What coil surface area does a heat pump cylinder need?

How much coil area does a heat pump cylinder need?

Between 2.5 and 3.0 m² for a typical domestic cylinder, and 3.0 m² is the figure most consulting engineers and heat pump manufacturers converge on. As a planning rule, allow about 1 m² of coil per 100 litres of stored water. A standard indirect cylinder built for a boiler carries 0.5–0.8 m² — three to four times less.

This single specification decides more of a heat pump's real-world performance than the brand on the outdoor unit, and it is the one number missing from most quotations.

Why is a heat pump coil three to four times bigger?

Because heat transfer is proportional to surface area multiplied by temperature difference, and the heat pump has thrown away most of the temperature difference on purpose. The governing relationship is Q = U × A × ΔT: the heat moved equals the heat transfer coefficient times the coil area times the mean driving temperature difference.

Work it through for a 10 kW charge:

• Boiler. Primary flows at 80 °C and returns at 60 °C while the tank averages 40 °C during the charge. Mean driving ΔT ≈ 30 K. With a realistic coil coefficient of 300–500 W/m²K, the required area is 0.7–1.1 m².
• Heat pump. Primary flows at 50 °C and returns at 45 °C while the tank averages 35 °C. Mean driving ΔT ≈ 12 K. The same 10 kW now needs 1.7–2.8 m².

And it gets tighter at the end of the charge. As the tank passes 45 °C the boiler still has 25 K of driving difference to work with; the heat pump has 3–5 K. That final stretch is what sets the recovery time, and it is why the industry rule lands at three times the area rather than the 2.5 times the average arithmetic suggests.

What does an undersized coil actually cost?

Three penalties, all of which land on the electricity bill:

1. A higher flow temperature. The control compensates for the small coil by raising the primary temperature. Above 45 °C, each degree costs roughly 2–2.5% of COP, so charging at 55 °C instead of 48 °C burns about 15% more electricity for the same hot water.
2. Immersion backup. When the tank cannot make its target inside the scheduled window, the electric element finishes the job at COP 1.0 — around three times the cost per kilowatt-hour.
3. Poorer stratification. A small coil concentrated low in the tank leaves a cold middle band, so the usable volume is smaller than the litres on the label suggest.

How do I check the coil on a datasheet?

Ask for four figures and treat a missing one as a red flag:

• Coil surface area in m². Not "large coil", not "heat pump compatible" — the number.
• Coil output in kW at a stated primary temperature. A rating quoted at 80 °C primary is meaningless for a heat pump; insist on 50 or 55 °C.
• Reheat time from 15 °C to 50 °C at the heat pump's rated output.
• The test basis. EN 12977-3 sets the performance test methods for solar and low-temperature stores, and EN 12897 covers unvented cylinder construction. Quoted numbers should say which standard they follow, and how the coil heat exchanger was rated.

Coil geometry matters as much as raw area. A tall coil that runs the full height of the tank charges the whole volume; a short coil crowded into the base leaves the upper half to the immersion heater. Finned and dimpled coils raise the effective heat transfer coefficient, so a well-designed 2.5 m² coil can outperform a plain 3 m² one — which is exactly the engineering Solimpeks puts into its enamel-coated and hygienic combination tanks.

Does the same rule apply to solar and combination systems?

The direction is the same and the numbers are more generous still. Solar collectors deliver at 25–60 °C, an even smaller driving difference, so the solar coil sits low in the tank with generous area, and a second smaller coil higher up serves the boiler or heat pump. In a twin-coil tank the position of each coil decides which part of the volume it heats — a detail covered in the thermal storage tanks guide.