Yes, in most homes — but the cylinder decides the job, not the roof. Solar needs its own heat exchanger low in the store, so a single-coil cylinder is normally swapped for a twin-coil solar cylinder or paired with a solar pre-heat store. Budget one to two extra days of work and roughly €1,000–2,000 for the cylinder itself.
Can solar thermal be added to an existing system?
Almost always. The collectors, pipework and solar pump station are indifferent to what heats your water today; the question is where the solar heat is going to land. Solar arrives at 25–70 °C and needs to charge the coldest water in the house, which means a dedicated exchanger in the bottom of a store. Three retrofit routes exist:
- Replace the cylinder with a twin-coil solar cylinder. The cleanest answer. Solar coil at the bottom, boiler or heat pump coil above it.
- Add a solar pre-heat store in series ahead of the existing cylinder or combi boiler. Cold mains enters the solar store first, then feeds the existing appliance.
- Fit a retrofit coil through the immersion boss. Cheap and quick, but the exchanger surface is a fraction of a factory coil, so yields are lower. Treat it as a last resort where the cylinder cannot be moved.
What has to change in the cylinder?
Surface area and position. A solar coil should offer roughly 0.15–0.2 m² of exchanger per m² of collector aperture and sit in the bottom third of the tank, where it meets the coldest water. That is what keeps the collector cool, and a cool collector is an efficient one — the collector efficiency curve falls away as the fluid temperature rises above ambient.
The second requirement is volume. A retrofit that keeps a 120 L cylinder and adds 4 m² of collector will overheat by mid-morning in June, then stagnate for the rest of the day. Plan 40–60 litres of store per m² of collector, which for a family of four usually means moving from 120–150 L to 250–300 L.
What does the retrofit look like for your setup?
| Existing setup | What the retrofit needs | Typical disruption |
|---|---|---|
| Vented cylinder + boiler, single coil | Swap for twin-coil vented cylinder | 1 day, plus collector install |
| Unvented cylinder, single coil | Swap for solar-rated twin-coil unvented cylinder; G3-qualified installer | 1–2 days |
| Combi boiler, no cylinder | Solar pre-heat store plus blending valve upstream of the boiler | 1–2 days, needs airing-cupboard space |
| Electric immersion cylinder only | Twin-coil cylinder, or retrofit coil via the immersion boss | 0.5–1 day |
| Heat pump cylinder | Twin-coil store with large upper coil for the heat pump, solar below | 1–2 days |
If you have a combi, the solar-plus-combi arrangement is a real and certified option — it just needs a pre-heat store and a valve that caps the boiler's inlet temperature.
What else does a retrofit involve besides the tank?
- A pipe route from roof to cylinder: 15–22 mm copper or stainless flexible pair, insulated with UV- and 170 °C-rated insulation, kept as short as possible.
- An expansion vessel sized for stagnation, not for a boiler circuit — it must swallow the entire collector content as steam.
- A solar controller with collector and store sensors, and a store limit set behind a thermostatic mixing valve.
- A structural check. Two flat-plate collectors add only around 70 kg, but the fixings must reach the rafters.
- Certified equipment. Grant schemes across Europe require Solar Keymark-certified collectors and an accredited installer — MCS in the UK, RGE in France, SEAI-registered in Ireland.
Is a solar retrofit worth doing on an old system?
The economics improve sharply when the cylinder was due for replacement anyway. A cylinder over 15 years old, a corroded magnesium anode or an immersion-only electric tank all mean you are paying for a new store regardless — and the incremental cost of choosing a twin-coil solar model instead of a plain one is small compared with the collector array.
The reverse is also true: adding solar to a two-year-old single-coil cylinder means scrapping a good tank, which can add 30–40% to the project cost and push payback out by years. In that case, the pre-heat store in series is usually the cheaper path, because nothing already installed is thrown away.
Retrofit cylinders are best specified from the collector aperture rather than from tank volume, since it is coil surface — not litres — that decides whether a retrofit array delivers what it was sized for.
Frequently asked questions
Can solar thermal be connected to my existing cylinder without changing it?
Only if the cylinder already has a spare coil low in the tank, or if you fit a retrofit exchanger through the immersion boss. Connecting solar to an existing upper boiler coil does not work: it heats the top of the tank where the water is already hot, so the collector runs hot and yields collapse.
How long does a solar thermal retrofit take?
Two to three days for a typical house: one day on the roof and pipe route, one day for the cylinder swap and commissioning. Water is usually off for only part of one day. Scaffolding, if needed, is often the longest lead item.
Does a retrofit need planning permission?
Rarely for roof-mounted collectors on a normal house, but listed buildings, conservation areas and ground-mounted arrays are the exceptions. Building regulations still apply, in particular the unvented hot water rules and any structural check.
Is it cheaper to retrofit solar thermal or PV with a diverter?
A diverter is cheaper if you already own PV, because it reuses surplus electricity you have already generated. Solar thermal delivers far more heat per square metre of roof, so it wins where roof space is the constraint or where hot water demand is high all year.