How to Connect Solar Collectors to Thermona Low-Temperature Boilers with a Tank

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Solar Collectors and Low-Temperature Boilers with Storage Tank from Thermona – Complete Connection Guide

Combining solar collectors with a low-temperature boiler with a built-in storage tank is one of the most practical energy solutions for family homes in Slovakia. Solar gain can cover 60–80% of the annual energy consumption for domestic hot water (DHW) heating during sunny months, with the boiler kicking in only when the sun isn't enough. It sounds simple, but in practice, connecting the two requires a precise understanding of tank hydraulics, control selection, and correct sizing of the whole system. In this article, we'll cover everything you need to know – from the physical principles through specific wiring diagrams to setting up the controls on Thermona boilers.

Why a Low-Temperature Boiler with Storage Tank – Benefits of Combining with Solar

Low-temperature boilers with a built-in storage tank, such as models from the Thermona THERM PRO 14 KX or Thermona THERM 28 LXZ.A 5 series, have the DHW tank directly integrated into the boiler body. This brings several advantages when integrating with a solar system:

One unit, fewer pipes: The tank and boiler are combined in a single device, simplifying the hydraulic connection to the solar circuit.
Stratification in the tank: These boilers' tanks are vertical, allowing temperature stratification – solar energy heats the lower part, while the boiler only tops up the upper zone when solar gain is insufficient.
Legionella protection: The boiler can automatically heat the tank to 60–65 °C at regular intervals, which is hygienically safe even when solar isn't enough for full heating.
Cost efficiency: Compared to a solution with an external tank and solar station, you save on materials and installation.

On the other hand, it's important to be realistic: not every boiler model with a storage tank is directly prepared for solar connection. The key question is whether the tank has a lower solar coil (a separate coil in the bottom part of the tank) or whether it is designed for solar connection at all. The TKX series models – such as Thermona THERM PRO 14 TKX and Thermona THERM PRO 14 TKX – are specifically designed for combined operation with a solar system and include a separate solar coil in the lower part of the tank. This is a fundamental difference from the regular KX models.

Types of Solar Systems – Which is Suitable for Connecting with a Thermona Boiler

Before connecting, you need to decide which type of solar system to use. For a family home with a Thermona boiler, three basic variants are relevant:

1. Flat-Plate Collector

The most common type in Slovakia. It works with a heat-transfer fluid (a mixture of water and propylene glycol) in a closed pressurized circuit. Outlet temperature of 50–80 °C on a sunny day. The advantage is robustness and lower cost, the disadvantage being greater losses at lower outdoor temperatures. For a family home with 4 people and a 150–200 liter tank, 2 collectors with an area of about 4–5 m² are sufficient.

2. Vacuum Tube Collector

More efficient at lower temperatures and diffuse radiation (cloudy days). Outlet temperature can exceed 130–160 °C in summer, placing higher demands on the relief valve and expansion vessel. When properly sized, it covers 10–20% more of the annual DHW needs than a flat-plate collector of the same area.

3. Drain-Back System

A special type where the fluid gravity-drains back into the tank when the pump stops (without glycol). Less common, but eliminates the risk of overheating and fluid degradation. Connecting to Thermona boilers is technically possible but requires a more precise design – recommended only for experienced installers.

For most projects, we recommend a flat-plate collector in a pressurized glycol circuit – it is proven, service is readily available, and the system price is acceptable.

Hydraulic Connection – How It Works Step by Step

The basic principle is simple: the solar system heats the lower part of the DHW tank via the solar heat exchanger. The boiler burner only kicks in when the tank temperature drops below a set minimum value (typically 45–55 °C). This process is controlled by a solar controller (differential thermostat), which controls the solar circuit pump based on the temperature difference between the collector and the lower part of the tank.

Conditions for Starting the Solar Pump

The solar controller (differential thermostat) monitors two temperatures – T1 at the collector and T2 in the lower part of the tank. The pump starts when T1 – T2 ≥ set delta (typically 6–10 °C) and stops when delta ≤ 2–4 °C. These values are adjustable, and it's important to set them correctly – too small a delta causes frequent short cycling of the pump, while too large a delta unnecessarily delays start-up and wastes solar gain.

Tank Overheating Protection

In summer, the solar system can overheat the tank above 90 °C, which is dangerous. This is why the solar controller includes an overheating protection function – when the maximum tank temperature is reached (e.g. 85 °C), the pump stops. Some controllers also have a night cooling function – the pump runs at night to dissipate heat from the collector into the ambient air.

Thermona Models Suitable for Solar Connection – What to Know Before Choosing

Not every Thermona boiler with a storage tank has a solar heat exchanger inside the tank. This is the most important thing to check before purchasing. Let's look at specific models:

THERM PRO 14 KX – Without Solar Heat Exchanger

The Thermona THERM PRO 14 KX is a compact boiler with an integrated DHW tank, but the tank has only one heat exchanger – the boiler coil. For solar connection, an external (plate) heat exchanger or a special hydraulic assembly would be needed. This increases installation complexity and cost. We therefore do not recommend this model as the primary choice when planning a solar system.

THERM PRO 14 TKX – Directly for Solar

The Thermona THERM PRO 14 TKX and its dual-boiler variant are designed for combination with a solar system. The tank includes a separate solar coil in the lower zone. Hydraulic connection of the solar circuit is therefore direct and simple – the solar station connects to the inlets and outlets of the solar heat exchanger. The tank typically has a volume of 130–160 liters, sufficient for a household of 3–5 people.

THERM 28 LXZ.A 5 – For Higher Output and Larger Tank

The Thermona THERM 28 LXZ.A 5 is intended for larger family homes with higher DHW consumption. The larger tank is suitable for 5–7 people. When sizing the solar system for this boiler, plan for 2–3 collectors (5–7 m² area) and a solar station with a flow capacity of at least 2–4 l/min.

Sizing the Solar System for Thermona Boilers

Correct sizing is the alpha and omega of the entire project. You will lose tens of percent of solar gain if the collector area and tank volume are not properly balanced.

Basic Sizing Rules

Tank volume vs. collector area: The rule of thumb is 50–80 liters of tank per 1 m² of collector area. For a 150 l tank, you therefore need 2–3 m² of collectors (typically 2 flat-plate collectors).
Number of people: Plan for 40–60 l/day of DHW at 45 °C per person. For a family of 4, that's 160–240 l/day, which, with a 150 l tank and solar gain, gives reasonable coverage in the summer months.
Collector tilt and orientation: The optimal tilt for year-round operation is 35–45°, oriented south. A deviation of 30° from south reduces yield by about 5–8%.
Shading: Even partial shading (tree, chimney) during critical hours (9:00–15:00) can reduce yield by 30–50%. Shading should be carefully avoided.

Practical Example: Family of 4, THERM PRO 14 TKX

A customer from central Slovakia, a family home from the 1990s, 4 people, central heating + DHW. Thermona THERM PRO 14 TKX boiler, 150 l tank. We install 2 flat-plate solar collectors on the roof with a total area of 4.2 m², tilt 42°, orientation SSW. Solar station with circulation pump, 18 l expansion vessel, safety valve. A Resol DeltaSol BS Plus controller monitors T1 (collector) and T2 (bottom of tank). Result: from April to September, solar covers 65–75% of DHW needs, with monthly gas savings of 8–12 m³. Payback on the investment (solar system ~€2,200 with installation) at a gas price of €0.08/kWh is about 7–9 years.

Solar Station – Components and Correct Selection

The solar station is a compact assembly that contains all the elements needed for circulation of the solar circuit. It is sold either as a ready-made assembly or made up of individual components.

What the Solar Station Must Include

Circulation pump: For most family homes, a pump with a flow rate of 1–4 l/min is sufficient. Note: solar pumps must be resistant to higher temperatures (up to 130 °C) and to propylene glycol.
Safety valve: Set to 6 bar for closed pressurized systems. Absolutely necessary – without it, there is a risk of dangerous overheating and pipe bursting.
Expansion vessel: Compensates for the volume of fluid during heating. For the solar circuit, special red solar expansion vessels resistant to high temperatures (up to 130 °C) are used. Vessel volume should be at least 10% of the total circuit volume.
Flow meter and ball valves: For setting and checking flow.
Air vent: An automatic air vent to remove air bubbles from the circuit.

Heat-Transfer Fluid – Glycol and Correct Concentration

The standard is a mixture of propylene glycol and distilled water. The concentration depends on the minimum winter temperature at the location. In Slovakia, we recommend 35–40% glycol, which protects down to –18 to –22 °C. A higher glycol concentration reduces heat transfer and increases viscosity – which is undesirable. Important: always use propylene glycol (not ethylene glycol, which is toxic). Glycol should be replaced every 5–7 years or when visible degradation occurs (darkening, sour smell).

Control and Boiler Setup for Solar Connection

Thermona boilers have their own control system that manages heating of the DHW tank. In combined operation with solar, it is essential to correctly set priorities and temperatures so that the boiler doesn't unnecessarily reheat what the solar system has already heated for free.

Setting the Minimum Tank Temperature

In the boiler, you set the minimum DHW tank temperature – typically 45–50 °C. The boiler turns on only when the tank temperature drops below this value. If solar keeps the tank at a higher temperature, the boiler won't start at all. This is the most important parameter for maximizing solar gain.

Legionella Protection Function

The boiler can periodically (e.g. once a week) heat the tank to 65 °C to eliminate Legionella pneumophila bacteria. This function is mandatory for health reasons and should be active even during solar operation. We recommend setting it to a time when DHW consumption is expected to be low (e.g. night hours).

Direct Boiler Start vs. Solar Priority

Modern solar controllers (e.g. Resol, Solare Datensysteme, Steca) have an output to suppress the boiler burner – as long as solar is heating the tank, the boiler is not allowed to fire. This signal is fed to the boiler's blocking input. On Thermona boilers, this can be implemented via an external demand terminal or directly through the boiler controller – the procedure depends on the specific model, and we recommend consulting the service documentation or an authorized Thermona service center.

Installation Procedure – Step by Step

Installation of a solar system may only be carried out by a person with appropriate qualifications (a heating and sanitary equipment installer). When working on a gas boiler, regulations on gas equipment safety also apply. The following procedure serves as an informational overview for the customer, not as instructions for non-professional work.

Roof and boiler room preparation: Check the load-bearing capacity of the roof structure (each collector weighs 25–35 kg), ensure pipe passage (min. 2× DN 18 Cu or stainless flexible pipe), install the mounting structure.
Collector mounting: Collectors are attached to the mounting structure, and supply and return pipes are connected. Route piping with a slight slope towards the venting point. All exterior joints must be thermally insulated (insulation resistant up to 150 °C).
Solar station installation in the boiler room: The solar station is placed as close as possible to the tank. Connection to the tank's solar heat exchanger (supply and return branches). Installation of venting and safety fittings.
Sensor placement: T1 sensor on the collector (immersion or contact type on the supply pipe), T2 sensor in the lower part of the tank (typically via an immersion sleeve – part of the TKX tank). Ensure proper thermal contact of the sensors and their insulation.
Filling and venting the circuit: The circuit is filled with the glycol mixture using a pump through filling valves. Flush until clean fluid without bubbles flows out. Set the operating pressure to 2–3 bar.
Controller setup: Set the switch-on delta (6 °C), switch-off delta (2–3 °C), maximum tank temperature (85 °C), overheating protection, and the legionella protection function.
Boiler setup: In the Thermona boiler menu, set the minimum tank temperature for boiler top-up heating (45–50 °C) and the anti-bacterial heating interval.
Pressure test and start-up: Perform a pressure test of the circuit (min. 1.5× operating pressure for 30 min). After a successful test, start up the system and monitor the controller's function during a sunny day.

Most Common Installation Mistakes and How to Avoid Them

In practice, we encounter several typical mistakes that reduce system efficiency or directly cause failures:

Undersized expansion vessel: During stagnation (collector overheated, pump stopped), the solar circuit reaches temperatures of 140–160 °C. The fluid volume increases significantly as a result. An undersized expansion vessel causes the safety valve to open and glycol to leak. Use only solar expansion vessels (red, certified up to 130–160 °C).
Standard expansion vessel instead of a solar one: Standard black/grey expansion vessels (for heating) do not have a membrane resistant to high temperatures – after a season they burst and the circuit loses pre-pressure.
Incorrectly placed T2 sensor: If the T2 sensor is not in the lower zone of the tank but, e.g., in the middle, the controller shuts off the pump before the tank is sufficiently charged. Always install T2 according to the tank manufacturer's instructions – on TKX models, the sensor position is clearly marked.
Uninsulated piping in the attic/exterior: Poor insulation causes heat losses and faster glycol degradation during stagnation. Minimum insulation thickness for solar piping is 25 mm (resistant up to 150 °C).
Incorrect glycol concentration: A too-diluted mixture will freeze, a too-strong one loses heat transfer. Measure with a refractometer and adjust before winter.
Neglecting solar priority in boiler control: Without correct setup, the boiler starts heating DHW before solar has had a chance to sufficiently charge the tank – wasting gas.

Operation, Monitoring and Servicing of the Solar System

A solar system is relatively low-maintenance, but several regular checks are essential for long service life and maintaining performance:

Annual Checks

Circuit pressure check: Operating pressure should be 2–3 bar. A pressure drop indicates a leak or a problem with the expansion vessel.
Visual inspection of collectors: Cracked glass, frame deformation, corrosion. Always check collectors after hail.
Glycol check: Measure the concentration and pH with a refractometer (pH < 6 = acidic glycol, needs replacement). Degraded glycol corrodes the circuit's metal parts.
Sensor and wiring check: Worn sensor cable insulation, loose contacts in the controller.

5-Year Checks

Glycol replacement (even if pH is fine, manufacturers recommend a maximum of 5–7 years).
Check and possible replacement of the expansion vessel membrane.
Check and lubrication of the circulation pump (if not maintenance-free).

You can find more about regular maintenance of heating equipment in the article Maintenance and servicing of a low-temperature boiler with storage tank: what to check and how often.

Economics and Payback – Real Numbers

The economic aspect is key for most customers. Let's look at specific figures:

A typical family system (2 flat-plate collectors of 4 m², 150 l tank, THERM PRO 14 TKX boiler) in the central regions of Slovakia produces about 1,500–2,000 kWh of thermal energy annually for DHW heating. At a gas price of €0.08/kWh (including fixed fees), the annual saving is €120–160. Investment in the solar system (collectors + station + installation, excluding the boiler) is typically €1,800–2,800 depending on collector quality and installation complexity. The simple payback period is therefore 12–18 years, which can drop to 7–10 years with grant support (e.g. subsidies from SIEA or Green Households).

Important aspect: a solar system doesn't just mean money savings, but also reduced load on the DHW tank – the boiler heats the tank less often, extending the lifespan of both the boiler and the tank. You can read more about tank lifespan in the article Rust, limescale and the tank anode – how to extend boiler lifespan.

Frequently Asked Questions (FAQ)

Can I connect solar collectors to the Thermona THERM PRO 14 KX model, which doesn't have a solar heat exchanger?

Technically yes, but in a significantly more complicated way. An external plate heat exchanger needs to be included in the circuit between the solar circuit and the DHW tank. This increases installation costs by €300–600 and reduces heat transfer efficiency. A much more practical choice is the Thermona THERM PRO 14 TKX model, which has a built-in solar heat exchanger directly in the tank.

What expansion vessel volume should I choose for the solar circuit?

Basic rule: expansion vessel volume = 1.5× to 2× the volume of fluid in the collectors + piping. For 2 flat-plate collectors, this is usually 18–25 liters of expansion vessel. For vacuum tube collectors, where the stagnation temperature is higher, plan for a larger volume – consult a designer. Always use expansion vessels certified for solar systems (min. 130 °C, red marking).

How can I tell that the solar system is working correctly together with the Thermona boiler?

Monitor the tank temperature before and after a sunny day. If the tank reaches 55–75 °C on a typical sunny day (April–September) without the boiler starting, the system is working correctly. If the boiler still tops up the heat even on sunny days, the problem is either in the controller's delta setting, poor sensor placement, or insufficient solar area. Solar controllers usually have temperature logging, where you can check the history and identify the problem.

Is it necessary to drain the solar circuit or shut it down in winter?

With a glycol circuit at the correct glycol concentration (min. 35%), winter shutdown or draining is not necessary. The system can operate year-round – it produces less energy in winter but without risk of freezing. The only thing to watch out for is regularly checking the glycol condition before winter (refractometer, pH check).

Can I use the solar system to support heating as well, not just DHW?

Yes, this is a so-called combi system (solar combi). The tank must be larger (300–500 l), the collector area larger (6–10 m²), and the control more complex. For low-temperature Thermona boilers with a built-in tank (volume 120–200 l), this variant is unsuitable – the tank is too small for a combi system. Consider a solution with an external buffer tank, to which the Thermona boiler would be assigned only as a supplementary source. You can find more about tank sizing in the article What DHW tank volume do I need for a family home or apartment.

What happens when the tank is fully heated and the sun is still shining – is there a risk of overheating?

Yes, overheating is a real risk in summer. When the maximum tank temperature is reached (e.g. 85 °C), the solar controller stops the pump – the system enters a stagnation state. The collector overheats to 120–160 °C, and the glycol in it may partially evaporate (creating a steam pocket). It is therefore important to: 1) properly size the expansion, 2) use quality glycol with a boiling point of at least 160 °C, 3) consider additional consumption (solar support for a pool, garden showers), 4) activate the night cooling function in the controller.

Conclusion – An Investment That Pays Off with Correct Implementation

Connecting solar collectors to a low-temperature Thermona boiler with a storage tank is a proven, practical, and – with correct installation – reliable solution. The key to success is choosing the right boiler model with a built-in solar heat exchanger (especially TKX series models such as Thermona THERM PRO 14 TKX), precise sizing of the solar area relative to the tank volume, a quality solar station with a correctly sized expansion vessel, and well-set-up control with coordination between the solar controller and the boiler.

Long-term maintenance should not be underestimated either – glycol needs regular checking, sensors need calibrating, pressure needs monitoring. A system given minimal attention will lose efficiency within 3–5 years. A system that receives an annual check will serve you without problems for 20 years or more. If you are planning an installation and want to be sure which boiler model is best suited to your situation, browse the full range in the low-temperature boilers with built-in tank category and compare the technical parameters of individual models.

Do you have a question on this topic?

Can't decide or dealing with a specific situation in your household? Write to us - we'll be happy to help.