What is the difference between a heat pump cylinder and a normal hot water cylinder?
The main difference between a heat pump cylinder and a normal hot water cylinder? The answer is the design and performance of the internal heat exchanger coil. A heat pump cylinder is specifically designed to transfer heat effectively from a low-temperature heat source, such as an air source heat pump or ground source heat pump, whereas a conventional hot water cylinder is often designed to work with a gas or oil boiler operating at higher flow temperatures.
Larger heat exchanger coil
Traditional boilers can commonly supply primary heating water at relatively high temperatures. Because there is a large temperature difference between the boiler water and the domestic hot water stored inside the cylinder, a conventional cylinder can often achieve adequate heat transfer through a comparatively small heat exchanger coil.
Heat pumps operate differently. An air source or ground source heat pump is generally most efficient when producing lower flow temperatures. The smaller temperature difference between the primary heating water and the stored domestic hot water means that a larger surface area for heat transfer is normally required.
For this reason, a heat pump hot water cylinder typically has a larger surface-area coil than a standard boiler cylinder. The larger heat exchanger allows more thermal energy to be transferred into the stored water without requiring the heat pump to operate at unnecessarily high flow temperatures.
This is important because raising the flow temperature of the heat pump can reduce its coefficient of performance, or COP, and therefore increase electricity consumption.
Heat pump cylinders are designed around lower flow temperatures.
A heat pump cylinder should be considered as part of the complete heat pump system rather than simply as a storage tank.
The cylinder’s heat exchanger needs to be matched to factors such as:
- heat pump output
- primary flow temperature
- coil surface area
- primary water flow rate
- required domestic hot water temperature
- cylinder capacity
- required recovery time
Installing a conventional boiler cylinder with an undersized coil may result in slower hot water recovery and could force the heat pump to operate at higher temperatures to achieve the required cylinder temperature.
Sensor and control arrangements can also differ
Heat pump cylinders may also have different sensor pocket positions compared with conventional cylinders.
Modern heat pump systems commonly use a temperature sensor or thermistor installed into the cylinder. This communicates with the heat pump controller so that the system knows when domestic hot water needs reheating and when the required temperature has been reached.
Correct positioning of this sensor is important because it influences how much of the cylinder is heated and when the heat pump starts a domestic hot water cycle.
Immersion heaters and temperature control
Many heat pump cylinders include an immersion heater connection. The immersion heater can provide backup heating and may also be used by some systems to raise the stored water temperature during a periodic high-temperature cycle.
However, immersion heating uses direct electrical resistance rather than the refrigeration cycle of the heat pump, so it should not generally be treated as a substitute for correctly sizing the cylinder and its heat exchanger.
Insulation and cylinder configuration
Heat pump cylinders are also commonly designed with high levels of insulation to minimise standing heat losses from stored domestic hot water.
Depending on the installation, they may be available as unvented or vented cylinders, with single or multiple heat exchanger coils and different combinations of immersion heater and sensor connections.
Ultimately, the key difference is that a heat pump cylinder is designed to transfer and store heat efficiently when connected to a lower-temperature heat source. A normal boiler cylinder may look very similar externally, but its heat exchanger may not provide the surface area or heat transfer performance required for an efficient heat pump installation.