How many collectors can the control station ZPS 6, ZPS 16 and ZPS 28 handle

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How many collectors can the control station ZPS 6, ZPS 16 and ZPS 28 handle – a comprehensive technical overview

One of the most frequently asked questions when designing a solar system is this seemingly simple one: how many collectors can I actually connect to a given control station? The answer is not as straightforward as it might seem at first glance – and that is precisely why this article was created. In practice, I have seen installations where the customer bought the largest station "just to be safe" and sold it five years later because the system never operated efficiently. I have also seen the opposite case – an undersized station that overheated in the first summer and the pump burned out.

The control stations ZPS 6, ZPS 16 and ZPS 28 (the number in the name indicates the pump performance in units related to its hydraulic characteristics) are designed for different size categories of solar installations – from a small family house up to a larger apartment building or commercial property. This article will guide you through all the relevant technical parameters, explain what in practice affects the maximum number of collectors, and help you make the right decision even before purchasing.

What actually determines the number of collectors – not just the pump

Before we look at the specific numbers for each model, it is important to understand one fundamental principle: the control station itself is not what limits the number of collectors. There are multiple limiting factors and each of them can be the "weakest link in the chain":

Hydraulic pump performance – must overcome the pressure losses of the entire circuit (pipes, fittings, collectors)
Medium flow rate – the solar circuit needs a certain minimum and maximum flow rate calculated per collector area
Thermal capacity of the storage tank – collectors are pointless if the tank cannot extract the heat
Controller in the station – must handle the system logic with the given number of sensors and circuits
Piping dimensioning – undersized piping is a throttling valve that devalues even a powerful station

In this article, we will focus mainly on the hydraulic aspect and real operational scenarios. For a more detailed look at the selection of regulation itself, we recommend the article How to choose regulation for a solar system: what to pay attention to, which you can also find in the Knowledge Centre.

Control station ZPS 6 – for small family houses and smaller installations

Control station ZPS 6 is dimensioned for small solar systems, typically for the needs of hot water (HW) in a family house with 2 to 5 people. The pump unit in this station is designed for a flow rate of approximately 6 to 10 liters per minute (360–600 l/h) at the typical pressure losses of a standard circuit.

This practically means that ZPS 6 is capable of hydraulically serving:

1 to 3 flat solar collectors with an area of 2 to 2.5 m² each (total active area up to approximately 6–7.5 m²)
1 to 2 vacuum tube collectors with a performance corresponding to an active area of up to approximately 6 m²

Why exactly this number? The recommended flow rate for solar systems ranges between 30 and 50 liters per hour for each m² of active absorption area. So, if we have three flat collectors with a total area of 7.5 m², we need a minimum flow rate of 225 l/h and ideally around 300–375 l/h. ZPS 6 can handle this easily – but with four identical collectors (10 m²), we would need 300–500 l/h, which is already on the edge. Problems arise when the pressure losses of the collectors themselves are added to the losses in the long supply pipe, in the return pipe, in the storage tank heat exchanger, and in all the fittings and valves. On a long circuit (e.g. collectors on a gable roof, storage tank in the basement – height difference of 8 meters, pipe 2× 15 meters), pressure losses can easily reach 60–100 kPa, which is already close to the limit of a smaller pump.

In practice, I have encountered a case of a family house near Trenčín, where the customer had three flat Viessmann collectors on a south-facing roof and a 200-liter storage tank in the boiler room. ZPS 6 was an ideal choice for this system – it worked for 7 years without problems, and the annual gas savings were consistently around 35%. When the customer later wanted to add a fourth collector and expand the tank to 300 liters, we recommended switching to ZPS 16.

Approximate limits of ZPS 6

Maximum recommended active collector area: 6–8 m²
Number of collectors (flat, 2.0–2.5 m²/pc): 2–3 pieces
Number of collectors (vacuum tube, performance equivalent 3–4 m²/pc): 1–2 pieces
Recommended storage tank size: 150–300 liters
Typical application: family house, 2–5 people, HW

Control Station ZPS 16 – Medium Category for Single-Family Homes and Recreational Buildings

Control Station ZPS 16 is the best-selling model in this range. The reason is simple – it covers the widest range of real installations. A more powerful pump, a more robust hydraulic assembly, and in some versions, a more advanced controller allow for significantly larger collector fields to be operated.

ZPS 16 is capable of handling flow rates typically between 10 and 25 liters per minute (600–1 500 l/h), with the pump's operating point depending on the actual pressure losses in the circuit. For a typical installation, this means:

4 to 8 flat collectors with a surface area of 2 to 2.5 m² each (total area approx. 8–20 m²)
3 to 6 vacuum tube collectors (depending on the specific model and its performance)
Installations for combined TÚV preparation and heating support

It is important to mention one practical aspect here: in systems supporting heating (solar heating), a significantly larger collector area is required compared to purely TÚV systems. As a rough estimate, it is recommended to have 0.8–1.5 m² of collector per 10 m² of heated area. For a house with 200 m², this could mean 16–30 m² of collector area, which is still within the reach of ZPS 16, but only with proper system dimensioning and a storage tank (minimum 500–1 000 liters).

From practice: in one project, I helped design a system for a cabin in operation from April to October. The customer had a south-facing roof and wanted to maximize solar gain for pool heating and TÚV. Six flat collectors in two rows of three, a 500-liter combined tank (TÚV + pool heat exchanger). ZPS 16 was ideal – sufficient flow for the parallel connection of two rows, and the controller handled the logic of two heat exchangers.

Series vs. Parallel Connection of Collectors – Impact on Station Selection

Here we need to take a short detour to a very practical topic: the way collectors are connected has a decisive impact on pressure losses, and thus on the choice of station. In practice, we encounter two basic schemes:

Parallel connection – the medium flows through each collector individually, pressure losses are the same as in one collector, but the flow must be multiplied by the number of collectors. For 6 collectors, you need 6× the flow.
Series connection – the medium flows through the collectors one after another, the flow is the same as in one collector, but pressure losses add up. For 4 series-connected collectors, pressure losses are 4× higher than in one.
Combined (series-parallel) – for example, two rows of three collectors each. This method is most common in practice for systems with multiple collectors.

Therefore, ZPS 16 is also suitable for a larger number of collectors in a parallel connection – the pump must provide a high flow, but pressure losses are not extreme. On the contrary, in a series connection of multiple collectors, a smaller flow is sufficient, but the pump must overcome a higher pressure. For ZPS 6, a series connection of 2–3 collectors is fine, while ZPS 16 can handle a combined connection of up to 6–8 collectors.

Control Station ZPS 28 – for Larger Buildings and Commercial Applications

Control Station ZPS 28 is the highest-performance member of this product range. The pump in this station is capable of providing a flow of up to 25–45 liters per minute (1 500–2 700 l/h), depending on the actual pressure losses in the circuit. This opens up possibilities for installations that are far beyond the scope of a typical single-family home:

10 to 20 flat collectors in a combined connection (total area 20–50 m²)
Apartment buildings with centralized TÚV preparation for 10–30 apartment units
Hotels, guesthouses, recreational centers with high TÚV consumption
Industrial preheating of process water
Larger systems combining TÚV, heating, and pool heating

From practice: a system for a guesthouse with 16 rooms, where we designed 14 flat collectors on a flat roof, a 2 000-liter tank with two heat exchangers, and a bivalent supplementary source. ZPS 28 was the only reasonable choice – nothing smaller would have handled the hydraulics of such an installation. The collectors were connected in four rows of three (combined series-parallel scheme) plus one spare collector for the second tank circuit. Total active area: 35 m².

With ZPS 28, it is also important to pay attention to the built-in controller in the station – advanced models allow control of multiple consumers (TÚV, heating, pool, tank 1 and tank 2), which is essential for larger systems. For comparison with simpler solutions, also see the article Euroster 813 Solar vs. control stations ZPS: which solution is better for your system in the Knowledge Center – there you will find a detailed comparison of the philosophies of both approaches.

Watch out for hydraulic stability with large fields

With a larger number of collectors, one specific technical problem arises – hydraulic unevenness in the flow distribution between individual branches of a parallel connection. If the system is not properly balanced (using regulating valves, appropriate pipe diameters), some collectors will receive more medium, others less. Result: collectors with low flow will overheat, leading to medium evaporation, pressure increase, and in extreme cases, damage to the collector or the station itself.

Solution: Tichelmann (so-called reverse distribution system) – the supply and return pipes are routed so that the hydraulic path to each collector is equally long. This ensures natural hydraulic balancing without the need for complicated adjustment of regulating valves. This is practically essential for ZPS with 28 or more collectors.

Summary comparison table of ZPS 6, ZPS 16 and ZPS 28

Parameter	ZPS 6	ZPS 16	ZPS 28
Max. flow (approx.)	up to 600 l/h	up to 1 500 l/h	up to 2 700 l/h
Max. active area	6–8 m²	10–20 m²	20–40+ m²
Number of collectors (flat)	2–3 pcs	4–8 pcs	10–20 pcs
Tank size	150–300 l	300–800 l	800–3 000+ l
Typical use	single-family house, DHW, 2–5 people	single-family house, DHW+heating, up to 8 people	apartment buildings, hotels, industry
Hydraulic connection	series / simple	parallel / combined	Tichelmann, multiple circuits

Practical procedure: How to calculate which station you need

Selecting a control station does not start with the product list, but with your actual needs. Here is a practical procedure I use when designing systems:

Step 1 – Determine daily DHW demand (or other needs). For a family of 4 people, calculate 40–50 liters of hot water per person and day at a temperature of 55 °C. For 4 people, this is 160–200 liters/day.

Step 2 – Determine the tank size. Recommended rule: 1.5–2 times the daily DHW demand. For 200 l/day, the tank should be 300–400 liters.

Step 3 – Determine the required collector area. Approximately 1–1.5 m² of active area per person for DHW in Central Europe. For 4 people, this is 4–6 m².

Step 4 – Check the hydraulic design of the proposed connection. Calculate (or estimate) the pressure losses of the entire circuit. For most standard installations with 2–4 collectors and DN 22 pipe over a distance of up to 15 m, the losses are in the range of 30–80 kPa.

Step 5 – Compare with the performance characteristics of the station. Based on the flow (liters/hour) and pressure losses (kPa), determine where your operating point is located in the pump's Q-H diagram. The operating point must lie on the pump curve, not outside of it.

A more detailed procedure with calculations and specific values can be found in the article How to set the differential controller for optimal solar system yield, where we discuss the topic of flow and differential parameter settings in more detail.

Typical scenarios from practice – where mistakes are made and how to avoid them

Scenario 1: Single-family house, 4 people, only DHW

This is the most classic case. A house on the outskirts of a city, a south-facing sloped roof, slope 35–40°. The customer wants to save on hot water heating. Requirement: approx. 200 liters of DHW per day, 300-liter storage tank. Recommended collector area: 4–6 m², i.e. 2–3 flat collectors.

Correct choice: ZPS 6. The pump handles the circuit without problems, the controller is sufficient for a simple differential control (collector – storage tank). The investment is appropriate for the savings. A mistake I often see: the customer buys a ZPS 16 "just to be safe", pays 150–200 EUR more, and the higher performance of the pump paradoxically reduces efficiency (the pump runs outside the optimal operating point, higher electricity consumption).

Scenario 2: Single-family house, DHW + heating support

House 180 m², floor heating, the customer wants to combine solar energy for DHW and preheating of heating during the transitional period. Requirement: 8–12 m² collector area, 500–800-liter storage tank (combined), or two storage tanks.

Correct choice: ZPS 16. The controller must handle the logic of two consumers (DHW storage tank and heating accumulator), which the basic ZPS 6 controller may not allow. Hydraulically, ZPS 16 is a necessity for 5–6 collectors in a parallel/combined connection.

Scenario 3: Apartment building, 12 apartments, central DHW

12 apartments, on average 2.5 people, DHW requirement approx. 750 liters per day. Storage tank 1,200–1,500 liters, collector area 18–25 m² (8–10 flat collectors in two rows of 4–5 units). Long pipe to the tank in the basement (20 m, height difference 12 m).

Correct choice: ZPS 28. Only this station ensures sufficient flow for such an area. The Tichelmann scheme is essential. The controller must handle the priority control of the DHW storage tank and possibly also a recirculation loop.

Scenario 4: Rooftop installation with long piping

This is a classic pitfall that customers often underestimate. Collectors on the roof, storage tank in the basement – height difference 8 meters. Total pipe length 2× 25 meters. Even for 3 collectors (ZPS 6 would normally be sufficient), the pressure losses can be so high that the pump's operating point shifts beyond the optimal flow. In this case, it is more reasonable to choose ZPS 16 even for a smaller number of collectors, but with demanding hydraulics.

This is an example of how the choice of a station is not only about the number of collectors, but about the overall hydraulic balance of the system. More on this topic can be found in the article Installation of solar system regulation: procedure and most common mistakes in the Knowledge Center.

Controller in the station – what it can and cannot do

ZPS control stations are not just hydraulic blocks – they also integrate a controller that regulates the pump operation based on temperature differences between the collector and the storage tank. This differential control is the basis of every solar system and its correct setting has a direct impact on energy gain.

The basic controller in ZPS 6 can:

Measure temperature on the collector and in the storage tank
Switch the pump on/off when the set temperature difference is reached (typically ΔT on = 5–8 °C, off = 2–3 °C)
Protect the storage tank from overheating (stop the pump when the maximum tank temperature is reached)
Protect the collector from freezing (possible reverse flow for circulation)

The more advanced controller in ZPS 16 and ZPS 28 can also:

Control multiple consumers (DHW storage tank, heating accumulator, swimming pool)
Prioritize between consumers
Record performance and energy (energy meter)
Communicate with a higher-level system (in some versions)
Regulate pump speed (EC motor) for flow optimization

If you have a simple system (1 collector → 1 DHW storage tank), there is no reason to pay for an advanced controller. However, if you plan to expand the system in the future with another storage tank or a swimming pool, it is reasonable to buy a station with a controller that allows it. An alternative is a separate controller – for example, Euroster 813 Solar, which can be installed independently of the hydraulic block and covers more advanced logic even in smaller systems.

What else you need to solve – a system view

The control station is the heart of the solar circuit, but it is not the whole system. In practice, customers sometimes buy a station and then find out that they are missing an expansion tank, a deaerator, solar fluid, heat transfer medium, or that they do not have the right storage tank with a double heat exchanger.

Standard equipment of a solar primary circuit:

Control station (pump, flow meter, manometer, controller)
Expansion tank – special for solar systems, heat-resistant up to 160 °C
Pressure relief valve – set to the maximum system pressure (usually 6 bar)
Deaerator – automatic, for solar medium
Solar fluid – glycol mixture for low temperatures, heat-resistant
Storage tank with heat exchanger – for the solar primary circuit, indirect heating via the heat exchanger is necessary
Pipe insulation – solar pipes exposed to the exterior (UV resistant)

The article Control station for a solar system: what is included and what you need to buy separately deals in detail with what is standard in a control station and what needs to be bought separately. I recommend reading it before making the final decision to avoid unpleasant surprises during installation.

Frequently asked questions (FAQ)

Can I connect 4 flat collectors to ZPS 6 if they are connected in series?

Connecting 4 flat collectors in series reduces the required flow, but significantly increases pressure losses. For most flat collectors, the pressure loss at the recommended flow of 30–50 l/h per collector is 15–30 kPa. Four serial collectors thus give a total loss of 60–120 kPa, which is at or beyond the hydraulic capacity of ZPS 6 – especially when you add the losses in the piping and the storage tank. The result is system undercooling and incomplete energy transfer. Therefore, I recommend switching to ZPS 16 for 4 collectors.

What happens if the station is not powerful enough for the number of collectors?

The pump runs at maximum speed, but the flow is still lower than optimal for the collector area. Consequence: the medium overheats in the collector, pressure in the system rises, the safety valve opens and discharges the expensive solar fluid. Collectors operate with low efficiency (performance of flat collectors drops more steeply at higher temperatures), overall energy gain is significantly lower than it could be. Long-term damage to the pump from overheating or cavitation is possible.

Is ZPS 28 suitable for a small system (2–3 collectors) – should I buy it "for the future"?

No, and this is an important point. The powerful pump in ZPS 28 operates outside its optimal operating point in a small system – it either runs too fast (flow is higher than suitable for the given area – the medium does not heat up enough) or the controller throttles it, which is also inefficient. In addition, the higher electrical consumption of the pump increases the energy consumption, which devalues the energy balance of a small system. Buy a station dimensioned for current needs. If you plan an expansion, it is a much better investment to have a professional prepare an overall project that includes the expansion in the original design.

Can I connect two different types of collectors (flat and vacuum) to one control station?

Technically yes, but in practice it brings complications. Flat and vacuum collectors have different pressure losses and temperature characteristics. If you connect them to one parallel circuit without hydraulic balancing, the flow will not be distributed evenly. Vacuum collectors with lower pressure losses will get too much flow, flat ones too little. In addition, vacuum collectors reach higher stagnation temperatures, which can be problematic in a shared circuit. If it is absolutely necessary, hydraulic balancing and thorough checking of the entire circuit's operating point are essential.

Does the length and diameter of the pipe affect the choice of station more than the number of collectors itself?

In some cases, yes. I have seen a system with two collectors, where the pipe (DN 15 instead of the recommended DN 22) over a 30-meter route caused such pressure losses that the ZPS 6 was insufficient – a ZPS 16 had to be installed. On the contrary, a system with five collectors in close proximity to the storage tank (5 meters of DN 22 pipe) worked well with a ZPS 16 with some reserve. It is therefore always necessary to assess the overall hydraulic balance, not just the rough number of collectors.

Is it necessary to change the station if I add one collector to an existing system?

It depends on where you currently are in terms of the capacity reserve of the existing station. If you are using a ZPS 6 with two collectors and want to add a third, you can usually do so without replacing the station – but it is necessary to verify the hydraulics. If you are using a ZPS 6 with three collectors and want to add a fourth, it is very likely a reason to upgrade to a ZPS 16. Before any system expansion, I recommend a hydraulic calculation or consultation with a specialist. Replacing the station is more expensive, but cheaper than repairing the system after several seasons of suboptimal operation.

Conclusion: Rules to simplify decision-making

After reading this article, you should have a clearer picture of how the size of the control station relates to the number of collectors. To conclude, we summarize a few practical rules that you can use directly:

ZPS 6 – for 2 to 3 flat collectors, total area up to 8 m², storage tank up to 300 liters, simple family house, DHW
ZPS 16 – for 4 to 8 flat collectors, total area up to 20 m², storage tank 300–800 liters, family house with heating support or a larger building
ZPS 28 – for 10 or more flat collectors, area 20–40+ m², storage tank over 800 liters, apartment building, hotel, commercial building
Always verify the hydraulics of the entire circuit, not just the number of collectors
Long pipe runs and small pipe diameters can push the selection up by one level
For combined systems (DHW + heating + pool), choose a station with advanced regulation

If you are still unsure about the selection, do not hesitate to look at other articles in the Knowledge Center – for example, Connecting solar regulation with an existing heating system or DHW storage tank or Common faults in solar regulation and how to eliminate them, where you will find more practical tips from real installations.

Do you have a question on this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.