Plans showing proposed building in Chester

EnerPHit Blog Article 10: Installation of the MVHR System.

Posted on 21/08/23

One of the five core principles of Passivhaus is mechanical ventilation and heat recovery (MVHR). This is essential to ensure high indoor air quality given the very high level of airtightness Passivhaus buildings are design and constructed to.

What is mechanical ventilation and heat recovery?

Most people will be familiar with mechanical ventilation and may assume that a Passivhaus ventilation system is therefore similar to other types of mechanical ventilation. This isn’t the case as I’ll outline below.

There are two main aspects of MVHR - the mechanical ventilation part and the heat recovery part.

The mechanical ventilation, in its most basic form, is a duct coming into the property, with a fan blowing air. As the system operates through a powered fan and, in most instances, some form of controls, the system is considered to be mechanically rather than naturally ventilated. In a Passivhaus system, this system is more complex, both supplying and extracting air at the same balanced rate. The air must also pass through specific filters which remove particles from the air before it is transferred down the lengths of duct.

Heat Recovery does exactly what it says it does. Heat is removed from the outgoing and stale air, then transferred, via a heat exchanger, to the colder incoming air. Typically, the inside air is warm because it has been intentionally heated in some way, or through incidental heat gains such as the escaping heat from an oven, or heat generated by appliances. External air is typically cooler, especially so at night. By using a heat exchanger, the typically cooler incoming air can be “pre-heated” to avoid the use of additional heating sources. As the MVHR system runs at a constant 365 days of the year, it is important to ensure that the incoming air is warmed as efficiently as possible. The two air paths never physically meet one another. Therefore, they don’t mix, so that any pollution or contamination in the stale outgoing air cannot be transferred to the fresh, incoming air.

As mentioned above, a Passivhaus MVHR system is a balanced system, with the same amount being supplied as is extracted. The airtight nature of a Passivhaus means that supply or extract only ventilation systems wouldn’t work effectively as air infiltration / air leakage would not be sufficient, nor can it be controlled.

The different parts of an MVHR system.

Somewhere on the external facade, two grills or duct hoods are installed. One is for the incoming supply air, the other is for the outgoing stale air. These terminals are both attached to an equal length of insulated, airtight duct that goes directly to the MVHR unit.

The MVHR unit is the brain and lungs of the system. Inside the unit, the incoming supply air passes through a filter to remove pollen, other particles and many forms of pollution. This filter protects the unit and particularly the heat exchanger from clogging up and also ensure that the supply air is clean and healthy for occupants.
After passing through the filter, the supply air, along with the outgoing exhaust air pass through a “counterflow” heat exchanger. These heat exchangers can recover anywhere from 75% to 95% of the heat from one side to another. For the majority of the year, this will pre heat the incoming air but in summer, when the air is already likely to be warm enough, it is normally possible to bypass the heat exchanger. This option is known as “summer bypass” and reduces any risk of the building overheating.

Within the MVHR unit, there are two small fans; one for the supply air and one for the extracted air. These are controlled by a clever system that monitors the flow rate of each to ensure they remain balanced. The system can also detect changes in humidity (e.g. if cooking or showering) to “boost” the system, or can be manually controlled by switches.

The fresh air has a duct from the unit to supply fresh air to the habitable rooms of the building. My own system is a radial system which means that the one duct goes to a manifold which then splits to the various individual supply 90mm ducts. These ducts each go directly to a single outlet in the rooms and are finished with a terminal fitted in either the ceiling or wall (or floor if required). Some terminals can be adjusted to help fine-tune the flow rate between different rooms.

The extraction system is almost exactly the same, but in reverse. Air is drawn into the ducts via terminals (which have additional filters located in them to stop dust etc clogging up the ducts). From the ducts, the air travels into a manifold and then exits via a single larger duct to the MVHR system. It then passes through another filter, before passing through the counterflow heat exchanger. From the heat exchanger it leaves the unit, via the duct to the outside terminal.

Why does a Passivhaus building require MVHR?

MVHR achieves three things in a Passivhaus: Comfort, control and energy efficiency.

MVHR provides the right amount of fresh filtered air at a comfortable temperature. It can help hay fever and allergen sufferer’s because the air is filtered. By supplying the correct amount, it removes moisture from the air & lowers indoor humidity, without making the air and the environment too dry which could aggravate some respiratory problems.

It removes odours, moisture, pollutants and stale air from the building. The drier air can eradicate issues such as mould and critters such as bed bugs. Moisture is one of the big potential health hazards in buildings and excess moisture can cause dampness, mould, and respiratory issues.

The control provided by a certified PH MVHR unit is excellent. It is possible to add many different sensors or switches, along with monitoring or remote access. I intentionally opted to keep the system very simple as I wanted it to manage itself and wasn’t interested in “tinkering” with it. Even so, with just the basic, built in humidity sensor, the system operates perfectly. It has a holiday mode that reduces the flow rates and energy consumption when the building is not being occupied, along with a standard mode for continuous running and a boost mode that is triggered by the built-in humidity sensor. Having been commissioned by an experienced specialist MVHR tradesman, the system is balanced and only supplies/extracts the calculated amount and effectively ”boosts”, when necessary, to manage increases in moisture. The summer bypass option means we’ve only had two days with overheating issues (deemed by the PH Institut as above 25°C) and this was only within in the kitchen and can be easily explained.

Energy efficiency is remarkable really, given how much we benefit from the system. As stated, the unit only has two small fans to push / pull the air flows. The heat exchanger efficiency of my unit can reach a maximum of 96% of heat in the out-going air. For the majority of the year, the fans use significantly less energy than the heat they reclaim which would otherwise have to be generated from other means. For the remaining summer period, it must be remembered that the system is still providing healthy and comfort benefits that it would be nigh on impossible to achieve by means of natural ventilation. To give you some context, with the heating off in the summer, and the heating system (as in solar thermal heating) not yet installed/contributing) we use approximately 5kW of electricity a day to run our appliances, run the SHP to have hot water, run the CCTV system, have TV’s etc on stand-by and occasionally work from home. Last winter (albeit we still have plenty to do before completing the retrofit of the property) we regularly used 35kW of electricity in a day. In the seven and a bit months the MVHR has been operating, it has used 44kW of energy – that is the equivalent of 9 summer days of energy use, or just more than one very cold winters day.

Our Real-World experiences.

Prior to the MVHR unit being commissioned, there was no mechanical ventilation in the house at all. The house was also incredibly expensive to heat, as it previously had electric panel heaters and had very little in the way of insulation. Whilst we tried to ventilate naturally, we did fall foul of not opening windows enough as it would cool the house so much. Even when we did ventilate well, the high humidity rooms - utility room, kitchen & bathroom – along with the window reveal’s in all rooms started to show signs of black mould. Basically, anywhere there was a cold surface or a cold bridge, condensation formed and mould would follow. One of our children has asthma and last year also spent nights in hospital, on two occasions, because they developed Pneumonia. Safe to say, a cold, damp and mould infested home was not a good place for them, or any of us to be. When the unit was first turned on, it determined the relative humidity of the indoor air of being in the region of 75%. Within 24 hours of being turned on it had dropped to 51% and within a week, it had dropped to a healthy 41%. Within days of the system being turned on, all condensation vanished from the house and, after a scrub down of any affected areas, we’ve not seen any recurrence of any mould and I fully expect not do so ever again. We don’t really notice the MVHR now as we have become used to it, but in the first few weeks after system was turned on, we all very definitely sensed, felt and tasted the difference in the quality of the air. When working from home now my concentration levels are better, because the quality of the air is better. Generally, our energy levels feel better throughout the day.


The installation of MVHR does provide many measured improvements for building occupiers. In a reasonably airtight building (or better) it will significantly improve air quality, and therefore the comfort and potentially the health of occupants. It will reduce the risk of any moisture-based damage to the building fabric itself. It will save significantly more energy from recovering heat than would be necessary to provide new heat to the building. It will remove indoor air pollutants including C02, VOC’s, damp dog smells etc. For a well-constructed, close-to-airtight, building, MVHR is an essential element of the building itself and of its management strategy.

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