How does a heat recovery unit work?

Around the world, heat recovery units remain a popular option among HVAC installations– in 2018, sales of residential solutions reached €409m across the EU.  Both heat recovery wheels and plate heat exchangers offer a cost-effective way to manage HVAC costs. They represent an environmentally-friendly choice, recovering up to 90% of sensible energy. They also allow for the effective ventilation of an indoor space to a far greater extent than simply opening a window, which is significant for several reasons.

The benefits of indoor ventilation

First, modern homes are built to increasingly stringent standards, so moisture from cooking, washing, showers and even breathing can remain trapped inside a house due to its airtightness. Mould, bacteria and dust mites are just some of the things that can thrive in moist environments. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has also been clear in its assessment of the value of HVAC systems, particularly in relation to the coronavirus pandemic. It categorically states that the ventilation and filtration HVAC systems provide is capable of reducing the airborne concentration of COVID-19, which in turn, lowers the risk of airborne transmission. Providing a healthy environment is crucial for public buildings where people continue to gather, such as supermarkets, banks, healthcare facilities, workplaces and universities.

ASHRAE adds that unconditioned spaces can cause both life-threatening thermal stress and lower a human’s resistance to infection. In its Guidance for Building Operations During the COVID-19 Pandemic, ASHRAE recommends increasing outdoor air ventilation instead of recirculating stale air that may contain airborne traces of virus. So heat recovery units are crucial in more ways than one. They help to regulate indoor temperatures and environments to an optimum for human activity, as well as lower energy usage and reduce energy bills. And in today’s world, they are a significant factor in preventing the further spread of COVID-19.

How much fresh air is needed?

Keeping a healthy level of fresh air is key to maintaining the long-term health of both a building and its occupants.In previous decades, relatively poor construction methods meant there was a natural flow of fresh air into most buildings. However, today’s improved standards and focus on energy efficiency continue to deliver buildings and houses that are more airtight than ever.

Since the 1970s, ASHRAE Standard 62.1 has been the global benchmark on ventilation for acceptable indoor air quality.

To be effective, ASHRAE says residential ventilation should represent at least .35 air changes per hour. As a minimum, ventilation should not fall below 15 cubic feet per minute (cfm) per person.

ASHRAE 62.2 lays out a specific calculation for residential homes.

This calculation represents the level at which healthy levels of oxygen are maintained–even in today’s more airtight homes.  It also facilitates the removal of small, invisible contaminants in the air that might otherwise lead to harmful effects on the people living in the space. Finally, it helps to ensure the building’s durability and avoid the build-up of mould by removing excess humidity.

This online calculator can give you precise ventilation requirements for your indoor environment.

So how does a heat recovery unit work?

Engineers design heat recovery units to minimise our environmental impact. The units work on a principle of renewable energy, extracting as much sensible energy as possible from the exhaust air leaving your building.

The theory is simple. As air from inside a building is replaced by air from outside, the heat energy of the exiting air is lost. But a heat recovery unit retains a significant portion of the exhaust air’s sensible heat energy when it passes through the unit’s heat-exchange core.

In plate heat exchangers, for example, the energy in the exhaust air is used to heat a series of either aluminium or polymer plates. At the same time, incoming air passes by the other side of the plates. The heat energy is exchanged from the stale outgoing air to the fresh incoming air.

In practice, like any piece of sophisticated engineering, the details are much more complex. Everything from the distance between the plates (2mm or 3mm in Swiss Rotors’ latest models) through to the construction material (aluminium or polymer plates) and cleaning methods is deliberately chosen to ensure a unit’s efficiency and performance complies with appropriate standards. 

Which heat recovery unit is the best choice for you?

When it comes to heat recovery in ventilation and air conditioning systems, the most popular solutions are heat recovery wheels and plate heat exchangers. Each has its own strengths and particular suitability for specific circumstances.

Why choose a heat recovery wheel?

Heat recovery wheels transfer both sensible and latent energy between exhaust and supply airflows. They are available in a range of different sizes, making them ideally suited for most applications. Heat recovery wheels from Swiss Rotors, for example, are available in diameters as small as 21” and as wide as 83”. Airflow ranges from 353m3/h to 40,715m3/h. At all sizes, the depth of the wheel remains consistent at just 4.5”.

Heat recovery wheels offer up to 87% energy recovery and are available in three different recovery types: sorption (total), enthalpy (hygroscopic) and condensation (non-hygroscopic).

• In the sorption model, both foil layers are desiccant coated. A 3A molecular sieve ensures the hygienic quality of the device. It also offers high latent recovery efficiency.
• In the enthalpy model, one foil layer is desiccant coated. The other is coated in standard aluminium. This combination lowers the cost of the wheel while still maintaining moderate latent heat recovery.
• In the condensation model, only sensible heat recovery is provided. No desiccant is used, meaning the recovery of moisture from the exhaust air is not possible.

Swiss Rotors’ heat recovery wheels are Eurovent and 1060 AHRI certified. They also hold VDI 6022 and SWKI VA104-01 hygiene certificates. These certifications provide assurance and peace of mind for our customers, who know our products will always live up to expectations. Each monolith wheel is made using a patented foil forming technology that mechanically interlocks foil layers to ensure unconstrained airflow.  The hub is constructed from extruded aluminium, reinforced with 45mm spokes when the wheel diameter exceeds 1,800mm. Permanently sealed and lubricated ball bearings guarantee more than 200,000 hours of operation. Finally, a double peripheral sealing system delivers roughly 2% higher recovery while maintaining very low air leakage. 

Why choose a plate heat exchanger?

The benefits of plate heat exchangers are also significant. They recover up to 90% of sensible energy. They also prevent cross-contamination of airflows, making them ideally suited to a world struggling to manage the coronavirus pandemic. They are the most efficient sensible energy recovery system and the only solution suitable for critical situations that can only operate with a guarantee of zero cross-contamination.

Swiss Rotors’ engineers have created a full range of sizes to suit all standard industry formats. The smallest model is 496mm x 271mm; the largest registers at 1,182mm x 959mm. Both aluminium and polymer exchangers are available (check this article to see how they compare).

Our HexWall system is designed to increase the volume of airflows. Internal casing separates the air streams (so they do not mix) and provides the load-bearing element of the structure. Each HexWall system is ready-built and takes just a few minutes to install. They also offer a much shorter footprint and are available on just a two-week lead time.

Double folded aluminium edges (in the aluminium core) and ultrasonic welding (in the polymer heat recovery unit) give Swiss Rotors’ plate heat exchangers the highest possible level of airtightness. 

Plate heat exchangers are also capable of reversible energy recovery. They can cool or heat incoming air, depending on the outdoor climate. This feature makes them ideal for hot countries such as the United Arab Emirates, where exhaust air can be used to cool incoming warmer air flows.

Just like with heat recovery wheels, Swiss Rotors’ plate heat exchanger models hold Eurovent, as well as VDI 6022 and SWKI VA104-01 certificates. They are tested according to EN308 and an automated manufacturing process ensures competitive pricing, Swiss precision and just two-week lead times. 

Make the right choice for you

Choosing the right heat recovery unit for your situation is a simple way to reduce ongoing costs and energy usage.

You can use the Swiss Rotors selection tool to find the right product for your project.

Or you can talk to a representative at Swiss Rotors for friendly advice about plate heat exchangers and heat recovery wheels. Contact us today.