For years now, it has been a common industry assumption in residential homebuilding not to install an MVHR system in a “draughty home”, or one with an intended or actual fabric airtightness level of 3 m3.hr.m2 @50Pa or higher (worse).
As such, MVHRs have only ever been considered in “airtight” new builds and retrofits.
Research by the Passivhaus Trust has shown why this received wisdom is actually incorrect, and proves without a doubt that a modern MVHR system will always result in significantly lower CO2 emissions at any reasonable level of fabric air permeability.
The aim of this post is to explain how and why the methodology for assessing the energy efficiency of MVHRs within UK Standard Assessment Procedure (SAP) should be modified, how it currently penalises what is the most efficient and comfortable way to ventilate a home, and what you should be considering if renovating or building a new home.
If designed, installed and operated correctly, MVHR systems provide exceptional levels of indoor air quality. In SAP, however, the quality of indoor air is not measurable, so the MVHR is evaluated only in terms of energy or carbon savings instead.
A common quirk of SAP design that most low energy designers have witnessed over the years is that unless the intended fabric airtightness of the building is set at 3 m3.hr.m2 @50Pa or better (ie, good airtightness), the final Energy Performance Certificate score (EPC) will worsen with MVHR compared to wet room extraction and natural ventilation (a system composed of trickle vents in windows and bathroom/kitchen extractor fans).
Basically, SAP forces (or even incentivizes) designers to only specify MVHRs in buildings with a high level of airtightness.
Disregarding the cold draughts, poor air quality and discomfort that comes with natural ventilation, to most low energy designers this never “felt right”. How could a moderately airtight home with MVHR, which brings in warmed, filtered air through a heat exchanger in winter, have a worse energy efficiency than the same moderately airtight home that used cold air trickle vents and extractor fans instead?
The reality is it can’t.
Above all else, the heating system in a naturally ventilated home has to work harder than one with warmed air via an MVHR.
The problem is one of mathematics.
Clever minds at the Passivhaus Trust looked into the inner maths behind SAP. What they found is that infiltration, or cold draughts, are treated differently whether one ticks MVHR or Natural Ventilation.
The Natural Ventilation calculation used a quadratic rather than linear curve. This produces a more gradual increase in heat loss due to infiltration, ultimately reducing the model’s infiltration heat loss rate compared to one with MVHR for all homes with an air permeability rate of between 3 and 20 m³/m².hour @50Pa or lower (better).
The formula for natural ventilation also assumed that residents would never open their windows. This is effectively trying to predict occupancy behaviour as a function of airtightness without any clear rationale. After all, even people in the draughtiest homes will still open their windows every once in a while.
The below graph shows what happens in SAP when one models a 90m2 semi-detached property with either natural ventilation or MVHR. The property’s specification stays the same, all that changes is the airtightness and ventilation strategy. The y-axis is the total regulated carbon dioxide emissions, which is directly related to total energy use. Notice how MVHR is a perfectly straight line (linear) whereas Natural Ventilation is a soft, curved line.
Logically, this doesn’t make much sense. Heat loss through infiltration should not be treated differently depending on your ventilation strategy. Air leaking through cracks in your building fabric does not know whether you’ve got an MVHR unit or an extractor fan upstairs.
When the Passivhaus Trust modelled the property using the same linear infiltration equation and assumptions for both ventilation strategies, this happened:
Across all levels of airtightness – even the leakiest homes – MVHR has a lower emissions rate than natural ventilation.
This is great news for those fighting the good fight for low carbon house-building in the UK – as long as one day in the future SAP updates its assumptions.
When calculating total carbon emissions, the PHT even accounted for the difference in carbon usage intensity between gas (powering the central heating) and electricity (powering the ventilation fans)*.
As the above graph shows, MVHR performance outperforms natural ventilation by between 20 and 30% in terms of carbon emissions for all levels of airtightness. The “MVHR 2020” line goes one step further by changing some out of date assumptions in SAP. The performance of the MVHR unit is improved to match products available in 2020, and carbon factors are reduced to represent our decarbonising electricity grid.
The carbon factors were changed to match the proposed SAP 10.1 grid intensity values of 136g/kWh for electricity and 210g/kWh for gas. The results still come out in favour of MVHR even at increasing levels of poor airtightness, and as the National Grid decarbonises further, this will only ever improve.
*Calculated from a fan power of 30W, building volume of 223m³ and ventilation air change rate of 0.5ACH @50Pa.
In conclusion, the common wisdom that said MVHR systems should only be installed in properties with an air permeability of 3 m³/m².hour @50Pa or lower (better) have been based on outdated information and flawed assumptions using a modelling system which was never intended to be used to compare ventilation systems.
For this reason, architects, low energy designers and engineers should always consider the suitability of MVHR across all buildings – whether new build or retrofit, leaky or airtight. Green Building Store and Enhabit now have hundreds of successfully installed and commissioned MVHR systems in homes all over the UK, and residents are enjoying healthy air quality, increased comfort, better sleep and lower energy bills as a result.
Patrick Chester, Project Manager and Passivhaus Consultant, Enhabit