Have you ever stepped into the shower, enjoying the hot water, when suddenly it turns ice-cold because someone else used up the entire tank earlier? Or perhaps you have a small holiday cottage or an Airbnb studio where there literally isn't a single spare square metre to hang a bulky water tank? The solution comes from Tankless Water Heaters (Instant Water Heaters). These units don't store any water - they heat it instantly as it flows through, delivering unlimited hot water.
A traditional water heater (with a tank) heats 80 litres and keeps them hot. If you don't shower, the water slowly cools (standby losses) and the element kicks in again, burning electricity for water you're not using. The tankless unit eliminates the tank entirely. It stays completely off. The moment you open a hot tap, a flow sensor detects the water movement and fires the heating source instantly. Cold water enters, passes through the heat exchanger, and comes out piping hot in 2 seconds. The moment you close the tap, the unit switches off completely. Result: you only burn energy for the water you actually use.
Unlike a traditional water heater that "runs out" when two people shower back-to-back, the tankless heater provides hot water for hours on end without ever leaving you cold. This makes it ideal for large families, Airbnb properties, and holiday homes with multiple guests.
Without a bulky 80-100 litre tank in the attic or bathroom, you reclaim valuable space. Electric tankless heaters mount on the wall like a book, while gas units sit on the balcony - inside the house you don't lose a single centimetre.
They're small (the size of a shoebox), sleek, mount inside the bathroom or under the kitchen sink, and are very cheap to buy. But here's the massive trap (physics): in winter, the mains water in Greece is freezing (around 10°C). For a comfortable shower, it needs to reach 40°C - meaning the unit must raise the temperature by 30 degrees, in a fraction of a second, while the water flows at 8-10 litres per minute.
If you buy a cheap single-phase tankless heater (5-8 kW) and plug it into a standard socket, in winter you'll get a lukewarm trickle. It's suitable only for washing dishes or summer showers (when mains water is already at 25°C). For a proper hot shower in winter, you need a unit rated at 18 kW to 24 kW.
Such a unit requires a three-phase power supply, heavy-duty cables run directly from the distribution board, and upgraded breakers. If your home has a single-phase meter, the cost of upgrading with the utility company and rewiring makes the purchase of a tankless heater completely uneconomical.
If you already have a natural gas connection (or LPG bottles), the gas tankless heater is by far the best option. It delivers 24 kW of thermal power effortlessly - you can shower while someone else washes dishes in the kitchen, with no drop in temperature. It plugs into a standard socket (just for the control board and ignition spark), never trips the breakers, and natural gas is cheaper than electricity.
Massive power (24 kW without electrical strain), zero cable stress on breakers and wiring, and low running costs thanks to gas pricing. In modern condensing models, efficiency reaches 94-98%, utilising almost every unit of energy burned.
Because it burns gas, it produces flue gases - it must be installed on the balcony (with a flue) and plumbing needs to be run to the bathroom. It can't be hidden under the washbasin. Additionally, it requires an annual inspection by a licensed gas technician, as with every gas appliance.
Whether you choose electric or gas, the tankless heater has one major vulnerability. Because water passes through extremely narrow channels and is heated abruptly to very high temperatures, limescale crystallises rapidly. In areas with "hard" water (islands, borehole-fed areas), the heat exchanger will clog, pressure will drop dramatically, and the unit will shut down on overheat protection. The solution: a polyphosphate filter fitted to the cold water inlet. Yes, choose one if: you have three-phase power (electric) or natural gas, you want to shower for hours without running out, and you want to free up space by eliminating bulky tanks. No, avoid it if: you have single-phase power and plan to buy a cheap 7 kW unit thinking you'll enjoy a hot shower in January - you'll be bitterly disappointed.
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