Blog Saving Energy - Close-in boiler or instant heater?

Saving Energy - Close-in boiler or instant heater?

In Europe, the price for electricity is going through the roof, peaking at rates of €0,90/kWh in The Netherlands. Suddenly, using appliances everyone took for granted are pushing up the bills by hundreds of euro’s. People usually look at things like the fridge, that big TV standing in the livingroom or the computer being turned on all day to save on energy consumption in the house. But what if I told you that the close-in boiler in your kitchen could be consuming more energy than your fridge, TV and computer combined?

The situation

Most Dutch households have a close-in boiler installed in their kitchen cabinets, it’s supposed to be more efficient than running hot water from the central heating unit, which is often installed in the attic. It’s great for comfort, you always have hot water ready for use in the kitchen and you don’t have to waste 5 liters of water before it actually becomes hot. This comfort comes with a price, that boiler’s water has to be kept at 50°C (sometimes even 70°C, depending on the settings) and will consume energy whether you use it or not. I think that’s a waste and with today’s pricing, a costly one.Plieger 10l

So how much does a standard popular close-in boiler consume you may ask? Well it depends… on a lot of things. To give you a range, a standard 10 litre close-in boiler will use anything between 300kWh and 700KWh per year depending on the usage. When we bought our house it had the (very popular) Plieger 10l boiler and it denotes a yearly usage of 526kWh on the EU Energy Label. Calculating with a (current date) conservative €0,5/kWh that’s €263 per year, just for hot water in the kitchen.

Possible solutions

The most ideal way to get hot water is by using a heat pump, it’s very efficient in warming up water and some heatpumps are already showing they can create upwards of 4kWh thermal energy from 1kWh electricity (COP ⧉). This solution will be able to provide the same 50°C of water at less than a quarter of the price any standard close-in boiler would. But not everyone has a heat pump these days and even if you have one, it’s not always possible to connect the kitchen’s hot water to the heat pump’s boiler.

Another solution, which this post is about, is the instant heater (also flow heater or instantaneous heater). For us Dutch people, this concept is quite unknown, when you go to one of our neighbouring countries however, instant heaters can be found everywhere and are very commonly applied from kitchen to bathroom. The instant heater has the same COP as a close-in boiler since it uses electricity to directly heat the water, which is 1:1 (every 1kWh used produces 1kWh of thermal energy). The main difference is that the instant heater does not store hot water but instead produces it when you turn on the tap. All those losses from a close-in boiler keeping the water at a constant temperature are non-existant. Due to having no buffer, the peak power consumption of the instant heater is a lot higher than that of a close-in boiler though and it could push the limits of the electrical circuits in the house. Instant Heater Peak Power Instant heater peak power consumption

A real world example

Obviously, all this talk about both solutions won’t have much weight without some actual data backing it. When we bought our current house almost four years ago, a 2-month old Plieger 10l close-in boiler came with the kitchen from the previous owners. As my goal was to limit my power consumption as much as possible to be able to run the house completely from the solar panel’s generated power, I kept a close eye on power consumption in the house during the first months of construction. One meter in particular popped out and, you guessed it, it was the boiler.

Given that our kitchen also has a dishwasher and we only use hot water in the kitchen maybe twice a day (less than 2 litres), it didn’t make sense to keep a boiler at temperature all the time. Apart from that, the boiler was connected to its own 16A circuit breaker which meant I was looking for an instant heater that would fit into that. The smallest one I could find actually required the full 16A when turned on and would deliver a measly 2 liters per minute of hot water, but for us, that’s more than enough in the kitchen.

Putting our situation into perspective with real data:

 Plieger 10l boilerClage MBH3 instant heater
Price (€)175130
Energy usage (kWh/year)43215
Max. water temp. (°C)76ΔT=25 (water temp +25, est. 45)
Max. Flow (l/m)Limited by tap2
Peak power (kW)23.5
Size (mm)290 x 275 x 450135 x 186 x 87
MountingNone, standingWall mount

As you can see, there are some limitations during use, like the maximum temperature and flow. In our situation, we don’t even notice it as we don’t require higher specs. The instant heater also has a very small body that can be bolted to the wall, our whole kitchen cabinet is free for storage. One added positive note for us, the temperature that’s reached is right at the edge of getting that burning-your-hands-feeling, it’s quite safe!

Does the instant heater save money?

The short answer is, yes, it will save money as it’s more efficient to only heat water when it’s required.

The long answer is still, it depends.

  • If you can live with less litres per minute flow and don’t want water temperatures above 50°C it’s a no brainer to get one and it can easily be swapped with your current boiler.
  • If you want higher flow rates and temperatures and do the dishes in the sink, you will most likely need an electrician to upgrade your circuits before you can use the instant heater for your requirements, this can be quite costly but can still make sense in the long run.
  • If you already have a heat pump connected, it won’t help you, keep the heat pump!

Based on my data, using the instant heater saves us 417kWh per year which is around €208 euro per year with the current electricity prices. The past 3-4 years combined has saved us around 1.5MWh of energy.


Swapping out the close-in boiler for an instant heater was one of the best early decisions I made for the house and my electricity bill. I hope this experience puts things into perspective an maybe help you make a decision or save on your monthly bills. Small tip if you want to buy an instant heater as a Dutch person, get it from Germany as the price for instant heaters is 50% of what Dutch shops are asking.

Let me know what you think in the comments or join our Discord for realtime chat ⧉! All feedback and tips are welcome, please do tell. You can contact me through the comments section, GitHub discussions or directly on GitHub. If you find any errors please open up an issue using this link create an issue ⧉ and I will have a look!

This post is licensed under CC BY 4.0 by the author.

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