Electric boiler SLOKOV and night rate - how to set up heating

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Electric boiler SLOKOV and night rate – complete guide to heating setup

An electric boiler combined with a night rate electricity tariff (also known as low tariff, LT) is one of the most common solutions that people in Slovakia inquire about when planning cheaper and more comfortable heating. The idea is attractive: electricity is significantly cheaper at night, the boiler runs intensively when the tariff drops, and the house is heated during the night hours to a temperature that will then "last" through the day. Reality is a bit more complex – it depends on the installation, thermostat settings, boiler configuration, tank size, and, of course, on the specific tariff that your distributor grants you. This article will give you a comprehensive view of what you need to know, set up, and check if you want to operate the SLOKOV SL ELEKTRO boiler with a night rate as efficiently as possible.

What is a night rate and how does it work in Slovakia

The night rate (low tariff – LT) is a time period during which the electricity distributor charges electricity at a significantly lower price than during the day period (high tariff – HT). In Slovakia, three regional distributors operate the distribution: ZSE, SSE, and VSE, each of which has its own specific times for LT and conditions under which it is granted.

The night rate is not automatic – you must actively request it and meet technical conditions. Most importantly, you must have a so-called "direct electric heating" appliance at home or an electric water heater with a minimum power (usually from 3.5 kW, or higher, depending on the tariff). The SLOKOV SL ELEKTRO electric boiler with power ratings of 10, 15, 18, 23, and 26 kW easily meets these conditions.

The most commonly used tariffs for electric heating in Slovakia are:

D2 (resp. D02d): Two-band tariff with 16 hours of LT per day. Typically, LT runs from 22:00 to 06:00 and during two blocks during the day (usually 2×4 hours). Suitable for electric floor heating or water heating with a storage tank.
D3 (resp. D03d): Three-band tariff, where LT can last up to 20 hours per day. The distributor controls switching via HDO (mass remote control). This tariff is ideal for electric boilers with heat accumulation – for example, when combined with a large storage tank.
D1 (resp. D01d): Single-band tariff – there is no LT, you pay the same rate all day. It is not optimal for electric heating in terms of savings.

For the SLOKOV electric boiler, which will be used as the primary heat source, we always recommend considering the D3 tariff. If the boiler is used only for preparing hot water (TUV) or as a supplementary source, D2 may be sufficient. Your electricity supplier or installer can advise you on the selection.

How HDO affects the operation of the SLOKOV boiler

HDO – mass remote control – is a system by which the electricity distributor sends a radio signal to your home electrical installation (in the past, a signal was used via the power line – so-called "ripple control"). This signal switches on and off the so-called HDO relay, which is part of your electricity meter box (distribution board). The relay directly controls the LT consumer circuit.

For the SLOKOV boiler, it is important to understand that the HDO relay does not switch the boiler itself, but the power supply for the LT tariff. If you want the boiler to use LT automatically, you must have the correct wiring – the boiler's power supply (or its corresponding "night heating" input) must be connected via the HDO relay. This topic is discussed in more detail in the article on the installation and wiring of the SLOKOV electric boiler.

In practice, this means that the SLOKOV SL ELEKTRO boiler can be connected in two ways:

Directly to the LT circuit: The entire boiler is powered via the HDO relay. The boiler operates only during the LT period. This option is suitable if you have sufficient thermal mass (e.g., a large TUV storage tank + radiators with a large water volume), or if the LT is long enough (D3 – 20 hours).
Combined connection (HT + LT input): The boiler has the ability in its control logic to receive a signal from an external thermostat or HDO input and switch between "night" (maximum) heating and "day" (reduced or off) mode accordingly. This is the preferred option for family homes, where you want to have heat during the day as well, but want to save on night heating.

Setting up the SLOKOV SL ELEKTRO boiler for operation with a night rate – step by step

Let's now go through the specific setup procedure. We assume that the boiler is already physically installed, has a properly dimensioned power supply, and is connected to an existing heating system. If not, we recommend reading the article Installation of the SLOKOV electric boiler – procedure and requirements.

Step 1: Checking the tariff and HDO signal

Before setting anything up on the boiler, check with your distributor what tariff you have approved and at what times the HDO signal is actually active. This can be easily checked on the HDO relay in the distribution board – when LT is active, the relay is engaged and there is voltage at the output terminals. Monitor for a few days exactly when the LT switches on and off. These times are needed for the correct setup of the boiler's controller.

Typical HDO switching times for individual distributors (approximate, always check the current times with your distributor):

ZSE (west Slovakia): LT typically 22:00–06:00 + 2 blocks during the day (e.g., 10:00–12:00 and 14:00–16:00) for D2
SSE (central Slovakia): LT may be distributed differently, D3 gives up to 20 hours of LT
VSE (east Slovakia): Similar to SSE, depends on the specific HDO circuit

Step 2: Setting the heating water temperature for night mode

The SLOKOV SL ELEKTRO boiler allows setting the heating water temperature in the range of typically 30 – 80 °C. For night operation in combination with radiators, we recommend setting the temperature according to the following logic:

Floor heating: water temperature maximum 45 – 50 °C (floor heating systems cannot tolerate higher temperatures)
Low-temperature radiators (panel, design): 55 – 65 °C in night mode
Classic cast iron radiators: 70 – 75 °C, or up to 80 °C if the house is poorly insulated
Combination of floor heating + radiators: the temperature is set according to the primary circuit; the floor heating circuit has its own mixing valve

In night mode (NT), set the boiler to the maximum operating temperature that your system can withstand. The goal is for the building structure and heating elements to absorb as much heat as possible during the cheaper electricity period. During the day, after NT ends, either turn off the boiler or switch it to a setback mode.

Step 3: Setting up an external thermostat / controller

The SLOKOV SL ELEKTRO boiler has an input for an external room thermostat. This input is crucial for comfortable and energy-efficient heating. The thermostat (or room temperature controller) can be:

A simple bimetallic/digital thermostat with one temperature setting
A programmable weekly thermostat with different temperature programs for day/night and work/weekend days
A smart WiFi thermostat controllable via an app (e.g., SALUS, Honeywell, Danfoss...)

For operation with NT, we recommend a programmable thermostat. Set it up as follows:

Night program (NT band): desired room temperature 20 – 22 °C. The thermostat will keep the boiler running until the set temperature is reached. If the house is well insulated, the boiler will restart and shut off during NT according to the room cooling.
Day setback program (VT band, you are at home): temperature 18 – 19 °C. The boiler will start only if the temperature drops below this level – it will run minimally, or not at all, if the house retains heat from the night heating.
Absence program (VT band, you are away from home): temperature 15 – 16 °C. Minimal operation, frost protection.

Step 4: Setting up the boiler's internal controller

SLOKOV SL ELEKTRO boilers have an internal controller with a display. On the display, you set the following:

Heating water temperature (VO): the maximum temperature to which the boiler will heat the water. For night heating, set it according to the heating system (see above).
Frost protection: the minimum temperature at which the boiler will start automatically, regardless of the thermostat. We recommend setting it to 5 – 8 °C of heating water temperature.
Temperature hysteresis: by how many degrees the temperature must drop below the set value for the boiler to restart. With a larger hysteresis band (e.g., 5 °C), the boiler cycles less, which saves switching contacts and prolongs its lifespan.
Pumping / circulation pump: setting the pump run time after the boiler is turned off. Most SLOKOV boilers have this setting automatic.

Step 5: Setting up the equithermal regulation (optional)

Certain models or external controllers allow so-called equithermal regulation – the heating water temperature automatically adjusts to the outside temperature (the colder it is outside, the higher the water temperature in the system). For efficient use of NT, equithermal regulation is beneficial: in freezing nights, the boiler automatically increases the water temperature, while in mildly cold nights it saves energy. If you have an external sensor and the controller supports it, we recommend activating this function.

Selecting the right SLOKOV SL ELEKTRO model for heating with NT

Selecting the boiler power for NT operation has its own specifics. Since the boiler runs only part of the day (NT band), it must deliver enough thermal energy for the entire day during that time. A simple calculation:

Example: A house with a heat loss of 8 kW (a well-insulated family house, approx. 150 m²). We need 8 kW × 24 h = 192 kWh of heat in 24 hours. If NT lasts 16 hours (tariff D2), the boiler must deliver 192 kWh in 16 hours. This means a minimum boiler power of 192 / 16 = 12 kW. In practice, however, we take losses into account and add a reserve – the recommended boiler power would be at least 15 kW. Therefore, a suitable model would be, for example, SLOKOV SL ELEKTRO 15.

Another example: An older masonry villa with a heat loss of 15 kW, NT only 10 hours a day (unfavorable case for D2 in some areas). Over 24 hours: 15 × 24 = 360 kWh. For 10 hours of NT: 360 / 10 = 36 kW – this is unrealistic without massive thermal accumulation. In such a case, NT heating alone is insufficient, and it is necessary to either add a thermal storage tank and higher insulation of the house, or to account for partial heating also in the VT band. In this case, we would recommend the SLOKOV SL ELEKTRO 26 in combination with a thermal storage tank of 1,000 – 2,000 liters and the D3 tariff.

More about selecting the power can be read in the article What power of SLOKOV electric boiler do I need for my house, where you will find more detailed tables and calculation methods.

Thermal storage tank as a key component of the NT system

If you want to effectively use the night tariff, a thermal storage tank (buffer tank) is almost essential for the system. Without it, the boiler heats water in the system only for "immediate" use, and when the NT is off, the system quickly cools down.

The thermal storage tank functions as a "thermal battery" – during NT, the boiler heats not only the water in the system but also the volume of water in the tank. This accumulated thermal energy is then released into the system throughout the day, when the boiler is mostly (or completely) not producing.

Approximate recommendations for the volume of the thermal storage tank for SLOKOV boilers:

10 kW boiler (SLOKOV SL ELEKTRO 10): thermal storage tank 300 – 500 liters
15 kW boiler (SLOKOV SL ELEKTRO 15): thermal storage tank 500 – 800 liters
18 – 23 kW boiler: thermal storage tank 800 – 1,200 liters
26 kW boiler (SLOKOV SL ELEKTRO 26): thermal storage tank 1,000 – 2,000 liters (for exclusive NT operation)

The tank should be thermally insulated (at least 80 – 100 mm insulation) to minimize heat losses. Placing the tank indoors (boiler room, technical room) is an advantage – any heat losses from the tank are "shared" with the neighbor-inside the house, so heat does not escape outside.

Practical tips and experiences from installations

Over the years of practice in installing electric boilers with night tariffs, we have gathered several recurring insights that can save you trouble:

Do not check NT only on paper: Customers often confuse guaranteed NT hours (according to the contract) with the actual HDO signal. Sometimes the HDO relay in a specific circuit switches differently than the tariff states. Always install an NT hour meter (available for a few euros), which records the actual NT time per month. We have found cases where a customer paid for the D3 tariff, but the HDO actually gave them only 12 hours of NT.
Watch out for the transitional period: In the fall and spring, when temperatures fluctuate, it can happen that during the NT period the boiler heats to maximum, but the house is overheated – thermostats turn off too early. Set the correct night temperature on the thermostat (not too high) and rely on hysteresis.
Circulation pump and NT: If you have a TÚV storage tank with circulation piping, the pump should run during the NT period – during night heating, hot water circulates also through the TÚV piping, and in the morning you have immediately available hot water. Set the circulation via a timer for the NT period + morning hours.
Legionella protection: For storage TÚV preparation, regular disinfection is mandatory – heating the tank to at least 70 °C at least once a week. Most modern boiler controllers have this function built-in, set it to the NT period (e.g., Saturday, 03:00 – 05:00).
Circuit breaker and cable sizing: For higher powers (18, 23, 26 kW), pay attention to proper sizing. A 26 kW boiler on a three-phase system draws ~40 A, the cable must be sized at least for this load (typically CYKY 4× 10 mm²) and the circuit breaker at least 3× 40 A (or 3× 50 A with thermal reserve). Undersized supply is the most common technical error we encounter in complaints.
Stepwise power regulation: If you have a smaller house and a 10 kW boiler is still too powerful for the transitional period, some SLOKOV models allow switching between heating elements (stepwise regulation). Use this feature – it will reduce boiler cycling and extend its lifespan.

Most common errors when setting up an electric boiler with night rate

During years of service practice we have identified several recurring errors that lead to inefficient operation or even failure:

Error #1: Setting too high a temperature of heating water for floor heating

Floor heating is a low-temperature system – the maximum supply temperature is 45 – 50 °C. If someone sets the boiler to 70 °C without a mixing valve before the floor heating manifold, the floor heating pipe can overheat, potentially damaging the piping, laminate floors or parquet. Always use a three-way mixing valve with a thermostatic head calibrated to max. 45 °C for floor heating circuits.

Error #2: Boiler connected directly to NT without a thermostat

When NT starts, the boiler starts heating at full power and no one can stop it until NT ends. The house overheats to 26 – 28 °C, which is uncomfortable and wastes energy. You must always have a thermostat! The boiler must have a control input from the thermostat, which will turn it off when the set temperature is reached, even if NT is still running.

Error #3: Neglecting hydraulic balancing of the system

If you have multiple circuits (e.g. upper floor + ground floor, or heating + DHW tank), without hydraulic balancing one circuit will receive too much heat and the other too little. The result is uneven heating and unnecessary energy consumption. Use balancing valves or regulating valves.

Error #4: Ignoring anti-frost protection

The customer was on vacation, the NT period changed or the HDO relay failed, the boiler did not heat for several days and the pipe froze in the house. Always activate anti-frost protection on the boiler (minimum 5 °C water temperature) and consider a GSM alarm that will notify you of a temperature drop in the house.

Economic evaluation: When does NT actually pay off?

Approximate comparison of heating costs for various scenarios for a house with an annual heat demand of 15,000 kWh (medium insulation, ~150 m²):

Scenario	Electricity price (€/kWh)	Annual costs
Single-rate D1 (all day HT)	~0.22 €/kWh	~3,300 €
Two-rate D2, 70 % NT / 30 % HT	~0.14 €/kWh (mix)	~2,100 €
Three-rate D3, 90 % NT / 10 % HT	~0.11 €/kWh (mix)	~1,650 €
Gas (for comparison, current prices)	~0.08 €/kWh (gas)	~1,200 – 1,500 €

The numbers are approximate and depend on current energy prices, the specific tariff and the efficiency of the system. From the table it is clear that D3 with maximum use of NT can economically approach gas heating – while eliminating dependence on gas, risks associated with combustion and service costs of a gas boiler. For customers who do not have access to gas or want to go fully "electric", D3 + SLOKOV + storage tank is a very serious alternative.

Compliance with technical standards and distributor requirements

When requesting the NT tariff, you must provide the distributor with a technical report (or installation protocol) confirming that you have an authorized electric heating consumer installed at the metering point. The SLOKOV SL ELEKTRO electric boiler meets the requirements for direct electric heating. For the purpose of the request, it is sufficient to provide:

Boiler technical specification sheet (power, power supply, type – provided by the seller)
Electrical installation inspection report (prepared by the inspection technician after installation)
Completed application form with the distributor (form available on the distributor's website)

The tariff change usually takes 1 – 4 weeks from the date of submission. The distributor will come to replace or configure the electricity meter (or install an HDO receiver if you don't have one).

Frequently asked questions (FAQ)

Can I operate the SLOKOV boiler exclusively on NT without a storage tank?

Yes, it is technically possible, but only if the NT period is long enough (ideally 18 – 20 hours, i.e. tariff D3), the house is well insulated and is able to retain heat even during short HT breaks. In practice, with a shorter NT period (8 – 12 hours) and no storage tank, the house may not warm up sufficiently or may cool down too much during HT and comfort will drop. A storage tank significantly improves the efficiency of the whole system and we recommend it for every installation with NT.

How long does it take for the NT tariff to "pay off" and return the cost of the boiler?

It depends on the specific situation, but when switching from direct electric heating (boilers, direct heating elements) to the SLOKOV boiler with D3 tariff and storage, the return on investment can be 3 – 6 years. When switching from a gas boiler (where you have a lower base for comparison), the return is longer – 8 – 12 years, depending on energy price development. The advantage of an electric installation is minimal maintenance and no annual service inspections at the level of 100 – 200 €, as is the case with gas.

Can the SLOKOV boiler work with a smart WiFi thermostat?

Yes. The boiler has input terminals for an external thermostat (no-voltage contact, electrically isolated). You can connect any modern WiFi thermostat with a relay output contact to these terminals – for example Salus iT600, Danfoss Ally (in a radio system), Honeywell T6R or many others. Thanks to the WiFi thermostat, you can control the boiler remotely via smartphone, set back heating during vacation or turn on heating before arriving home.

Will my HDO times change if I move or change my electricity supplier?

The HDO signal and its times are the responsibility of the distributor (ZSE, SSE, VSE), not the commercial electricity supplier. The NT times can change if the distributor updates its HDO map. When changing electricity supplier, the tariff (and thus the NT times) remains the same, provided you choose the same distribution tariff with the new supplier. We recommend verifying the actual HDO times directly by measuring at the HDO relay after changing the supplier.

What if the HDO relay fails and the boiler is not powered during NT?

The HDO relay is an electromechanical device and like any relay can fail over time. Signs of failure: the boiler does not start at night, the electricity meter does not show consumption in the NT period, the relay "clicks" irregularly. The relay is located in the electricity meter or distribution board and its replacement is the responsibility of an electrician, or the distributor. We recommend installing a simple NT clock (an electromechanical timer as a backup solution), so the boiler will at least run in a basic night period in case of HDO failure. More on diagnostics and troubleshooting can be found in the article Common SLOKOV electric boiler faults and their solutions.

Is it necessary to change the boiler settings when switching from heating to summer mode (only DHW)?

Yes, when switching to summer mode we recommend turning off the heating function on the boiler (or reducing the temperature to a minimum) and leaving only the DHW tank heating active. Most SLOKOV SL ELEKTRO boilers have separate regulation of the heating (central heating) and DHW temperatures. In summer, set the heating circuit temperature to a minimum or deactivate it, set the DHW temperature to 55 – 60 °C and run DHW heating only during the NT period (at night). This minimizes consumption during the HT period.

Conclusion: SLOKOV SL ELEKTRO and night rate – a meaningful combination with the correct settings

The electric boiler SLOKOV SL ELEKTRO in combination with the night rate – especially D3 with a long NT band – represents an economically and technically well-functioning solution for family homes and smaller buildings, particularly where natural gas is not available or where the customer has opted for a fully electric system. The key to success lies in three things: correct thermostat setting (day/night/away mode), sufficient thermal storage capacity (tank), and verifying the actual HDO band from your distributor.

If you are considering choosing a specific boiler model, take a look at the full range of SLOKOV electric boilers on atria.sk, including the models SLOKOV SL ELEKTRO 18 and SLOKOV SL ELEKTRO 24.