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  • Mechanical Ventilation with Heat Recovery (MVHR) systems recycle the heat from exhaust air by passing it to incoming air. This is a passive process that relies on heat exchangers with large surface areas, and it requires no additional energy input.
  • Such a system is an advantage in winter, but in summer months there is a risk that the process will continue to return heat to building interiors, increasing the risk of overheating.
  • It is recommended that house builders should specify systems with a summer by.pass control. This overcomes the problem by ensuring that exhaust air is not passed over the heat exchanger.
  • Controls which modulate the system fan power can also reduce over heating risk, as well as reducing system energy demands and CO2 emissions.
  • Changes to Part F in 2010 mean that the uptake of MVHR will be more common for buildings with infiltration below 5m3/hm2.

MVHR and the Current Building Regulations Part F

Part L of the Building Regulations considers heat losses associated with uncontrolled ventilation. This is ‘infiltration’ measured in volume of air per square metre of building per hour (m3/hm2).

Part F of the Building Regulations is concerned with adequate means of ventilation in buildings through purpose.provided ventilation. This is ‘ventilation’ measured in litres per second (l/s).

Three kinds of ventilation are considered under Part F:

  • Whole Building (Background) – To ensure regular air changes
  • Extract (Intermittent or Continuous) – To extract moisture and pollutants
  • Purge (Rapid) – For rapid dilution and removal of pollutants

MVHR systems provide both ‘extract’ ventilation and ‘whole building’ ventilation. They are different in this regard from the other systems, where whole building ventilation is provided passively, for example by trickle vents. They may also have a purge function, though that requirement is most often be satisfied by having windows.

MVHR systems are continuous mechanical supply systems. They are categorised as a ‘System 4’ strategies and have their own minimum requirements for supply of background ventilation.

Summer Overheating and MVHR

  • MVHR systems are intended to work continuously year round. Where they are providing background ventilation they must do so in order to ensure sufficient air changes.
  • Heat is recovered from exhaust air by passing it over finned heat exchangers with large surface areas; a passive process requiring no energy input.
  • This process maintains indoor temperature in winter and reduces heating bills. However, there is a risk that in summer months that these systems will keep returning heat into the building. This would compound rather than reduce overheating risk.
  • This issue is being realised by house builders who are receiving complaints of overheating from homeowners, particularly in well insulted modern dwellings with low infiltration rates.
  • House builders should specify MVHR systems which include measures to overcome this issue.

Three approaches to reducing summer overheating

Purge ventilation

  • The Part F requirement for purge ventilation is intended to ensure that any build-up of pollutants can be quickly evacuated. It is not intended for reducing summer overheating under the Building Regulations.
  • Many MVHR systems have onboard purge ventilation or boost. When systems are in boost mode they cannot recover heat efficiently due to the volume of air flow through them.
  • Purge cannot be the best solution for reducing summer overheating because it would have to be controlled, requiring either intervention by occupants; or triggering automatically through sensors, which would has both energy use and noise implications.


  • Manual control of an MVHR boost function is common. This is often achieved through bathroom switches with timers. Remote hand.held devices are available for high specification systems.
  • Occupant control of MVHR systems for anything other than boost of extract from wet rooms is generally undesirable.
  • If control of MVHR for reducing overheating is given to occupants, systems are often switched off to save perceived running costs.
  • Automatic control of MVHR systems may take the form of sensors which modulate their performance. Humidity and CO2 sensors can perform this function. Such sophisticated demand controls of domestic ventilation systems are, however, not a large part of the product market.
  • Control of ventilation systems has a part to play in reducing summer overheating, but must be weighed against the goal of limiting occupant interaction.

Summer By-Pass

  • It is recommended that house builders should specify systems with Summer By-Pass. This overcomes the issue of returning hot air to the dwelling by ensuring that it is not passed over the heat exchanger.
  • Diversion of exhaust air is usually achieved by directional louvers, and these can be adjusted either manually at the  unit or remotely.
  • Where summer by-pass is applied manually, the occupant must take the decision that they are entering the summer heating season, and to remove the heat recovery from their unit. This raises a question of what to in interim periods when outdoor temperatures are fluctuating.
  • There are systems available with automatic by-pass. For example systems monitor both internal and external air temperatures. Bypass is initiated when internal temperature is above a set comfort level, or is higher than external temperatures.
  • Such a system would offer the most desirable solution to this problem, but obviously with higher installed costs.