Unstable temperature: a myth from the 1990s
July 3, 2026
‘Flow-through heaters give you unstable temperature, cold and hot by turns.’ The experience is real, but it comes from old hydraulic units. Modern sensor-controlled electronics solve the problem. Here is the difference.
The reputation that lingers
Mention a to someone who tried one twenty years ago and the objection often comes straight away: ‘they give you unstable temperature, it goes cold and hot by turns’. That is a real experience. It is simply no longer true, and the explanation lies in the difference between two ways of controlling the heat.
How the old ones worked: hydraulic control
The first flow-through heaters were . They had no electronics measuring anything, they simply responded mechanically to the water pressure: open the tap and the heating element switched on at full strength.
The problem is that reality is not constant. The water pressure varies when a neighbour draws water, the inlet temperature swings between summer and winter, and you adjust the tap yourself. A hydraulic heater could take none of this into account. The result was exactly what the reputation says: if someone else turned on a tap, the pressure changed, and the water suddenly became hotter or colder. And because it only had on and off, it often delivered water that was too hot, which you then had to blend down by hand, wasting energy in the process.
These are the units the reputation comes from. And the reputation was deserved.
How they work now: electronic control
A modern flow-through heater measures its way forward instead of guessing. Small sensors continuously monitor how hot the water is going in, how much is flowing, and how hot it is coming out, many times a second.
From this the electronics adjust the power output continuously, , not just on and off. You set it to 38 degrees, and the control delivers 38 degrees:
- If the inlet temperature falls in winter, it increases the power output to compensate.
- If the water pressure changes because someone is drawing water elsewhere, it adjusts immediately.
- If you turn up the volume, the power output follows so the temperature holds steady.
Where the old unit responded mechanically to pressure, the new one actively controls towards a temperature you have chosen. That is the difference between a switch and a thermostat.
| Hydraulic (old) | Electronic (modern) | |
|---|---|---|
| Control | mechanical, on/off | sensor-controlled, stepless |
| Responds to pressure changes | no, the temperature jumps | yes, compensates automatically |
| Handles a fluctuating inlet temperature | no | yes |
| Set temperature | approximate | accurate to the degree |
| Energy use | wasted by overheating | delivers only what is needed |
Why this is worth knowing
The objection about unstable temperature is one of the most common reasons people dismiss flow-through heating. But it rests on a technology that is barely sold any more. Judging a modern, electronically controlled heater by a hydraulic one from the 1990s is like judging a new electric car by a lead-acid golf buggy.
That does not mean everything on the market is the same, control comes at different levels, from simple electronics with a few fixed steps to fully electronic regulation accurate to the degree. But the basic instability of the hydraulic days is a solved problem.
If you want to understand how power output and water volume fit together in practice, see How much hot water do you actually use, or find a model that suits you in the choose a model guide.