Connecting the SLOKOV electric boiler into an existing system

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Connecting the SLOKOV electric boiler into an existing system – complete technical guide

The SLOKOV electric boiler is one of the solutions we encounter more and more often in practice – not because it's a fashion trend, but because customers realize that replacing the original heat source with an electric boiler is sometimes much simpler than it seems. The most common scenario we see: someone has an old gas boiler, or a solid fuel boiler, a system with well-established piping, radiators or floor heating, and wants to switch to clean electric operation without having to demolish walls or change the entire installation. This is precisely the moment when proper connection plays a decisive role.

In this article, we will cover all the essential aspects of connecting the SLOKOV SL ELEKTRO boiler into an existing system – from the initial diagnosis of the old piping, through hydraulic and electrical requirements, to setting up the regulation and commissioning. If you are in the planning process or you are a technician who is to carry out the connection, you will find everything you need to know here.

1. What does "existing system" actually mean – diagnosis before replacement

Before you even order the boiler and plan the installation, you need to have a clear idea of what you are working with. Existing heating systems differ in several critical parameters that directly affect how easy or difficult it is to replace the heat source.

Type of circulation and method of heat distribution

The first thing to find out: is it a system with forced circulation (pump), or a gravity system? Older family homes built before 1990 often had gravity piping – that is, without a circulation pump, where water circulated by the physical principle of density difference between hot and cold water. These systems are characterized by much larger pipe diameters (DN 25–32 mm for main lines) and specific routing of risers.

The SLOKOV SL ELEKTRO series boilers, from SLOKOV SL ELEKTRO 10 to SLOKOV SL ELEKTRO 26, are designed for systems with forced circulation and an integrated circulation pump. If you are taking over a gravity system, you need to assess whether the pump can be integrated into the existing route without excessive pressure resistance – larger diameters are not a problem paradoxically, but an old radiator system without thermostatic valves may have different specifications.

Condition of existing piping and radiators

When replacing a boiler, it is equally important to assess the overall condition of the installation. Old steel or cast iron piping can be partially clogged with deposits (limescale, corrosion, magnetite) after decades of operation. When you connect a new electric boiler to such a system, the dirt will be released due to changes in flow conditions and start to circulate – and that is a situation you do not want. It will damage the circulation pump and the boiler body itself.

Practical recommendation: always flush the system before connecting a new boiler, ideally chemically (neutralization and flushing), and add water with a corrosion inhibitor. For systems with mixed piping materials (steel + copper + aluminum), the inhibitor is absolutely essential, otherwise galvanic corrosion occurs.

Pressure test and expansion tank

Every closed heating system must have a functional expansion tank. With old installations, we encounter either an open expansion tank (located in the basement) or a closed membrane tank. SLOKOV boilers operate in a closed pressure system – the operating pressure is typically 1.0–2.5 bar. If the existing system still has an open expansion tank, the system must be upgraded to a closed pressure system with a membrane expansion tank and a safety valve.

The safety valve in a closed system must be set to the correct pressure – typically 3 bar for apartments and family homes. Do not leave the old safety valve without inspection; after years of operation, it may be stuck or non-functional.

2. Hydraulic connection of the SLOKOV boiler into an existing system

The hydraulic connection itself is usually less complicated than it seems – as long as the existing piping is functional and passable. SLOKOV SL ELEKTRO boilers have an inlet/outlet for heating water with a G 1" thread (inch external thread), which is a standard size compatible with the vast majority of existing installations.

Step-by-step procedure for hydraulic connection

We recommend the following procedure in practice:

Close and drain the system: Close the supply of make-up water, open the drain valve at the boiler or at the lowest point of the system. Before draining, record the pressure in the expansion tank (reference value).
Disassembly of the old boiler: Cut or unscrew the original boiler connections, leave enough pipe length for new connections. If the old boiler was gas, do not forget to have the gas piping closed by a qualified person.
Preparation of the connection site: Ensure that the pipe at the connection points is straight, without kinks and mechanically stable. The boiler must not be a suspension for the pipe – it must be hung on the wall with separate anchoring elements.
Installation of ball valves: Install full-bore ball valves DN 25 (1") on the boiler inlet and outlet. They allow for future boiler disassembly without draining the entire system.
Connection of the expansion tank and safety valve: If the system does not have a closed expansion tank, install a membrane tank on the return pipe before the boiler, a safety valve on the outlet pipe – never behind a ball valve.
Filling and air venting: Slowly fill the system with water, gradually vent air from the highest points (radiators) downward. The target filling pressure is 1.5 bar cold.
Pressure test: After filling, check all connections, raise the pressure to 2.5 bar, wait 30 minutes – no connection must show a leak.

Important: flow direction and pump position

SLOKOV boilers have an integrated circulation pump – this is an advantage compared to some other solutions where the pump must be installed externally. The internal pump is dimensioned for standard installations with appropriate hydraulic resistance. However, if the system contains long pipe runs or a large number of heating units, it may be necessary to assess whether the internal pump can cover the system's hydraulic loss – if not, an external auxiliary pump may be required.

Approximate values: the internal pump in SL ELEKTRO boilers usually covers systems with an average hydraulic loss of about 20–25 kPa (0.2–0.25 bar). For larger systems, or systems with floor heating with long loops, it is advisable to consult a service technician.

3. Electrical connection – conditions, protection, wiring

This section is critical – and at the same time the most common source of problems in practice. An electric boiler is not like a standard electrical appliance. It is a three-phase appliance with relatively high power, which requires a dedicated electrical circuit, properly dimensioned wiring and corresponding protective and safety devices.

Power categories and power consumption

The SLOKOV SL ELEKTRO series covers power outputs from 10 to 26 kW, and in this series we find:

SLOKOV SL ELEKTRO 10 – power 10 kW, current draw at 400 V ~ 14.4 A
SLOKOV SL ELEKTRO 15 – power 15 kW, current draw ~ 21.7 A
SLOKOV SL ELEKTRO 18 – power 18 kW, current draw ~ 26 A
SLOKOV SL ELEKTRO 23 – power 23 kW, current draw ~ 33.2 A
SLOKOV SL ELEKTRO 26 – power 26 kW, current draw ~ 37.6 A

All models operate on three-phase power supply 3×400 V / 50 Hz. Single-phase power supply is not possible. This is an important piece of information when replacing an old (e.g., gas) boiler – if the household does not have a three-phase power supply available at the boiler installation location, it is necessary to plan the electrical installation in advance.

Wiring and circuit breaker dimensioning

A rule that has proven effective in practice: the cross-section of the supply cable and the circuit breaker rating are selected with a minimum 25 % reserve over the nominal current draw. Different requirements apply to each model:

Model	Power	Nominal current	Minimum cable cross-section	Recommended circuit breaker
SL ELEKTRO 10	10 kW	14.4 A	3×2.5 mm²	3×20 A
SL ELEKTRO 15	15 kW	21.7 A	3×4 mm²	3×25 A
SL ELEKTRO 18	18 kW	26 A	3×4 mm²	3×32 A
SL ELEKTRO 23	23 kW	33.2 A	3×6 mm²	3×40 A
SL ELEKTRO 26	26 kW	37.6 A	3×10 mm²	3×40 A

The values in the table are approximate – always verify the exact requirements in the technical documentation of the specific model and also consider the length of the supply cable (longer cable = larger cross-section due to voltage drop). The connection must be carried out by an electrician with the appropriate qualification (certificate § 21 or § 22 according to Ordinance 508/2009 Coll.).

Protective devices and earthing

In addition to the circuit breaker, installation of a residual current device (RCD) with a maximum residual current of 30 mA is mandatory for circuits with an electric boiler in households. The boiler must be properly earthed – the PE terminal must be directly connected to the protective conductor in the distribution board, not "taped" to the pipe. Older installations sometimes did not have a protective conductor (only phase + neutral), which is now completely unacceptable – such an installation must be replaced.

What if the household is supplied with single-phase power only?

This is a situation we often encounter in older rural houses and cottages. When a customer wants to install an electric boiler SLOKOV, but their building is only supplied with a single-phase 1×230 V connection, they must first address the issue of increasing the circuit breaker and introducing a three-phase supply through the distribution network (DSO, or the relevant distribution company). The process of changing the tariff group or increasing the circuit breaker capacity can take several weeks to months – plan accordingly with this timeframe in mind.

4. Connection to control and thermostat

SLOKOV SL ELEKTRO boilers are equipped with an internal built-in controller that allows setting the desired water temperature, or connecting an external room thermostat. This area is crucial for comfort and efficient operation when connecting to an existing system.

Internal boiler regulation

The boiler is equipped with a built-in thermostat that allows setting the temperature of the heating water (output temperature). This regulation is simple and reliable – the boiler heats the water to the set temperature and maintains it. In practice, this is sufficient for simple systems where control is done manually or via an external thermostat.

External room thermostat

For more comfortable control, we recommend connecting an external room thermostat (room thermostat). The SLOKOV boiler has two-wire low-voltage terminals (dry contact) for this purpose. The thermostat controls the boiler via its switching contact: when the room temperature drops below the set value, the thermostat closes and the boiler turns on. This method of control is more energy-efficient, as the boiler reacts to the actual heat demand in the interior, not just to the water temperature.

When replacing an old boiler, it is very common that the existing old thermostat (two-wire dry contact wiring) is fully compatible with the new SLOKOV boiler – it is sufficient to connect it to the appropriate terminals. Verification: if the original boiler (gas, solid fuel) was controlled by a room thermostat via two-wire wiring without power supply (only switching contact), practically 100% compatibility is guaranteed.

Programmable thermostats and smart control

Modern programmable thermostats (WiFi, smart, e.g., with a smartphone app) are also fully compatible – it is sufficient for them to have an output in the form of a dry contact (relay output). Such thermostats allow programming weekly schedules – ideal for use with night-time electricity tariffs. We will discuss this topic in more detail in the article "Electric boiler SLOKOV and night tariff – how to set up heating".

5. Integration with underfloor heating

Underfloor heating is typically a low-temperature system – it operates with water temperatures of 30–45 °C, up to a maximum of 55 °C. SLOKOV SL ELEKTRO boilers are capable of operating in a low-temperature mode as well, but there are a few important facts to keep in mind.

Mixer and hydraulic separator

If the existing system includes underfloor heating and at the same time radiators (a combined system), it is essential to use a mixing unit or a hydraulic separator (hydraulic balancer). Radiators typically require a temperature of 60–70 °C (older cast iron radiators), while underfloor heating requires 35–45 °C. A direct connection without mixing would mean that the underfloor loops are exposed to excessively high temperatures – this can damage the underfloor heating (cracking, floor deformation) and compromise comfort (too hot floor).

Solution: a three-way mixing valve with an actuator (actuator) or a thermostatic head in the underfloor heating manifold, while the boiler operates at a higher temperature for radiators and the mixer reduces the temperature for underfloor heating. This is a standard configuration that we implement in most combined systems.

6. Typical scenarios from practice – replacing specific types of boilers

Scenario A: Replacing an old gas boiler

This is the most common case. The customer has a gas boiler (e.g., an old wall-mounted boiler with a 20-year lifespan), the gas consumption is no longer cost-effective or they plan to disconnect the gas connection. The system is functional – radiators, piping, expansion tank. The procedure is relatively straightforward: dismount the gas boiler, seal the gas piping (authorized company), install the SLOKOV SL ELEKTRO in the same place (or nearby). Since most wall-mounted gas boilers have G ¾" or G 1" connections, it is necessary to use reducers in some cases. The electrical connection is done from scratch – new cable, new circuit breaker, RCD.

Scenario B: Replacing an old solid fuel boiler (solid fuel boiler)

The solid fuel boiler was usually installed in the boiler room, the system was either gravitational or with an additional pump, with larger pipe diameters. Here is a key advantage: larger pipe diameters do not cause problems for the new electric boiler – hydraulic resistance is lower, the integrated pump works without issues. More problematic is usually the electrical supply to the boiler room, where the old boiler had no electrical supply (or only a socket for the pump). A full three-phase supply must be brought in.

Scenario C: Backup heat source next to the existing boiler

Some customers do not want or cannot completely abandon the original heat source (e.g., a stove insert with a water jacket, heat pump) and want to connect the SLOKOV SL ELEKTRO as a backup or supplementary source. In this case, we are talking about a bivalent system with two heat sources. Hydraulic integration requires the installation of safety valves and check valves on each heat source to prevent reverse flow. The control must ensure that both sources do not operate simultaneously (unless it is intentionally as a bivalent parallel system).

Bivalent systems are technically more complex – we always recommend consulting with a designer or experienced service technician. More on choosing the right model can be found in the article "How to choose an electric boiler SLOKOV – what to pay attention to".

Scenario D: New installation in a renovated house

Renovation of an older house with additional thermal insulation reduces heat losses – and thus the boiler's required output. When a customer insulates a 1960s family house, heat losses can drop from the original 20–25 kW to 8–12 kW. In such a case, the SLOKOV SL ELEKTRO 10 may be fully sufficient where a boiler of double the output was previously installed. Correctly determining the output is crucial – we address it in the article "What output of the SLOKOV electric boiler do I need for my house".

7. System air venting and first heating

After hydraulic and electrical connection comes a phase that many underestimate: proper air venting and the first heating of the system. Air in the system is the enemy of the electric boiler – it causes noise, corrosion, and in extreme cases, damage to the circulation pump (cavitation).

Procedure for the first heating:

Fill the system to a pressure of 1.5 bar when cold.
Start the boiler at a low temperature (e.g., 40 °C) and let the pump run for 10–15 minutes with circulation without heating (if the boiler allows), or immediately with gentle heating.
Gradually vent all radiators from the top floor down. It is sufficient when water starts to flow without bubbles.
Check the pressure – after venting it may drop, refill to 1.5 bar.
Gradually increase the set temperature to the operating value and monitor the system for another 30 minutes.
After the full heating, check the pressure again – at full operating temperature (approx. 70 °C), it should not exceed 2.5 bar. If it does, the expansion tank is oversized or improperly set.

If you hear cracking or bubbling in the radiators after the first heating, vent again. Sometimes it is necessary to vent 2–3 times during the first day of operation until all air is removed from the system.

8. Most common mistakes during installation – what we have seen in practice

After years of work on customer sites, the same mistakes keep repeating. We list them here so you can avoid them:

Safety valve behind a ball valve: The valve is closed, the boiler overheats without the possibility of pressure release. The safety valve MUST always be directly on the pipe without the possibility of being closed before it.
Undersized cabling: A 2.5 mm² cable for a 23kW boiler is risky – overheating of insulation, fire hazard. Always dimension according to the table with a reserve.
Forgotten corrosion inhibitor: A new boiler in an old system without an inhibitor – in a year the heat exchanger is corroded and the circulation pump is damaged by magnetite.
Incorrect boiler position (below the level of radiators): The boiler must be installed so that system venting is possible during operation.
Missing check valve in a bivalent system: Without a check valve, one heat source can heat the other – with a gas boiler this can cause condensation and corrosion, with an electric boiler it can cause sensor malfunction.
Outdated regulator settings: After replacing the boiler, the original temperature settings for the old (larger) boiler remain – the boiler unnecessarily operates at maximum output and overheats the rooms.

9. Legislation, inspections and commissioning

Connection of an electric boiler must comply with current Slovak legislation – primarily STN 33 2000 (electrical standards), STN EN 12828 (heating systems), and Government Regulation No. 508/2009 Coll. on the safety of electrical equipment.

After installation, an expert inspection and test (OPaS) of the electrical installation must be carried out by a person with the appropriate certificate. The result is an OPaS report – a document you will need in the case of an insurance incident or when selling real estate. Without this document, the installation is illegal from an insurance perspective, even if it is technically in good condition.

For the electricity tariff: if you plan to use the boiler in combination with night rate (HDO), you must have your consumption point registered with the appropriate tariff with your distributor. The installation of the HDO receiver and its correct connection to the boiler control is a separate topic – more in the article "Electric boiler SLOKOV and night rate – how to set up heating".

Most frequently asked questions (FAQ)

Can I connect SLOKOV SL ELEKTRO to the same location where an old gas boiler was installed, without modifying the hydraulics?

In most cases, yes – provided the original system has a closed expansion tank, a functional safety valve, and pipework with forced circulation. The diameter of the connections (SLOKOV has G 1") and the condition of the pipework must be verified. The gas pipe must be sealed and sealed by an authorized company. The electrical supply is always new – the old supply for the gas boiler (230 V, 16 A) is insufficient for an electric boiler with a power of 10–26 kW.

Do I have to replace the entire pipework if I have an old system with cast iron radiators in my house?

No, cast iron radiators are hydraulically compatible with the SLOKOV electric boiler. They do have a larger water volume (longer warm-up time), but they work reliably. It is important to check the condition of the joints and seals, flush the system, and add an inhibitor. Replace the pipework only if it is physically damaged or so clogged that flushing is not sufficient.

What if I want to connect SLOKOV with a hot water storage tank (DHW)?

SLOKOV SL ELEKTRO boilers are primarily intended for heating. For DHW preparation, an external storage heater with electric or heat exchanger heating can be used. If you want one source for heating and DHW, a typical solution is a storage tank with indirect heating (coiled heat exchanger), connected to the boiler circuit via a three-way valve with DHW priority. This solution requires proper hydraulic design.

What pressure should the system have during operation?

Cold (water temperature below 30 °C), the pressure should be in the range of 1.2–1.5 bar. At full operating temperature (60–75 °C), the pressure increases due to thermal expansion of water – typically to 1.8–2.3 bar. If the pressure drops below 1 bar when cold, the system has a leak or the expansion tank has low pre-pressure. If the pressure rises above 2.5 bar at full temperature, the expansion tank is either too small or incorrectly set up.

Do you have a question about this topic?

Not sure or dealing with a specific situation in your household? Write to us – we are happy to help.