How many collectors can the ZPS 6, ZPS 16, and ZPS 28 regulation handle
How many collectors can the ZPS 6, ZPS 16 and ZPS 28 regulation handle?
When a customer chooses a regulation for a solar system, one of the first practical questions is: "How many collectors can this station handle at all?" At first glance, it seems simple – after all, it's just a pump and some sensors. In reality, it is a much more complex matter, where the hydraulic resistance of the collector field, the performance of the integrated circulation pump, the pipe diameter, the height of the collector installation above the station, and a whole range of other factors play a role. In this article, we will go into detail so that you know exactly what to expect from the control station ZPS 6, ZPS 16 and ZPS 28 – and what you cannot expect from them.
Why it is not enough to just count collectors – basics of the solar circuit hydraulics
A standard flat collector or vacuum tube collector has a certain hydraulic resistance – this means how much pressure is needed to pass the medium (usually a non-freezing mixture of glycol and water) at the required flow rate. A standard value for one flat collector with dimensions around 2 m² is in the range of 20–60 mbar at a flow rate of 40–50 l/h. When connected in series (one after another), these resistances add up, and when connected in parallel branches, the hydraulic resistance decreases, but the total flow requirement increases.
The control station is essentially an integrated unit that contains a circulation pump, ball valves, check valves, filling and draining valves, a thermometer and a manometer, and sometimes a flow meter or flow regulator. The heart of the entire device is the pump – and it is precisely its hydraulic characteristic (Q-H curve, i.e., the dependence of flow on the head) that determines how many collectors the station is actually able to serve in practice.
From the graph, it is clearly visible why a parallel connection of branches is chosen for a larger number of collectors – the total hydraulic resistance of the field remains relatively stable, but the required flow changes. This has a direct impact on the choice of pump in the control station.
ZPS 6 – for which collector field is it intended?
Control station ZPS 6 is designed for small residential solar systems. The integrated pump with a performance class corresponding to groups of pumps with low to medium head (typically around 2–3 meters of water column at nominal flow) is able to serve a collector field with a maximum flow capacity of about 6 l/min (360 l/h).
In practice, this means the following: if we calculate with the recommended specific flow rate of 40–50 l/h per one m² of collector aperture area, ZPS 6 can jointly serve a collector field with a total active area of about 6–8 m². Converted to standard flat collectors with dimensions of 2.0 × 1.0 m (active area of about 1.9–2.0 m²), this comes to 3 to 4 collectors. When connecting 2 collectors in series and 2 branches in parallel – i.e., 4 collectors in total – ZPS 6 is still hydraulically suitable, provided that the total geodetic height (the height difference between the station and the highest point of the field) does not exceed about 5–7 meters.
When the height increases, for example, with collectors on a steep sloped roof with a height difference of 10 meters, the pump must overcome the hydrostatic pressure difference as well – and then the capacity of ZPS 6 narrows. In an extreme case, the limit may already be only 2–3 collectors, because the pump has to spend part of its energy overcoming the height and has less reserve for flow through the collectors. This is one of the things that customers most often forget when choosing a regulation.
ZPS 16 – medium category for larger family homes
Control station ZPS 16 is hydraulically dimensioned for a higher flow and greater head. The pump integrated in ZPS 16 achieves significantly higher performance than ZPS 6, with a nominal flow at the load of the collector field being approximately in the range of 8–16 l/min (480–960 l/h). This corresponds to a collector field with a total active area of about 10–20 m².
In concrete numbers: with flat collectors with an area of approx. 2 m²/pc, the ZPS 16 is capable of serving 5 to 8 collectors, assuming a standard installation geometry (height of the collector field above the station up to 10 m, pipe Cu 22 mm or stainless steel hose 20 mm). With vacuum tube collectors, which have a slightly different hydraulic profile (depending on the manufacturer and specific model), this number may be a bit lower or higher – it is always important to calculate with real values of hydraulic resistance from the collector's technical data sheet.
ZPS 16 most commonly appears in practice in family homes with a higher demand for hot water – for example, a family of 5–6 people, or a combination of hot water heating and pool preheating. In such cases, the collector field consists of 6 collectors connected in two parallel branches of 3 collectors each – a 3×2 connection. This configuration is hydraulically and thermally balanced, and the ZPS 16 handles it without any problems.
Important practical warning: if you plan to expand the field in the future (e.g., add two more collectors for floor heating), it is better to directly opt for ZPS 28. Replacing the station is several times more expensive and labor-intensive than purchasing a more powerful unit from the beginning. This is a classic mistake we see among customers who wanted to "save money" at the beginning.
ZPS 28 – for large systems and collective installations
Control station ZPS 28 represents the top of the range and is intended for large solar systems. Its integrated pump is capable of delivering a flow rate in the range of approx. 15 to 28 l/min (900–1680 l/h), which corresponds to a collector field with a total active area of about 22–40 m².
In practice, this means that ZPS 28 can handle 10 to 14 flat collectors with an area of 2 m²/pc, connected in a suitable combination of parallel branches. For example, 4 branches with 3 collectors each – i.e., 12 collectors in total. Such a solution is typically found in larger family homes with a pool, in guesthouses, small hotels, apartment buildings, or in industrial preheating of technical water.
With ZPS 28, proper hydraulic balanced design of the entire circuit is particularly important. If the individual parallel branches are not hydraulically balanced (using balancing valves or a Tichelmann series), the flow may be distributed unevenly – some branches may be over-dimensioned and others under-dimensioned. The regulation will start the pump correctly, but the system as a whole will operate inefficiently. Do not forget about hydraulic balancing in larger installations.
How to correctly determine the required pump performance – calculation procedure
If you want to be precise when choosing a control station, it is necessary to go through the following calculation procedure. It is not rocket science, but it does require at least basic patience and a technical data sheet from the collectors.
Step 1: Determine the total active area of the collector field. For example, 6 collectors × 2.1 m² = 12.6 m².
Step 2: Calculate the required flow rate. The recommended value for flat collectors is 40–50 l/(h·m²). At 12.6 m² × 45 l/h = 567 l/h, which is approx. 9.5 l/min. For vacuum collectors, a lower specific flow rate is sometimes recommended (25–35 l/(h·m²)), so at the same area only 315–441 l/h.
Step 3: Determine the total hydraulic resistance of the circuit. This includes the resistance of the collectors (as "pressure drop at nominal flow" in the technical data sheet), the resistance of the piping (approx. 10–15 mbar per meter, depending on the diameter and flow velocity), the resistance of fittings (bends, ball valves, check valves), and the geodetic height of the system (each meter of height = approx. 100 mbar in a closed circuit, but in a pressurized closed system, hydrostatic pressure is compensated – in reality, it only affects friction losses).
Step 4: Find a pump whose Q-H curve covers your operating point. The operating point is the intersection of the required flow rate and the total hydraulic resistance. The pump integrated in the ZPS station must have sufficient reserve – ideally at least 20 % above the calculated operating point.
This procedure is only approximate – for larger or non-standard installations, it is always recommended to consult with a designer or supplier.
Influence of collector type on station capacity
Not all collectors are equal from a hydraulic point of view. Flat collectors with a meander-type absorber surface have a different hydraulic profile than collectors with parallel piping (harp-type). Vacuum tube collectors (Heat Pipe or Direct Flow) have yet different characteristics.
Concrete examples from practice:
- Flat collectors – meander type: Hydraulic resistance of one collector at a flow rate of 50 l/h is typically 40–80 mbar. At 3 collectors in series = 120–240 mbar. ZPS 6 can still handle this, 4 in series (320 mbar) is on the edge.
- Flat collectors – harp type: Lower resistance, approx. 20–40 mbar per collector. You can connect one more collector in series with the same station.
- Vacuum Heat Pipe collectors: Resistance is low (only through the headers), but a lower minimum flow rate is required (sometimes only 20–30 l/(h·m²)). On one hand, this allows serving a larger area, but at low flow rates, overheating is a risk, so it is essential to follow the manufacturer's minimum flow rate.
- Vacuum Direct Flow collectors: Highest hydraulic resistance of all types, sometimes even 100 mbar per tube at the recommended flow rate. With this type, you must be especially careful when choosing a station, and ZPS 6 may be insufficient in capacity even with 2–3 collectors.
Controller versus control unit – what is considered in regulation
It is important to note that control units ZPS 6, ZPS 16 and ZPS 28 are integrated hydraulic units – they include a pump and valves, and regulation (electronics, sensors, switching logic) is either integrated or connected externally. The regulation itself – that is, the "brain" of the system, which decides when to turn the pump on and off – does not directly limit the capacity of the collector field. The pump primarily limits it.
For small systems with one to four collectors, a simpler differential thermostat can be a full-fledged alternative, for example Euroster 813 Solar, which you operate together with an external pump that you dimension yourself. The advantage is greater flexibility in choosing the pump, the disadvantage is more complex installation and the need for more components. The differences between these approaches are discussed in more detail in the article Euroster 813 Solar vs. control units ZPS: comparison of functions and use and also in the article How to choose regulation for a solar system: differential thermostat vs. control unit.
Practical scenarios and examples from real customer orders
Scenario A: Family house, 4 people, domestic hot water heating
Typical customer order – a family house with a consumption of about 200–250 liters of hot water per day. Collector field: 3 flat collectors (each 2.15 m², meander type). Connected in series. Height of collectors above the station: 6 m. Pipe Cu 18 mm, circuit length about 20 m. Hydraulic resistance of the entire circuit including pipes and valves: about 180–220 mbar. Required flow: about 3 × 2.15 × 45 = 290 l/h. For this case, ZPS 6 is the ideal choice with sufficient reserve.
Scenario B: Family house with a swimming pool, 5 people
A house with higher domestic hot water consumption and preheating of a 30 m³ swimming pool. Collector field: 7 flat collectors in two branches (2+3+2 not, but 3 and 4). Height above the station: 8 m. Pipe Cu 22 mm. Total flow: 7 × 2.0 × 45 = 630 l/h. Hydraulic resistance (parallel branches, maximum 4 collectors in one branch): about 200–270 mbar. In this case, ZPS 16 is the clear choice.
Scenario C: Guest house, 15 rooms, domestic hot water heating and heating
A larger commercial installation. Collector field: 12 vacuum Heat Pipe collectors (active area 2.3 m²/pc) in four branches of 3 each. Total area: 27.6 m². Height above the station: 12 m, stainless steel pipe DN25, circuit length 35 m. Flow: 27.6 × 30 l/(h·m²) = 828 l/h (for Heat Pipe lower specific flow). Hydraulic resistance: about 150–200 mbar for parallel configuration. Choice: ZPS 28 with reserve.
Scenario D: Cottage, 2 people, simple water heating
A cottage without year-round occupancy, summer operation. 2 flat collectors in series, 150 l storage tank. Height above the station: 4 m, short Cu 15 mm pipe. Total flow: 2 × 2.0 × 45 = 180 l/h. ZPS 6 is more than sufficient, in fact it would even be possible to consider Euroster 813 Solar with a cheaper external pump – it depends on the owner's preferences and installation costs.
Other factors that affect the capacity of the station
Viscosity of the antifreeze mixture
The solar circuit usually does not contain pure water, but a mixture of water with propylene glycol or ethylene glycol in a ratio of about 40–50 %. This mixture has significantly higher viscosity than water, which increases the hydraulic resistance of the entire circuit – especially at low temperatures (starting the system after a night in spring/autumn). In practice, this means that at 40 % glycol concentration, the hydraulic resistance can be 20–35 % higher than with pure water. The pump in the station must take this into account.
Pipe diameter and material
Copper 15 mm for circuits with flow up to about 200 l/h, copper 18 mm up to about 400 l/h, copper 22 mm up to about 700 l/h. Stainless steel hoses (Flexwell) have slightly higher resistance than copper pipes of the same diameter, but with proper dimensioning the difference is negligible. For long supply pipes (over 20 m in one direction), it is necessary to choose a larger diameter, otherwise pipe losses dominate the total pressure loss and the capacity of the station is effectively reduced.
Number and type of fittings
Each elbow, each T-piece, each ball valve adds resistance. A ball valve in fully open position typically has a resistance equivalent to 0.5–1 m of straight pipe, a check valve 2–4 m of pipe, a thermostatic valve can be up to 10–20 m of equivalent pipe. If the route runs through many fittings, the total resistance of the pipe route can be higher than the resistance of the collectors themselves – which is a situation where even a stronger station will not help if the piping is poorly designed.
Circuit configuration in combined systems
If the solar circuit is connected via a heat exchanger to a storage tank or boiler, the exchanger itself adds hydraulic resistance (plate exchanger typically 50–300 mbar, depending on size and flow). This is another factor that must be included in the overall hydraulic calculation. The topic of connecting solar system regulation with a boiler or DHW tank is discussed in detail in the article How to connect solar system regulation with a boiler or DHW tank.
Summary of approximate values in a table
| Parameter | ZPS 6 | ZPS 16 | ZPS 28 |
|---|---|---|---|
| Max. flow (approximate) | up to 360 l/h | up to 960 l/h | up to 1680 l/h |
| Collectors (flat, 2 m²) | 2 – 4 pcs | 5 – 8 pcs | 9 – 14 pcs |
| Active area of collector field | up to 6–8 m² | up to 10–20 m² | up to 22–40 m² |
| Typical use | Family house, 2–4 people, DHW | Larger house, pool, DHW+heating | Guest house, apartment building, industrial |
| Recommended pipe | Cu 15–18 mm | Cu 18–22 mm | Cu 22–28 mm / DN25 |
| Max. height of collectors above the station (approximate) | 5–7 m | 8–12 m | 12–18 m |
The values in the table are approximate and apply to flat meander collectors with standard hydraulic resistance, a non-freezing mixture with 40% glycol concentration, and a standard length of supply and return piping up to 25 m (one-way). Deviations from these conditions can reduce the capacity by up to 20–30%. For more information on selecting a solar controller, see the article How to choose a controller for a solar system: what to pay attention to.
Most common mistakes when dimensioning a control station
- Underestimating the hydraulic resistance of piping: The customer calculates only the resistance of the collectors and forgets about the piping, valves, and heat exchanger. As a result, the pump operates at the edge of its capabilities, and the system is less efficient overall.
- Ignoring the viscosity of glycol: The calculation is done for pure water, and the resulting capacity seems acceptable. In real operation with a glycol mixture, the pump is insufficient, and the solar system does not achieve the planned yield.
- Series instead of parallel connection when using a larger number of collectors: Five collectors in series have a hydraulic resistance that even a ZPS 16 cannot handle. The correct solution is two parallel branches with 2–3 collectors each.
- Selecting a ZPS 6 with plans for future expansion: The customer plans for 3 collectors but has space on the roof. In two years, they add another 2, reaching 5 collectors – the capacity of ZPS 6 is exhausted, and the station must be replaced.
- Not considering installation height: A collector on a steeply sloped roof 12 m above the station significantly loads the pump. With ZPS 6, this can already be a problem with 2 collectors if the circuit is long.
- Forgetting the resistance of the plate heat exchanger: The combination of "station + heat exchanger + collectors" can show a total resistance of 400–600 mbar, where ZPS 6 is absolutely insufficient, and a ZPS 16 or ZPS 28 is required.
Most frequently asked questions (FAQ)
Can I connect 5 collectors to a ZPS 6?
In practice, it is not recommended. ZPS 6 is hydraulically dimensioned for a maximum of 4 flat collectors with an area of about 2 m²/each under normal conditions. With 5 collectors, the pump would operate at or beyond its limits, leading to reduced flow, insufficient cooling of the collectors, and accelerated pump wear. If you plan to use 5 or more collectors, choose a ZPS 16.
Is the capacity of the station the same for vacuum tube collectors as for flat ones?
Not always. It depends on the specific model. Heat Pipe collectors usually have lower hydraulic resistance, so you can connect more units to one branch. Direct Flow vacuum collectors, on the other hand, have higher resistance, so a ZPS 6 may be insufficient with the same number of units where a ZPS 6 would be sufficient for flat collectors. Always check the technical data sheet of the collector and verify the pressure drop at the recommended flow rate.
How does the length of piping affect the selection of a control station?
Significantly. Each meter of Cu 18 mm piping at a flow rate of 400 l/h adds about 10–15 mbar of resistance. If the supply piping is 30 m and the return is 30 m, the total resistance of the piping route is about 600–900 mbar – more than the resistance of the collectors themselves. In such a case, it is necessary to either increase the pipe diameter (e.g., to Cu 22 mm) or choose a more powerful station. This is a very common situation in installations where the collectors are far from the technical room.
What happens if the pump in the station is undersized?
The flow through the collectors will be too low. The medium will overheat in the collectors above the designed temperature, evaporation and "knocking" in the circuit may occur. The regulation will incorrectly evaluate the temperatures, and in extreme cases, stagnation may occur – meaning a complete stop of circulation and extremely high temperatures in the collectors (even above 180–200 °C), which accelerates the degradation of the antifreeze mixture and can damage the collectors, piping, and the station itself.
Can I connect two control stations to one collector field?
Theoretically yes, but practically it is the wrong approach. Two pumps in one hydraulic circuit influence each other, leading to flow instability and hydraulic vibrations. The correct solution is always to dimension one suitable station, or in the case of very large systems, use an external pump with a frequency inverter that covers the entire system need.
How can I tell if the control station is insufficient for my system?
Typical symptoms: the system does not achieve the designed temperature gain, the collector overheats even with a running pump, the thermometer on the outlet pipe (after the collectors) shows excessively high temperatures, the medium in the circuit degrades quickly (browning, sour smell). Another signal is that the regulation stops the pump (the pump was running, but the temperature in the storage tank does not rise). In such a case, it is necessary to check the flow with a flow meter and compare it with the recommended flow for the collector field. For more information on faults and their symptoms, see the article Common faults in solar controllers: error messages, pump failures, and inaccurate sensors.
Conclusion: approximate numbers are not enough – calculate with real values
The short answer to the question "how many collectors can a ZPS 6, ZPS 16, and ZPS 28 handle?" is: ZPS 6 covers 2–4 flat collectors with a total area up to about 8 m², ZPS 16 covers 5–8 collectors with an area up to 20 m², and ZPS 28 can handle 9–14 collectors with an area up to 40 m². These numbers are valid only under normal installation conditions.
In practice, it depends on each individual factor – the type and hydraulic resistance of the collector, the length and diameter of the piping, the number of valves, the installation height, the glycol concentration, and the presence of a plate heat exchanger. If these factors deviate from the norm, you may find yourself in a situation where ZPS 6 is insufficient even for 2 collectors, or conversely, ZPS 16 easily handles 9 collectors with favorable hydraulics. Proper dimensioning always starts with a hydraulic calculation, not just approximate tables.
If you are unsure whether your planned setup is correct, consult a specialist before purchasing. Replacing a control station after installation is costly and unnecessary – and no customer should experience it just because they saved time on a proper calculation.
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