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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 £303 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 just 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 detetected 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. 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 options if my ST pump has ceased pumping?

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. 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 (unlikely) event of your pump being irreparable then there are 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!

So what are the choices?

1/ Get a replacement from the new distributors -OVER £300 (OUCH!) Not so attractive? How about trading the old pump in against a new one: If a new pump is the only way forward then I currently have access to a small stock of brand new, unused Solartwin pumps at a very major discount, and also some reconditioned pumps, 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 £25.00 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.

As of April 2017 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 now unit.. Email me for details.

3/ The newer systems already have a Controller, so this can be used to operate a relay to switch on a Mains/DC adaptor, allowing a range of inexpensive pumps to be considered . Contact me for more information.

4/ My preferred option will always be as shown above in 2/ - However I have been testing a specialised centrifugal pump which runs well on the small Solartwin PV panel. This could be a cheaper option (around £50) for a straight replacement for the original pump. If , as described above, you subsequently find you have problems with air locks then I can take the pump back against another solution. If you are able to add an extra 5Watt PV panel (£20) it will work even better. Email me for details!

As yet I have not listed these new centrifugal pumps in my web shop as I would like to offer good personal support to make sure any buyer is well briefed on what needs to be done. 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


PV Panel




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