Ideal humidity home thanks to Jablotron Futura ventilation

Ideal humidity home thanks to Jablotron Futura ventilation

Published 17. 04. 2023

In addition to the need for heating, the winter season is also associated with relatively lower indoor humidity.

In this article we explain:

  • Why do we have dry air at home in winter?
  • How is beer related to the Mollier diagram?
  • How does a Jablotron ventilation unit provide humidified indoor air?

Before reading, you can recall the function of the unique controlled enthalpy exchanger in the Jablotron Futura heat recovery unit here.

How are temperature and humidity related?

First we need to recall a little physics. Don’t worry, it’s not rocket science.

It is a law of physics that if we heat the air, its relative humidity will decrease. This relationship is expressed in the so-called Mollier h-x diagram (see end of article).

The explanation is simple. Air contains water, i.e. some amount of water, which can be counted in grams or H2O molecules, for example. We can also express this amount of water in relative terms, as a percentage.

As the air warms up, its volume increases significantly. The initial amount of water in grams remains the same, but spreads out into a larger volume. Thus the relative amount of water as a percentage of volume decreases.

Of course, the reverse is also true: if we cool the air, its relative humidity increases.

The threshold of 100% relative humidity is defined as the point when, if the air is cooled further, the water in it will become so concentrated that it will start to become liquid = condense out of the air.

We are all very familiar with condensation, like a wet glass of beer or foggy glasses when we walk inside from outside in the winter. Condensation is associated with the cooling of air and relative humidity exceeding 100%.

Why does ventilation dehumidify the interior in winter?

When you ventilate in the winter, cold outside air enters the interior. If this fresh air is warmed up, its relative humidity drops.


Outside air at 0°C has relative humidity rH of 75%.

If this air is heated to 22°C, its relative humidity drops to 18%.

In other words, by ventilating under these conditions, the humidity in our home will gradually decrease towards 18% rH. This is a law of the heating season that has always applied. Some readers may recall the ceramic evaporators on radiators that are still commonly sold in home improvement stores.

Source: https://www.proalergiky.cz/


But the neighbour has high humidity even in winter…

Paradoxically, we see the opposite extreme in new buildings that are insulated and poorly ventilated. Water from construction processes is essentially sealed into the building by the insulation and tight envelope and has no way to escape.

In some cases we have documented values of over 80% relative humidity after moving in, which is completely unacceptable. Such conditions are direct cause of mould and other structural defects.

For a comfortable and, above all, healthy environment, it is essential to ventilate the house constantly. Too high relative humidity in new buildings is always associated with insufficient ventilation and therefore an unhealthy environment.

Can a compromise be reached?

So how do we ensure a sufficiently ventilated interior and at the same time optimal value of relative humidity?

This is the question that led to the birth of the Futura heat recovery unit.

The solution is the so-called controlled enthalpy exchanger, which transfers not only heat but also humidity to the freshly supplied air. In the controlled enthalpy exchanger, the H2O molecules, i.e. 100% pure water, first condense. This pure water is then used to humidify the fresh air.

Example: Futura ventilation unit in the reference house, 20. January 2023

The outside air has a relative humidity rH of 75% at 0°C.

Ventilation without a controlled enthalpy exchanger would bring in fresh air that would have relative humidity of only 18% at 22°C.

Ventilation with the Futura unit supplied fresh air that had relative humidity of 28% at 22°C.

Simply said, in this example, the Futura delivers fresh air 1.6 times more humid than if it was ventilated through windows or a ventilation unit without a controlled enthalpy exchanger.


  1. The resulting humidity in the house is always a compromise between perfectly ventilated air and optimal humidity. Higher humidity can be achieved by limiting ventilation, at the cost of stale and unhealthy air.
  2. Futura offers a fantastic price/performance ratio because it humidifies fresh air without additional energy. Any active humidification consumes energy and water.
  3. Futura only returns back the humidity that is generated in the home, with an efficiency of around 75%. Futura can’t work miracles either and therefore can’t return moisture if it is not being generated in the home.
  4. Normal household activities such as cooking, showering, drying laundry, or watering flowers contribute to the resulting humidity. If you still find the resulting humidity low and decide to use an active humidifier, always use distilled water. Regular water from the tap can 100% clog the filters in the ventilation unit within 2-4 days!
  5. In freezing weather, when outdoor humidity is also very low, it is almost impossible to achieve an indoor relative humidity above 40% with affordable technologies without intensive additional active humidification. Higher humidity is almost always a sign of inadequate, or even worse, no ventilation.


For the technically curious: Mollier h-x diagram

The current outdoor and fresh air values are available to every customer either on the Alfa wall-mounted controller (room values) or in the mobile app (values directly inside the ventilation unit at the heat exchanger).

Those who want to verify the above statements can use the h-x diagram to do so.

The chart looks cluttered at first glance, but you can just focus on two values:

  • Air temperature on the vertical axis, the figure shows values between -15°C and 35°C.
  • Relative humidity is indicated by the hyperbolic curves marked in the range 10% to 90% (the 0% and 100% curves border the graph).

If we find the intersection of the current temperature and the relative humidity curve on the graph, we have a starting point. As we warm up the air, we move vertically up from this point to the new temperature value. The new hyperbolic curve of lower relative humidity passes through this point.

In this way, you can easily determine how much better your Futura is than conventional ventilation with no humidity recovery.