Hydraulics and Hot Water · 3 lessons
Hot water with a heat pump
Most heat pump homes store hot water in a cylinder. The heat pump heats it through a coil, at lower temperatures than a boiler, so the cylinder has to suit it.
Have a go
0Which cylinder feature most needs checking when the primary circuit runs at a lower temperature?
Tap an answer
Why the coil matters
In an indirect cylinder, heating water runs through a coil and warms the stored water around it; the two never mix. Because a heat pump's water is cooler, the coil needs a bigger surface to pass the heat across. A cylinder chosen by litres alone may have the wrong coil.
Legionella
HSE guidance says hot water should be stored at 60 °C or above, reach 50 °C at outlets within a minute and cold water be kept below 20 °C where possible. That's guidance, not a thermostat setting fixed in law for every home. A heat pump may run a planned hotter cycle, set by the system's design and instructions.
Scalding
Hotter stored water scalds. A thermostatic mixing valve at the tap or bath blends it down to a safer temperature without cooling the store.
Unvented cylinders
An unvented cylinder runs at mains pressure, with a temperature control, an independent overheat cut-out, expansion space and a relief valve whose discharge shows at a tundish. Our lessons teach you to recognise these safeguards and hand over the work: in England and Wales a store over 15 litres goes through building control or a competent person scheme, with a competent installer; Scotland and Northern Ireland have their own rules.
Key numbers
- 60 °C
- HSE guidance for stored hot water where temperature is the legionella control.
- 50 °C
- HSE guidance for hot water at outlets, reached within a minute.
- 15 litres
- Above this, an unvented hot water store in England and Wales falls outside the small-system exemption.
Common mistakes
- Choosing a heat pump cylinder by its size in litres and ignoring the coil.
- Calling HSE's 60 °C guidance a legal setting for every home.
- Capping a relief valve that keeps discharging instead of getting it investigated.
The lessons this guide comes from
- Cylinder coils
- Legionella and pasteurisation
- Unvented: recognise and hand over
Keep learning
- Heat pump topics
- Heat pumps: learn the water side
- Heat pump cylinder
- Legionella cycle
- Unvented cylinder
- Heat pump practice questions
- How does a heat pump work?
- Heat loss: how big a heat pump does a house need?
- Do radiators work with a heat pump?
- Commissioning a heat pump system
- Heat pump grants and MCS, nation by nation
Sources
Sources checked 31 October 2026. Guidance, grants and official pages can change.
- HSE: Legionnaires' disease, things to considerHSE's guidance on hot water stored at 60 °C or above, reaching 50 °C within a minute at outlets, and cold water kept below 20 °C.
- Government guidance: Building regulations approvalInstalling or replacing a heating system needs building regulations approval in England and Wales unless a competent person scheme member does it; Scotland and Northern Ireland have their own rules.