Glossary for domestic ventilation and other services

ADF - Approved document F

English and Welsh Building Standards for domestic ventilation.
The ADF is not applicable in Scotland.

A/C - Air conditioning

Air conditioning is a device to adjust the air temperature in a room. These operate with active heating or cooling elements that increase or decrease the air temperature whilst they re-circulate the air. Therefore it is important to close all windows and doors, for the effective operation of the system. If at all, fresh air is introduced only in smaller quantities.

MVHR systems operate in a complete different way as these usually not actively heat or cool, nor re-circulate air.

Ach - Air exchange rate

The air exchange rate is the amount of controlled supply of fresh air or extraction of stale air in relation to the internal volume of a room or the whole building. Often the volume of double height spaces is capped off at 2.5m or 3m. It is expressed in m3 (of air supplied or extracted)/ (hr*m3 (of space)).

Air tightness test / Air permeability test

Since the 2010 Building Regulations all new built homes in Scotland have to be tested for their air tightness (also called air permeability test). The test verifies the target air tightness and is to be carried out when the building is finished. It excludes all controlled ventilation openings (e.g. background ventilators, extract fans, chimney flues). It is expressed in how much air escapes the building per hours (under 50Pa pressurisation) in relation to the surface envelope area (q50).

See q50 and n50.

Appendix Q

It contains independently verified product performance information for the SAP calculations, e.g. on the heat recovery rate and energy use of MVHR systems.

ASHP - Air source heat pump a heat pump that generates hot water from the heat energy in either external air or internal air.

Background ventilators

So called trickle vents, mainly used in the upper window frame help to provide natural ventilation for a dwelling. In modern buildings these are not sufficient to maintain a good indoor air quality and need to be used in conjunction with manually operated cross-ventilation through regular window opening, e.g. bedroom windows tilted at night and during the day every 2-3 hours for 5-10min.

BPEC - British Plumbing Employers Council

A UK charity that aims to raise the knowledge and skills of those who work in the UK plumbing and heating industry.

The BPEC accredited training for domestic ventilation installers is mandatory in England and Wales, but not in Scotland.

BESA - Building Engineering Services Association

The UK's leading trade organisation for building engineering services contractors – representing the interests of firms active in the design, installation, commissioning, maintenance, control and management of engineering systems and services in buildings.

Cascade fan

Internal demand controlled fan, which creates and air flow through a room without ducting. As such it links into the fresh air supply of a cascade ventilation system.

These are often installed in bedrooms and exhaust the air e.g. into a hallway, thus forcing a fresh air supply into the room through the door undercut.

Cascade fans are also called active transfer fans.

CEPH - Certified European Passivhaus Designer Course

A stringent course for experienced professionals to be certified PH Designers. The course includes all aspects of the PH methodology and knowledge to use the PHPP.

CHP - Combined heat and power

A replacement for boilers with economic and environmental benefits, simultaneously producing heat and power.

Commissioning Certificate

The commissioning certificate of a mechanical ventilation system shows the measured ventilation rates at nominal level and should also show if all components have been installed and are working correctly.
Most MV systems, unless fully demand controlled need commissioning.

CO2 - Carbondioxide

Despite the negative press in relation to the greenhouse effect, it is a relatively harmless gas that is found in our atmosphere to the degree of 350-450 ppm (parts per million). Flames will extinguish with levels above 100,000 ppm and no life is possible above 200,000 ppm.

Human and animal respiration turns O2 (Oxygen) into CO2 and releases it into the atmosphere. Exhaled breath contains about 40,000 ppm of CO2.

Indoors increased CO2 levels flag up decreased Oxygen levels and low air exchange rates to the atmosphere. It is easily measured and therefor widely used as a tracer gas to test the indoor air quality.

COP - Coefficient of performance

It shows the efficiency of heat pumps, etc. E.g. a COP of 3.5 means that (at e.g. at 10ºC outside temperature and DHW temperature of 40ºC) the device (e.g. heat pump) will use 1kW for every 3.5kW put out.

Counter-flow principle

By sending two media that exchange heat over a certain distance in opposite direction, up to 100% of heat can be transferred from the one to the other medium (e.g. water or air).

Cross ventilation

Cross ventilation is caused by wind which causes pressure differences between one side of a building and the other.

Defroster / Defrost pre-heater

A heating element on the intake leg of MVHR systems, used for frost -protection. It ensures that the system operates on maximum efficiency even in the coldest time of the year.

DER - Dwelling emmission rate

(annual CO2 per sqm) due to space heating, water heating, ventilation and internal lighting minus any CO2 emissons saved by any generation of electricity.

EHX - Enthalpy heat exchanger

A heat exchanger for MVHR systems which transfers heat and moisture. With a moisture recovery rate of up to 75%, it prevents the over- de-humidification of dwellings, when the internal humidity gains are low compared to the necessary ventilation rates.

The EHX increases slightly the thermal performance of the dwelling through latent gains.

EPE / EPP - Extruded Polyethylene / Extruded Polypropylene

A closed cell insulation material, used for foam ducting or ventilation components that need to be vapour-proof.
EPE is relatively soft.
EPP is more rigid than EPE.

Fire damper / fire & smoke damper

A fire damper is a closure mechanism within a ventilation duct in between a fire compartment wall or ceiling. It is often mechanically or intumescently activated by heat or fire.
Intumescent fire valves are used in ceiling or wall applications.
Fire dampers can typically not be used against the circulation of cold smoke.

Fire/ smoke dampers are motorised valves that are actuated by direct fire & heat and are additionally triggered by the fire & smoke alarm.

Free Area

The free area of a ventilation component equates to the available space that air can travel through. It is usually expressed in mm2 or cm2. (1 cm2 is 100 mm2)

GHX - Ground heat exchanger

Ground heat exchangers on the intake leg of MVHR systems are used for frost protection and gentle additional summer cooling. There are two types in use:

A) Ground-to-air GHX: These were the first of their kind. Typically a 30-40m long tube of 200-250mm dia., buried 1-1.5m in the ground. Ideally these are silver lined on the inside to prevent growth.
B) Ground-to-brine GHX with a brine-to-air heat exchanger. These are pump driven devices, similar to a ground source heat pump.

HX - Heat exchanger

Heat exchangers are elements that transfer heat from one medium to another in a passive way. In MVHR systems these recovery the heat from the extract air into the fresh incoming air.

The first HX were plate heat exchangers with recovery rates of 65-75%. Modern cross-counterflow heat exchangers can recovery 85-95% of heat.

Heat Pumps

A device for generating hot water by extracting heat from a source of a lower level of temperature (e.g. outside air or ground) and ”pumping it up” to a higher, more useful level for space heating and DHW. With some, the process can be reversed to extract heat from the building (air conditioning). Typical applications are air source heat pumps and ground source heat pumps.

HVAC - Heating, Ventilation & Air Conditioning

The technology of indoor and vehicular environmental comfort. Its goal is to provide thermal comfort and acceptable indoor air quality.

Hydroscopic materials

Building materials that can buffer (absorb and release) moisture. E.g. Lime render, which was used internally in traditional dwellings had hygroscopic qualities, which regulated the indoor humidity.

IAQ - Indoor Air Quality

The indoor air quality depends mainly on:
- the quality of the outdoor air
- air exchange rates per room
- air exchange rates per dwelling
- internal pollution sources
- internal humidity levels & condensation
- cold bridging
- ingress of Radon

IDA - Indoor Air Quality Levels

EN13379 shows four categories of indoor air quality. Please note that the values in the EN are to be seen in relation to the outdoor CO2 levels. We have added these in the following table:
IDA 1- High IAQ: Max 750 - 850ppm
IDA 2- Medium IAQ: Max 850-1050ppm
IDA 3- Moderate IAQ: Max 1350-1450ppm
IDA 4- Low IAQ: Over 1350-1450ppm

Internal heat gains

All emitted heat from technical equipment, e.g. fridges, cooker, TV, heat from baths and showers, as well as body heat from occupants and pets.

Passivhaus buildings mainly use internal heat gains, solar gains to provide space heating. For periods of cold weather without solar gains, a backup heater or central heating system is needed.

Infiltration rate

See natural infiltration rate.

Latent gains

The physical effect of evaporation of water is that a small amount of heat energy is lost in the process. Reduced humidity levels will increase the evaporation rate.

Out of this reason enthalpy heat exchangers will prevent lower internal humidity levels and therefore decrease the evaporation heat losses.

Latent heat cells

High-performance thermal stores. Ideal for small units or where there is not much space for a heat store (can e.g. be placed in the corner of a roof).

Mass flow constant operation

Like volume flow constant operation, this feature prevents the dis-balancing of MVHR systems or a drop in ventilation rates through increased resistance, e.g. soiled filters. In comparison to volume flow constant operation it is 7-8% more accurate, due to the seasonal differences in air volumes (cold air has less volume than warm air).

Natural infiltration rate

The natural infiltration rate is varying with the weather conditions (mainly wind speeds and temperature). It depends on the air tightness of the building, its wind exposure and shielding, internal segmentation, curtains or blinds, etc. It is difficult to calculate the real infiltration rate of a building.

n50 (air tightness)

The European standard for measuring the air permeability of buildings is in relation to the internal volume of the building (in contrast to q50). It is expressed in m3/(hr m3) at 50 Pa.

In Europe a pressurisation and de-pressurisation test is needed.

Nominal ventilation rate

The ventilation level set forth which is needed for the design occupancy of a building. A mechanical ventilation system is commissioned at this level. E.g. MVHR level 2.
In real life this level may be used only at certain times when the house is fully occupied or at night.

NZEBs - Nearly Zero Energy Buildings

Buildings with a very high energy performance (Fabric First approach). The low remaining energy demand is covered mainly by renewable sources.

ODA - Outdoor Air Quality Levels

There are three outdoor air quality categories with different concentration levels of CO2, NO2, SO2 and PM10:
ODA 1: Rural areas
ODA 2: Small towns
ODA 3: City centres

Overheating prevention

Structural measures for preventing overheating through solar gains are the most effective ones: e.g. shading, overhangs, thermal mass.

Secondly cross-ventilation is needed if there is a risk of overheating.

Although most ventilation systems will help to bring in cooler air to a certain degree, these should not be planned as overheating prevention.

PH - Passivhaus

A building methodology to minimise the heating consumption of buildings (domestic and non-domestic), as developed by Dr. Feist from Germany. It optimises the building physics by incorporating high levels of insulation and air tightness, as well as MVHR. The basis is a planning tool called PHPP (Passivhaus Planning Package), which accurately predicts the heat energy consumption of a building.

PHPP - Passivhaus Planning Package

Very accurate but also quite time consuming energy assessment. Used to certify passive houses.

Pollen filter (F7)

A finer grade of filtration than the standard G4 dust filters. It is recommended to be used for the intake of MVHR systems to prevent finer dust to enter the ducting system and dwelling.

Post heater

A post heater is used in some MVHR systems to provide or supplement space heating through the air handling system.

There are warm water based or electrically operated post heaters.

PTC heating element (Positive Temperature Co-efficient)

A self-limiting heating element with high power density, used for quality defrost pre-heaters or electrical post heaters.

q50 (air tightness)

The UK standard for measuring the air permeability of buildings is in relation to the surface area of the building (in contrast to n50). It is expressed in m3/(hr m2) at 50 Pa.

In the UK only a pressurisation test is needed.

rh - Relative Humidity

The relative humidity of air depends on its moisture content (absolute humidity) and air temperature.
Ideally internal humidity levels should be between 40 and 60% rh at 20 deg. C. For limited times the levels can range between 30 and 70 % rh.
Regular humidity levels above 70% rh can promote mould growth.

SAP - Standard Assessment Procedure

for new homes. Independed of location (using a ”standard location”) and including lights, heating and hot water but not cooking and appliances. Rating scale is from 1 to 100+ with the higher the number the lower the ful running costs. 100 corresponds to zero running costs, so the rating can be over 100 if surplus energy is exported. SAP rating provides the basis for the ECPs, which place the rating on an A-G scale (similat to what you would find on white goods).

SAP energy use predictions for dwellings are to be seen with a pinch of salt as they can vary from the real performance by up to 40%.

Sensor control

Form of automated operation of a MVHR system with the help of CO2 and/ or humidity sensors. In contrast to time control.

See also demand controlled ventilation.

SBS - Sick Building Syndrom

It is a medical condition where people in a building suffer from symptoms of illness or feel unwell for no apparent reason. Symptoms can be headaches, eye, nose, and throat irritation, fatigue, and dizziness and nausea. These symptoms appear to be linked to time spent in a building, though no specific illness or cause can be identified.

A 1984 World Health Organization (WHO) report suggested up to 30% of new and remodeled buildings worldwide may be subject of complaints related to poor indoor air quality. (Source: Wikipedia)

Solar gains

Solar gains is heat energy that is introduced into a building by sunshine, mainly through windows. Depending on the size and orientation of windows solar gains can be very powerful. Typically 1kW of heat can be gained through one normal sized window with full sun exposure.

Modern dwellings with large glazing and light weight building structures (timber frame) with hardly any thermal mass suffer especially from overheating through solar gains.

See overheating prevention.

Summer bypass

The summer bypass helps through passive cooling to a certain degree against overheating. It is typically an automatically operated flap that opens when the internal temperatures (measured with the extract air) rise above a certain threshold. This threshold can be adjusted according to the needs of the occupants.

Some systems have a summer bypass simulation, which switches the extract fan off when the internal temperatures rise above a certain threshold.

Target Air Tightness

The target air tightness of a dwelling is set forth by the designer at Building Warrant stage and is to be verified through an air tightness test at the end of the build.

TER - Target Emmission Rate

The DER must not exceed the Target Emission Rate (CO2) for a notional dwelling of the same size and shape.

Thermal bridge

A thermally weak part of a structural element, which drains heat out of the building.

Thermal bypass

Inconsistently applied insulation can cause cold air to pass underneath the insulation and defeat its properties.

Thermal mass

The heat that is contained in the structure of the building, e.g. in internally exposed stone or concrete floors or walls. By absorbing solar gains, it helps to level out in-door temperatures and reduces the risk of overheating in summer.

Thermal store

Used to store heat for hot water and/or space heating.

(Air) Transfer gaps or openings

These are typically internal door undercuts of 10-15mm to allow air circulation within the dwelling. Noise attenuated transfer units in internal walls are available, too.
The size of these gaps or the free area of transfer openings depend on the design transfer air flow.

Ventilation heat losses

Heat energy that is lost to the atmosphere through natural infiltration, manual cross-ventilation and mechanical ventilation.
Typically this is 25-35% of the overall heat demand of a dwelling, if it is correctly ventilated. This can be drastically reduced through heat recovery ventilation (MVHR).

Ventilation Strategy

Part of the design of a house, which sets forth the ventilation demand and the system used to meet that demand.

Volume flow

The volume flow of a ventilation system is either expressed in m3/h or l/s or m3/sec.

1 m3/h = 0.278 l/s
1 l/s = 3.6 m3/h
1 m3/sec = 3,600 m3/h

Volume flow constant operation

The volume flow constant operation of MVHR systems will adjust the fan power to keep the air flow rates at the same level. Increased resistance, e.g. from soiled filters will not lead to dropped ventilation rates or a dis-balanced system. This feature helps to keep the system at peak efficiency at all times. See also mass flow constant operation.



The g-value of windows indicates how much solar energy can travel through the glazing. The higher the value the more solar gains can enter into the dwelling.

Lambda value (λ)

The Lambda value is the intrinsic figure for the thermal conductivity of a material. The unit is W/(mK). Is the figure high, e.g. 220W/(mK) for Aluminium, the material is a good conductor of heat. Is it low, e.g. 0.035W/(mK) for silk, the material makes a good thermal insulation.

Psi value (Ψ)

The Psi-value is related to the lambda-value - the unit is also W/(mK) -, but in this case it denotes a thermal conductivity of an assembly of materials, e.g. the edge of a window glazing panel, where glass sheets, one or more spacers and the window frame with one or more different materials come together. It describes therfore the thermal bridging of that detail. The higher the figure, the higher the heat loss through that detail. Figures of 0.01W/(mK) and smaller are considered "thermal-bridge-free" and this is what detailed planning of a Passivhaus is aiming for. In some special cases (e.g. for detais where a well-insulated wall meets a well-insulated floor slab), the value might go even below 0, i.e. negative.


A measure of the rate of heat transfer of a building element. Units = W/(m2K).
When comparing U-values, the whole building element U-value should be considered, not just the value of a individual component, e.g. the U-value of a window (frame, glass & spacers) is less than the value for the glass alone.