A direct cylinder heats water with an electric immersion heater inside the tank; an indirect cylinder heats it through a coil fed by a boiler, heat pump or solar collector. The difference is speed: a 3 kW immersion needs 3.9 hours to raise 200 litres by 50 °C, while a 24 kW boiler coil does the same job in under 30 minutes.
What is the difference between a direct and an indirect cylinder?
A direct cylinder has no heat exchanger. The heat source sits in the water itself — usually one or two electric immersion heaters, occasionally a district heating supply. An indirect cylinder keeps the heat source separate: hot primary water from a boiler, heat pump or solar loop circulates through a sealed coil heat exchanger inside the tank and never mixes with the drinking water.
Indirect is the standard for any home with a wet heat source, and the only sensible option for solar thermal or a heat pump, because those sources produce heat as hot water rather than as electricity.
How long does each take to heat a tank?
Heating 200 litres by 50 °C takes 11.6 kWh — one kilowatt-hour raises 17.2 litres by 50 degrees. Divide that by the input power and the difference between the two designs becomes obvious.
| Heat input | Power delivered to the tank | Time to heat 200 L by 50 °C |
|---|---|---|
| Single 3 kW immersion (direct) | 3 kW | 3 h 52 min |
| Two 3 kW immersions (direct) | 6 kW | 1 h 56 min |
| Heat pump via a 2.5 m² coil | 5–7 kW | 1 h 40 min – 2 h 20 min |
| Boiler via a 1.0 m² coil | 15 kW | 47 min |
| Boiler via a 1.5 m² coil | 24 kW | 29 min |
| 4 m² solar array in good sun | 2–2.5 kW | 5–6 h of collection |
The coil is what sets the indirect figures, not the boiler. A 30 kW boiler feeding a 0.6 m² coil still only transfers about 12 kW, which is why coil surface area, not burner size, decides reheat time.
When does a direct cylinder make sense?
Where there is no wet heat source at all, or where electricity is genuinely cheap at the moment of heating. Three cases:
- All-electric flats and apartments with no boiler and nowhere to put an outdoor unit.
- Off-peak or time-of-use tariffs, where a direct cylinder on a night rate acts as a simple thermal battery. A 250 litre tank charged overnight stores roughly 14 kWh.
- Solar PV surplus diversion, where an immersion controller dumps otherwise-exported generation into the tank.
The penalty is efficiency. Direct heating converts one kilowatt-hour of electricity into one kilowatt-hour of heat; a heat pump feeding an indirect cylinder converts the same kilowatt-hour into three or more, so the running cost is a third or less even before any solar contribution.
Why do heat pumps and solar need an indirect cylinder?
Because both deliver heat at low temperature, and low temperature demands a large surface. A boiler coil sees a 20–25 K difference between the 75 °C primary water and the tank; a heat pump at 50 °C sees barely 10 K once the tank warms up. Heat transfer is proportional to that difference, so the coil has to grow to compensate — typically 2.5–3.0 m² against the 0.5–0.8 m² of a boiler cylinder, as explained in what coil surface area a heat pump cylinder needs.
Solar thermal has the same requirement plus one more: the coil must sit low in the tank so the collectors always meet the coldest water available. Solimpeks builds its solar and heat pump tanks around that principle, with the primary coil in the lower part of the tank and dedicated solar-coil (TSE-VS) and heat-pump-coil (TSE-VS HP) versions for low-temperature duty.
Can a cylinder be both direct and indirect?
Yes, and most good tanks are. An indirect cylinder with an immersion heater in the upper third gives you a coil for the main heat source plus an electric backup that can raise just the top 60–80 litres — useful for a legionella pasteurisation cycle, for PV surplus, or for the week the boiler fails.
A twin coil cylinder takes the same idea further, with two indirect coils at different heights so solar and a boiler can share one tank without fighting each other. The immersion heater in that arrangement is a third input, not a substitute for either coil.
Frequently asked questions
Which is better, a direct or an indirect cylinder?
Indirect, for any home with a boiler, heat pump or solar collectors, because it heats far faster and runs on cheaper heat. A direct cylinder only makes sense in all-electric properties, on off-peak tariffs, or as a solar PV surplus store.
Can I tell from the outside whether a cylinder is indirect?
Yes. An indirect cylinder has two extra tappings labelled primary flow and primary return, with pipes running back to the boiler or heat pump. A direct cylinder has only cold in, hot out and one or two immersion boss covers.
How long does a 3 kW immersion take to heat a full tank?
About 3 hours 52 minutes for 200 litres raised by 50 °C, and just under 3 hours for 150 litres. Heating only the top of the tank with an upper immersion is much quicker — roughly 70 minutes for 60 litres.
Does an indirect cylinder still need an immersion heater?
Not strictly, but it is worth having. An upper immersion provides backup if the heat source fails, can run a weekly legionella cycle on a heat pump system, and can absorb surplus solar PV generation.
Sources & further reading
- 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
- Approved Document G — Sanitation, hot water safety and water efficiency (UK Government)