Adiabatic cooling: what is it and how does it work?

An MVHR unit keeps your home supplied with fresh air all year round. But did you know that you can also use the same ventilation system to cool your home on warm days? With adiabatic cooling, also known as evaporative cooling, you use water to cool the air. Your MVHR unit does the rest. This creates a pleasant, cool indoor climate in your home on warm days, without needing a separate cooling system with an outdoor unit. Handy, right?

In this blog, we explain exactly what adiabatic cooling is and how an adiabatic cooler works together with your MVHR unit. In a worked example halfway through this article, you'll see how much cooler the fresh air is when it enters your home. You'll also find out which type of MVHR unit adiabatic cooling works with, where to install the cooler and how the cooler decides for itself when cooling is useful/cost-effective.

What is adiabatic cooling?

Adiabatic cooling is a method of cooling in which water evaporates in an airflow. Water needs heat to evaporate. The water takes that heat from the air itself. As a result, the temperature of the air drops, while the humidity rises.

The adiabatic process in brief

The word adiabatic means that no heat is added or removed from outside. In an adiabatic process, the total amount of energy in the air therefore stays the same. The energy simply shifts: part of the heat in the air is used to evaporate water. The result is air that is cooler and more humid. Adding water vapour in this way is called adiabatic humidification. You notice the same effect when your wet skin dries in the wind: the evaporating water feels cool.

Good to know: the lower the relative humidity (RH), the more water can evaporate and the more the air cools down. Relative humidity indicates what percentage of the maximum amount of water vapour is already in the air. For a comfortable and healthy indoor climate, a relative humidity of 50 to 60% is recommended. If the humidity in your home stays within this range, adiabatic cooling works well. If the air in your home is more humid than 60% RH, less water can evaporate and the MVHR cooler cools the air less effectively.

How adiabatic cooling works with an MVHR unit

An MVHR unit (a mechanical ventilation system with heat recovery) extracts stale air from your home and blows fresh outdoor air in. Both airflows pass through a heat exchanger without mixing. Heat always moves from the warmer airflow to the cooler one. In winter, the extracted indoor air warms up the cold outdoor air in this way. On a hot summer's day, the opposite happens: the warm outdoor air then transfers part of its heat to the cooler indoor air that is leaving the house.

An adiabatic cooler, also known as an MVHR cooler, makes use of this summer effect. The MVHR cooler is fitted on the extract duct on the house side of the MVHR unit. This is the ventilation duct that carries the extracted air from your home to the MVHR unit. Inside the MVHR cooler, this 'extract air' flows through a wetted matrix. This is a structure that is kept wet at all times.

Good to know: the water evaporates and the extract air cools down before the extract air reaches the heat exchanger. The colder this extract air, the more heat the extract air can absorb from the warm outdoor air. This means cooler fresh air enters your home.

The extra moisture is carried outside together with the extract air. The fresh air entering your home therefore picks up no extra water vapour. That is why this is also called indirect adiabatic cooling.

Worked example: how much cooler does adiabatic cooling make the fresh air?

How much cooling with MVHR achieves depends on the efficiency of your MVHR unit and the temperature difference between the outdoor air and the extract air. How exactly does that work? We'd like to explain it with a simple worked example. Suppose you have an MVHR unit with a cross-flow heat exchanger and an efficiency of 90%. It is 30 °C outside and 21 °C indoors. With an efficiency of 90%, the heat exchanger transfers 90% of the temperature difference between the outdoor air and the extract air.

Without an MVHR cooler, the fresh air in this example enters your home at 21.9 °C. With the MVHR cooler, it is 17.4 °C, in other words 4.5 degrees cooler. On a warm day, you notice this temperature difference straight away.

Why does the MVHR cooler always belong on the extract duct?

You might think it would be more convenient to cool the fresh air directly by fitting the MVHR cooler on the supply duct. We strongly advise against this! On the supply duct on the house side, the MVHR cooler would blow the water vapour straight into your home. In summer, outdoor air is often already humid. When extra water vapour is added to it, the air indoors becomes (too) clammy. Excessive humidity can lead to mould on walls, ceilings and in cupboards.

A different device belongs on the supply duct on the house side: the MVHR humidifier. It also works with adiabatic humidification, but with a different purpose. In winter, the air indoors is often very dry. The MVHR humidifier then adds moisture to the fresh air, so that the humidity in your home stays comfortable.

Which MVHR units does adiabatic cooling work with?

An adiabatic cooler is a standalone device. This means you can combine the cooler with any brand of MVHR unit. There is one important condition, though: adiabatic cooling only works well with an MVHR unit with a cross-flow heat exchanger.

A cross-flow heat exchanger only transfers heat between the two airflows. The moisture that the MVHR cooler adds to the extract air is therefore simply carried outside. An enthalpy exchanger transfers moisture as well as heat. With an MVHR unit with an enthalpy exchanger, the moisture from the MVHR cooler would therefore end up in the fresh air entering your home. The humidity indoors then becomes very high and the air feels clammy and unpleasant. You can check the specifications of an MVHR unit to see whether the product has a cross-flow heat exchanger. Is information missing or are you unsure? Then get in touch with our specialists! They'll be happy to help.

Cooling with your MVHR system: the MVHR cooler knows when cooling is useful

Cooling a house with an MVHR unit is not always useful. That is why the MVHR cooler measures the temperature with built-in sensors. These are located, for example, in your home and in the duct that carries outdoor air to the MVHR unit. On the MVHR cooler, you set the desired indoor temperature, for example 21 °C. Based on these measurements, the MVHR cooler decides for itself whether cooling is useful. Here are four situations:

Why does the MVHR cooler switch off when the bypass is open?

The bypass is a damper in the MVHR unit that routes the fresh outdoor air around the heat exchanger. This lets the cool night air enter your home directly, without the warmer extract air heating it up. Because the fresh air then does not pass through the heat exchanger, cooled extract air cannot cool the fresh air either. An MVHR cooler that kept running would only waste water and energy.

Advantages and points to consider for adiabatic cooling in your home

Considering adiabatic cooling for your home? Below you can see at a glance what it offers and what you need to bear in mind.

How to choose the right solution for your MVHR unit

Do you already have an MVHR unit with a cross-flow heat exchanger? Then the HomEvap MVHR cooler is a logical addition. Would you like to control the MVHR cooler with your own control system, such as a smart home system? Then choose the HomEvap MVHR cooler suitable for external control.

Would you like a comfortable indoor climate all year round, with cooling in summer and humidification in winter? Then the HomEvap MVHR All Season humidifier and cooler combines both functions: the cooler on the extract duct and the humidifier on the supply duct on the house side.