outlet point).

If you are interested in the difference between pressure and non-pressure systems in more detail, I recommend an article in our Knowledge Center: Pressure vs. non-pressure water heater: what is the difference and what to choose? There you will also find a comparison with devices such as flow heater PM 4.5 kW with a non-pressure system, which operates on a completely different principle.

Regular maintenance after installation

A storage water heater is not a device you "install and forget". It requires regular maintenance, especially in areas with hard water (and most water supply areas in Slovakia have water hardness of 2–3 mmol/l, which is medium-hard to hard water).

• Check the safety valve (once a year): Manually open the safety valve (lever or screw cap) and check whether water flows and the valve can be closed again. A clogged or calcified valve must be replaced.
• Replacement of the magnesium sacrificial anode (every 2–3 years, depending on water hardness): The anode protects the tank from electrochemical corrosion. When the anode is consumed, the tank starts to rust from the inside. Anode replacement is the most important service procedure for the tank.
• Descaling of the tank (every 2–5 years): In hard water, limescale accumulates on the heating element, which reduces heating efficiency and shortens the element's lifespan. More about this in the article Maintenance and descaling of an electric water heater: how to extend its lifespan.

Storage water heater vs. instantaneous: a quick comparison for decision-making

If you are still deciding whether to choose a storage or an instantaneous water heater, here is a short practical comparison. An instantaneous water heater, such as MK1 4.5 kW with electronic switching, heats water instantly without a tank, but requires a significantly higher power (3.5–5.5 kW) and a stronger electrical installation. A storage water heater operates with lower power, but stores hot water in advance and requires time to heat it up.

For a small bathroom with one faucet and limited electrical installation (an old apartment), a 30–50 L storage water heater is ideal. For a new apartment with the need to supply a shower and a sink simultaneously, an instantaneous water heater with sufficient power or a larger storage unit is more advantageous. Read more on this topic in the article How to choose an electric water heater: storage or instantaneous?

Frequently asked questions (FAQ)

Can I install a top-heated storage water heater vertically with the head upwards?

No. A top-heated storage tank is specifically designed for installation with the head downwards – under a sink or a countertop. If you install it upside down, the hydraulic connections will be in the wrong positions, hot water will not rise properly, and the heating element may operate out of water. For vertical installation with the head upwards, always choose a storage tank with bottom heating.

Do I need to call an electrician, or can I do the electrical wiring myself?

According to Slovak law (Decree No. 508/2009 Coll.), electrical installations in residential areas must be installed by a person with the appropriate professional qualification. In the bathroom, stricter safety requirements apply (zones, RCD). You can install the plumbing (water connection) yourself, but the electrical wiring and inspection should be done by an electrician – both for safety and to maintain the validity of your property insurance.

Why does water occasionally drip from the safety valve – is it a fault?

No, this is normal and correct. Every time the water is heated, the volume of water in the tank expands (water expands by about 3–4% when heated). This excess volume is released through the safety valve. If it only drips during or shortly after heating, the valve is working properly. If it drips continuously even without heating, the valve is damaged or the pressure in the system is too high – in such a case, you should install a pressure-reducing valve and replace the safety valve.

What temperature should I set the thermostat to?

The recommended minimum temperature is 60 °C – at this temperature, Legionella bacteria, which can cause serious respiratory illness, are killed. Most storage tanks have a maximum of 75–85 °C. A higher temperature means more limescale buildup, but also a greater amount of usable hot water (after mixing with cold water). In practice, 65–70 °C is a good compromise between hygiene, energy efficiency, and the device's lifespan.

What is the lifespan of a storage water heater?

With proper installation and regular maintenance (valve inspection, anode replacement), the realistic lifespan is 10–15 years. Without anode replacement, the tank can corrode from the inside in 5–7 years, especially in areas with hard or aggressive water. The heating element is also a replaceable part – if it becomes clogged with limescale, a service technician can replace it for a few dozen euros.

Can I connect a storage water heater to a solar heating system?

A standard electric storage water heater (single jacketed, with only an electric heating element) is not directly suitable for combination with a solar collector – for that, bivalent tanks with two heat exchangers are intended. An electric storage tank can be used as a booster behind a solar tank, but not as the primary tank in a solar circuit. For combining solar systems with electric boosting, it is recommended to consult a heating system designer.

Conclusion: installing a storage water heater is not rocket science, but it does require order

Installing a storage water heater is a manageable task if you know what you are doing and what to pay attention to. The key points are: choosing the correct type (top vs. bottom heating according to the installation position), reliable mounting for the weight of a full tank, mandatory safety valve with a drain, a dedicated electrical circuit with RCD 30 mA, and filling the tank with water before the first switch-on. If you are unsure about anything – especially the electrical part – call an electrician. The cost of an electrical inspection and professional wiring is a fraction of what it would cost to repair damage from a poorly connected appliance.

If you are interested in how the instantaneous alternatives work and when they are more cost-effective than a storage tank, see the article Installation of an electric instantaneous water heater: procedure, connection, and requirements for electrical installation or Hydraulic vs. electronic switching for an instantaneous water heater: what it means and which is better? in our Knowledge Centre.

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