How Heat Pumps Heat Water
An air source heat pump heats your domestic hot water in exactly the same way a gas boiler with a hot water cylinder does — by circulating heated water through a coil inside a cylinder. The difference is the water temperature.
A gas boiler sends 65–75°C water to the cylinder. A heat pump typically sends 45–55°C water (though it can go up to 75°C when needed). At these slightly lower temperatures, the heat pump’s efficiency (COP) is at its best.
Why the Cylinder Matters
Not all hot water cylinders are suitable for heat pumps. A standard vented copper cylinder designed for a gas boiler has a small coil that cannot transfer heat fast enough at the lower temperatures a heat pump operates at.
This is why we install the Heat Geek Super Cylinder. It features:
- Oversized coil surface area — Up to 4x larger than standard cylinders, allowing faster heat transfer at low flow temperatures.
- Superior insulation — Minimal standby heat loss.
- Optimised for heat pump operation — Designed from the ground up for the conditions a heat pump creates.
- Fast recovery times — A full reheat typically takes 1–2 hours depending on cylinder size, rather than the 3–4 hours that lesser cylinders require.
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Legionella Protection
UK regulations require hot water to be stored at or periodically heated to 60°C to prevent Legionella bacteria growth. The Vaillant aroTHERM Plus can produce water at up to 75°C, so it handles this automatically with a scheduled weekly pasteurisation cycle.
Some cheaper heat pump brands cannot reach these temperatures without a backup electric immersion heater, which is less efficient. It is one of the reasons we specify Vaillant.
Sizing Your Cylinder
The right cylinder size depends on your household’s hot water usage:
- 1–2 people: 150–180 litres
- 3–4 people: 200–250 litres
- 5+ people or multiple bathrooms: 250–300 litres
We assess your specific needs during the survey and recommend the right size. Oversizing wastes energy; undersizing leaves you running out of hot water.
Frequently asked questions
Yes. A heat pump heats a hot water cylinder just like a boiler does. The Vaillant aroTHERM Plus reaches up to 75°C, so it meets household demand without relying on a backup immersion heater.
Day to day it heats the cylinder to 45–55°C for best efficiency, and runs a scheduled weekly cycle to 60°C+ for legionella protection. It can reach 75°C when needed.
Roughly 150–180 litres for 1–2 people, 200–250 litres for 3–4, and 250–300 litres for larger households or multiple bathrooms. We size it to your actual usage during the survey.
Yes — a scheduled weekly pasteurisation cycle heats the cylinder above 60°C to kill bacteria, fully compliant with UK regulations. The Heat Geek Super Cylinder we fit recovers quickly thanks to its oversized coil.
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