Setting Up and Commissioning the Thermona THERM EL Boiler – First Start-Up Step by Step

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Setting Up and Commissioning the Thermona THERM EL Boiler – Step-by-Step First Start-Up

The Thermona THERM EL electric boiler is among the technically less demanding devices to install, yet the first start-up still requires thorough preparation and a systematic approach. Mistakes made during commissioning are among the most common causes of premature service calls, damage to the heating element, or unnecessary heating outages at inconvenient times. This article will guide you through the entire process – from pre-start checks, through control setup, to verifying correct operation. You'll find specific values, diagrams, and practical notes from real-world jobs.

If you're still at the stage of choosing a boiler or planning the installation, we recommend first reading Installing the Thermona THERM EL Electric Boiler – Procedure, Electrical Installation Requirements and Inspection as well as Connecting the Thermona THERM EL to the Heating System – Wiring Diagrams and Hydraulic Requirements. This article assumes the boiler is physically installed, hydraulically connected, and the electrical installation is complete and inspected.

Why the First Start-Up Is a Critical Phase

Unlike a gas boiler, where a technician usually performs the manufacturer's mandatory commissioning, with the Thermona THERM EL electric boiler, installation and first start-up are often handled by an electrical installation company or an experienced owner themselves. This offers flexibility, but also carries risk – no one will automatically point out what you forgot to check.

Starting up the boiler without water in the system or without bleeding the circuit can lead to immediate damage to the heating element. The electric resistance heating element in the Thermona THERM EL is designed to operate submerged in water. If it starts "dry" or air bubbles remain in the system, local overheating can irreversibly damage the element – and such a failure is not covered by the warranty, since it constitutes a demonstrable mechanical failure caused by improper operation.

Another practical reason for a careful first start-up: this is when all operating parameters are set – heating water temperature, thermostat hysteresis, and possibly the connection to an external room thermostat or zone controller. The parameters set during the first start-up determine the economics of operation for years to come.

Checklist Before First Start-Up

Before you even touch the main circuit breaker, systematically go through the following points. We recommend actually checking them off – not just reading them.

Hydraulic Side – Water in the System

The system is filled with water – the minimum operating pressure is 1.0 bar when cold (i.e., at ambient temperature, before heating). The optimal cold pressure for most installations is 1.2–1.5 bar. If the pressure is below 0.8 bar, the boiler will display an error code and refuse to start.
The system has been bled – on all radiators as well as on the boiler's air vent valve. Air in the system is the most common cause of noise during first operation and insufficient heating.
The expansion vessel is correctly pressurized – the nitrogen pre-charge pressure in the expansion vessel should correspond to the system's static height. For a typical family house with a height of 5–6 m, this is usually 0.5–0.6 bar. This pre-charge pressure is set before filling the system with water, not after.
The safety valve is functional and not blocked – typically set to 3 bar, and must not be flooded or mechanically blocked.
All shut-off valves are open – including valves on the boiler's outlet and return, on the manifold, and at the radiators.
The circulation pump is functional – manually rotate the shaft to verify it isn't stuck after a long period of inactivity.

Electrical Side

The electrical installation has been inspected – a valid inspection report is a prerequisite for warranty and insurance validity. Without it, we do not recommend starting the boiler at all.
The supply cable cross-section matches the boiler's output – for the Thermona THERM EL 8 (8 kW, three-phase), a minimum of 4 × 2.5 mm²; for the Thermona THERM EL 23 (23 kW), a minimum of 5 × 6 mm²; for the Thermona THERM EL 38 (38 kW), a minimum of 5 × 10 mm².
The circuit breaker matches the output – the manufacturer specifies exact breaker values for each model; check the technical data sheet supplied with the boiler.
Grounding is functional and verified by inspection.
All terminals in the boiler are tightened – terminals may loosen during transport. Before the first start-up, manually check all terminal blocks, including power terminals.

First Start-Up – Step-by-Step Procedure

Step 1 – Switching On the Main Breaker and Checking the Display

Switch on the boiler's main circuit breaker. The boiler will go through a brief initialization cycle – the display will light up all segments (display test), then show the current water temperature in the system or the currently set point. If the display remains dark, check the power supply and the correctness of the wiring at the L/N/PE terminal block (for single-phase models) or L1/L2/L3/N/PE (for three-phase models).

Immediately after starting, check whether the boiler displays an error code. The most common ones during first start-up are pressure-related codes (low water pressure) and temperature interruption codes (insufficient bleeding or a closed valve). For more on error codes, see the article Common Faults of Thermona THERM EL Boilers – Causes, Error Codes and Solutions.

Step 2 – Setting the Desired Heating Water Temperature

The basic setting on the Thermona THERM EL is the heating water temperature – i.e., the temperature to which the boiler heats water in the primary circuit. This value is set directly on the front panel using the ▲/▼ buttons.

What temperature is correct? It depends on the type of heating system:

Underfloor heating – the maximum permissible primary circuit temperature is usually 45–55 °C, typically set to 40–45 °C. Higher temperatures damage the floor structure and are not necessary.
Low-temperature radiators (modern panel radiators) – optimal temperature 55–65 °C. With correctly sized radiators, 55 °C is often sufficient.
Classic steel or cast-iron radiators (older installations) – usually 70–80 °C. If the radiators are undersized or the building is poorly insulated, a higher temperature is necessary.

Practical tip: for the first start-up, set the temperature to 60 °C as a safe middle ground. After the system warms up and the radiator function is verified, you can optimize the temperature downward – every 1 °C reduction saves approximately 1.5–2% of electrical energy.

Step 3 – Bleeding the System While Hot

Even if you bled the system before starting, additional air bubbles dissolved in the water are usually released during the first heating cycle. Let the boiler heat the system to at least 50 °C and then go through all radiators again and bleed them. Repeat the process until clean, bubble-free water flows from the bleed valves.

After each bleeding, check the system pressure and, if necessary, top it up to 1.2–1.5 bar via the filling valve. If the pressure drops below 1.0 bar after bleeding, the boiler may stop with a low-pressure error.

Step 4 – Setting the Thermostat Hysteresis

Thermostat hysteresis determines how many degrees the temperature must drop below the set point before the boiler switches on again. The standard value is 5 °C – meaning that if you set 60 °C, the boiler will switch off at 60 °C and switch on again at 55 °C.

Why does this matter? Too small a hysteresis (1–2 °C) causes so-called "cycling" – the boiler switches on and off very frequently, which unnecessarily wears out relays and heating elements. Too large a hysteresis (8–10 °C) causes temperature fluctuations in the rooms. For a typical heating system, the optimal value is 4–6 °C.

The hysteresis setting depends on the specific model – see the technical manual for your boiler. On some models, it is set directly via the service menu; on others, it is fixed from the factory.

Step 5 – Setting the Maximum Temperature (Safety Limit)

In addition to the working temperature, the boiler also has a set maximum safety temperature (STB – Safety Temperature Breaker). This value is usually 95 °C and is not intended for regular adjustment – it protects the boiler and the system from overheating. If the STB trips, the boiler shuts down and requires a manual reset (RESET button on the panel or a reset button opening under the cover).

If the STB trips repeatedly, the problem is not in the settings but in the system – it is usually caused by a stopped circulation pump, a closed valve, or extremely clogged filters. Do not reset the boiler repeatedly without identifying the cause.

Step 6 – Connecting and Setting Up an External Room Thermostat

The Thermona THERM EL is standardly equipped with terminals for connecting an external room thermostat (usually labeled "TA" or similar – see the wiring diagram in the manual). If no thermostat is connected, the terminals must be bridged with a short-circuit jumper (supplied from the factory), otherwise the boiler will not heat.

When first starting up with an external thermostat:

Set the room thermostat to 20–21 °C.
Check that the thermostat is calling for heat (the contact is closed).
Verify that the boiler responds to the thermostat contact opening – once the set room temperature is reached, the thermostat should open the contact and the boiler should switch to standby mode (maintaining a minimum temperature or switching off, depending on the setting).

Note: some installers bridge the thermostat terminals "permanently," reasoning that "it's simpler." The result is a boiler that regulates only according to water temperature, not the actual room temperature – significantly and unnecessarily increasing operating costs.

Step 7 – Verifying Stepped Output Regulation

One of the main technical advantages of Thermona THERM EL boilers is stepped output regulation. Instead of always switching to full power, the boiler activates the heating elements progressively – in several stages. For example, the Thermona THERM EL 15 with an output of 15 kW can operate in stages of 5 kW – 10 kW – 15 kW, depending on current heat demand.

During the first start-up, it is worthwhile to verify that the staging works correctly. Monitor the boiler's power consumption (if you have access to real-time consumption measurement) or watch the display – some models show the active power stage. In practice, this means: on a cold start, the boiler switches on maximum output, and once the working temperature is reached, it reduces to lower stages. If the boiler always operates at full power regardless of staging, this may indicate a fault in the control electronics. This topic is covered in more detail in the article Stepped Output Regulation in Thermona THERM EL Boilers – How It Works and Why It Matters.

Setting Service Parameters – Overview

The Thermona THERM EL allows access to a service menu where advanced parameters can be configured. Access to the service menu varies by model – usually a combination of holding a button for a long time or entering a PIN code (4444 or 1111 – check the current manual). Note: the service menu is not intended for the ordinary user. Incorrect settings can cause malfunction or damage to the boiler.

For the purposes of first start-up, the following service parameters are relevant:

Minimum frost protection temperature – typically 5–8 °C. At this temperature, the boiler switches on automatically, even in off mode, to prevent the system from freezing. Do not leave the boiler physically disconnected from power during winter.
Circulation pump settings – some models allow setting a delay for the pump to continue running after the boiler switches off (post-circulation). The recommended value is 3–5 minutes.
Pump protection function – during longer periods when the boiler is off (summer break), the boiler briefly runs the pump once every 24 hours to prevent it from seizing up. This function should always be active.

First Hours of Operation – What to Monitor

After a successful first start-up, let the boiler run for at least 2–3 hours and monitor these parameters:

System Pressure

During the first heating cycle from 10 °C to 70 °C, the system pressure will rise by approximately 0.3–0.6 bar (depending on the volume of water in the system and the capacity of the expansion vessel). If the resulting hot pressure exceeds 2.5 bar, the expansion vessel is either undersized or has a low pre-charge pressure. If the pressure drops (falling below 0.8 bar after cooling), the system has a leak or the expansion vessel is defective. In both cases, professional inspection is required.

Noise and Vibration

Some noise during the first start-up is normal – this includes the sound of flowing water, minor air bubbles, and thermal expansion of pipes. If you hear loud knocking, strong buzzing, or irregular banging, there is likely still air in the system, or the circulation pump is working against resistance (closed valve, clogged filter).

Even Heating of Radiators

After an hour of operation, all radiators should be heating evenly. If a radiator remains cold despite a hot inlet, either the thermostatic valve is closed, or the radiator still contains air. If a radiator is hot at the top and cold at the bottom, this indicates sediment buildup – a problem typical of older systems, resolved by chemical flushing.

Setting the Time Program and Setback Mode

For economical operation, it is essential to set a heating time program. The Thermona THERM EL has an internal timer switch, or you can connect an external weekly programmer.

Typical recommended settings for a family house:

Weekdays 6:00 – 8:00 – comfort temperature (e.g., 22 °C in rooms, 60–65 °C heating water)
Weekdays 8:00 – 15:30 – setback temperature (nobody home) – reduce to 18 °C / 50 °C heating water, or full setback with frost protection
Weekdays 15:30 – 22:00 – comfort temperature
Night 22:00 – 6:00 – night setback 16–17 °C / 45 °C heating water
Weekend – all-day comfort temperature or individually as needed

Important: for an electric boiler, the night setback should not go too low (below 15 °C indoors). Raising the temperature from 12 °C to 22 °C in the morning costs significantly more energy than maintaining 16 °C overnight. This is one of the most common mistakes we see when analyzing consumption for new customers – they try to save energy by switching off the boiler overnight, but end up consuming more in the morning than they saved.

Specific Considerations for Different THERM EL Models at First Start-Up

The basic first start-up procedure is the same across the entire range, but some practical details differ depending on output and model:

Thermona THERM EL 8 (8 kW) – the smallest model in the range. Suitable for smaller houses or apartments. Single-phase power supply, so at full output it draws ~35 A. Check that the breaker and cable correspond accordingly. For first start-up in a small apartment, we recommend setting the temperature to a maximum of 55 °C – a higher temperature would unnecessarily shorten operating cycles given the small radiator volume.

Thermona THERM EL 15 (15 kW) – the best-selling model, suitable for family houses of 100–150 m². Three-phase power input. During first start-up, it is important to verify phase load balance – each phase should be loaded evenly (±10%).

Thermona THERM EL 23 (23 kW) and Thermona THERM EL 30 (30 kW) – mid-range models for larger houses or duplex installations. With a larger system volume, the initial filling and bleeding takes longer – expect 2–4 rounds of bleeding before the system is completely free of air.

Thermona THERM EL 38 (38 kW) – the most powerful model, intended for large buildings, industrial use, or multi-unit apartment buildings. Here, the first start-up is usually part of a more complex job involving hydraulic balancing of the system. We recommend entrusting the first start-up to a qualified technician.

Documentation After First Start-Up

After a successful first start-up, record the following values – they will be valuable for any future inspection or service intervention:

Date of first start-up
Set heating water temperature
Cold pressure in the system (at ambient temperature)
Hot pressure in the system (at maximum working temperature)
Expansion vessel pre-charge pressure
Set control parameters (hysteresis, minimum protection temperature)
Type and setting of the external thermostat/controller
Electrical installation inspection report number

Write these values in the manual or on a label stuck inside the boiler cover. It may seem unnecessary at first glance, but during a service call three years later, when you call a technician, this note can save an hour of work (and therefore costs too).

Frequently Asked Questions (FAQ)

The boiler switches on, but the water doesn't heat up – what's the cause?

The most common causes are: a missing thermostat terminal bridge (the contact is open), the set temperature is lower than the current water temperature, or the boiler is in protective mode after the safety thermostat (STB) has tripped. Check whether the display shows an error code. If not, try resetting the boiler using the RESET button. If the condition recurs, check the thermostat wiring.

Pipes crack and knock after the first start-up – is this normal?

Some cracking during the first heating cycle is normal – it results from thermal expansion of the pipes and their movement within the clips. If the sounds are pronounced or persist even after several hours of operation, the pipes are likely routed without sufficient expansion sections or are secured in rigid clips without damping inserts. Persistent loud knocking may also indicate air in the system or water hammer – in which case a hydraulic inspection is needed.

The system pressure drops quickly after the first fill – do I have a leak?

Not necessarily. During the first fill, dissolved air is released from the water, causing an apparent pressure drop. Bleed the system and top up the pressure. If the drop continues even after complete bleeding (for example, 0.3 bar per day), it really is a leak, and it needs to be located and repaired. A small leak is not always visible – it may be a micro-leak at a thread or under insulation.

Can I let the boiler run at maximum output all night during the first start-up?

Yes, that's fine – provided the system is correctly filled, bled, and the pressure is within normal range. Running the first start-up at full power is even beneficial, as it helps "flush out" air from the system and speeds up the thermal expansion of the pipes. What's more important is that you're present for at least the first hour so you can respond to any problems.

Is a service inspection needed after the first start-up?

Manufacturer warranties do not require regular service inspections at the same intervals as gas boilers (which require a mandatory annual inspection). Nevertheless, we recommend checking the system pressure, filter condition, circulation pump function, and overall boiler condition after the first heating season (i.e., after about 6 months of operation). For details, see the article Maintenance and Servicing of the Thermona THERM EL Electric Boiler – What and How Often to Check.

Can the owner perform the first start-up themselves, or is a service technician required?

The hydraulic part (filling, bleeding, temperature setting) can be performed by a technically skilled owner. The electrical part – wiring, checking the circuit breaker and grounding – must be performed and inspected by a person with the appropriate electrotechnical qualification. The first switch-on of the boiler after the electrical installation has been checked can be done by the owner themselves. If you're unsure about any step, contact the installation company or an authorized Thermona service technician.

Conclusion – First Start-Up as the Foundation for Long-Term Trouble-Free Operation

The first start-up of the Thermona THERM EL boiler is not just a formality. It's the moment that determines how the boiler will operate for the next 15–20 years. A correctly set heating water temperature, functional controls, a bled system, and documented parameters – these are the foundations that determine energy consumption, system noise, component lifespan, and heating comfort.

If, after reading this article, you feel confident about the procedure but still have questions about certain steps, we recommend checking out related articles in our Knowledge Center: Frequently Asked Questions About Thermona THERM EL Boilers offers clear answers to the most common dilemmas, and Connecting the Thermona THERM EL to the Heating System discusses hydraulic diagrams for various types of installations in detail. If you're considering which model is right for you, the article How to Choose a Thermona THERM EL Electric Boiler – Output, Space Size and Other Criteria will help.

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