Hot water is typically the second largest energy use in a home after space heating, and it is the one households think about least. It is also where a genuine safety issue sits alongside the efficiency question, because the temperature that discourages bacteria is hotter than the temperature that is safe at a tap.
Getting both right is not complicated once the principles are clear.
The three arrangements, and how they differ
A combination boiler heats water on demand as it flows, with no storage. Nothing is kept hot, so there are no standing losses, and flow rate is limited by the boiler's output — which is why a combi can struggle to run two showers. There is nothing to schedule and no cylinder to insulate.
A cylinder heated by a boiler stores a tank of hot water, reheating on a schedule or on demand. Good flow rate, a finite quantity, and it loses heat continuously through the cylinder walls. This is where scheduling and insulation matter.
A heat pump cylinder stores water heated far more efficiently, and typically at a lower temperature — often around 50 °C rather than 60 or higher — because a heat pump's efficiency falls as the target temperature rises. That lower temperature is the reason legionella management needs explicit attention, covered below.
An immersion heater is a simple electric element in a cylinder. Inefficient as a primary source, and extremely useful as a controllable electric load for solar surplus or cheap-rate hours.
The safety point, stated clearly
Legionella bacteria occur naturally in water and multiply in the range of roughly 20 to 45 °C. Inhaled as an aerosol — from a shower, a spray tap, a hot tub — they can cause Legionnaires' disease, a serious pneumonia. Risk is higher for older people, smokers and anyone immunosuppressed.
Standard control in a stored system is to keep the cylinder above 60 °C, at least periodically, and to ensure the whole volume reaches that temperature rather than just the top. A cylinder held permanently at 45 °C to save energy is a genuinely poor idea.
The complication is that water above about 50 °C scalds, and quickly — at 60 °C a serious burn takes only seconds, which matters enormously for children and older adults.
The resolution is standard practice: store hot, deliver warm. Keep the cylinder at 60 °C or above, and fit thermostatic mixing valves that blend it down to a safe temperature at the outlet — typically around 43 °C for a bath or shower, sometimes lower for vulnerable users. Many jurisdictions require these on baths in new work.
Where the energy goes, and how to reduce it
Standing losses. A cylinder loses heat continuously. A good factory-insulated cylinder loses relatively little; an old one with a jacket loses a great deal. Adding or upgrading insulation on an older cylinder is among the best-value energy measures available, and insulating the first couple of metres of pipe leaving the cylinder is nearly free and surprisingly effective.
Dead legs. The water sitting in the pipe between cylinder and tap goes cold between uses, so every use wastes both that water and the energy in it. Long pipe runs to a distant bathroom are the worst case, and the fix at refurbishment time is shorter runs or a small point-of-use heater. Insulating accessible hot pipework helps.
Secondary circulation. Some houses have a pump keeping hot water circulating so taps run hot instantly. Convenient, and a continuous energy loss. If you have one, put it on a schedule matching when you actually use hot water, or on demand via a button or occupancy. Running it 24 hours a day is a substantial waste.
Reheat timing. A cylinder reheated in the morning has all day to lose heat before the evening. Heating closer to use, or twice in smaller amounts, loses less.
Volume actually used. Showers dominate hot water use in most homes, so a lower-flow shower head is a hot water measure as much as a water measure.
Scheduling and shifting, which is where automation earns its place
A hot water cylinder is a thermal store, which makes it one of the most useful flexible loads in a house.
Heat on cheap electricity. On a time-varying tariff, running the immersion or the heat pump during the cheapest hours and coasting through the expensive ones is straightforward and worthwhile.
Absorb solar surplus. If you have panels and export earns little, diverting surplus into the cylinder is one of the best uses available — it is a battery you already own. Dedicated solar diverters do this proportionally, matching the element to available surplus rather than switching on and off.
Schedule around real use. Most households use hot water in two clusters. A schedule matching them beats continuous availability.
Away mode. Drop to a frost-protection minimum when nobody is home, and run a pasteurisation cycle on return before anyone showers. Water that has stood warm and stagnant for a fortnight deserves that cycle.
Keep the legionella cycle whatever else you automate. Weekly to 60 °C throughout the cylinder is the usual guidance, and it should survive any optimisation you build.
Monitoring worth having
Cylinder temperature, top and bottom if possible. The difference tells you how much usable hot water remains and whether the whole volume is reaching pasteurisation temperature — the bottom is what matters there, since stratification means the top can be hot while the bottom is not.
Energy into the cylinder, so you can see what hot water actually costs and spot an immersion running when it should not be. A failed cylinder thermostat with a stuck immersion is an expensive and common fault that monitoring catches immediately.
A leak sensor underneath. Cylinders fail, usually without warning, and they contain a lot of water.
Reheat duration trends. A cylinder taking progressively longer to reach temperature suggests scale on the element or a failing coil, which is a fault you can catch before it becomes a cold shower.
Scale, briefly
In hard water areas, calcium carbonate deposits on heating surfaces. Scale is an insulator, so a scaled immersion element or heat exchanger uses more energy for the same result, and eventually fails.
Two notes. Scale forms much faster above about 60 °C, which is a real tension with legionella control — the usual resolution is to store at 60 rather than higher, and accept periodic descaling. And a water softener protects the system effectively, with the caveat that softened water is normally not plumbed to the drinking tap.
What we would do
- Check your cylinder temperature. If it is below 60 °C, understand why before leaving it there.
- Fit thermostatic mixing valves so you can store hot and deliver warm safely.
- Insulate the cylinder and the first pipe runs. Cheap, immediate.
- Put any secondary circulation pump on a schedule or a demand button.
- Schedule reheating close to use, in two clusters rather than continuously.
- Shift heating to cheap hours or solar surplus with a diverter or a controlled immersion.
- Monitor temperature top and bottom, plus energy in, and put a leak sensor underneath.
- Keep a weekly 60 °C cycle, always.
Steps two and eight are the ones we would not compromise on, because they are the two that are about safety rather than money.