Heat recovery in airtight and energy efficient buildings! All year around!
June 20, 2019

Heat recovery in airtight and energy efficient buildings! All year around!

nZEB, low-energy and passive buildings

All buildings need to be ventilated for the health and comfort of the people inside. New building, especially nZEB (near Zero Energy), low-energy and passive buildings, are built as airtight and therefore there needs to be focus on adequate ventilation to provide good air quality inside.

Airtightness has a very substantial part in energy efficiency, a building’s airtightness is an indicator not just of its energy efficiency, but its build quality. In terms of airtightness, a maximum of 0.6 air changes per hour at 50 Pascal pressure (ACH50), as verified with an onsite pressure test (in both pressurized and depressurized states).

Mechanical system with heat recovery!

Increasingly popular strategy to provide good indoor air quality and thermal comfort, is mechanical ventilation with heat recovery. For example, the passive houses, in cold and moderate climates, require a mechanical ventilation with heat recovery to ensure the thermal comfort criteria is met year-round (e.g. fresh air supply must be no lower than 16°C).

Mechanical ventilation with heat recovery can only help to maintain comfort conditions if they are fitted in an airtight building. The reason for this is to ensure that as much air as possible, into and out of the building, goes through the heat exchanger.

Mechanical ventilation with heat recovery should recover sufficient amounts of heat from the extracted air to make their specification worthwhile in terms of comfort, but also investment. That is stated as heat recovery efficiency with a minimum of 75%, but preferably somewhere between 80-90% on average.


The efficiency of an installed system is a function of the heat exchanger efficiency, the airtightness of both the building envelope and the ventilation system with heat recovery (including ducting), and the correct system air flow setting and balance at commissioning. The ventilation unit including ducting must be installed inside.

Ian Mawditt, the ventilation expert, states: „Not only should MVHR systems exchange thermal energy as efficiently as possible, they should also be efficient to run. As such, a further criterion for passive house systems is that they have a specific fan power (SFP) of no more than 0.45 Wh/m3, i.e. for every cubic metre of air it exchanges per hour, it should use no more than 45 Watts of electrical energy. You will find many systems on the market now have much better SFP, often below 0.25 Wh/m2. But, as with heat recovery efficiency, the actual SFP of a system will depend on the volume air flow, fan performance characteristics, and the total pressure of the connected ducting.“

All year around!

It is important that the ventilation unit works efficiently all year around. Sometimes in hot summer, the heat can be rejected rather then recovered, this is called summer by-pass function. But some ventilation system can switch off the supply fan and thus the system becomes only extract air, but you need to open the windows to get fresh air to flow inside the building.

In winter some systems will automatically shut down to protect the heat exchanger when the external temperature approaches freezing. This is because the condensation forming and maybe even freezing, therefore not providing the good air quality when it is needed. Designers will need to account for these functional limitations: selecting a unit based upon its volume flow rate capacity is simply not good enough. Ancillary equipment, such as a pre-heater on the incoming air stream is a must where this is not integral to the ventilation system with heat recovery.

Read more!

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