Stuck or low-flow solar pump – causes and solutions

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Stuck or weakly pumping solar pump – causes and solutions

The solar pump is the heart of every active solar system. When it stops working properly – whether it is completely stuck or pumping weakly and inefficiently – the entire system ceases to perform its function. The storage tank does not heat up, solar energy is not utilized, and in the worst case, other components of the entire circuit can be damaged. From practice, we know that pump problems are among the most common service cases for installed solar systems – and yet most of them have a relatively simple cause that can be resolved by a technically capable owner without calling in a service technician.

This article is primarily intended for installers, technically capable homeowners, and anyone who wants to understand what is happening "behind the façade" of their solar system. We will cover all common causes of problems – from the simplest (air in the system) to more elusive electronic faults or mechanical damage to the circulation. For each cause, we will provide a specific diagnostic and solution procedure.

How a solar pump works – the basis for understanding faults

Before we get into the faults, it is important to know how the solar pump is connected and what it is supposed to ensure. Unlike a boiler circulation pump, a solar pump operates in a specific environment: the medium is not pure water, but an antifreeze mixture (usually propylene glycol or ethylene glycol, typically in a 1:1 ratio with water). This mixture has a higher viscosity than water, which increases the hydraulic resistance of the circuit by itself.

The pump is usually located in a pump unit – a compact block that also contains ball valves, a check valve, pressure gauges, a safety valve, an expansion tank, and a controller. The controller tells the pump to start when the temperature in the collector is a defined difference higher than the temperature in the storage tank (typically Δt = 5–10 °C). When the difference drops below the lower limit (usually 2–3 °C), the pump stops.

Each element in this circuit can be a source of a problem that manifests as "weak pumping" or complete stoppage. Let's go through them systematically.

Cause #1: Air in the solar circuit (most common problem)

Air in the pipes is by far the most common reason why the solar pump is running (you can hear it humming or buzzing), but it is not pumping the medium or only pumping a minimal amount. In an air lock, the pump is essentially just spinning air instead of liquid – it is running in vain and not transferring heat.

Air gets into the system most often during the initial filling (inadequate air venting), after a repair and refilling, or gradually through sealing elements if the pressure in the system is too low. Another source can be leaks or a damaged membrane in the expansion tank.

How to recognize air in the circuit

The pump is running, but the storage tank is not heating up – the medium temperature at the collector outlet rises, but the tank remains cold
During full operation of the pump, you hear unusual gurgling, cracking, or thunder-like sounds in the pipes
The pressure gauge on the pump unit shows zero or significantly lower pressure than at start-up (normal: 1.5–2.5 bar in cold state)
After stopping the pump, the pressure fluctuates significantly

Procedure for venting the solar circuit

Venting the solar circuit is a bit more demanding than in a standard heating system, as you are working with an antifreeze mixture, not water. Procedure:

Check the pressure in the system – if it is below 1.0 bar, add medium through the filling valve of the pump unit
Turn on the pump at maximum speed (manual override on the controller or directly on the pump)
Gradually open the vent valves at the highest points of the route – at the collector, or on the pump unit
Let the pump run for at least 15–20 minutes at full capacity until only liquid without bubbles comes out of the vent valve
After venting, check and set the operating pressure (cold system: 1.5 bar, at maximum temperature it must not exceed the safety valve setting minus 10 %)

If air keeps returning to the system repeatedly, look for leaks or check the expansion tank – more on that in the section below.

Cause #2: Stuck (blocked) circulation pump

The pump can become mechanically stuck – the rotating part (rotor) stops responding to the electrical signal. This happens especially after a long system downtime (e.g., after the winter break or a vacation) or when impurities settle in the medium, blocking the pump housing.

A typical symptom is that the pump emits a buzzing sound (coils are receiving current), but the medium is not flowing – the flow meter on the unit shows zero. When touching the pump body, you feel vibration, but real circulation does not occur.

Manual Pump Unlocking

Most circulation pumps for solar systems have a bleed/unlock valve on the front side – usually with a slot for a flathead screwdriver, located in the center of the pump's front face. Procedure:

Turn off the pump power (circuit breaker, or possibly turn off the entire regulation)
Place a cloth or container under the pump – a little medium may drip out
Carefully unscrew the valve cap (not the whole valve – just the cap)
Insert a flathead screwdriver into the slot and gently turn the rotor shaft (usually a turn of 10–20° is enough)
You should feel the rotor loosening – it will turn more easily
Screw the cap back on, turn on the power and check the operation

If the rotor still does not start turning after this attempt, the cause is likely elsewhere (electronics, burned coil), and the pump needs to be replaced. In older systems after 8–12 years of operation, this is not unusual.

Cause No. 3: Low operating pressure or damaged expansion tank

The solar loop must be under pressure at all times – typically 1.5 to 2.5 bar in a cold state, depending on the system height. Sizing rule: minimum pressure = static height (in meters) × 0.1 bar + 0.5 bar safety margin. So for a collector 8 m above the pump: 8 × 0.1 + 0.5 = 1.3 bar minimum.

If the pressure drops too low, several problems occur at once: the medium starts to boil at a lower temperature, vapor pockets form in the system, and the pump effectively pumps vapor instead of liquid – effective cavitation, which leads to noisy operation, zero heat transfer, and rapid pump wear.

Checking and charging the expansion tank

The expansion tank has a rubber membrane that separates the air and liquid compartments. If the membrane breaks or the air from the gas chamber is lost, the tank will no longer perform its function – the pressure in the system will be too low in a cold state and will jump above the safety valve setting when heated, causing the safety valve to open and release the medium.

Check: Disconnect the expansion tank (close the shut-off valves), and check the gas pressure via the valve (like in a car). It should be 0.5 bar lower than the system operating pressure. If it is zero or you detect air pushing out the liquid, the membrane is likely damaged and the tank needs to be replaced.

Cause No. 4: Dirty filter or medium stagnation

Professional circulation units – for example, solar circulation unit IVAR.SOLAR K with regulation IVAR.SOLAR IMTDC – have a built-in mesh filter (sludge trap) to protect the pump and other components. This filter captures sediments, pieces of rust, metal shavings from the pipe, and degraded remnants of the antifreeze mixture.

After several years of operation, the filter can become clogged to such an extent that the flow drops to a fraction of the nominal value. The flow meter may show, for example, 2 l/min instead of the usual 8–12 l/min. The system "seems" to be working, but heat transfer is minimal and the storage tank does not heat up even under strong sunlight.

Filter cleaning

Close the ball valves before and after the circulation unit
Depressurize this section via the bleed valve
Unscrew the filter body (usually a plastic or brass nut with an O-ring)
Wash the screen under running water or with a soft brush
Check the condition of the O-ring – apply a thin layer of silicone grease before remounting
Reinstall, open the valves, check the tightness and operating pressure

We recommend performing this task annually during the system's service inspection. More on this can be found in the article Maintenance and service of a solar system – what to check every year.

Cause No. 5: Degradation of the antifreeze – increased viscosity and corrosion

The antifreeze mixture (glycol mixture) is not eternal. Propylene glycol or ethylene glycol degrades over time – due to the effect of high temperatures (stagnation during summer overheating of the collector can lead to temperatures of 180–200 °C), UV radiation, and catalytic effects of metals. Degraded mixture:

Has an acidic pH (drops below 7) and becomes corrosive – it attacks copper pipes, brass fittings, and aluminum surfaces of the collector
Increases its viscosity, increasing the hydraulic resistance of the entire loop and forcing the pump to work harder
Forms deposits (dark sludge), which clog the filter, the heat exchanger in the storage tank, and the pump itself
Loses its corrosion and freeze inhibitors, increasing the risk of freezing in winter months

The result is that the pump operates at an increased power consumption, but heat transfer is minimal. The pressure at the pump's inlet and outlet differs more than it should according to the manufacturer's nomograms – the pump is overloaded.

When to replace the filling

Manufacturers recommend replacing the glycol every 4–6 years. Faster replacement is necessary if the pH of the mixture drops below 7.5 (measured with litmus paper or a pH meter) or if the mixture has changed color to dark brown. Before a new filling, it is necessary to flush the system, otherwise the new mixture will degrade just as quickly.

Cause No. 6: Regulation failure – the pump does not start or runs continuously

The regulation (differential thermostat) controls when the pump starts and stops. A regulation failure is manifested in two ways:

The pump does not start at all – even though the collector is hot and the storage tank is cold. Causes: failure of the collector or storage tank temperature sensor, broken cable, failure of the control unit, or a burned fuse.

The pump runs continuously – even at night, in bad weather, without a temperature difference. This leads to the cooling of the storage tank by the collector (night radiation) and unnecessary electricity consumption.

Thermal sensor diagnostics

Thermal sensors in solar systems are the most vulnerable part of the regulation. A faulty sensor gives the regulation incorrect information and the whole system behaves irrationally. Proper sensors must fit tightly in the well, be protected from moisture, and have an undamaged cable.

For collectors with temperatures up to 180 °C, it is suitable to use temperature sensor for well – 180 °C with 2 m cable, which can withstand even extreme summer temperatures during collector stagnation. For measuring the temperature of the storage tank, where temperatures do not exceed 95 °C, sensor for well with PVC cable 4 m and maximum temperature 95 °C is sufficient – a longer cable allows convenient routing to the regulation without extension joints, which are a source of further problems.

Resistance measurement of the sensor

Most solar regulators use NTC sensors with a nominal resistance of 10 kΩ at 25 °C. Using an ohmmeter (Ω), you can quickly verify whether the sensor is working correctly:

At a temperature of 20–25 °C: the resistance should be 9 800 – 10 200 Ω
At a temperature of 50 °C: resistance approx. 3 600 Ω
At a temperature of 80 °C: resistance approx. 1 500 Ω
If the multimeter shows "OL" (open circuit) or 0 Ω (short circuit) – the sensor is damaged and needs to be replaced

Read more about diagnostics and sensor replacement in the article Sensors and solar system regulation – how they work and when to replace them.

Reason No. 7: Thermal stagnation and steam locks in summer

Summer stagnation is a specific problem of solar systems. When the storage tank is fully heated and the collectors are still producing heat, the regulator stops the pump. In the collector – for example in the flat frame solar collector IVAR.SOLAR 210 M5 – the absorber temperature can reach 180–210 °C in summer. The medium in the collector begins to boil and steam is pushed into the supply pipe.

When the regulator then restarts the pump again (e.g. in the morning or after cloud cover), the pump has to overcome a steam plug in the upper part of the pipe. If the system pressure is insufficient or the pump start ramp is too short, it often happens that the steam is not pushed through and the pump cavitates.

Solutions for stagnation problems

Increase the system operating pressure to the upper limit (2.5 bar) – higher pressure increases the boiling point and suppresses steam formation
Check whether the pipes are correctly sloped – the medium must be able to drain by gravity from the collector back to the tank when the pump is stopped
Ensure that the supply pipe has good thermal insulation – reduces the amount of steam formed in the pipe during stagnation. An ideal solution is stainless steel pipe in rubber insulation double with cable 2× pr. 16 mm / 10 m, which withstands stagnation temperatures and at the same time minimizes heat losses
If the system stagnates repeatedly every summer, consider passive tank cooling (night circulation) or shading of the collectors in critical months

Reason No. 8: Incorrect Δt setting on the regulator or incorrect pump speed stage

This is a cause that does not appear at first glance as a fault, but leads to a significant reduction in system efficiency. The regulator compares the temperature of the collector (T1) and the tank (T2). If the starting Δt is set too high (e.g. 15 °C instead of optimal 7–8 °C), the pump starts too late and a large part of the production is not used on a hot day.

On the other hand, too small a Δt (e.g. 3 °C) causes the pump to start and stop every few seconds – so-called chattering. With each pump start, cold medium from the tank is pumped into the collector, unnecessarily cooling the tank and shortening the pump's lifespan.

Optimal settings for most Slovak conditions:

Start difference Δt ON: 6–8 °C
Turn-off difference Δt OFF: 3–4 °C
Maximum tank temperature (overheat protection): 60–65 °C for a standard tank, max. 70 °C
Minimum collector temperature for starting: 10–15 °C (prevents night circulation)

The pump speed stage affects the flow. For most domestic solar systems (2–4 collectors, 200–400 l tank), the optimal flow is 40–60 liters per hour per m² of collector area. Too high a flow leads to a small temperature difference between the supply and return, which reduces the heat exchanger in the tank. Too low a flow, on the other hand, causes local overheating and reduces performance.

More detailed calculations for dimensioning the pump unit can be found in the article Dimensioning solar piping and pump unit – how to do it.

Reason No. 9: Mechanical damage to pipes or fittings

Solar piping is exposed to extreme temperature differences – in summer, the stagnation temperature is 180+ °C, in winter, possibly negative temperatures. This thermal fatigue can lead to gasket cracking in fittings, cracking of PVC sensor cables, or deformation of flexible piping.

If the pipe route is incorrectly laid (without thermal expansion compensators), mechanical stress can occur, which can bend the pipe or loosen the fittings. Result: pressure loss, air in the system, performance drop. For long runs (over 15 m), it is always advisable to use calculated compensating loops or expansion joints.

When building and inspecting the piping, make sure that each connection is accessible for inspection (not embedded in concrete without inspection openings) and that the insulation is intact and resistant to UV radiation.

Step by step: Systematic diagnostics of a solar pump in practice

In practice, the following procedure has proven effective when arriving at a customer with the complaint "solar is not working":

Visual inspection – Look at the regulator display: what is the current collector temperature (T1) and tank temperature (T2)? Is the pump running? If T1 > T2 + Δt ON and the pump does not start – the problem is in the regulator or the pump.
Pressure check – Look at the pressure gauge. Below 1.0 bar? Top up the medium and look for the cause of the leak.
Manual pump start – Activate the override on the regulator. Do you hear a buzz? Do you see movement on the flow meter? If not – mechanical blockage or electronic pump failure.
Deaeration – Even if the flow looks OK, perform deaeration – air can get trapped in hard-to-see places.
Filter check – Remove and inspect the filter mesh.
Measurements on sensors – Measure the resistance of sensors T1 and T2 with a multimeter. Compare with the NTC 10k characteristic.
Glycol check – Take a sample and measure pH and freezing point with a refractometer.
Conclusion and documentation – Record the measured flow (l/min), pressure (bar), temperatures T1/T2 and DP, glycol condition. Take a photo of the regulator status display.

Prevention: How to avoid pump jamming

As with any technical device, prevention is cheaper than repair. A few specific measures that significantly extend the lifespan of a solar pump and the entire system in practice:

Annual service inspection in spring months (before the main solar season) – check pressure, glycol, filter, electrodes and tightness
Replacement of antifreeze mixture every 5 years regardless of visual condition – degradation occurs internally and is not always visible
Maintaining correct operating pressure – check the pressure gauge at least once a month
Checking the condition of pipe insulation every 2–3 years – especially external parts where UV radiation damages rubber insulation
Ensuring collector shading in summer, if the storage tank does not have sufficient volume for full-day sunshine – prevents extreme stagnation
Installation of an automatic air vent valve at the highest point of the route – gradually releases small amounts of air without the need for manual intervention

A detailed checklist for regular maintenance can be found in the article Maintenance and service of a solar system – what to check every year.

Replacing the pump – when is it worth repairing and when to replace

A solar circulation pump has a lifespan of 15–20 years with proper operation. If the pump is older and the failure recurs, it is sometimes more economical to replace it rather than repeatedly repair it. A rough guideline:

Situation	Recommendation
Pump younger than 8 years, first failure	Repair / unblock, replace the seal
Pump 8–15 years old, repeated failure	Consider replacement, repair may be uneconomical
Pump older than 15 years	Replace the entire pump unit including the regulation
Burned electronic board (burnt smell)	Replace the pump, check the electrical installation
Mechanically cracked pump housing	Replacement, repair is not possible

When replacing the pump unit, also consider that a new pump may have different hydraulic parameters and its setting (speed level, Δt) will need to be checked and possibly adjusted. More about choosing the right system can be read in the article How to choose a solar water heating system for a family home.

Most frequently asked questions (FAQ)

Pump buzzes, but water in the tank does not heat up – what to do?

The most likely cause is air in the solar circuit – the pump is running, but it is not pumping liquid, only air. Procedure: check the operating pressure on the pressure gauge of the pump unit (it should be at least 1.5 bar in a cold state), turn the pump to full power and bleed the system at all the highest points of the route. If the flow meter starts to show normal values after bleeding (typically 6–12 l/min), the problem was air. If not, check for mechanical rotor blockage using the unblocking screw on the front of the pump.

How long does antifreeze last in a solar system?

Under normal conditions (one to two stagnations per season, correct operating pressure), propylene glycol lasts 4–6 years. If the system stagnates every summer repeatedly at extreme temperatures, the lifespan of the mixture is reduced to 2–3 years. We recommend measuring the pH of the mixture every 2 years (using a refractometer or litmus strips) – if it drops below 7.5, replace the mixture without waiting for the scheduled time. Degraded mixture is corrosive and damages the pump, heat exchanger and valves.

Regulation shows correct temperatures, but the pump does not start – where to look for the problem?

If the regulation displays T1 (collector) and T2 (tank) values and the difference T1 – T2 is greater than the set Δt for starting, and the pump still does not start, check these things: 1) Whether the pump is physically connected to the regulation (230 V output terminal on the regulation – measure the voltage when the condition is met). 2) Whether the fuse on the regulation or in the electrical installation is not burned out. 3) Whether the pump is set to automatic (Auto) mode and not to "0" (off). 4) Check the setting of the minimum collector temperature for starting – if it is set too high (e.g. 30 °C) and the collector is in the shade or in the morning, the pump will not start.

Can I add regular water instead of glycol to the solar system?

No, this is one of the worst interventions you can make. Water dilutes the glycol mixture, which shifts the freezing point upward – in winter, freezing and cracking of the collector, pipe or tank is at risk. In addition, water without corrosion inhibitors aggressively attacks metal parts of the system. When adding any medium, it is necessary to use the same type of glycol in the correct ratio with distilled water, or directly prepared mixture with the appropriate freezing point (for Slovakia at least –25 °C, ideally –28 to –35 °C).

How can I determine the flow rate of my solar system – without a flow meter?

If your pump unit does not have a built-in flow meter with a rotor, you can estimate the flow by measuring temperatures: measure the temperature of the medium at the inlet to the tank heat exchanger (return from the collector, colder) and at the outlet (supply to the collector, warmer). The temperature difference at optimal flow should be 7–12 °C under normal conditions. If the difference is less than 5 °C, the flow is likely too high. If it is more than 15 °C, the flow is likely too low.

Do you have a question about this topic?

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