How an indirect water heater works
A coil — a heat exchanger — runs inside the tank. Hot heating water from the boiler flows through it and heats the water in the tank. The shower water itself never mixes with the water from the heating system.
Hence the name: the water is heated not directly by a burner or an electric element but via the heating water. Most of the energy is supplied by the boiler, which is running for heating anyway.
When you need a water heater rather than a combi boiler
- The house has two or more bathrooms.
- You need stable pressure and temperature at several outlets at once.
- Hot water has to travel far from the boiler, and recirculation is needed.
- The boiler is already a system, floor-standing or solid fuel boiler.
If the house has one bathroom, a combi boiler is usually enough. Read more in our article “Combi boiler: how to choose”.
What capacity to choose
| Family and house | Recommended capacity |
|---|---|
| 2-3 people, 1 bathroom | 100 l |
| 3-4 people, 2 bathrooms | 150-200 l |
| 5 or more people, 3 bathrooms, bathtub | 200-300 l |
| Large house, guest bathrooms, SPA | 300-500 l or several tanks |
The guideline is 40-50 l per person. If the household uses a bathtub, not just a shower, choose a larger capacity.
How to match it to the boiler
The coil output must match the boiler output. If the coil is more powerful than the boiler can deliver, the tank will heat up slowly. If the boiler is much more powerful than the coil, part of its output goes unused.
To connect it to a system boiler, a three-way valve or a separate pump is used. The boiler controls give priority to hot water: while the tank is heating, space heating switches off for a short time. The house does not have time to cool down in 20-30 minutes.
What matters during installation
- Safety group on the cold water inlet: a check valve and a safety valve.
- A potable water expansion vessel on the hot water circuit, separate from the heating vessel.
- A magnesium anode protects the tank from corrosion. Check it once a year.
- Recirculation is needed if there is more than 8-10 m of pipe to the furthest tap. Hot water arrives immediately, without running off cold water first.
- Temperature. Once a week, heat the water to 60 °C or higher. This prevents bacteria from growing in the tank.
Floor-standing, wall-hung or combined
Vertical floor-standing tanks of 100-500 l are the main option for a boiler room. Wall-hung ones are more compact, but their capacity is limited. Combined water heaters have both a coil and an electric element: in summer, when the boiler is not needed, water can be heated electrically.
Important: the tank capacity, coil output and charging pump are selected together with the boiler. Otherwise there will not be enough hot water even with a large tank.
Tell ThermoHouse how many bathrooms and occupants your house has. We will select a 100-500 l FERROLI ECOUNIT water heater and a connection diagram for your boiler. Contact us →
