Ventilating a building is essential: regulations require it, occupant health demands it and indoor air quality depends on it. But ventilation carries an enormous energy cost that is rarely quantified: every cubic metre of fresh air that enters the building must be heated in winter and cooled in summer. In an industrial facility with high air-change rates, the energy consumed in conditioning ventilation air can represent 30% to 50% of total HVAC consumption.
Heat recovery addresses this problem at its root: it captures the energy from the air being extracted from the building and uses it to precondition the incoming fresh air, dramatically reducing the workload of the heating and cooling systems. This article explains how it works, which types of heat exchangers are available and in which situations the investment clearly pays for itself.
How it works
A heat recovery unit is a thermal exchanger that brings two airstreams into contact without mixing them:
- Extract air: the stale air being expelled from the building. In winter it is warm (20-22 degrees C); in summer it is cool (24-26 degrees C).
- Supply air: the fresh air being drawn in from outside. In winter it is cold (5-10 degrees C in Valencia); in summer it is hot (30-35 degrees C or higher).
The exchanger transfers a portion of the energy from the extract airstream to the supply airstream. In winter, cold outdoor air is preheated before reaching the heating system. In summer, hot outdoor air is precooled before reaching the cooling system. The result is that the HVAC system needs to contribute far less energy to bring the air to the desired temperature.
Types of heat recovery units
Cross-flow plate heat exchanger
This is the simplest and most robust type. It consists of a stack of metal or plastic plates through which extract and supply airstreams pass alternately without ever mixing.
- Typical efficiency: 50-75%.
- Advantages: no moving parts, zero risk of cross-contamination between airstreams, very simple maintenance (periodic plate cleaning).
- Drawbacks: lower efficiency than other types, larger footprint for the same capacity.
- Applications: installations where total separation of airstreams is a requirement (pharmaceutical, food processing, hospitals).
Counter-flow plate heat exchanger
Similar to the cross-flow design but engineered to maximise the exchange surface by running the airstreams in opposite directions. Achieves significantly higher efficiencies.
- Typical efficiency: 75-90%.
- Advantages: high efficiency in a compact form factor, no moving parts.
- Drawbacks: higher cost than the basic cross-flow design.
- Applications: the majority of commercial and industrial installations.
Rotary heat exchanger (thermal wheel)
A large-diameter wheel with a matrix of hygroscopic or metallic material rotates slowly between the two airstreams. As it turns, it absorbs heat from the extract air and releases it into the supply air.
- Typical efficiency: 75-85%, and up to 80-90% when the wheel also recovers moisture (enthalpy wheel).
- Advantages: high efficiency, recovers both sensible and latent heat (moisture), compact size.
- Drawbacks: has moving parts (wheel drive motor), a small percentage of cross-contamination between airstreams (normally below 5%), requires motor and bearing maintenance.
- Applications: office buildings, hotels, shopping centres, facilities with high comfort requirements.
Run-around coil system
This uses two heat-exchange coils (one in the extract duct and one in the supply duct) connected by a closed glycol-water circuit. It does not require the supply and extract ducts to be adjacent, making it ideal for existing installations where it is not practical to bring both ducts to the same point.
- Typical efficiency: 45-65%.
- Advantages: complete flexibility in coil location, zero cross-contamination.
- Drawbacks: lower efficiency than other types, requires a circulation pump, more complex installation.
- Applications: refurbishments of existing buildings, installations where extract and supply points are far apart.
Regulatory requirements
Spain’s Regulation on Thermal Installations in Buildings (RITE), in Technical Instruction IT 1.2.4.5.2, mandates heat recovery in certain conditions:
- Extract air volume exceeding 0.5 m3/s (1,800 m3/h) and more than 1,000 operating hours per year.
- The minimum heat recovery efficiency depends on the airflow rate and operating hours, ranging from 44% to 70%.
In practice, this means that the vast majority of mechanical ventilation installations in commercial and industrial buildings are required to incorporate heat recovery. Similar requirements exist across the EU under the Energy Performance of Buildings Directive and national implementing regulations.
Energy savings: how much can you save?
The savings delivered by a heat recovery unit depend on three main factors:
- Unit efficiency: higher efficiency means greater savings.
- Indoor-outdoor temperature difference: the greater the difference, the more energy is recovered.
- Operating hours: the more hours the ventilation system runs, the more hours the recovery unit works.
In Valencia’s climate, with long hot summers, the heat recovery unit delivers significant benefits in both winter (preheating outdoor air) and summer (precooling outdoor air). For an industrial installation with 5,000 m3/h ventilation running 10 hours a day, a recovery unit with 75% efficiency can deliver annual HVAC savings of between EUR 3,000 and EUR 6,000, depending on the energy source.
Design considerations
Pressure drop
Every heat recovery unit introduces additional resistance to airflow (pressure drop) that must be overcome by the fans. This pressure drop creates additional fan electricity consumption that must be factored into the net savings calculation. A well-sized recovery unit has a pressure drop of 150 to 300 Pa; units with higher losses can erode a significant portion of the energy savings.
Bypass system
The recovery unit should have a bypass that allows recovery to be deactivated when it is not needed or when it would be counterproductive. For example, on summer nights when the outdoor temperature is below the indoor temperature (night-time free-cooling), it is preferable to introduce outdoor air directly without passing it through the recovery unit.
Frost protection
In climates with very low outdoor temperatures, there is a risk of ice formation on the exchanger when moisture in the extract air condenses and freezes on contact with cold surfaces. In Valencia this risk is marginal, but systems should be designed to handle it on the few nights of the year when temperatures drop below 0 degrees C.
Filtration
Air passing through the recovery unit must be filtered beforehand to prevent dirt accumulation on the exchanger, which reduces efficiency and increases pressure drop. Filters must be inspected and replaced at the appropriate intervals.
Maintenance
Heat recovery unit maintenance is straightforward but essential:
- Exchanger cleaning: at least once a year, or more frequently in environments with high dust loads.
- Filter replacement: according to the manufacturer’s schedule or when the pressure drop exceeds the recommended maximum.
- Efficiency verification: measure inlet and outlet temperatures on both airstreams to calculate actual efficiency and compare it with the rated value.
- Bypass and actuator inspection.
- Motor and bearing servicing on rotary units.
A dirty recovery unit or one with saturated filters does not merely lose efficiency — it forces the fans to work harder, increasing electricity consumption and reducing motor service life.
At Acoval Technical Installations we design and install ventilation and HVAC systems with heat recovery for commercial and industrial buildings in Valencia. If you want to assess whether your current installation could benefit from a heat recovery system, or if you are planning a new installation, get in touch through our contact page. We will analyse your situation and present a proposal with estimated savings.
