Yes, and it is the cheapest way to buy into solar. Within one manufacturer's range a twin coil cylinder costs roughly 10–20% more than the single coil version, while replacing the tank later costs the whole cylinder again plus a day of labour and disruption. Cap the spare coil, fit the sensor pocket now, and the solar connection becomes a two-hour job.
Is it worth fitting the second coil before you need it?
Almost always. The second coil is a piece of steel or copper tube welded in during manufacture; adding it later means buying a new tank. Within a single product range the twin coil version typically carries a 10–20% premium over the equivalent single coil cylinder, against a retrofit cost of the full tank price plus draining, disconnection, disposal, refill and commissioning.
The decision is also about disruption. Swapping a cylinder in an occupied house means a day without hot water, often new pipework, and on an unvented system a fresh safety group and notification. Fitting the right tank once avoids all of it.
What has to be specified now, not later?
Five things are fixed at manufacture and cannot be added afterwards:
- Lower coil surface area, sized for the solar array you might install — 0.2–0.35 m² of coil per m² of collector aperture, so 1.0–1.4 m² for a typical 4 m² array.
- Coil positions. The solar coil must sit in the bottom third and the backup coil in the upper third, for the reasons covered in which coil heats which part of the tank.
- Sensor pockets, one level with the lower coil for the solar controller, one at the upper coil for the backup thermostat.
- Tank volume. Solar needs 50–70 litres per m² of collector, which usually means one size larger than a boiler-only tank — 250–300 litres rather than 180–210.
- Temperature and pressure rating. A solar-charged tank can see 90 °C-plus during a summer overrun, so the vessel, insulation and controls have to be rated for it.
Only the pipework, pump station, collectors and controller are genuinely retrofittable.
What has to be done with the unused coil?
Leave it filled, sealed and insulated — not open. An empty coil corrodes internally from condensation, and an open one connected to cold pipework can act as a cooling loop, quietly drawing heat out of the tank by natural convection. Three steps:
- Fill the coil with clean water plus inhibitor, or with the glycol mix you intend to use later, and cap both tappings with sealed fittings rather than plastic transit plugs.
- Fit isolating valves at the tappings if the pipework stub is being run in advance, so the future installer never has to drain the tank.
- Insulate the stub pipework to the same standard as the rest of the tank. An uninsulated 300 mm tail on a coil tapping is a permanent standing loss.
Does the same logic apply to a future heat pump?
Yes, but the numbers are harsher. A heat pump needs 2.5–3.0 m² of coil against the 0.5–0.8 m² in a boiler cylinder, so a tank chosen only for a boiler will never be adequate later. If a heat pump is a realistic prospect within the tank's 15–25 year life, buy a heat pump ready cylinder now and run the boiler through it — an oversized coil costs a boiler nothing.
Where both solar and a heat pump are possible, the usual answer is a large lower coil for the heat pump, an upper coil for the boiler, and the solar loop routed to the lower coil when it arrives, or a three-connection combination tank.
What does the future-proofed specification look like?
| Item | Specify now | Cost if added at build | Cost if retrofitted |
|---|---|---|---|
| Second (lower) coil | 1.0–1.4 m² for 4 m² of collector | 10–20% of tank price | Full tank replacement |
| Larger tank volume | 250–300 L instead of 180–210 L | One size step | Full tank replacement |
| Sensor pockets | Two, at lower and upper coil level | Negligible | Strap-on sensors, poor accuracy |
| Coil tappings and stubs | Valved and capped | An hour of pipework | Draining and refilling the tank |
| Controller | SG-Ready or solar-capable | Small | Straightforward |
Solimpeks builds its Enamel TSE-VD twin coil tanks to this pattern, with the solar coil low, the backup coil high and G ½ sensor pockets, so the collectors can be added years after the tank goes in.
Frequently asked questions
How much more does a twin coil cylinder cost?
Within one manufacturer's range, roughly 10–20% more than the single coil equivalent of the same volume. That premium buys a component that cannot be retrofitted, so it is usually cheaper than any later upgrade path.
Can a second coil be added to an existing cylinder?
No. Coils are welded in during manufacture and the vessel is then enamelled or passivated as a unit. Adding a heat source to a single coil tank means either replacing the tank or using an external plate heat exchanger with a pumped loop, which is less efficient and adds controls.
What should I do with the spare coil until the solar is installed?
Fill it with inhibited water or the future glycol mix, cap both tappings with sealed fittings, and insulate any pipe stubs. Never leave it open to cold pipework, because natural convection will turn it into a heat leak.
Should I future-proof for a heat pump as well as solar?
If a heat pump is plausible within the tank's lifetime, yes — and it changes the specification more than solar does. A heat pump needs 2.5–3.0 m² of coil, three to four times a boiler coil, so the lower coil should be sized for the heat pump from the start.
Sources & further reading
- EN 12977-3:2018 — Thermal solar systems and components: performance test methods for solar water heater stores
- EN 12897:2016+A1:2020 — Water supply: specification for indirectly heated unvented (closed) storage water heaters
- Commission Regulation (EU) No 814/2013 — Ecodesign requirements for water heaters and hot water storage tanks