Central or decentralised hot water: where does the electricity go?

The journey from the basement to the shower

In most Norwegian houses the hot water lives centrally: one conventional water heater in the basement or utility room heats all the water, and pipes carry it out to the kitchen, bathroom and shower. That sounds efficient. But on the way from the tank to the tap, electricity you have paid for disappears in three places.

Loss 1: The tank works around the clock

The conventional water heater keeps 200 litres hot around the clock, even when the house is empty. However good the insulation is, the heat seeps out, and the element kicks in to compensate. For a typical conventional water heater this amounts to 1–2 kWh a day. That calculation has been given an article of its own.

Loss 2: The pipes are full of water that goes cold

Between the hot water tank and the shower there may be 10–15 metres of pipe. Depending on the pipe dimension, each metre holds 0,1–0,3 litres of water. When you turn off the tap, the water sits in the pipe and cools down. The next time you open it, all that cooled water has to go down the drain before the hot water arrives.

That costs you three times over:

• Waiting time. With 1–4 litres in the pipe and a tap delivering 4–6 litres per minute, you wait 15–60 seconds for hot water.
• Heated water down the drain. The water that went cold in the pipe had already been heated once.
• Water and wastewater. If you have a water meter, you pay for every litre, twice.

Here is the anchor figure the rest of the arithmetic rests on:

Heating one litre of water from 8 to 65 degrees costs approx. 0,07 kWh.Coax, own calculation

A cautious example: 8 draw-offs a day where the pipe has had time to go cold, with 1,5 litres in the pipe, gives 12 litres of heated water straight down the drain every day. That is around 0,8 kWh a day, or approx. 300 kWh a year, in pipe losses alone.

Loss 3: The tank heats more than you need

The hot water tank stores the water at 65–75 degrees, partly because of legionella. You shower at 38. The hotter the tank is kept, the greater the heat loss from tank and pipes. More on that temperature mismatch in a later article.

Decentralised: the heater sits where the water is used

A flow-through heater turns the whole model around. It is mounted at the draw-off point and heats the water the moment it flows through, to the temperature you are actually going to use.

Then the loss points disappear, not because the technology is magic, but because they never arise in the first place:

What does that mean in kroner?

Let us add it up, with the assumptions on show:

• 200 litre conventional water heater with a standby loss of 1,5 kWh per day: approx. 550 kWh per year
• Pipe losses as in the example above: approx. 300 kWh per year
• Electricity price of 1,5 kr per kWh including grid rent and taxes

That gives around 850 kWh, or approx. 1 300 kroner a year, in electricity that never became hot water in the shower. The figures vary with the house, the pipe lengths and the water heater, and they have deliberately been kept moderate. The point holds either way: with central hot water you pay around the clock, with decentralised you pay for it when you use it.

When is central still fine?

A new, well insulated conventional water heater placed close to the draw-off points is no disaster, and nobody replaces working equipment on pure reflex. But know the numbers: the next time the water heater is due for replacement, or you are building a bathroom, cabin or extension, it is worth working out whether the heat should be made where it is used.

Wondering what your own consumption costs? Try the consumption calculator. If you want to see what suits your home, you will find the models here or you can use the choose a model guide.