Circulation Pump for Heat Pump or Solar System

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Circulation pump for heat pump or solar system – complete technical guide

Heat pumps and solar thermal systems are today among the most widespread renewable energy technologies in residential homes. Both technologies share one key property: for proper operation, they necessarily require a reliable circulation of the heat transfer fluid. This is where the circulation pump comes into play – a component that is unobtrusive at first glance, but its correct selection, dimensioning and setting can determine whether the entire system operates efficiently and trouble-free for decades, or will repeatedly malfunction and unnecessarily increase operating costs.

In this article, we will look at the specifications that distinguish circulation pumps intended for heat pumps and solar systems from standard heating circulation pumps. We will explain why it is not possible to simply use any pump, which parameters need to be monitored, what a correct hydraulic scheme looks like, and what is most often the source of problems in practice. If you have not yet read our article How to choose a circulation pump for central heating, we recommend starting there – we build on it here and go deeper into special applications.

Why heat pumps and solar systems are a special case

A standard circulation pump for central heating operates with water temperatures of 50–75 °C, with clean water or water with corrosion inhibitors and in a relatively simple and predictable pressure-flow range. Heat pumps and solar systems bring a whole range of deviations from this "norm":

Heat transfer fluid with antifreeze mixture: In the primary circuit of a heat pump (ground-water, water-water) or in the solar circuit, a mixture of water and propylene glycol or ethylene glycol is almost always used. These mixtures have a different viscosity, density and chemical properties than pure water, which directly affects the performance of the pump and the requirements for material composition.
Extreme temperatures: A solar thermal collector can reach temperatures above 180 °C in the liquid during stagnation in summer. In winter, the temperature in the circuit can be below zero at start-up. The pump must withstand this temperature range without damage.
Low operating temperatures: The primary ground circuit of a heat pump typically operates at temperatures of −5 to +10 °C. This places requirements on the materials of seals and bearings.
High energy efficiency requirements: A heat pump is a technology built on energy savings. If the circulation pump consumes too much electricity, it worsens the COP (coefficient of performance) of the entire system. In a ground-water primary circuit, the pump can consume 30–80 W continuously – this can amount to 260–700 kWh per year.
Long operating hours: A solar pump can run 2,000–4,000 hours per year (during daylight in the season). A heat pump pump can operate 4,000–6,000 hours per year. This is several times more than a standard heating pump.

These factors together mean that the selection of a pump for these applications requires much more careful attention than in a standard heating system.

Circulation pump in a heat pump – distinguishing circuits

A ground-water (geothermal) heat pump typically operates with three hydraulic circuits, each of which can have its own pump with different requirements. It is important to distinguish these circuits from each other, because confusion or incorrect selection can have serious consequences.

Primary circuit (ground collector or well circuit)

The primary circuit transports heat from nature (from the ground, subsurface or water) to the evaporator of the heat pump. It operates with temperatures typically in the range of −5 °C to +15 °C depending on the season and the depth of the collector. The heat transfer fluid is always a mixture of water with propylene glycol, usually in a concentration of 25–35 % (protects up to about −15 °C). The flow in the primary circuit is dimensioned according to the performance of the heat pump – roughly 0.25–0.35 liters per minute for each kilowatt of thermal power of the pump.

We need a pump for the primary circuit that:

Can work with glycol mixtures (materials resistant to propylene glycols)
Has sufficient head to overcome the resistance of a long pipe system (ground collectors can have 200–600 m of total pipe length)
Is energy efficient, as it runs almost continuously during the heating season
Has at least an energy efficiency class A (ideally a pump with an EC motor and variable speed function)

Secondary circuit (heating circuit)

The secondary circuit is essentially a classic heating circuit – it transports heat from the condenser of the heat pump to the heating system (floor heating, radiators, fancoils). Here we can use a standard circulation pump for central heating, but with conditions – the water temperature is lower than in boiler heating (35–55 °C for floor heating, up to 65 °C for radiators), which increases the requirements for precise dimensioning due to lower temperature differences, but does not mean any special material requirements.

If the heat pump also works with domestic hot water (DHW tank), there may be another pump for the tank heating circuit in the system. More on the dimensioning of the secondary circuit can be found in the article What circulation pump power do I need for my house.

Direct built-in circuit of the heat pump

Most modern monoblock and split heat pumps air-water or ground-water have some pumps built in directly inside the unit. This greatly simplifies installation, but it is good to know that these built-in pumps usually have fixed or limited adjustable parameters and may not be sufficient in the case of very long supply pipes or complicated hydraulic schemes – in such cases, it is necessary to add an external pump or hydraulic module.

Pump for solar thermal system – special category

A solar thermal system is in many ways even more demanding for a pump than a heat pump. The reason is the extreme temperatures that occur during stagnation (collectors without heat removal can overheat above 180–200 °C) and also the large temperature range between winter and summer.

Specific requirements for solar circulation pump

The following technical requirements apply to the solar circuit, which must be respected when selecting a pump:

Temperature resistance: The pump must withstand short-term temperature peaks up to 130–140 °C (some solar pumps are certified up to 160 °C). Standard heating pumps are only rated for 110 °C.
Resistance to glycols: The solar circuit uses heat transfer media based on propylene glycol or special solar fluids. The sealing materials (EPDM, PTFE) and the pump body must be chemically compatible.
Resistance to stagnation: During stagnation, the fluid in the heat exchanger and pump may temporarily enter the vapor phase. The pump must be able to handle system drainage and re-filling after cooling without mechanical damage.
Low power consumption: A solar pump should have as low an electrical power consumption as possible, since its operating time is directly subtracted from the solar gain. Modern solar pump stations with EC motors have a power consumption of 5–25 W instead of 50–100 W for older asynchronous motors.
Speed regulation: The solar system controller (differential controller) usually directly controls the pump speed via PWM or an analog 0–10 V signal. The pump must have the corresponding input.

Solar pump station vs. standalone pump

In practice, solar pumps are most often part of so-called solar pump stations (groups) – compact devices that include not only the pump but also an expansion tank, safety valve, flow meter, shut-off valves, and thermometers. Such a station is more convenient for installation, but when selecting it, it must be verified whether the pump in the station is suitable for the system (collector performance, length and dimensions of the piping, type of heat transfer fluid).

A standalone circulation pump for a solar system is more suitable for larger installations or renovations, where other components (expansion tank, valve) are already built in.

Pump sizing – specific calculations and rules

Correct pump sizing for a heat pump or solar system requires knowledge of two basic parameters: required flow rate (in m³/h or l/min) and required head (in meters of water column). These two values determine the operating point of the pump, based on which the pump is selected from the characteristic (Q-H curves).

Flow rate calculation for the primary circuit of a heat pump

The flow rate in the primary circuit of a heat pump is calculated based on the power absorbed from the ground (not the total thermal output of the pump). The formula is:

Q = P_absorp / (c × ρ × ΔT)

where P_absorp is the power extracted from the ground collector (typically 60–70% of the total thermal output of the heat pump, the rest is the compressor's electrical power), c is the specific heat capacity of the glycol mixture (approx. 3.8 kJ/kg·K for a 30% propylene glycol mixture), ρ is the density of the mixture (approx. 1 040 kg/m³), and ΔT is the temperature difference between the inlet and outlet of the collector (typically 3–5 K).

Example: A heat pump with a total thermal output of 10 kW, COP = 4. Compressor electrical input = 2.5 kW. Power extracted from the ground = 7.5 kW. With ΔT = 4 K and a 30% glycol mixture: Q = 7 500 / (3 800 × 1 040 × 4) = 0.000473 m³/s = 1.7 m³/h ≈ 28 l/min. This is a relatively high flow rate, for which a pump with sufficient capacity is needed.

Head calculation

The head depends on the hydraulic resistance of the piping network. For a ground collector, we must consider:

Resistance of the supply and return pipes from the building to the collector (depends on diameter, length, and flow velocity)
Resistance of the collector pipes themselves (typically 200–600 m in a horizontal collector or 50–300 m in geothermal boreholes)
Resistance of the heat pump evaporator (provided by the manufacturer, typically 1–3 m)
Resistance of valves, filters, and dirt traps

Approximately: for a horizontal ground collector at a house with a 2×30 m layout and a 400 m collector at a diameter of PE 32 mm, the total head is in the range of 4–8 m. For geothermal boreholes with smaller probe diameters (PE 25 mm, total length 2×150 m), the resistance can reach 10–16 m, which requires a more powerful pump.

Correction for glycol mixture

This is a point that is most often overlooked in practice. Pump characteristics (Q-H curves) are always given for pure water at 20 °C. Glycol has a higher viscosity (at 0 °C, it is about 1.5–3× higher than water), which reduces the actual flow rate and increases the required head for the same point. For a 30% propylene glycol mixture at 0 °C, a correction factor of about 1.1–1.2 for the required head and 0.9–0.95 for the flow rate compared to the values from the water curve should be considered.

In other words: a pump that is just on the edge according to the water curve will likely not be sufficient with glycol. Always allow at least 15–20% reserve. More about selecting the correct parameters can be found in the article What circulation pump power do I need for my house?.

Energy efficiency – why it pays to invest in EEI class A

Modern circulation pumps are divided according to the energy efficiency index (EEI – Energy Efficiency Index). Since 2015, the sale of new circulation pumps for heating systems with an EEI higher than 0.23 has been banned in the EU. Since 2020, a stricter limit of 0.20 has applied. The best available pumps with electronically commutated (EC) motors achieve EEI < 0.10, which represents 5–10 times lower consumption compared to old asynchronous motors.

This has an even greater practical impact for heat pump and solar system applications. Let's calculate an example:

A primary pump for a heat pump (10 kW) operates 4 500 hours per year. An old pump with an asynchronous motor: 80 W × 4 500 h = 360 kWh/year. A modern EC pump: 20 W × 4 500 h = 90 kWh/year. The difference is 270 kWh/year at an electricity price of 0.20 €/kWh = 54 € annual savings just on the pump. Over 15 years of lifespan, this is more than 800 €. And in addition: every watt saved on the pump improves the overall COP of the system – less electricity is input, more heat is output.

This is one of the reasons why it is absolutely justified to invest in a premium pump with an EC motor instead of a cheap solution for heat pumps. More about setting the speed and efficient operation can be found in the article Setting the speed and power of a circulation pump.

Material requirements and compatibility

Special requirements are placed on the materials of the pump for the primary circuit of a heat pump and for the solar circuit:

Pump body and impeller

Common heating pumps have a body made of cast iron or bronze – both are suitable for glycol. A problem can be aluminum: propylene glycol mixtures with an unsuitable pH (below 7) can corrode aluminum components. In solar systems, where pumps with aluminum bodies are commonly used (common in solar groups), it is essential to use a heat transfer medium with the correct pH (7.5–8.5) and to regularly check its condition.

Seals and O-rings

The key material is the seal: for glycol mixtures and solar fluids, EPDM (ethylene-propylene-diene rubber) or PTFE (teflon) are suitable. Nitrile (NBR) rubbers are not suitable, as they swell and degrade in glycols. Always check the pump's technical documentation to verify compatibility with the media.

Magnetic coupling vs. mechanical seal

Wet-rotor pumps (the most common type for these applications) have a motor and rotor cooled and lubricated by the heat transfer fluid itself, without an external shaft seal. This is advantageous in terms of compatibility with glycols, as it eliminates the problem of shaft seal degradation. More on the differences between pump types can be found in the article Difference between wet-rotor and dry-rotor circulation pump.

Regulation and control of the pump in the system

Modern heat pumps and solar controllers require speed control of the circulation pump. There are these types of regulation:

Proportional pressure (Δp-prop)

The most common setting for the secondary (heating) circuit. The pump automatically increases or decreases speed to maintain a pressure differential proportional to the current flow. The result is quiet, efficient operation at partial load – a typical state for heat pumps.

Constant pressure (Δp-const)

Suitable for systems with large flow changes (radiators are opening and closing). Less suitable for solar circuits, where flow is intentionally regulated.

External control signal (0–10 V or PWM)

The solar controller directly controls the pump speed via an external signal. This allows optimization of flow according to current solar radiation intensity – slower flow at low sun, faster at high sun. Some algorithms even optimize flow for maximum solar gain, not just for maximum temperature.

Protective functions

Pumps for heat pumps should have built-in protection against rotor blockage (anti-seize function – automatic short run after long inactivity), dry-run protection, and an automatic air-venting sequence. These functions prolong the pump's lifespan and prevent failures after long summer shutdowns.

Installation tips from practice

Over the years of working with heat pump and solar system installations, we have gathered several practical insights that distinguish a correct installation from a problematic one:

Always mount the pump with a horizontal rotor axis, not vertically. Vertical mounting shortens bearing life and can cause noise.
Install a dirt trap (filter, magnetic separator) before the pump. The primary circuit of the heat pump can be filled with impurities from the ground or pipes during the first filling – without a filter, these will enter the pump and the evaporator.
Use only pumps specifically designed for solar applications in the solar circuit – not standard heating pumps of the "same size". The difference lies in the temperature resistance of materials and certification for solar fluids.
Thoroughly deaerate the primary circuit before starting. Air bubbles in glycol-based systems are much more stable than in pure water and harder to remove. Use automatic air vents and check for noise in the first few days of operation.
Set the correct glycol mixture concentration and record it in the documentation. Check it with a refractometer during each annual inspection. Degraded glycol corrodes metal parts and damages seals.
Install a check valve after the pump in the solar system – prevents night-time reverse circulation (heat from the storage tank is lost to the cold collector during the night).

A detailed installation procedure including typical mistakes can be found in the article Installation of a circulation pump step by step.

Typical faults and problems in practice

Based on service interventions in heat pumps and solar systems, the most common problems with circulation pumps are as follows:

Blocked rotor after summer shutdown: Pumps in solar systems or primary circuits of heat pumps that have been idle for several months can become blocked due to deposits on the rotor. Most modern pumps solve this with automatic anti-seize cycles, but older models require manual cleaning (unscrewing and cleaning with a service screw) before the season. Preventive solution: set the pump for short daily operation even outside the season.

Noisy operation (cavitation): If the pump hums or makes crackling sounds, the cause may be insufficient inlet pressure (NPSH – net positive suction head). In solar systems, the fluid can partially boil into vapor on a hot summer day at high temperatures. Solution: increase system pressure (add an expansion tank, increase pre-charge pressure), or reduce temperature (shade collectors, night cooling).

Low flow despite the pump running: A typical symptom of a clogged filter, closed valve, or improperly low pump setting. In heat pumps, this is indicated by the heat pump tripping on a "low flow" or "condenser overheating" fault.

Further typical problems are analyzed in the article Common circulation pump faults and their solutions.

Most frequently asked questions (FAQ)

Can I use a standard heating pump for the solar circuit?

In most cases, no. Standard heating pumps are dimensioned for a maximum temperature of 110 °C and tested with pure water. A solar circuit during stagnation can reach 130–180 °C and uses a special heat transfer fluid (glycol or synthetic oils). Using an unsuitable pump can cause seal damage, plastic components of the rotor cracking, and fluid leakage. Always use pumps explicitly certified for solar applications.

What is the difference between a primary heat pump pump and a standard heating pump?

The primary pump of a heat pump operates with a glycol mixture at low temperatures (−5 to +15 °C), must run thousands of hours per year, and must have the lowest possible electrical consumption due to its impact on the system's COP. A standard heating pump is optimized for higher temperatures and shorter operating hours. In practice, some high-quality heating pumps can be used for the primary circuit of a heat pump, but always verify material compatibility with glycol and adjust the performance according to the actual pressure curve for the glycol mixture.

How do I know if the pump in the primary circuit of a heat pump is too weak?

Signs include: the heat pump trips on a protective fault "low flow" or "low suction temperature," the temperature difference between the inlet and outlet of the primary circuit on the heat pump control display is large (more than 6–7 K under normal operation), and the system may emit cavitation noise. Solution: measure the actual flow using flow meters or based on temperature differences and compare with the required values from the heat pump documentation.

Why does the solar pump run, but the collector does not heat up?

This phenomenon can have multiple causes: (1) air in the system is blocking circulation (the pump is running, but the fluid is not moving – "the pump is running dry"), (2) the check valve is stuck in the "closed" position, (3) the pump is too weak to overcome the system resistance, (4) the differential controller does not start the pump at the correct temperature differential – check the ΔT activation setting (typically 5–8 K) and ΔT shutdown setting (typically 2–4 K). A systematic approach to problem solving can be found in the article Common circulation pump faults and their solutions.

How long does a circulation pump in the primary circuit of a heat pump last?

Modern wet-rotor pumps with EC motors can last 15–25 years with proper installation and regular maintenance (fluid checks, filters) even with a high number of operating hours. A critical factor is the quality of the heat transfer fluid – degraded glycol with low pH significantly shortens bearing and seal life. Annual pH and inhibitor checks of the glycol mixture are a low-cost preventive investment. More on maintenance can be found in the article Maintenance and service of a circulation pump.

Do I need to replace the pump when installing a heat pump instead of a boiler?

For the secondary (heating) circuit, it depends on the specific parameters: a heat pump operates at lower temperatures and has a higher flow rate for floor heating compared to a boiler system with radiators. The existing pump may be undersized (low flow) or oversized (unnecessary noise and consumption). We always recommend recalculating the hydraulics of the secondary circuit when changing the heat source. For the primary circuit (ground collector), the pump is always new, as the primary ground circuit does not exist in boiler heating.

Conclusion – the pump as the heart of the renewable energy system

The circulation pump in a heat pump or solar system is not just a standard auxiliary component – it is one of the key devices that determines the final efficiency, reliability, and lifespan of the entire investment. An error in selection or dimensioning will manifest as regular system failures, higher electrical consumption, or premature wear.

The correct procedure is always the same: first, accurately determine the required flow and head for the specific system, take into account corrections for glycol mix and operating temperatures, select a pump certified for the given application (solar, primary circuit of a heat pump, heating circuit) with energy efficiency class A and variable speed function, and ensure proper installation and regular maintenance.

If you are unsure about choosing the right pump for your specific system, take a look at the range of circulation pumps on atria.sk – in the category you will find pumps for all types of applications, including special solar and primary pumps for heat pumps, with precise technical parameters and expert advice.

Do you have a question about this topic?

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