A coil heat exchanger is a spiral tube — smooth steel, enamelled or corrugated stainless — immersed inside a storage tank to transfer heat between two water circuits without mixing them. Solar practice sizes the coil at roughly 0.2–0.3 m² of exchanger surface per m² of collector area; heat pump cylinders need around three times the coil surface of an equivalent boiler cylinder.
What does a coil heat exchanger do in a storage tank?
An indirect coil separates two water circuits while letting heat flow between them. The primary circuit — solar fluid, heat pump heating water or boiler water — is pumped through the coil; the stored water around it warms by natural convection across the coil wall. The circuits never mix, which is essential when the primary side carries propylene glycol antifreeze or non-potable heating water while the tank stores drinking water.
Indirect coils are the standard heat-transfer method in European cylinders. EN 12897, the standard for indirectly heated unvented storage water heaters, defines how these tanks and their exchangers are specified and tested, and EN 12977-3 tests how well solar stores charge and discharge through them.
What types of coil are used in hot water tanks?
- Smooth-tube steel coil — the classic solar or boiler coil, enamelled together with the tank in glass-lined cylinders.
- Corrugated stainless-steel coil — corrugation packs more surface into each metre of tube; used as the instantaneous drinking-water coil in a hygienic tank.
- Double-wall safety coil — two tube walls with a leak-detection gap, required by some plumbing codes where solar fluid and potable water meet.
- Twin-coil arrangements — a solar coil low in the tank plus a boiler or heat pump coil higher up, the standard configuration for solar combi cylinders.
How much coil surface does a tank need?
| Application | Sizing rule of thumb | Example |
|---|---|---|
| Solar DHW coil | 0.2–0.3 m² per m² of collector | 5 m² of collectors → 1.0–1.5 m² coil |
| Boiler reheat coil | ~0.5–1.0 m² for a family cylinder | 200–300 L cylinder |
| Heat pump DHW coil | ~0.25–0.3 m² per kW of output | 8 kW heat pump → 2.5–3 m² coil |
| Hygienic DHW coil | Sized to peak draw rate | Corrugated stainless, full tank height |
Undersizing is the classic field error: a too-small coil cannot deliver the source's full power, so charging slows and the source runs hotter than necessary.
Why do heat pumps need larger coils?
Heat transfer across a coil is driven by the temperature difference between primary water and stored water. A boiler at 80 °C charging a 60 °C cylinder has a 20 K driving force; a heat pump producing 55 °C water has perhaps 5–10 K. To move the same power with a quarter of the temperature difference, the coil needs several times the surface. An undersized coil forces the heat pump to raise its flow temperature, and each extra 1 °C of condensing temperature costs roughly 2–3% COP (EHPA design guidance). This is why dedicated heat pump cylinders carry 2.5–4 m² coils where boiler cylinders manage with about 1 m².
Where do the coils sit inside the tank — and why does it matter?
Position follows stratification. The solar coil sits at the very bottom, where the tank is coldest, because collector efficiency rises sharply when the collector is fed cold water. The boiler or heat pump coil sits in the middle-to-upper zone so backup heat warms only the standby volume instead of destroying the cold zone the solar circuit needs. A hygienic tank's drinking-water coil runs nearly the full height, harvesting the temperature ladder from cold inlet to hot outlet. In enamelled potable cylinders the coil is enamelled with the vessel and protected by the same magnesium anode. Solimpeks manufactures single- and twin-coil cylinders and hygienic stores at its Konya factory, matching coil surface to collector field or heat pump capacity.
Frequently asked questions
What is the difference between a single-coil and a twin-coil cylinder?
A single-coil cylinder has one exchanger for one heat source, usually a boiler or heat pump. A twin-coil cylinder adds a second, lower coil for solar collectors, letting solar preheat the whole volume while the upper coil tops up. Twin-coil is the standard choice for solar DHW systems.
Coil-in-tank or external plate heat exchanger — which is better?
Internal coils are simpler: no extra pump, no controls, no fouling-prone narrow gaps. External plate exchangers transfer far more power per unit size and suit large collector fields and fresh-water stations. For residential systems the internal coil is usually the right answer.
Can a failed internal coil be replaced?
No — coils are welded into the vessel and are not serviceable parts. Failures are rare because the coil lives in the same protected environment as the tank; if one does fail, the practical repair is an external plate heat exchanger or tank replacement.
Does the coil destroy stratification when it runs?
A correctly placed coil supports stratification: heat released at one level rises gently to its density-matched layer. Mixing damage comes from high-velocity direct inlets, not from coil surfaces.