If you’re used to a combi boiler, the idea of switching to a heat pump raises an obvious question: where does the hot water come from? A combi gives you instant hot water on demand — no cylinder, no wait. A heat pump works completely differently, and understanding how it handles hot water is important before making the switch.
Heat pumps and hot water: the basics
An air source heat pump heats water indirectly, storing it in a hot water cylinder rather than heating it on demand. The heat pump heats water in the cylinder to a set temperature — typically 45°C to 55°C — and the cylinder keeps it warm until you need it.
This is fundamentally the same principle as a system or regular boiler with a hot water tank. If you’ve lived in a property with a separate hot water cylinder before, the change from a combi won’t be dramatic. If you’ve only ever had a combi, it requires a small mental adjustment — but in practice, most households barely notice the difference once the system is set up correctly.
What temperature does the hot water need to be?
For comfortable bathing and washing, you need hot water at around 40°C to 45°C at the tap. A cylinder heated to 50°C provides this comfortably, with a little mixing from the cold supply.
There’s also a hygiene consideration. Legionella bacteria can multiply in water stored between 20°C and 45°C. To control this risk, the cylinder should reach 60°C at least once a week — a process known as a legionella pasteurisation cycle. Most heat pump controls run this cycle automatically, usually at a scheduled time (often overnight when electricity may be cheaper on time-of-use tariffs).
Running the pasteurisation cycle uses more electricity than the normal heating cycle because the heat pump is working at a higher output temperature. Some systems use an immersion heater element within the cylinder to do this more efficiently, rather than running the heat pump at a stretch.
Will you run out of hot water?
This is the practical concern most people have when moving from a combi. The answer depends on cylinder size. A correctly sized cylinder for your household should provide more than enough hot water for normal use — and in many cases, more than a combi would.
A common guide for cylinder sizing is about 50 litres per person per day for typical domestic use. A family of four might therefore need a 200-litre cylinder. Many heat pump installations use cylinders of 200 to 300 litres, which provides a comfortable buffer.
Where households do sometimes notice a constraint is when everyone showers in quick succession, or there’s an unusually high demand event. The heat pump can reheat the cylinder relatively quickly, but it’s not instantaneous. Good cylinder sizing and sensible scheduling — setting the heat pump to run before peak morning use — resolves this for almost all households.
How does running cost compare to an immersion heater?
An immersion heater is essentially a large electric element inside a cylinder — it converts electricity directly to heat at roughly 100% efficiency. Running it costs whatever your electricity tariff rate is per unit.
A heat pump heating the same cylinder operates at a COP of around 2.5 to 3.5 even when working at hot water temperatures. That means for the same volume of hot water, you’re using roughly a third of the electricity an immersion heater would consume. Over the course of a year, this is a substantial difference.
The immersion heater element that most heat pump cylinders include is there as a backup — for legionella cycles, for when the heat pump is being serviced, or for peak demand periods. It shouldn’t be used as the primary means of heating water in normal operation.
Solar and hot water: a natural combination
If you have solar panels as well as a heat pump, there’s a particularly neat opportunity. Smart controls can direct surplus solar electricity to run the heat pump (or immersion heater) during the day, effectively pre-heating your cylinder on free electricity before evening demand arrives.
Several heat pump controllers and home energy management systems support this kind of solar diversion. It’s one of the genuinely satisfying aspects of combining technologies — the whole system starts working together rather than as separate parts.
What cylinder do you need?
Not just any hot water cylinder works with a heat pump. You need one that’s designed for low-temperature operation — specifically with a larger coil inside (to transfer heat efficiently at lower flow temperatures) and good insulation (to retain heat for longer between heating cycles).
Many existing cylinders aren’t suitable and will need replacing when a heat pump is installed. This is a cost to factor in — a good quality heat-pump-compatible cylinder typically costs £600 to £1,200 plus installation. Your installer should be clear about this in any quote.
The cylinder is also where your heat pump and any solar immersion diverter integrate. Getting the specification right at the point of installation avoids complications later.
Wise Energy Solutions designs complete heating and hot water systems — not just the heat pump unit — for homes across Blackpool and the Fylde Coast, Preston and South Ribble, and Burnley and Pendle. We’ll make sure the cylinder, controls, and integration with any existing solar are all specified correctly from the start.
📞 Ready to understand what a heat pump system would look like for your home? Call 01253804151 or visit wiseenergy.co.uk.