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If you have a Solartwin system then this page might be interesting, especially if it just stopped working! :-

(please note we have no connection to Solartwin or its associated Companies and provide these opinions without any legal duties or accountability)

My SolarTwin pump has stopped working and I have been quoted £312 for a new one - HELP!

Don't Panic! We now have spares and a repair service that means you no longer have to pay this. Read on....

Although we are nothing to do with the Company we are often asked for advice on fixing broken Solar Twin systems so the following information details the research findings, possible cause of problems and economic solutions relating to this type of layout.

What is a Solartwin system?

Solar Twin Ltd (ST) was a Solar Installer Company based in the UK which developed a simple and effective hot water solar panel system that was fitted to many British homes in the late 1990s. It appears that it went into liquidation in 2012.

The right to use the trading name was then bought out by another Company, whose press release informs us, do not accept any liability for warranty issues from the former organisation. This new company continues to market the new systems and will provide spare parts, but at a price that wipes out your energy savings!

If you have even very basic DIY skills and just been quoted £303 to supply just for the simple replacement pump then you may like to read about alternative, affordable solutions.

Technical Stuff:

A typical ST installation comprises a solar collector array of around 3.5 sq mtrs. The panel is a flat sheet type with silicone pipe in a serpentine layout in an upwards flow. The relative inefficiency of the collector (when compared with modern Evacuated Tube panels) is made up for by the larger surface area and the risk of damage through frost is avoided by the use of the stretchy pipework. Thus it can be connected in a "Direct Layout" to the water tank, keeping the system very simple.

Direct layout gif

Older systems even avoided using any form of Temperature Differential Controller, by connecting the pump to a PV panel. When the sun shines, the pump is powered, when it doesn't the pump stops. It wasn't ever perfect and there can be occasions that the pump would run when no heat was available, cooling the tank, but overall it worked OK.

Early Example of ST pump

Old Solartwin Pump circa 1999 in for repair and on test.

The critical part of this 'simple' design is matching the pump characteristics with the PV panel. And here is the challenge if wanting to avoid the very high cost of the installer's replacement pump. This is made even harder by the fact that ST chose to use a low power PV panel at just 5Watts. Very few pumps are available that will run on such a low current.

As the system uses the 'direct layout' it is constantly heating fresh, aerated water. So the tendency to form air bubbles in the panel can lead to air locks forming unless the pump can push hard enough to force it through the pipework. ST chose a diaphragm pump as these create a higher pressure which will overcome the airlocks. Finding a low wattage diaphragm pump that will handle the high temperatures has been difficult.

How can I check if my pump is working or not? Inspecting your pump for function:

Maybe you are finding the water isn't getting as hot as it used to? Can you hear the pump running?

Note the inlet and output ports on the pump, which are indicated by arrows on the square metal plate on the top of the pump. In this photo you can see the inlet is on the right. The pipe for this comes from the base of your hot water tank. The water then flows up to the panel from the outlet (LHS) and back into the top of your hot tank.

Inlet and Outlet Markings ............................................. house circuit

1/ If the sun is shining and you can hear the pump running, squeeze the outlet pipe closed and listen for a change in pump speed. The extra resistance you have created will slow a healthy pump and the difference in tone will be obvious. If no change is detected when either pipe is restricted then it probably has failed valves (which can be replaced, see Option 2 below) .

Please note that you may have the same symptoms when an airlock is stopping the flow, this is only likely to be the case after having removed and refitted a pump. Failed valves are typical in pumps over 6 years old.

2/ Flip the end cap off the crank case section to reveal the cam and piston assembly. This should be dry and without any visible corrosion. You will see the crank spinning, operating the piston assembly to the diaphragm. If the pump isn't running try turning the flywheel clockwise. Does it start now? If not, check the voltage across the pump leads.

3/ Electrical Test - Using a voltmeter set to 10Amps, connect either power wire through the voltmeter to the pump. A healthy pump will momentarily draw around 0.3A and then settle at circa 0.15Amp. Anything higher than about 0.22 Amp indicates a faulty motor. This exercise assumes you have bright sun shining directly on to the PV panel.

4/Check the voltage across the pump terminals while it is running. The voltage created by the unconnected PV will be around 18V, but as soon as a pump is connected this will drop to closer to 6V due to the current being drawn. 0 volts indicates a motor or a PV problem.

5/ It is unlikely to be a PV problem and this can be checked by disconnecting one pump wire and placing the DVM across the PV leads while set to 10A. A healthy PV will register around 0.25 Amps when shorted in this way in bright sunshine.

Cover removed

6/ If it is necessary to remove the pump:

  1. Disconnect at least one of the wires from the PV panel, noting that red is Positive and Black is Negative. NEVER allow these to be reversed, these pumps will be irreparably damaged by incorrect polarity. Mark the wires for later reconnection.
  2. Fold both pipes over twice and tape them securely to stop leakage, disconnect the pipes from the pump.
  3. Blow into the inlet port and then the outlet port. You should feel much greater resistance when blowing in to the outlet port as the valves will seal against the flow. If they are similar it is time for new valves.

Re-fitting the pump after checking

  1. Attach the inlet pipe to the pump and allow water to flow out of the outlet port in order to expel air. Leaving air in the system will prevent the circulation becoming established, these pumps are only good at pumping water and need to be primed first, in this way. These pumps are often located just above the level of the cold water tank so it may be necessary to lower it briefly at allow the water the enter the pump.
  2. Trying to avoid air entering the outlet pipe, slip it over the outlet port and secure.
  3. Reattach the power leads, carefully noting the polarity. Check if unsure!!
  4. Leave the pump to run for a few minutes and repeat the squeeze test to see if the water is circulating successfully.
  5. Air locks can form in the panel while the pipework was open, the pump pressure may be insufficient to overcome gravity and so will not clear a strong airlock automatically. Usually a healthy pump with a panel in direct sun will create enough power to push the air through. But if you find you have a problem then attaching a hose to the outlet pipe of the circuit and GENTLY pushing about 2 gallons of mains pressure water through should blow the air out and into the hot tank, from where it will vent automatically. Replace the pipe onto the outlet port and retest.
    A cut-down nozzle from a mastic dispenser makes a useful adaptor for connecting a garden hose to your pipework. Hose can be softened in hot water to assist assembly of the adaptor. Make sure you have all the air out of the hose before attaching it to your pipework, otherwise you will be adding more air .

Nozzle adaptor



So what are the choices if my Pump has stopped working?

1/ Get a replacement from the new distributors -OVER £300 (OUCH!) Not so attractive? How about trading the old pump in against a replacement. If another pump is the only way forward then I currently have access to a small stock of reconditioned Solartwin pumps at a very reasonable price, so you can be back up and running tomorrow. Email me for details.

2/ Cheapest and simplest! Fix the the old pump!

If you can hear that the motor is still running then I can usually offer a same-day repair service to diagnose and replace the worn/faulty parts and get you up and running again for £27.00 including postage back to you in the UK. This is 'no-fix no fee' so definitely worth checking out before resorting to spending £££ on a new one. You pay nothing until the working pump is back with you and you are fully satisfied.

3/ Until recently a broken motor meant a scrap pump. I have a small stock of original old motors, or I am now able to offer a non-OEM replacement motor if the old one has failed. Previously this had been impossible and meant a complete new unit..This solution will cost in the region of £60 to £80 including postage Email me for details.

4/ Less Favoured Options - We have tested many different pumps from a range of top manufacturers and have not yet found a straightforward, 'perfect' direct substitute to replace the pumps supplied with your system. Now that we are able to replace motors if necessary these other ideas are rarely better than fixing the original pumps.

For example, many owners have reported good results using a centrifugal style pump, whilst others have found a tendency for this style of pump to be prone to air-locks forming overnight. This seems to depend on your particular installation and how aerated your mains water supply is.

In the (very unlikely) event of your pump being completely irreparable then there are still alternatives to the huge cost of a new one. The limitation has been the low power available from the tiny 5 watt PV panels supplied with the ST systems. However, I can supply a power supply and a circuit that monitors that PV power and thereby connects a mains power adaptor to run a higher wattage pump at a speed to match the level of sunlight. This solution is around £80 for all the parts and is a simple task to fit.

The actual electricity saved by SolarTwin using the PV amounts to around £2.50 per annum so there is no need to worry about the additional cost of powering from a mains adaptor!

I like to offer good personal support to make sure any buyer is clearly briefed on their options and what needs to be done. So if you are interested in learning more about your choices then please email me... sales@solarproject.co.uk

I am always on the look out for broken ST pumps to use for spares, so I may be able to offer you something for your old pump against the replacement, if it cannot be fixed .

I hope this has been helpful?

I'll be adding more to this section from time to time and will be pleased for any learning you can contribute to this subject


SolarTwin Controllers

ST Controller

Newer SolarTwin systems included a Differential Controller, powered by the PV panel. Its function is to ensure the pump only runs when there is heat available in the panel. This controller delivers improved efficiency versus the early system whereby the pump runs whenever there is enough light hitting the PV, irrespective of heat being available.

In this photo the arrows pointing down at the panel icon at top left of display indicate the pump has been switched on to capture heat. The temperature displayed is at the tank top, indicating the stored water temperature.

We have seen several of these controllers breaking down after a few years, usually due to the failure of two key components which prevents the controller storing sufficient power to run itself. If you have a faulty controller then contact us to explore whether it may be repaired.

This Loss of Display fault is due to a couple of the components on the circuit board breaking down. Contact us if you'd like us to fix this, it will require removing the front cover and sending it to us. Meanwhile the pump can be connected directly to the PV panel for the few days it will be away and the pump will run normally during that time, that is how the original systems were installed before the controllers were added. This Controller fix is inexpensive at £23 inc UK postage.

The panel temperature probes seem to suffer from inaccuracy in some installations. These are a PT1000 type sensor on a very long length of co-ax cable so I wonder if fitting a small capacitor (around 0.1uF) across the two wires as they enter the controller casing would help avoid erratic reads. I'd be pleased to hear from anybody who could try this fix.


Other common faults include a failing panel temperature probe. Spares are available on request but they are fitted into the rear of the panel so replacement involves climbing on the roof to lift the panel in order to gain access to the cable entry point.

New controllers are hard to find, though I do currently have one in stock if yours cannot be fixed.


Setup mode

The default display on these units shows only the temperature of the hot water tank, measured at the top. There is an alternative display mode which is useful for setting a system up, whereby all three probe temperatures are shown on the same screen. Accessing this is supposed to be by opening the controller case and pressing UP DOWN and SET buttons together for 30 seconds.

We found this did not work on our sample, so try the following sequence to change the display:

1/ Hold down SET and then press RESET briefly. The display shows "EEP" with "000" beneath it.

2/ Now press SET once more.

The three temps will show, the upper one is the panel temp, middle is the tank top temp and the third is the tank base measurement. On the unit I tested this also switched the pump on, irrespective of temperature delta, so is only useful for de-bugging sensors, not for constant display use.

Return to the default display by pressing RESET.

I have learned some useful things about this controller - It doesn't seem to like being asked to switch when supply voltage is below 7.5V, although it continues to read and display Ok at this low voltage. A faulty pump with high current draw might cause the input voltage to drop so try disconnecting one pump wire to see if that corrects a controller error. The controller passes whatever the input V is on to the pump, hence the variable speed.

Similarly, the two supercapacitors often fail, try unplugging them completely in case they are pulling the voltage down with a short circuit. These are replaceable if failed.

The controller compares the panel temp with the tank top temp to decide when to run the pump (not the tank base as with every other type of controller!) There is a time delay of 30 seconds or more before the unit switches the pump after detecting the heat delta.

The panel probe is a type PT1000. At room temperature it will have a resistance of circa 1068 Ohms. The two tank sensors are NTC10K and have a room temp resistance of around 13k Ohms. If you are getting implausible readings then checking that the resistances across the disconnected wires of the sensor is a good idea. The resistances at different temps can be found by googling for a sensor chart for that type of sensor.

On the display : the inward pointing arrows are shown when the controller has passed power to the pump. If the tank top exceeds 60 degC and the panel is less than that temp then the pump is switched on to export heat and cool the tank. Outward arrows show when in this state.

Possibly Useful Solartwin reference documents

Original Installation Instructions to installers

Solartwin User Guide


Noisy Pumps!

These pumps have a rotary piston action so are inherently quite noisy and subject to vibration. If mounted incorrectly the vibration can be transmitted through the fabric of the house and create an unacceptable background sound, especially in full sun conditions.

If this over-revving in bright sun is a new problem it may indicate that the valves are failing as this reduces the resistance to revolving as less water will be being pumped, in which case you may also be able to see the panel temperatures rising to higher levels than previously due to the decreased transfer of a slow flow rate. If so then it might be time to get the pump checked out, which we are happy to assist you with and you will see notes on that on this page.

The pumps are designed to increase speed with the strength of the sun hitting the PV panel, with the logic that therefore the water panel should also be creating more heat which needs transferring faster. This is good, but the pumps can get a bit over-enthusiastic in direct sun, with a noticable increase in sound level.

Mounting method


......Recommended Pump Mounting Method to avoid sound transmittance.

If you have checked the mounting method is not allowing vibration to conduct to the fabric and are still bothered by the noise then I have found a method of capping the peak voltage coming from the panel to place a cap on the motor's top speed, as this is when most people notice the noise.

In this maximum power state the flow rate is probably faster than is really required anyway.




Speed ControllerSpeed Control .......


These circuits are available for around £5 or less on eBay, search for "LM317 Speed Control" or try this link: They are very easy to fit, feed the pump power in to one side and out to the pump on the other side.

The peak voltage can then be set by ear on a sunny day. From fully clockwise, Turn the power down until you get an acceptable noise level.

Keep an eye on the water panel temperature to check that the new flow rate is still enough to shift the heat away to your tank without the panel boiling.



PV Panels and System Checks PV Panel

There isn't much to go wrong with a PV panel, but if you have no power output then check the wiring carefully before seeking a new panel, it is much more likely to be a wiring fault than a panel failure. I have some spare panels if you need one.

If you Suspect your PV panel may be faulty then there are a couple of tests you can carry out. You will need a digital voltmeter, there are plenty of suitable models on eBay for little over £5.

STEP 1. On a bright sunny day with the panel facing the sun - Disconnect one of the wires that lead from the PV panel to the controller so that no current can be being drawn by the pump etc.

STEP 2. Switch your voltmeter to the 10Amp setting. (You may need to locate one probe lead to the 10A socket on the meter for this function)

STEP 3. Place the meter leads on each PV wire to measure the PV output in a 'dead short' . I would expect a reading of 0.25Amp or more in strong sun. If you are getting much less 0.2Amps then consider looking for a wiring breakdown before concluding that the panel is broken.

If the problem you identified was the pump running too slowly or not pumping properly then reconnect the PV to the controller and remove one lead from the pump. Now bridge the gap from pump lead to the controller terminal with the meter probes, so that you are now measuring the current being drawn by the pump.

A healthy pump will draw between 0.2 and 0.3 Amps to give a good flow rate. Now try squeezing first the inlet pipe then the outlet pipe to restrict the flow. You should hear the pumps note change under the increased flow resistance and the current will increase on your meter. If the pump note does not change then the pump may need servicing, please see my service offer above.


go to HOMEPAGE or here to go to SOLAR FAQ page Q&A or here for INSTALLATION GUIDE








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