How do ventilation units with heat recovery work? An in-depth guide

zoned HRV ventilation

A Heat Recovery Ventilation (HRV) system is a form of mechanical ventilation that can significantly improve the indoor air quality and energy efficiency of your home. An HRV system is basically designed to provide air movement in an airtight home and should be considered when designing a new build. The principle is to extract stale air at room temperature and introduce fresh, filtered air from outside. As the air passes through a heat exchange element, the fresh air that replaces the extracted air is close to the same temperature as the removed air. How heat recovery ventilation units work and what are the benefits of using them?

Exploring the mechanics behind heat recovery ventilation systems

Rising energy costs and global warming have highlighted the need to develop improved energy systems to increase energy efficiency while reducing greenhouse gas emissions. One of the most effective ways to reduce energy demand is to use energy more efficiently. Waste heat recovery has therefore become increasingly popular in recent years.

HRV uses heat from the extracted air and transfers it to the air being filtered in from outside, rather than simply extracting air and replacing it with outside air. During the cooling season, the system works to cool and dehumidify the incoming outside air. To do this, the system takes the rejected heat and sends it into the exhaust air stream. This air then cools the condenser coil at a lower temperature than if the rejected heat had not entered the exhaust air stream. The system works in reverse during the heating season. Instead of releasing heat into the extract airstream, the system draws heat from the extract flow to preheat the supply air.

The key components of ventilation units  with heat recovery

Mechanical ventilation heat recovery (MVHR) basically consists of a core unit, ducts, energy efficient an fans EC, counterflow heat or enthalpy heat exchanger, and an acoustically insulated structure made of EPP material. The internal exhaust air circuit passes through the heat exchanger without mixing supply and exhaust air streams. A heat recovery unit allows the internal air renewal of the premises, preserving and recovering the heat. The components of heat recovery ventilation are selected for each installation according to the specific requirements of the building.

The removal of moisture and condensation in turn reduces the risk of damp and mould throughout the home, so a heat recovery system can certainly reduce energy consumption and keep you warmer in the winter. New homes can save up to 30% on heating costs. Understanding heat exchange in ventilation is very important as it is a cost effective, sustainable and quick way to reduce global energy consumption and provide better indoor air quality (IAQ).

How often should ventilation be cleaned?

The frequency of ventilation system cleaning depends on factors like building type, occupant count, outdoor air pollution, pets, smokers, and overall air quality and performance. As a general guideline, residential ventilation systems should be cleaned every 3 to 5 years, while commercial or industrial systems may require more frequent cleaning due to higher usage and air pollution levels.

Understanding the heat exchange process: a closer look

A heat recovery system works via a heat recovery ventilation unit which is usually located in the attic, roof space or plant room of a building. The extract and supply air do not flow within the same ducts  and there would be no cross contamination of the different air flows. The heat recovery unit is connected to room air valves via a network of ducting throughout the building.
Mechanics of heat recovery systems is quite simple. For example, if the indoor temperature is 20 degrees and the outdoor temperature is 0, the warm air is extracted and passes through the heat exchanger component, heating the cool incoming air to the point where the fresh incoming air is approximately 18 degrees. These figures are for a heat recovery unit with 90% efficiency. Needless to say, this is a huge difference to an open window letting the 0 degree unfiltered air into the house.

The efficiency of heat recovery in modern ventilation systems

When you install a heat recovery system it will extract stale air from all the rooms in your home and replace it with clean, filtered, fresh, in all your habitable rooms and bedrooms without letting the heat escape. Your home will be fully ventilated all year round, with up to 40% of the heat recovered, giving you significant energy savings. By installing a heat recovery system, your home will remain airtight with no need for window vents or bathroom extractors, creating a healthier, cleaner and quieter environment.

Enclosed spaces require a regular supply of fresh air. There are variables that determine the required extraction rate, such as the size of the living space, the number of extraction points and the number of people in the house. With a centralised system it is also important to consider the sensation of draught, so it is necessary to limit the extraction rate per outlet. As a very general rule of thumb, the minimum number of air changes per hour could be 0.4 - 0.5 times the volume of the area.

Comparing counter-flow and cross flow heat exchangers in ventilation units

The counterflow heat exchanger has two zones where heat transfer takes place. A crossflow zone (triangular sections) where the air flows in the same way as in a crossflow heat exchanger, and a counter-flow zone (rectangular sections) with genuine counter-flow. Using the same plate spacing this type of heat exchanger can be significantly smaller than the crossflow equivalent. Taking into consideration the allowable pressure drop values, this means that the side of a crossflow heat exchanger must be at least 800 mm long. Such a solution corresponds to a flow rate of approximately 3,500 m3/h.

It is in the counterflow section of the counterflow heat exchanger that the miracle of high efficiency occurs. The longer the section, the greater the efficiency - this type of heat exchanger is unbeatable when used on recovery units with flow rates up to 5,000 m3/h and comes into its own at rates up to 10,000 m3/h. The shape of the counterflow plate is designed to maximise the heat transfer area, as this part of the heat exchanger contributes up to 70% of the overall efficiency. It is also appropriate to use some overlap of the fins so that the plates can better withstand pressure differences. The purpose of the cross-flow zone is to move the air into the counter-flow zone as quickly as possible, with as little pressure drop as possible.



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