Connecting Thermona THERM EL to the Heating System – Wiring Diagrams and Hydraulic Requirements

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Connecting the Thermona THERM EL Boiler to the Heating System – Wiring Diagrams and Hydraulic Requirements

The Thermona THERM EL electric boiler is a compact appliance with an integrated pump, expansion vessel, and safety valve – yet incorporating it correctly into a heating system is not a trivial matter. In my experience, most problems that appear during the first heating season don't originate in the boiler itself, but in the hydraulic connection: an insufficiently vented system, too small a water volume, an incorrectly positioned expansion vessel, or a missing or oversized circulation pump. In this article, I cover all the key areas – from choosing a wiring diagram through specific pipe dimensions to setting the safety valve and topping up water in the system.

If you're looking for information on selecting boiler output or comparing models, I recommend checking out the articles How to Choose a Thermona THERM EL Electric Boiler and What Output of the Thermona THERM EL Electric Boiler Do I Need for My House? in the Knowledge Center. Here, we focus exclusively on hydraulics and connection.

What's Built Into the Boiler and What Needs to Be Added Externally

Before we get into the diagrams, it's essential to know what the Thermona THERM EL includes as standard, as this directly affects what you need to address during installation.

All models in the THERM EL series (i.e. THERM EL 8, THERM EL 15, THERM EL 23, THERM EL 30, and THERM EL 38) include as standard:

Circulation pump (factory-preset speed, usually 3-speed)
Internal expansion vessel with a volume of 8 liters and a pre-charge pressure of 1.0 bar
Safety valve set to 3.0 bar (opening pressure)
Air vent (automatic float type) on the outlet port
Pressure gauge visible from the front
Return and flow temperature sensors
Flow sensor (flow switch) – protection against dry running

What the boiler does not include and must be added during installation:

Filling and drain valve (valve with check valve)
Dirt filter (magnetic separator or mesh filter) before the return port
Thermostatic valve or mixing unit (in the case of underfloor heating)
Additional expansion vessel for large systems (water volume above approx. 80 liters)
Balancing valves on branches, in the case of a system with multiple circuits

Basic Single-Circuit Diagram – Direct Connection to a Radiator System

This is the most common and simplest configuration: the boiler is connected directly to the radiator system without a mixing valve. Suitable for most family houses with standard panel or column radiators and a temperature drop of 70/50 °C or 60/40 °C.

The following hydraulic rules apply to this scheme: the boiler's outlet port (DN 25, G 1" female thread) is connected directly to the main distribution pipe. The return pipe is routed back before the boiler's pump. A dirt filter is mandatory and is placed on the return pipe as close to the boiler as possible – it protects the pump and the flow sensor from magnetite and sludge. A filling valve (a combination of a filling cock and a check valve, or possibly an automatic top-up unit) is also installed on the return pipe.

Pipe and Flow Sizing – Specific Values

One of the most common mistakes when installing electric boilers is undersizing the supply pipe, or conversely oversizing it for small outputs. The boiler does have an internal pump, but its maximum delivery pressure is limited – usually 150–200 Pa of available pressure after deducting the boiler's own hydraulic losses.

Recommended minimum pipe diameters according to boiler output and a temperature drop of 20 K (i.e. a difference of 70/50 °C):

Model	Max. output (kW)	Min. flow rate (l/min)	Recommended pipe size	Boiler ports
THERM EL 8	8 kW	≥ 5.7 l/min	DN 20 (3/4")	G 1" (DN 25)
THERM EL 15	15 kW	≥ 10.7 l/min	DN 20–25	G 1" (DN 25)
THERM EL 23	23 kW	≥ 16.4 l/min	DN 25	G 1" (DN 25)
THERM EL 30	30 kW	≥ 21.5 l/min	DN 25–32	G 1" (DN 25)
THERM EL 38	38 kW	≥ 27.2 l/min	DN 32	G 1" (DN 25)

Note: the boiler ports are G 1" even on larger models. If you're designing DN 32 piping, you must reduce it right after the boiler port and make sure the expansion zone after the reduction isn't too short – otherwise flow and air issues may occur.

The minimum flow rate shown in the table is calculated using the formula: Q (l/min) = P (kW) × 860 / (ΔT × 60), where ΔT is the temperature drop in °C. At a drop of 20 K and an output of 8 kW, this gives approximately 5.7 l/min. The boiler has a built-in flow switch that shuts the boiler down if the flow drops below the minimum – meaning that if the system is hydraulically poorly balanced, the boiler may cycle on and off repeatedly.

Two-Circuit Diagram – Radiators and Underfloor Heating (Mixing Unit)

In new builds and renovations, a high-temperature radiator circuit is increasingly combined with a low-temperature underfloor heating circuit. This isn't a problem for the THERM EL boiler, but a thermostatic three-way mixing valve with its own circulation pump for the underfloor circuit must be included in the diagram.

With this scheme, hydraulic scatter needs to be addressed: the boiler pump (P1, integrated in the boiler) supplies both the radiator circuit and the primary side of the mixing valve. The secondary pump of the underfloor circuit (P2) is separate and controlled by the underfloor heating thermostat. The mixing valve regulates the water temperature for the floor typically to 30–45 °C according to outdoor compensation or a room thermostat.

Practical note from experience: With a THERM EL 23 installed in an older house with underfloor heating additionally installed on the ground floor, I repeatedly encountered the problem of insufficient flow through the radiator circuit after pump P2 switched on. The cause: both pumps worked against each other and the boiler's internal pump wasn't enough. The solution was installing a hydraulic separator (low-loss header) between the boiler and both circuits – I discuss this variant in the next section.

Hydraulic Separator – When and Why to Use One

A hydraulic separator (also called a low-loss header) is a short manifold connecting the primary (boiler) circuit and the secondary (consumer) circuits so that their flows don't affect each other. For the THERM EL, using a separator is mandatory whenever:

There's more than one secondary circuit with its own pump in the system
The total flow of the secondary circuits could exceed the flow of the boiler's internal pump
The system includes underfloor heating with a larger area (area > 80 m² of floor)
A domestic hot water (DHW) storage tank with a charging pump is connected
You're combining the THERM EL with a buffer tank

The hydraulic separator must be sized so that the flow velocity inside it does not exceed 0.1–0.15 m/s. This ensures that the thermal stratification inside the separator isn't lost and the secondary circuits get water at temperatures matching their needs. For the THERM EL 38, I recommend a separator with a diameter of at least DN 50 and a length of at least 400 mm.

Expansion Vessel – When the Internal One Isn't Enough and an External One Must Be Added

The boiler's integrated expansion vessel has a volume of 8 liters and is pre-set to a pressure of 1.0 bar. This capacity is sufficient for a system with a total water volume of up to approximately 70–80 liters. If the system is larger, you need to add an external membrane expansion vessel.

The procedure for calculating the required expansion vessel volume is as follows: multiply the water volume in the system (V_s) by the water expansion coefficient (approximately 0.0287 for a temperature range of 10–80 °C), and divide the result by the vessel's efficiency (which depends on the pre-charge pressure and maximum operating pressure). A simple formula for a rough estimate: V_EN (l) = V_s × 0.05 – meaning that for every 100 liters of water in the system, you need at least 5 liters of expansion capacity. The internal 8 l vessel therefore covers a system with a volume of approximately 160 liters (at a ratio of 0.05), which in practice corresponds to a typical radiator system of a house with approximately 100–150 m² of floor area.

For larger houses or systems with underfloor heating (where there's a large amount of water in the loops), the need for an external vessel arises very often. The external vessel is installed on the return pipe, as close to the boiler as possible, before the pump. Its pre-charge pressure must correspond to the static height of the heating system: 0.1 bar for each meter of height, plus a safety margin of 0.2 bar. In a 2-story house with a system height of 5 m, set the pre-charge pressure to 0.5 + 0.2 = 0.7 bar.

Safety Valve – Correct Installation and Venting

The built-in 3.0 bar safety valve is located on the boiler's outlet pipe. Its discharge pipe must be routed to a safe area – ideally above a sink or into a drain. This is a point that's sometimes overlooked during installation, and the installer simply leaves the discharge pipe hanging in the air above the boiler. When the valve opens (pressure > 3 bar), hot water discharges – without a drain, this can cause damage or injury.

The safety valve must not have any shut-off element before it. It also must not be connected directly to the sewer without an air gap – waste water could be sucked back into the system under negative pressure.

Connection to a Buffer Tank – Diagram with a Buffer

An electric boiler combined with photovoltaic panels or a time-of-use tariff (dual tariff) efficiently uses a thermal buffer tank. During a cheap tariff period or a surplus of solar energy, the buffer is charged to maximum, and the boiler can then remain off longer while heat is drawn from the tank. This configuration is becoming increasingly common in installations of THERM EL 30 and higher outputs.

The scheme is analogous to a hydraulic separator: the boiler is connected to the primary circuit, the buffer tank (300–1,000 liters) serves as a hydraulic separator, and the secondary circuits (radiators, underfloor, DHW) are connected downstream of the tank. The water temperature in the tank is monitored by two sensors (top and bottom), and the boiler's control responds to the tank's charge state.

Important practical note: If the buffer tank is larger (≥ 500 l), the water volume in the system rises significantly – the internal 8-liter expansion vessel is far from sufficient. With a 500-liter buffer and a typical distribution system, expect a total water volume of 600–800 liters, which requires an external expansion vessel of 30–50 liters.

Venting the System – Procedure and Common Mistakes

Electric boilers like the Thermona THERM EL are more sensitive to air than gas boilers with a larger heat exchanger. Air in the system causes:

Noisy flow (bubbling, gurgling in the pipes)
Flow sensor malfunction – the boiler thinks water isn't flowing and shuts down on a protective fault
Local overheating of the heating element cartridge, shortening its service life
Corrosion processes in the presence of oxygen dissolved in the water

Correct venting procedure after the initial fill:

Fill the system with cold water via the filling valve to a pressure of 1.5 bar
Open the vent valves on all radiators – start with the lowest one, finish with the highest
Start the boiler at low output (lowest stage) and let the pump run for 10–15 minutes
Vent all radiators again – air is released when heated
Check the system pressure and top up water if it has dropped
Repeat steps 3–5 once more, then close the filling valve
Switch the boiler to operating output and monitor the pressure gauge for 30 minutes

Automatic air vents on the boiler, and possibly on the underfloor heating manifold, should always be left open during the first few days of operation, and can then be sealed (if the vent type allows closing).

Water Quality and Corrosion Protection

In its documentation, Thermona recommends that the fill water meet the parameters for heating systems according to standard EN 14868. In practice, this means:

Total water hardness: max. 2.0 mmol/l (approx. 20 °dH) – harder water must be softened
pH: 7.5 – 9.0 at a temperature of 25 °C
Chloride content: max. 50 mg/l
Oxygen content: max. 0.1 mg/l (achievable through degassing or properly sealing the system)

In areas with hard water (Bratislava, parts of central Slovakia), a mains water hardness of 3–5 mmol/l is common. Scale deposits on heating cartridges (tubes) dramatically shorten their service life – a thin 1 mm layer of limescale increases the cartridge's surface temperature by dozens of degrees, and the cartridge cracks or burns out. The solution is a mixing station with a softener, or a one-time fill with distilled or DI water followed by adding a corrosion inhibitor.

Initial Operating Pressure and System Setup

The operating pressure of the cold system (20 °C) should be 1.0–1.5 bar. When heated to 70–80 °C, the pressure in the closed system rises by approximately 0.4–0.8 bar (depending on the water volume and the expansion vessel). The resulting operating pressure of the hot system should be 1.5–2.5 bar. If you set the cold system's pressure to 2.0 bar, it may exceed 3.0 bar when heated, and the safety valve will open – this puts unnecessary strain on the valve and water is lost.

Practical rule: fill the system to a pressure equal to the expansion vessel's pre-charge pressure + 0.2 bar. For a vessel with a pre-charge pressure of 1.0 bar, fill to 1.2 bar. A pressure drop below 0.8 bar in the cold system indicates water loss or air in the expansion vessel.

Typical Hydraulic Connection Mistakes – Practical Experience

Over the years of working with customers and installers, I've encountered several recurring problems:

Shut-off valves on the safety valve: The installer mounted a ball valve before the safety valve to make it easy to remove. When the closed valve was forgotten, the safety valve was blocked – when the pressure rose, the boiler suffered a failure with cracked heat exchanger flanges.
Absence of a dirt filter: An older steel distribution system without a magnetic separator supplied the boiler with a massive amount of magnetite (black sludge). The flow switch clogged every few weeks, and the boiler displayed a P5 fault. A magnetic filter resolved this definitively.
Pipe too thin at the outlet: During a house renovation, the installer kept the original 1/2" piping and just connected the new electric boiler. The THERM EL 23 couldn't reach the minimum flow rate, the heating element repeatedly overheated, and the boiler kept shutting down due to an outlet temperature > 85 °C.
Incorrect placement of the return thermometer: If the return temperature sensor is located too far from the boiler and there's a large uninsulated section of pipe in the route, the boiler shuts down prematurely (the return temperature is lower than actual, and the reading is distorted).
Missing pipe insulation: An electric boiler is more expensive to run than a gas one – every watt of loss in uninsulated piping is paid for from electrical energy. Insulating the distribution pipes is particularly important with an electric boiler room.

Special Requirements for Underfloor Heating

Underfloor heating requires a maximum water temperature of 55 °C (per most screed and floor covering manufacturers), while the typical operating temperature is 30–45 °C. The THERM EL boiler without a mixing unit cannot set the outlet temperature to 35 °C – the minimum adjustable temperature, depending on the model, is 30–45 °C, and regulation occurs via stepwise output switching.

If the THERM EL is intended exclusively for an underfloor circuit without radiators, you can set the maximum outlet temperature directly on the boiler to 50 °C and use a thermostat to control the boiler's start/stop. This is a functional solution for small underfloor heating areas (up to 80–100 m²). For larger areas or a combination with radiators, a three-way mixing valve with a secondary pump is mandatory.

Underfloor circuits must be hydraulically balanced using regulating valves on the manifold. The maximum length of a single circuit is 100–120 m (depending on pipe diameter); with longer circuits, the pressure drop is too high and the flow becomes uneven. PE-RT or PEX pipes with a diameter of 16×2 or 20×2 mm are commonly used, at a spacing of 10–20 cm.

Connecting an External Thermostat and Control Unit

The Thermona THERM EL has terminals for connecting a room thermostat (terminals marked TA in the boiler's wiring box, voltage-free contact) and terminals for outdoor compensation (OpenTherm or a weather-compensating controller). Using weather compensation (outlet temperature depends on outdoor temperature) is hydraulically advantageous: in mild outdoor temperatures, the boiler heats the water only to 45–50 °C, reducing losses and extending the service life of the whole system.

From a hydraulic point of view, it's important that the underfloor heating circuit thermostat is electrically linked to the secondary pump so that the pump runs for at least 5 minutes after the thermostat closes – a so-called pump overrun. This prevents heat spikes in the underfloor manifold when the flow stops quickly.

Summary of Hydraulic Requirements – Checklist

✓ Piping sized for minimum flow according to output (see table above)
✓ Magnetic or mesh filter before the boiler's return port
✓ Filling and drain valve on the return pipe
✓ Vent valve on every radiator and at the highest point of the distribution system
✓ Expansion vessel – external, if water volume > 80 l or the system includes buffering
✓ Safety valve with a free discharge pipe to a drain
✓ Hydraulic separator with multiple secondary circuits
✓ Thermostatic mixer for the underfloor circuit (max. 55 °C)
✓ Fill water quality (hardness, pH, chlorides) within the standard range
✓ Insulation of all distribution pipes outside heated spaces
✓ Balancing valves on branches with a pressure drop difference > 20%

Frequently Asked Questions (FAQ)

Can I connect the THERM EL directly to an underfloor circuit without a mixing valve?

Only if it's a small area (up to approximately 60–80 m²) and you set the boiler's maximum temperature to 50 °C. For larger areas or a sensitive floor covering (wood, vinyl), a thermostatic mixing valve is mandatory. Without a mixing valve, there's a risk of overheating the screed and cracks in the floor covering.

What pressure should I set when filling the system?

Fill the system with cold water to a pressure equal to the expansion vessel's pre-charge pressure + 0.2 bar. The integrated vessel has a pre-charge pressure of 1.0 bar, so the correct filling pressure is 1.2 bar. The pressure reading at an operating temperature of 70 °C should be in the range of 1.6–2.4 bar. If the cold system pressure is below 0.8 bar or above 2.0 bar, correction is needed.

Do I need a hydraulic separator if I only have radiators and a THERM EL boiler?

No, with a single radiator circuit without its own pump, a hydraulic separator isn't necessary. The boiler's integrated pump is sufficient for a simple radiator system in a typical family house. Only consider a separator once you add another circuit (underfloor, DHW, buffer) with its own pump.

I forgot the dirt filter – what could happen?

In a system with older steel piping, enough magnetite and sludge accumulates within a few months to clog the flow switch or the pump's circulation cartridge. The boiler will report a fault (typically E5, P5, or a similar code), or the flow may decrease enough that the boiler overheats and trips the safety thermostat. The filter should be added at the next suitable opportunity – ideally right away.

Can I connect the boiler to a system with a DHW tank?

Yes, but a hydraulic separator or a switching three-way valve with a timer is needed. The DHW tank has a charging pump that must be hydraulically separated from the heating circuit, otherwise the flow through the radiators drops when the tank is being charged. Alternatively, you can use a simpler system with a three-way valve that, when DHW priority is active, switches the entire flow to the tank and temporarily stops heating.

What is the maximum distance from the boiler to the underfloor heating manifold?

There's no fixed maximum, but every meter of uncompensated piping represents a heat loss. I recommend a maximum distance of 5–8 meters with well-insulated piping. If the boiler is in a distant utility room and the manifold is at the opposite end of the house, consider a hydraulic separator directly at the boiler and pressure piping with insulation class T2 (loss < 10 W/m).

Conclusion

Hydraulically connecting the Thermona THERM EL boiler isn't complicated if you follow a few basic rules: correct pipe sizing, a mandatory filter before the return port, sufficient expansion capacity, and – with multiple circuits – a hydraulic separator. Most of the faults I've seen over years of installations didn't originate in the boiler itself, but in its surroundings. The boiler is essentially just an intelligent heater with electronics; everything else depends on the quality of the hydraulic design.

If you're interested in learning more about the electrical side of the installation, we

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