The heat exchanger in a heat recovery unit is the most important component of the entire mechanical ventilation system, and it is this component that determines how much thermal energy will actually be returned to the building, whether the system will frost up during cold weather, and ultimately, what quality of air you will breathe every day.
Anyone who decides to install mechanical ventilation with heat recovery sooner or later faces a dilemma that is most keenly felt during the winter season. On one hand, the aim is to reduce heating bills as much as possible; on the other, it is important to avoid the persistent problem of excessively dry indoor air.
In this article we break down the types of heat exchangers in detail to help you choose the technology that will ensure trouble-free operation and the highest standard of comfort.
Key conclusions
- The heat exchanger in a heat recovery unit enables energy to be recovered from exhaust air and transferred to the supply air. The two airstreams do not mix, which prevents the transfer of contaminants, viruses, and odours.
- Counter-flow heat exchangers offer the highest thermal efficiency (often exceeding 90%), which drastically reduces heating energy demand, but they are more susceptible to frosting during cold weather.
- Enthalpy heat exchangers recover not only heat but also moisture. This eliminates the need for condensate drainage from the unit and allows significant reductions in building operating costs, particularly in winter.
- The choice of unit must be guided by the specific characteristics of the building. Thermal efficiency is the key parameter for conventional heat exchangers, whilst enthalpy efficiency describes the total energy recovery (heat and moisture).
Why does the type of heat exchanger in a heat recovery unit matter so much?
Before examining the detailed breakdown, we need to cover the fundamentals. The primary purpose of a heat recovery unit is to supply fresh outdoor air whilst minimising heat losses that, in conventional gravity ventilation, escape through flues. This mechanism is based on the laws of physics: warm, stale exhaust air drawn from rooms transfers its energy to the cool, fresh air taken from outside. The entire energy exchange process takes place across the working plates of the heat exchanger.
To make an informed choice of heat recovery unit, you need to understand two parameters that manufacturers state in their specifications:
- Thermal efficiency – this indicator shows how much of the energy associated with the temperature difference the unit is capable of recovering (the heat you feel on your skin and measure with a thermometer). A higher percentage means lower heating bills in winter.
- Enthalpy efficiency – this parameter applies exclusively to units with enthalpy heat exchangers. It describes the system’s ability to recover total energy, encompassing both sensible heat and latent heat, i.e. the energy stored in water vapour.
Bear in mind: from the perspective of day-to-day building use, it is enthalpy efficiency and the ability to manage moisture that have the greatest influence on preventing dry mucous membranes in winter and on the general wellbeing of occupants and employees.
Classification by geometry: how does airflow influence heat recovery?
The design of the arrangement in which the airstreams meet directly determines the performance of the heat recovery unit. On the market you will encounter two principal solutions, which differ in the path the air takes through the unit.
Cross-flow heat exchangers
In this classic arrangement, the airstreams (supply air from outside and exhaust air from the building) cross each other at right angles as they pass through the channels of the unit.
Technical features:
Simple, compact construction in the shape of a rectangular cuboid with a square base. The path the air travels is relatively short.
Advantage:
The unit takes up considerably less space. It is characterised by compact dimensions and lower air resistance, which reduces the load on the fans.
Benefit for the user:
Choosing a unit with a cross-flow heat exchanger facilitates installation in confined spaces (e.g. low roof spaces, narrow plant rooms) and is generally associated with a lower purchase price. It should be noted, however, that heat recovery efficiency is lower (in the range of 50–70%). In practice, this means that during severe frost, the supply air may require more substantial reheating by the central heating system.
Counter-flow heat exchangers
From a technical standpoint, these are most commonly cross-counterflow designs. The airstreams cross at the inlet and outlet edges, but along the main, elongated section of the heat exchanger they run in parallel, in opposite directions.
Technical features:
Considerably elongated, in the shape of a prism with a rhombic or hexagonal base. The air has significantly more “time” to transfer energy.
Advantage:
Maximum heat exchange. The thermal efficiency of such units can exceed 90% under certain conditions.
Benefit for the user:
Drastic reduction of ventilation heat losses. The unit recovers the vast majority of energy from the exhaust air, which translates directly into noticeable savings on gas or electricity bills.
Operational challenge:
High efficiency comes at a physical cost. Because the heat exchanger extracts maximum heat from the exhaust stream, this stream undergoes severe cooling, often falling below zero degrees in winter. This causes moisture to condense and freeze on the heat exchanger plates (frosting). Counter-flow heat recovery units therefore absolutely require an effective anti-freeze system, most commonly in the form of an electric pre-heater that activates at sub-zero temperatures, in accordance with the manufacturer’s recommendations.
Classification by function: standard heat recovery vs. enthalpy heat exchanger
The shape of the heat exchanger is only one side of the coin. The second, equally important classification derives directly from the material from which the component is constructed. It is the material that determines precisely what passes through the walls of the unit.
Conventional heat exchangers (aluminium or polymer)
In this case the physics is straightforward: only heat is exchanged through the solid, impermeable walls. The plates are manufactured from rigid plastic (e.g. polystyrene) or aluminium.
Technical features:
The walls act as a 100% barrier to moisture. Warm, humid exhaust air from the rooms transfers heat on contact with the cold plate, and excess moisture condenses on the surface of the heat exchanger (condensation). The unit requires the installation of a condensate tray and drainage to the wastewater system.
Advantage:
Very high efficiency in the exchange of sensible heat alone. These materials are extremely durable, airtight, and can be easily cleaned with water.
Benefit for the user:
This is the optimal and safest solution in buildings where active dehumidification of the air is absolutely essential and where very large quantities of water vapour are generated (e.g. indoor swimming pools, drying rooms, or buildings under fit-out works). If your building has a problem with excessive humidity, a conventional heat exchanger will remove it effectively.
Enthalpy heat exchangers
This is a solution that takes ventilation to the next level. Through specially designed walls, not only heat but also moisture infiltrates from the airstream with higher relative humidity to the stream with lower humidity. The plates are constructed from specialist vapour-permeable polymer membranes, polyethylene, or refined cellulose.
Technical features:
The membrane allows water vapour molecules to pass through at the molecular level. In winter, moisture from the exhaust air does not condense to form liquid water, but instead permeates through the membrane into the dry outdoor supply air, humidifying it before it is delivered to the rooms.
Advantage:
Self-regulating moisture recovery. The absence of condensed water means no condensate, and consequently, minimal risk of the heat exchanger freezing in typical frost conditions.
Benefit for the user:
You supply your bedroom or living room with better-prepared, naturally humidified air. This eliminates the winter problems of stinging eyes, dry skin, and shrinking wooden floorboards. In many cases it also saves money on the purchase and maintenance of external air humidifiers. From an installation perspective, the absence of a condensate drainage connection simplifies installation in locations where access to the drainage system is difficult.
Which heat exchanger to choose? Heat exchanger comparison
To facilitate a rapid analysis before making a decision, we have compiled a summary of the key parameters of 3 heat exchangers (cross-flow, counter-flow, and enthalpy):
| Parameter / Feature | Cross-flow Heat Exchanger (Standard heat recovery) | Counter-flow Heat Exchanger (Standard heat recovery) | Enthalpy Heat Exchanger (Cross-flow or counter-flow) |
| Thermal efficiency | Lower / Medium (50–70%) | Very high (85–95%) | High (70–85%) |
| Moisture recovery from exhaust air | None | None | Yes |
| Condensate drainage to wastewater system | Required | Required | Not required |
| Risk of frosting in winter | Medium | Very high | Minimal |
| Primary application | Buildings with a limited budget and no dedicated plant room | Passive houses and buildings focused on maximum heating cost reduction | Buildings prioritising the highest comfort (no dry air) and reliable operation in frosty conditions |
Summary
How to match a heat recovery unit to your requirements?
A correctly selected ventilation unit should operate quietly, reliably, and effectively reduce your costs. For a heat recovery unit to fulfil its purpose, the heat exchanger must be matched to the specific characteristics of your building and your personal expectations. There is no single “best” type of heat exchanger for every situation.
If your priority is to achieve the maximum possible reduction in supply air heating costs and you are building to passive house standard, a counter-flow heat exchanger is the right choice – you simply need to account for the requirement of condensate drainage and the operation of a pre-heater.
If, however, your priority is comprehensive comfort, you wish to avoid the problem of dry air during the heating season, and you want to minimise the risk of failures caused by condensate freezing, an enthalpy heat exchanger with moisture recovery will perform considerably better.
Still unsure which unit will suit your needs best? Contact our team of technical advisers. We will help you choose a solution appropriate to your actual requirements, ensuring fresh air and many years of trouble-free system operation.
FAQ
1. Does an enthalpy heat exchanger transfer unpleasant odours from the bathroom and kitchen back into the living room?
This is not possible. The modern membranes used in enthalpy heat exchangers operate at the molecular level. They are permeable exclusively to the very small molecules of water vapour. Larger gas molecules (responsible for odours), as well as contaminants, fungi, viruses, and bacteria, are blocked, ensuring the full hygienic quality of the supply air.
2. How should the heat exchanger in a heat recovery unit be kept clean? Can it be washed?
This depends on the material. Conventional cross-flow and counter-flow heat exchangers made from plastics (e.g. PET) or aluminium can safely be removed from the unit and rinsed with lukewarm water and a mild detergent. In the case of enthalpy heat exchangers, particular care is required. Modern polymer membranes can be carefully rinsed with water (in accordance with the manufacturer’s instructions); however, older cellulose-based heat exchangers must under no circumstances be wetted and may only be cleaned by blowing with compressed air.
3. At exactly what point does the anti-freeze system (defrosting) activate?
In standard counter-flow heat exchangers, condensation occurs almost throughout the entire winter. When the temperature of the air drawn from outside falls below zero, this water freezes. The heat recovery unit’s control system then activates the pre-heater to warm the incoming air, or temporarily reduces the supply airflow, defrosting the heat exchanger using exhaust air. Enthalpy heat exchangers, by minimising the phenomenon of liquid condensation, shift this freezing point and are capable of operating without defrosting down to -7°C / -10°C, thereby saving electrical energy.
