Hot water
Domestic hot water is the water for your taps, shower and washing machine. In a well-insulated house it is a smaller load than space heating, but it runs all year, so it is a noticeable part of the electricity bill. EnergyFlow models it as its own storage tank, separate from the heating loop.
Two ways to heat the tank
Heat pump. The tank has its own small heat pump water heater of about 2 kW. It takes heat from the outdoor air and puts it into the tank water, the way a fridge moves heat out of its box but pointed the other way. A 3 kW electric element in the tank is the backup. The unit has no connection to the heating loop, so hot water and heating do not compete for the same compressor, the part of a heat pump that does the work. Its efficiency is the COP, the coefficient of performance: heat delivered per unit of electricity used. A COP of 3 means 3 kWh of heat for 1 kWh of electricity.
Electric resistance. The 3 kW element heats the water directly, like a kettle. Its COP is 1.
The tank holds 200 litres. The setpoint, the temperature the controller aims to hold, is 55 °C.
Heated by the boiler or the heat pump
The two options above keep the tank apart from the heating loop. A third choice, available with a boiler or an air-source or ground-source heat pump, does not. In an oil-heated house the boiler usually heats the hot water too, through a coil in a cylinder, and a heat pump normally takes over that job the same way. One machine heats either the house or the cylinder, never both at once, and hot water comes first: the radiators pause while the cylinder charges. A boiler charges it quickly, so the pause is short. An 8 kW heat pump needs 45 to 90 minutes for a 200 litre cylinder, and on a cold morning after a shower the radiators get no new heat for a while. The results show how much of the boiler’s fuel, or of the heat pump’s electricity, went to hot water.
Solar collectors
You can add solar thermal collectors on the roof. Unlike PV panels they heat a liquid directly. You choose the type (a glazed flat box, or vacuum-insulated glass tubes, which lose less heat in cold weather), the area, and the tilt (angle from horizontal) and compass direction they face. The model works out the heat collected hour by hour from the same weather as PV, for a generic collector. It uses a separate solar preheat tank, modeled at 50 litres per square metre of collector. This tank warms the feed to the hot-water tank, whose water heater or element supplies the remaining demand. The results show the heat delivered and the solar fraction, the share of your hot water heat that came from the sun.
How the tank is controlled
The heat pump water heater starts when the tank falls 5 °C below the setpoint. The element works to a setpoint 7 °C lower, so it only comes on when the heat pump has not kept up, for example after a large draw. The compressor stops when the outdoor air is colder than -7 °C, and the element then carries the load. How much the heat pump delivers depends on the outdoor air and the tank water temperature. The model uses the standard EnergyPlus performance curves for this type of unit: formulas for how its output and efficiency change with those two temperatures.
The daily draw
The model draws water on a fixed daily pattern: a morning draw, a short midday draw and an evening peak, shaped differently on weekdays and weekends. For four people that adds up to roughly 200 litres a day. The volume follows the household size you enter in the schedules step, so it is about 100 litres for two people and about 300 for six. The cold mains (the street supply) follows EnergyPlus’s seasonal formula, given a 10 °C yearly average and an 8 °C gap between the warmest and coldest month, so heating the water takes more energy in winter.
A four-pipe heat pump (two machines in one, one side heating while the other cools) gets a separate small water heater like the one above. Its cold feed is preheated to 40 °C from the hot buffer tank, the large insulated tank of hot water between the heat pump and the heating circuit. That unit draws a fixed 200 litres a day, whatever the household size.
What you see in the results
Hot water electricity is its own “Hot water” category in the monthly electricity chart. PV and the battery can supply it like any other load in the hour-by-hour sharing of electricity (dispatch). It is not in the heat pump’s SCOP, the seasonal COP (the year’s heat per unit of electricity), which covers space heating only.
Legionella
Legionella is a bacterium that multiplies in warm stored water and can cause a serious lung infection when breathed in as droplets, for example in a shower. The model has no periodic high-temperature disinfection cycle. The tank is held at its setpoint and nothing more, so the extra electricity of such a cycle is not in the results.
Requirements depend on the country and the size of the system. In 2011 Germany’s federal environment agency (the Umweltbundesamt) wrote that the rule DVGW W 551 applies to large systems, meaning more than 400 litres of storage or more than 3 litres of heated water in the pipes. One- and two-family houses are excluded, and for small systems it is a recommendation. If your installer calls for a regular hygiene cycle, ask what it involves.
Limits of the model
- The tank volume does not change with household size. A large household on a small tank runs short of hot water, and the tank then runs cooler than its setpoint.
- Hot water heating is not moved to hours with PV surplus. The tank is heated whenever it calls for it.
- The tank loses heat at 2.5 W/K, meaning 2.5 watts per degree between the tank and its surroundings. With the heat pump the model puts those surroundings at outdoor air temperature, so the loss never warms the house. With the electric element the surroundings are set to the tank’s own setpoint, so that tank loses almost no heat and its electricity figure is on the low side.
- The heat pump water heater is a generic unit, not a specific product.
- A fresh water station (it heats tap water on demand through a heat exchanger fed from a buffer tank) is not a separate option. There are no circulation loops, the pipes that keep hot water moving past the taps, and no pipe losses.