We Support Architects

There are a number of different calculation methods used, which often differ greatly, depending in which European country you live in.

·       Here in the UK, in England and Wales the Approved Document F (ADF) defines the domestic ventilation rates and is a binding standard. Similar regulations are applicable in Northern Ireland.

·       In Scotland however, there is only very loose, non-binding guidance given in the Technical Handbook – Domestic.

·       We find that these regulations lack minimum standards for air quality in various habitable rooms. Also the extract rates for bathrooms and kitchens are on the low side. We therefore counter-check all calculations with a time-proven, well documented alternative standard (Passivhaus Guidelines), which is more demand oriented, as it bases the ventilation rates on more than one criteria.

Extract ventilation as per Building Standards:

How much extract air is needed depends on the type of room and can also vary with the occupant behaviour. Following are the minimum extract rates for intermittent and continuous extract ventilation:

Supply ventilation:

Building standards set a two-tier specification standard for the whole dwelling ventilation rate at nominal level. This is applicable for continous ventilation systems: MEV, dMEV and MVHR.

A) Based on maximum design occupancy:
– Scotland: the whole dwelling ventilation rate depends on the number of individual habitable rooms (apartments) – see image
– England: the whole house ventilation rate depends on the number of bedrooms – see image

B) Based on the internal floor area: floor area x 0.3 l/s m2 (1.08 m3/h m2). Example: a house with 100m2 floor area would require 30 l/s (108 m3/h) ventilation rate.

The higher value of A or B will determine the minimum whole dwelling ventilation rate, which is the nominal ventilation when then house is occupied.

This ventilation rate is then divided into the various habitable rooms according to their room volume. The room volume is capped at 3m height (Scotland).

However, these values doe not take the into consideration the design occupancy level of each room. often bedrooms, especially when they are smaller will not get enough fresh air, in order to keep the indoor air quality acceptable.

We therefore consider also the Passivhaus guidelines for supply ventilation as below.

Following chart shows the fresh air requirements in relation to different activity levels. We are normally aiming at a medium air quality (IDA 2), which keeps the CO2 typically below 1000ppm.

The standard values are: 30 m3/h (8.3 l/s) fresh air supply per person during the day and
20 m3/h (5.6 l/s) during the night.
These values are especially important for rooms where we spend a lot of time, e.g. occupied bedrooms. We can conclude that single bedrooms should normally have 20 m3/h fresh air supply and double bedrooms 40 m3/h.

The overall supply ventilation rate of a dwelling should also be based on the number of occupants 30 m3/h per person. Whereas the system capacity needs to cater for the maximum number of occupants, the operation of the system can be reduced to the real number of occupants.
Unfortunately, these values are neither reflected in the English nor in the Scottish standards.

We will show you the difference:

As you see, the different standards only produce similar results for average sized homes. For smaller or larger dwellings, the English and Scottish standards will result in too little or too much ventilation. By the way, more is not better, as too much ventilation will cost unnecessarily more energy and will probably result in too dry air in winter, which affects the well-being and health of the occupants. Whilst the Scottish guidance proposes to proportion the supply rates according to their volume, the ADF proposes to divide the supply air evenly among the supply rooms. Neither of those approaches is taking the ventilation demand per room into account.

The Passivhaus standard for ventilation is much more oriented on the real demand of each room and overall. Thus it will consistently produce good air quality throughout the dwelling, and most importantly, in the bedrooms. 

We therefore verify the calculations for all dwellings against this methodology, irregardless of their energy standard.

The dimensioning of MVHR systems should always be based on the maximum occupancy of a dwelling. However, when it comes to the commissioning of MVHR systems, this can be based on the real use of the building, especially if there is a larger difference between the number of occupants and the size of the home. If, after a few years the occupancy level changes, the system can be re-calibrated to the changed needs of the dwelling.

Please be aware of the purge ventilation requirment for habitable rooms. It should be able to deal with overheating by providing a ventilation rate of min. 4 air exchanges per hour.

Such purge provision is normally achieved by openable windows and doors. The openable window and door size needs to be 1/20th (5%) of the floor area of the habitable room. If the windows do not open more than 30%, than the openable window size needs to be 1/10th (10%) of the floor area. Windows that open less than 15% do not count.

If the habitable room leads into another habitable room, e.g. conservatory, and it does not have enough openable windows or doors, then both rooms can be combined and the floor area of both rooms is counted together.

If there are not enough openable windows and doors or non-openable windows, then the purge ventilation provision needs to be achieved with a mechanical purge ventilation system (DPV) that achieves 4 air exchanges of the respective habitable room.

It is not recommended to connect the cooker hood into a MVHR system as grease still permeates the filters in the cooker hood and can get deposited in the extract ductwork. Grease deposits in small ductwork are very difficult to remove. It is far better to avoid these in the first place.
We recommend to install a re-circulating cooker hood with an activated charcoal filter, which filters out grease and some of the smells.
Close by should be the MVHR extract terminal with its own filter to protect the ductwork. According to our experience the filter should be visible and not be hidden inside the terminal (e.g. filter cones), as these get easily forgotten.

We are often asked how MVHR works with wood stoves. Generally there is no conflict with installing both of them. Following are the various aspects of the specification:

1. Air tightness: 
   Most wood stoves with adjustable vents can be reasonably air tight, so that the uncontrolled ventilation is not interfering much with the controlled ventilation of an MVHR system.
   We recommend to consider a room-air independent wood stove, which takes its combustion air from a direct air intake into the combustion chamber. These can be fairly well sealed to the room environment and can therefore be used for Passivhaus dwellings.
2. Fresh air supply for combustion:
   Correctly specified MVHR systems will provide enough fresh air for the combustion process of a wood stove. However Building Standards have not specified the fresh air requirement of wood stoves as such, but only prescribe the opening sizes of external walls to allow enough fresh air to enter the room with the combustion appliance. Typically wood stoves up to 5kW of heat output do not need a ‘hole in the wall’ as fresh air supply.
   Ventilation systems have not been officially recognised in the specification of the fresh air provision. Therefore wood stoves with an output over 5kW still need the required whole in the wall.
3. Possibility of combustion air to be drawn into the dwelling:
   Under certain circumstances, e.g. weather conditions, wood stoves present a risk of leaking combustion air into the dwelling. The danger arises when toxic Carbon monoxides or other harmful gases enter the dwelling.
   If the room with the combustion appliance is under negative pressure, this risk increases. Conventional ventilation systems with cooker hoods and extract fans can create such negative pressure, especially if the background ventilators (trickle vents) are closed.
   However, MVHR systems are typically balanced, thus avoiding any negative pressure build-up. Better than this, the room with a combustion appliance, e.g. a lounge or living room, is typically pressurised through the MVHR supply air, thus reducing the risk of combustion fumes being drawn into the dwelling even further.
   As such MVHR is the safest ventilation strategy when a wood stove is specified.
4. Heat distribution
   We recommend to install a air supply close to the stove, which will help distributing heat around the house better, than an extract point would be able to do. Also due to the considerations in the previous point, we would not advise to install an extract point close to a wood stove.
   Please note that the MVHR system, although it helps distributing heat, is not a heat distribution system, which will equalize temperatures throughout the dwelling, unless it is specifically designed to do so.

Apart from the Passivhaus Designer Course, very little training is available in the UK on how to design MVHR systems. Architects and installers usually rely on the MVHR suppliers to design their ductwork and components. It is not surprising that the quality of the designs varies and some common mistakes appear frequently. Please see below the top 10 mistakes when designing MVHR systems: