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Solar thermal - DIY flush/refill using mains water

Started by solardiy, December 04, 2023, 05:20:40 PM

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Solar thermal for domestic hot water seems to be less popular nowadays (compared to solar PV), and finding someone to maintain them can be difficult. But there are good savings to be had if you can maintain it a low budget - possible to do yourself with DIY skills, as I found after our system ground to a halt. I'm posting here my experiences getting our system going again (hoping it may help others).

We had 11 years of reliable operation, providing hot water maybe 2/3 of the year. But following a summer heatwave during which it vented steam, it must have sucked air in causing an airlock that stopped the pump flow. It's a pretty standard Kingspan pump station setup, with Velux flat panels on the roof, 25L expansion vessel, 6bar PRV (pressure relief valve), de-aerator (no air bleed in the roof), and SC100 Controller. From installation it was running at a low pressure, near zero when cold (which may have contributed to it failing).

To get it up and running again, first thought of course was to find a solar installer. Of about a dozen in our region, some had swapped over to solar PV, some only serviced their own installations, some never returned calls, leaving just two willing to come out. One said £100 for the first hour just to check the system, with servicing/topup £200-£300 excluding fluid flush/replacement. Costly quotes which would probably wipe out much of the energy cost savings.
As an aside, an interesting analysis of how maintenance costs affect cost-effectiveness is here (based on 4p/kWh, the current cost ~7p/kWh would tend to increase savings):

So I next looked into the DIY route, starting with online videos. Most show using a powerful 'Solar Flushing Pump' (eg this one with a standard setup like ours: However none were available for hire near me (apart from one place where you have to create an account to get quote!). Some people have used drill pumps, but this video ( showed pressure testing/flushing from mains water via a hosepipe, so I decided to give that a try to get up & running.
Aside #2, while trawling for advice I also read that old 'burnt' glycol can become viscous and block the pipework - fortunately not the case (at least for us), our long neglected fluid was dark with black particles, but flowed normally.

First to test if it would work, by running a garden hosepipe into the upper/fill port on the side of the pump station, turning the isolation valve (between ports) horizontal, then connecting a drain pipe to the lower/drain port. Result was that mains pressure was fine to inject a decent flow rate of water (nb our mains water runs through a pressure reducing valve so is at about 2bar). This flushed out airlock and old fluid, after which the system ran again. A second test showed it was possible with just the solar pump to suck water out of one bucket and drain into another (taking care to avoid air getting in) - hence it would be possible to refill this way with new glycol.

So... a few weeks later I'd got together some solar fluid and better hosepipes to do the job properly. Other approaches available of course(!) but here are my detailed notes for doing a full flush & refill:

1) Parts: pipework, air pump
A hosepipe is needed from a mains cold water feed into the upper fill port of the pumping station. In my case I had a central heating refill loop nearby, but a washing machine cold water outlet would be ideal, otherwise a garden tap. Water regulations require a one-way check valve to prevent backflow (I did), though in reality backflow is unlikely. Finding fittings that withstand the mains pressure may be the trickiest bit, a washing machine hose might suit, with a jubilee clip over any push fit joints. The pipe from the drain port into a bucket has no pressure so any spare bit of pipe will do. Expansion vessels normally have a car-type schrader valve on top, so to pressurise it you also need a tyre pump (ideally electric) & pressure gauge.

2) Mains flush.
Connect the mains pressure pipe into the upper side/fill port, a drain pipe into the lower side/drain port, and turn the isolation valve (typically between ports) so the slot is horizontal (this swaps upward flow to be from the fill port instead of from the solar loop). Turn on mains pressure (checking pipe joints don't leak), then turn on the side port valves together. Water should flow into the fill port and out the drain port into a waste bucket, at a good rate that flushes out any air in the pipes on the roof. Keep going until the drain water runs clear with no bubbles, at which point you should be able to estimate the total fluid capacity from how much old/coloured fluid you've drained. Continue if you want a better flush. Then turn off first the drain port valve, pressure should rise in the system, turn off fill port valve when pressure is 1-2bar. Turn off mains tap and you can remove the pipes. Turn the isolation valve back to vertical, and you can test running the solar pump to circulate water round the circuit (on the SC100 controller there is a maintenance switch on the left side allowing you to manually operate the pump). Providing there's no risk of frosts, the system can now run normally using plain water, I did this for several weeks.

3) Glycol refill
Buy sufficient propylene glycol for the system fluid capacity (eg 10-15L), available widely from plumbing chains, eBay etc, either concentrate or premixed. Likely to cost £50-100. A second clean bucket is needed to be filled with glycol, and a different length of fill pipe to go from the bottom of this glycol bucket into the fill port (doesn't need to take mains pressure). Set the expansion vessel pressure to a lowish pressure with a tyre pump/gauge (eg 1bar, less than your mains pressure).

This step is basically to run the solar pump to suck glycol from the clean bucket into the system, replacing water which drains out into the other bucket until it changes to the glycol colour. However precautions are needed to avoid getting air into the system, as we don't have mains pressure to flush air out. To do this, raise the fill pipe above the fill port, open the fill port valve slightly so water comes out slowly until it has filled the full pipe length up to the end, hold your finger over the pipe end while lowering it into the filled glycol bucket, then take your finger off. Now you should have no trapped air in this pipe. (You may want to clip a weight to the pipe to keep it below the glycol level in the bucket). Turn the isolation valve horizontal for filling; both port valves should be closed.

When ready, start the solar pump manually via the controller maintenance switch (no flow yet as valves closed). Open both fill & drain port valves together, glycol should pump up into the loop and water drain out. Be ready to close both port valves together (then stop the pump) when the glycol level falls low, it's important the glycol level never falls below the fill pipe inlet (so no air is sucked in). You may need to pause several times to refill the glycol bucket. Also watch the drain fluid colour; once it eventually changes from water to glycol colour you're nearly done. Close first the drain port valve, let the pressure build as far as possible (I got 0.7bar) to maximise the amount of glycol in the system. Close fill port valve & stop pump again.

Before finishing with glycol, we ideally want the expansion vessel part filled (eg 1/5). This is so the expansion tank contains both a) mostly air, to compress and take in excess fluid when it expands in the hot summer (without system pressure reaching the PRV limit, eg 6bar), but b) some excess fluid to avoid any future fluid loss resulting in an empty expansion tank (with negative system pressure sucking in air). You should be able to judge roughly how full it is by loosening its mounting and wobbling it or feeling its weight. To get glycol into the expansion tank, release some of its air pressure, then start the solar pump and open the fill valve.

4) Pressurise
Finally we need to have the system at the desired cold pressure (eg 2bar). Check your panel specifications to see if they are rated for high pressures (eg 6bar).

If you couldn't get glycol into the expansion tank, you can reconnect the mains pressure fill pipe again. First remove air from the mains fill pipe: with both fill & inlet valves off, turn mains water on, loosen the top fitting at the inlet valve slightly so pressure pushes air out, re-tighten it once only water is leaking out. Now to pressurise, open the fill valve slightly so mains water fills the system into the expansion tank, raising the pressure. Stop once you can tell the expansion tank has a few litres of water in it. Note this dilutes your glycol concentration slightly (hence better to fill it in step 3, esp if you live in a frost prone area).

Turn the isolation valve back vertical for normal operation. Use a tyre pump to pressurise the expansion tank to the desired working pressure, eg 2bar. This is a trade off as higher pressures raise the fluid boiling point on hot sunny days, but we want to stay well below the PRV max pressure (eg 6bar). This air pressure can always be adjusted the next summer.

That's it - a rejuvenated solar system that will hopefully chug away again for years - just needing an occasional check the fluid flow & pressure are ok. 


Welcome to our forum and thanks for the write-up of your experience with a solar thermal installation! really useful content for our forum users!

We get a lot of enquiries from people looking for solar thermal maintenance and it's often hard to find companies who can carry out the works. As you stated, there has been a move away from solar thermal systems in recent years so our geographical coverage has dwindled. There's still a fair few on our books though.

All the best!