Solar hot water — serviced, repaired, and worth keeping.
I look after solar thermal (solar hot water) systems across Luton, Watford, St Albans and the wider Bedfordshire, Hertfordshire and Buckinghamshire area — servicing them, fixing them when they stop performing, and getting the most out of them, including the cylinder when it’s time to move to a heat pump. Most domestic systems in this country went in between about 2005 and 2012, so a lot of them are now well past their first service life, and many of the firms that installed them have since closed. If yours has gone quiet, lost pressure, or you simply don’t know whether it’s still doing anything, I can tell you exactly where it stands and what’s worth doing about it.
One engineer for the pump, the controller, the glycol and the cylinder.
A solar hot water system is plumbing, electrics and controls working together, feeding a cylinder that has to be safe in its own right. I’m trained in solar thermal and hold the G3 qualification for unvented hot water cylinders, I’m F-Gas registered, a member of the CIPHE and WaterSafe-registered, and I hold 18th Edition and Level 3 electrical qualifications. So when I look at your system I can read the whole of it — the collectors, the pump station and controller, the glycol circuit, and the cylinder it all feeds — and I understand what I’m seeing.
That matters most when something has gone wrong and nobody can tell you why. Rather than condemning the system or guessing at parts, I test it, find the actual fault, and give you a straight answer on whether it’s worth repairing, recommissioning, or moving the cylinder onto a heat pump.
One thing I’m straight about up front: almost everything that needs attention on a solar system is indoors or in the loft — the pump, controller, expansion vessel, glycol circuit and cylinder — and that’s where the servicing and the great majority of faults sit. I work on all of it. What I won’t do is go onto a roof without safe, proper access in place, and I don’t use roof ladders. So if a fault is in the collector itself on the roof, that needs safe access — a tower or scaffold — arranged first. It’s a safety line I won’t cross, and I’d rather tell you than pretend otherwise.
What usually goes wrong on an older system.
Solar collectors themselves tend to last for decades. It’s the parts around them — the fluid, the pump, the controller and the expansion vessel — that wear out, and most “my solar hot water isn’t working” calls come down to one of these.
Degraded or lost antifreeze
The glycol that carries heat down from the roof breaks down over the years, and can be pushed out through a relief valve when the expansion vessel fails. Old or missing glycol means little or no heat reaching the cylinder.
Failed or noisy pump
The circulating pump moves heat from collector to cylinder. Worn bearings, seizing or an electrical fault stop it circulating, and the tank stays cold even in full sun.
Waterlogged or flat vessel
A failed expansion vessel lets the pressure swing too far as the system heats, which forces fluid out and causes repeated pressure loss — one of the most common underlying faults, and easy to miss.
Lost system pressure
A system that keeps dropping pressure usually points to glycol loss, a tired expansion vessel or a small leak. Topping it up without finding the cause just hides the problem until next time.
Controller and sensor faults
Faulty temperature sensors or a failing controller can stop the pump running when it should, or run it at the wrong times — including the classic night-time cycling caused by a leaking check valve.
Scaling and weeping joints
In a hard-water area like this one, scale builds up in the cylinder coil and fittings, while joints and seals can weep over time. Both quietly reduce performance and need putting right.
Why the glycol needs changing — and what’s happening inside it.
The fluid in a solar system isn’t plain water. It’s a glycol antifreeze, mixed with water and a package of corrosion inhibitors, and it does three jobs at once: it stops the fluid freezing and bursting a pipe or collector in winter, it carries the heat down from the roof to the cylinder, and the inhibitors keep the metalwork from corroding.
Solar collectors get extremely hot. When the cylinder is already full on a sunny day and the pump stops, the fluid sitting up in the collector can reach 150°C or more. Year after year, that heat slowly cooks the glycol: the inhibitors get used up, the fluid turns acidic, and in a system that has overheated badly it can darken or even turn to a sludge. That’s why manufacturers ask for the glycol to be tested and changed every few years — typically around every five — rather than left in for the life of the system.
A healthy system
Full freeze protection through winter. The inhibitors are intact and the fluid is close to pH-neutral, so the pump, collector and cylinder coil are protected. The fluid is thin and moves easily, so the pump shifts heat efficiently and the cylinder gets the most of the sun.
Quietly wearing itself out
The inhibitors are largely spent and the fluid is turning acidic, so it starts to corrode the pump, fittings and coil from the inside. Freeze protection drops, raising the winter risk. The fluid thickens and darkens, the pump works harder, flow falls, and less of the sun’s heat reaches the tank.
A glycol change is cheap insurance. It restores the freeze protection and the corrosion protection, and gets the heat moving properly again. Running on old fluid saves money today, but a corroded pump, a leak or a frozen collector costs far more than the fluid did — so the choice is yours to make, with the real trade-off in front of you rather than hidden.
Hard water, scale and the solar coil
There’s a second thing working against an older system, and it’s specific to hard-water areas like this one. The solar coil inside your cylinder runs hot — it’s where the heat from the roof is handed over to your water — and hot surfaces scale up fast when the incoming water is hard. Scale is an insulator: a furred-up coil passes heat far less well than a clean one, so even a perfectly healthy solar system delivers less and less hot water as the coil scales over.
Softening the cold water feeding the cylinder prevents that. A water softener keeps the coil clean, so the heat transfer stays high and the cylinder — and your boiler’s coil too — lasts longer and works better. If you’re already spending to keep a solar system running, softening the water is often the single change that does most for its performance, and I can fit and set one up as part of the same visit.
Three clear options — and I’ll tell you which one fits.
Not every system is worth saving, and I won’t push a repair that doesn’t add up. Once I’ve tested yours, you’ll get a plain recommendation along one of these lines.
Repair and keep it running
If the system is sound and the fault is a pump, glycol, vessel or sensor, repairing it is usually the sensible, lower-cost route — and you get years more free summer hot water out of it.
Recommission a dormant system
Plenty of systems have simply been switched off and forgotten. Often they can be flushed, refilled, recharged and brought back into use for far less than people expect.
Reuse the cylinder, or remove
Where a system is beyond economic repair, the cylinder is often still excellent — I can pair it with a heat pump, or remove a dead system entirely and reinstate normal hot water.
If you’re weighing up a wider change, I also repair and install heat pumps, so the hot-water side can be planned as part of one job rather than two.
Keeping the cylinder: pairing it with a heat pump.
When a solar system has reached the end of its life, the part most worth keeping is usually the cylinder. Most solar cylinders were supplied with very long warranties — often 25 years, sometimes lifetime — and they’re twin-coil, which makes them ideal for a heat pump. I connect the cylinder in parallel with the heat pump and use both coils, so you get the fast reheat from one and the large heat-transfer surface area a heat pump needs to run efficiently at its lower flow temperatures. Reusing a good cylinder this way also keeps the heat pump installation cheaper, because there’s no new cylinder to buy and fit.
It’s also why I don’t fit brand-new solar thermal systems from scratch. Over a year, a new solar system would save only around £50 more than simply heating the water with a heat pump, and the cost of a full new installation is too high to justify against that. So my work is keeping the systems already out there running well, and making the most of the cylinder when the time comes to change. If you’re thinking about a heat pump, see my heat pump repair and installation page.
What a solar hot water system is actually worth over a summer.
Most sites quote a big annual saving and leave it there. Here’s a worked example you can check for yourself, so you can decide whether keeping yours running is worth the money. It’s for a typical family of four in a three-bedroom house, with a roughly 4 m² collector facing south-east — close to the average real-world install around here. The assumptions: about 150 litres of hot water a day, heated through a 45°C rise; June to August (92 days); and a well-sized system covering around 90% of summer hot-water demand. On those figures the panels deliver roughly 650 kWh of useful heat across the summer, and a south-east roof is only a few percent behind due south in summer, because the morning sun favours it.
What changes the answer is not the solar — it’s what you’d otherwise heat that water with. Here’s the cost of putting the same 650 kWh of heat into the cylinder by each method, at summer 2026 prices.
| Method | How the water gets heated | Energy needed | Summer cost (Jun–Aug) |
|---|---|---|---|
| Solar thermal | Free heat from the roof | ~17 kWh of pump electricity | ~£5 |
| Gas boiler | About 85% efficient on summer hot water | ~765 kWh of gas | ~£56 |
| Heat pump | COP around 3 | ~217 kWh of electricity | ~£57 |
| Electric immersion | 100% efficient at the tap | 650 kWh of electricity | ~£170 |
So across the summer the panels put in around 650 kWh of hot water for about £5 of pump electricity. What that saves you depends entirely on what you’d otherwise use: roughly £165 over the summer if you’d be heating water on an electric immersion, but only about £50 if you have mains gas or an efficient heat pump. That’s the reason solar hot water pays best in homes on expensive electric immersion or off the gas grid — and it’s worth knowing before you spend money on a repair.
Costs use the Ofgem price cap for 1 July to 30 September 2026 (26.11p per kWh for electricity, 7.33p for gas, including VAT). Prices change every quarter, so treat these as a guide rather than a quote. Over a full year a typical system supplies around half of a home’s hot water, so the annual saving runs a little above double the summer figure, with winter adding little. For more on trimming heating and hot-water costs, see my guide to heating your home efficiently.
What you can safely check before calling anyone.
Some solar hot water problems can be narrowed down, or even fixed, by a confident DIYer. A few ground rules first: always switch off and isolate the power before touching the controller or any electrics; never work on the gas supply, the fixed wiring or beyond the safe pressures marked on the system; and don’t go on the roof. With that said, here’s where to start.
If a check points to a pump, a vessel, the glycol, a leak, or anything up on the roof, that’s where I come in — and you’ll already have narrowed it down, which saves time and cost on the visit.
Not sure what you’re looking at, or it’s not on this list? Send me a few photos and a short description through my contact page, and when I get a chance I’ll take a look and give you a steer — whether it’s something you can sort yourself, or something that needs a visit. There’s no charge for that, and no rush to book anything.
The manuals for the common controllers.
If you want to dig into the settings yourself, here are the official manuals for the controllers and systems most commonly fitted in UK homes. They open on the manufacturers’ own sites, so you always get the current version.
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RESOL DeltaSol controllersBS, SL, AL, MX/BX and CS — and many rebadged versions sold under other brands. Search your model under Downloads.
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Worcester Bosch GreenskiesSolar-Lifestyle on-roof installation and servicing instructions.
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Viessmann Vitosol / VitosolicFlat-plate and vacuum-tube collector technical guide.
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Vaillant auroTHERM / auroSTEPCollectors and the auroMATIC solar control. Search your model under Downloads.
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Kingspan / ThermomaxSC200 control, Twin Line pump station and Varisol / HP vacuum tubes. Search your model.
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NavitronEvacuated-tube systems and controllers. Search your model under support.
Always isolate the power before opening a controller, and anything involving the cylinder, the gas supply or fixed wiring should be left to a qualified engineer. Need the controls or wiring side looked at? See my electrical fault-finding and controls page.
Straightforward about price.
A charged survey, then a free written quote.
For a repair or service I can usually give you a price over the phone once you describe the system. For anything bigger — a recommission, a larger repair, or reusing the cylinder with a heat pump — I carry out a charged survey so the quote is based on what is actually there, and the written quote itself is then free. I’m not the cheapest engineer, and I don’t aim to be: you’re paying for one person who can read the whole system, do the work properly, and stand behind it with a 24-month workmanship warranty.
Luton, Watford, St Albans and across the home counties.
I’m based in Luton and cover Bedfordshire, Hertfordshire, Buckinghamshire and north London as standard — Watford very much included, as it has plenty of solar thermal and is well within reach. Distance is rarely the issue; if your system is worth looking at, I’ll come to it.
- Bedfordshire — Luton, Dunstable, Houghton Regis and the surrounding towns.
- Hertfordshire — Watford, St Albans, Harpenden, Hemel Hempstead, Welwyn Garden City, Hitchin and nearby.
- Buckinghamshire & north London — Leighton Buzzard, Aylesbury, Barnet, Finchley and the neighbouring areas.
- Further afield — larger or specialist jobs anywhere worth the journey, with travel agreed up front.
Tell me about your solar hot water.
Describe what’s happening — no heat, lost pressure, a system that’s been off for years, or whether yours is worth keeping — and I’ll tell you what’s likely going on and what it costs to put right. The easiest way is to send a few photos and a short description through my contact form; if you’ve been quoted to recharge the glycol and aren’t sure it’s worth it, send me that too and I’ll give you a clear view.
Call 07864 757929
Send photos & details
Email office@bson247.co.uk
Building Services ON 247 — Luton. Solar thermal service, repair and cylinder reuse across Bedfordshire, Hertfordshire and Buckinghamshire.