A heat recovery unit is a ventilation system that allows you to renew indoor air in a home without losing much of the energy that has already been used to heat or cool it. Put simply, it helps ventilate the house, but prevents conditioned air from being wasted. This is especially important in well insulated homes, where opening the windows for a long time can cause temperature losses, higher energy consumption and lower comfort.
So, let us look at what a heat recovery unit is, as it is becoming increasingly common in efficient buildings, low energy homes, passive houses and energy renovation projects. It not only improves thermal comfort, but also helps maintain better indoor air quality.
What is a heat recovery unit?
A heat recovery unit is a device that forms part of a mechanical ventilation system, and its main function is to renew the indoor air in a home, premises or building while making use of part of the energy from the air being extracted. In a home without this system, when we ventilate by opening windows, the indoor air goes directly outside, so if it is winter, we lose heat, and if it is summer, we lose cool air. In both cases, the climate control system has to work harder to recover the comfort temperature.
The heat recovery unit avoids a large part of that loss. To do this, it crosses two airflows: the indoor air being extracted and the outdoor air entering the property. These two airflows do not mix, but they exchange temperature through a heat exchanger. So, in winter, the warm air leaving the house preheats the cold air entering it and, in summer, the cool indoor air helps reduce the temperature of the warm air coming from outside.
How a heat recovery unit works
To understand how a heat recovery unit works, you need to imagine two airflows working at the same time. On one side, the system extracts stale indoor air, which usually comes from rooms such as bathrooms, kitchens, laundry rooms or damp areas, where odours, humidity and particles build up.
On the other side, the unit introduces clean outdoor air. Before entering the main rooms, such as bedrooms or the living room, that air passes through the heat recovery unit and receives part of the energy from the air being expelled. The key lies in the heat exchanger, a component that allows both airflows to come close enough to transfer temperature, but without mixing. In other words, the used air does not return to the home, it simply transfers part of its energy to the new air.
The process would be as follows:
- The stale indoor air is extracted from the home.
- The clean outdoor air enters the system.
- Both airflows pass through the exchanger.
- The outgoing air transfers part of its heat or coolness.
- The incoming air reaches the home at a temperature closer to the indoor temperature.
- The climate control system needs less energy to maintain comfort.
This operation allows continuous ventilation without relying solely on opening windows, with the home breathing, but without throwing away the energy that has already been consumed.
Types of heat recovery units
There are several types of heat recovery units, and each one works in a slightly different way. The choice depends on the type of building, the required airflow, the desired efficiency, the available space and the budget.
Cross flow
Cross flow heat recovery units are one of the most common systems in homes and small buildings. In them, the extract air and supply air cross inside the exchanger, but without mixing.
Their design is relatively simple, reliable and efficient. The air leaving the home transfers part of its energy to the air entering from outside, achieving significant heat recovery.
They are a good option in residential installations where a balance between efficiency, cost and maintenance is sought. They also tend to have a compact structure, which makes them easier to integrate into homes.
Parallel flow
In parallel flow heat recovery units, the air currents circulate in the same direction or in opposite directions inside the exchanger. When they work in counterflow, meaning in opposite directions, they can achieve very high levels of efficiency. This type of heat recovery unit allows a longer thermal exchange between both airflows, which is why it is usually more efficient than simpler designs.
Parallel flow or counterflow systems are common in high energy efficiency homes, well insulated buildings and projects where the aim is to reduce climate control demand as much as possible.
Rotary heat recovery
A rotary heat recovery unit works by means of a thermal wheel that rotates between the extract airflow and the supply airflow. This wheel accumulates energy from the outgoing air and transfers it to the incoming air.
One of its advantages is that it can recover both sensible heat and part of the humidity, depending on the type of rotor. This can be useful in buildings where it is important to control not only temperature, but also indoor humidity.
They are more commonly used in tertiary buildings, offices, commercial premises or installations with higher ventilation airflows. In residential homes they are less common, although they can be found in specific projects.
Thermodynamic heat recovery
A thermodynamic heat recovery unit combines ventilation with a heat pump. In this case, the system does not only make use of the passive exchange between the air leaving and the air entering, but uses a thermodynamic cycle to improve energy use.
This type of system can provide heating, cooling or support for domestic hot water production, depending on the design of the unit. It is a more advanced solution and can be interesting in homes or buildings where greater integration between ventilation and climate control is desired. Its installation is usually more complex and requires a more detailed technical study, but it can offer good results when designed correctly.
Efficiency of a heat recovery unit
The efficiency of a heat recovery unit indicates what percentage of the energy from the expelled air is recovered before it leaves the building. The higher this efficiency, the lower the heat loss during ventilation.
In many residential units, efficiency can reach high ranges, especially in counterflow heat recovery units, which means that a significant part of the heat from the indoor air is used to temper the outdoor air before introducing it into the home. For example, if in winter the indoor temperature is 21ºC and the outdoor temperature is 5ºC, the air entering does not come in directly at 5 ºC. Thanks to the heat recovery unit, it can enter much warmer, reducing the effort required from the heating system.
However, real efficiency depends on several factors:
- Type of exchanger.
- Air flow rate.
- Outdoor and indoor temperature.
- Quality of the installation.
- Duct insulation.
- Filter maintenance.
- Balancing of the ventilation system.
An efficient heat recovery unit is of little use if it is poorly installed, if the ducts have leaks or if the filters are dirty, so you will need to choose a good unit, but the entire installation must also be properly designed.
Advantages of heat recovery units
Heat recovery units offer advantages both in terms of energy and in terms of comfort and indoor health. Bear in mind that they are not just a technical add on, they can significantly change the way a home feels.
One of their main advantages is energy savings. By recovering part of the energy from the air being expelled, the heating or air conditioning has to work less, meaning lower consumption and more stable climate control.
They also improve indoor air quality. By renewing the air in a controlled way, they help reduce humidity, odours, CO2 and indoor pollutants and, in addition, many systems include filters that retain dust, pollen and particles from outside.
Another important advantage is thermal comfort. Ventilating by opening windows can create cold draughts in winter or let hot air in during summer, but with a heat recovery unit, air renewal is more gradual and the indoor temperature remains more stable.
They also help prevent problems with humidity and condensation, especially in very well insulated homes or homes with poor ventilation, where humidity can easily build up. A ventilation system with heat recovery allows air to be renewed without such a high penalty on energy efficiency.
When is it necessary to install a heat recovery unit?
A heat recovery unit is especially recommended when the home has a good level of insulation and airtightness. The more efficient a house is, the more important it becomes to ventilate properly without losing energy.
In older homes with many air leaks, air enters and leaves in an uncontrolled way through gaps, windows or weak points in the building envelope. This is not efficient, but it does create a certain amount of natural air renewal. By contrast, in modern homes or energy renovated properties, the envelope is much more airtight, and this is where mechanical ventilation with heat recovery makes more sense.
It is also recommended to install a heat recovery unit in the following cases:
- New build homes.
- Passive houses or nearly zero energy homes.
- Renovations with a major improvement in insulation.
- Homes with humidity or condensation problems.
- Homes where the aim is to improve air quality.
- Buildings with efficient climate control using air source heat pumps.
- Homes where the aim is to reduce energy consumption.
It is not always compulsory in all existing homes, but it can be highly advisable when efficiency, comfort and constant air renewal are sought. In addition, if the home already has systems such as air source heat pumps, underfloor heating or high efficiency climate control, installing a heat recovery unit can help the whole system work better.
What is the difference between a heat pump and heat recovery?
Although both concepts are related to energy efficiency, they are not the same. A heat pump is an active climate control system. Its function is to generate heating, cooling or domestic hot water by making use of energy from the air, water or ground. Air source heating, for example, is a heat pump that uses outdoor air as an energy source.
A heat recovery unit, by contrast, does not generate heat by itself in the traditional sense, as its function is to make use of part of the energy from the air being expelled from the home to condition the new air coming in.
In other words, the heat pump produces useful energy for climate control, while the heat recovery unit prevents energy from being lost during ventilation. Both systems can work perfectly together. In fact, in efficient homes they are a very interesting combination. The heat pump handles climate control and domestic hot water production, while the heat recovery unit allows the air to be renewed without greatly increasing energy demand.
In short, understanding what a heat recovery unit is and how a heat recovery unit works helps to appreciate its importance within an efficient home. It is not just a simple accessory, but a key component for better ventilation, energy savings and maintaining a healthier indoor environment throughout the year.
