So why do we need ventilation?
Building regulations require a minimum rate of ventilation approximately equivalent to half an air change per hour for the whole building. Even if building regulations did not require this, a minimum level of ventilation is such a good idea as it helps prevent condensation and the respiratory problems associated with stale air.
General improvements in house construction, building regulations and much higher levels of insulation in modern dwellings mean that houses are becoming almost airtight. Consequently, the air inside becomes stale with a build up of moisture and smells. The moisture can result in condensation and mould, and a generally ‘stuffy’ atmosphere.
What types of ventilation are available?
1) Single room extractors
Traditionally, ventilation has been provided by individual extract fans fitted to bathrooms and other ‘wet’ rooms. These provide short term ventilation, usually very noisily. The extracted air is replaced by fresh air entering the house through trickle vents in the window frames. These are neither attractive nor particularly sensible (having spent a lot of money on a well insulated house with high specification double glazing, a hole is made in the window frame in order to create a draught!)
2) Central Extract Ventilation
As modern houses have more wet rooms, so it becomes more economical to replace several individual extractors with one Central Extract Ventilation unit. With a CEV unit, only one vent to the exterior is required (via wall grill or roof vent) in order to expel the stale air and only one electrical connection with the obvious savings in installation costs over individual extract fans.
Using central extract ventilation, the replacement fresh air still enters the house through trickle vents. The fan runs continuously, allowing a gentle and imperceptible flow of air through the building. A CEV unit will have an override boost setting for the occasions when there are unpleasant smells or excessive moisture which need to be extracted. Whilst this is the minimum ventilation required by building regulations, it does mean that warm (stale) air is expelled from the building and is replaced cold fresh air through the trickle vents, resulting in a loss of heat.
Poor ventilation allows:
a build up of humidity resulting in condensation
the growth of mould in damp areas
the accumulation of unpleasant smells
a build up of carbon dioxide and other noxious substances
conditions in which dust mites thrive
By removing the stagnant damp air from a building, central extract unit will prevent a ‘stuffy’ atmosphere and the problems associated with poor ventilation can be eliminated. Good ventilation is of great benefit to those who suffer from asthma and other respiratory complaints
3) Heat Recovery Ventilation
From the Building Regulations 1995 Part F, amended 2010
“Ventilation systems in buildings result in energy being used to heat fresh air taken in from outside and, in mechanical ventilation systems, to move air into, out of and/or around the building. Energy efficiency is dealt with under Part L of the building regulations but consideration should be given to mitigation of ventilation energy use, where applicable, by employing heat recovery devices, efficient types of fan motor and/or energy saving control devices in the ventilation system.”
The diagram on the right shows the internal arrangement of a fairly sophisticated heat recovery unit.
This uses a counter-flow heat exchanger (the most efficent type). This particular model uses high-efficiency 190mm dia backward curved fans which result in low sound levels for any given airflow.
The (warm) stale air from the wet rooms and the (cold) fresh air from the exterior both pass through a heat exchanger, in opposite directions and without mixing.
The heat contained in the extract air is transferred to the incoming fresh air, which is introduced back into the living areas of the building.
Heat Recovery Ventilation does not produce heat, but minimises the amount of heat which would otherwise be lost through ventilation, with degrees of efficiency varying between 70 - 90%, depending on the model of MVHR unit.
In doing so, it reduces the amount of heat that needs to be produced to keep a house warm.
Heat Recovery Ventilation systems are designed to run continuously, virtually silently, providing clean fresh air throughout the building. Normal operation is at a very low speed, allowing the gentle movement of air from dry rooms to wet rooms and then to the exterior. Heat recovery units have two or more speed settings so that larger quantities of air can be extracted as the need arises. Depending on which system is chosen, humidistats, digital programmers, CO2 sensors and timers can also be used to control the ventilation.
Heat Recovery Ventilation offers the added bonus of filtering the incoming air so that there is less dust internally (and therefore less dusting to be done).
Pollen filters are available, which is a real boon to those who suffer from allergies, asthma and hay fever.
An HRV system can also be used to extract air from or supply air to rooms which get uncomfortably warm (garden or sun rooms, kitchens with range cookers, etc.) and redistribute the heat from this air, via the heat exchanger, to other rooms which are not so warm.
Building regulations do not permit air to be extracted under any circumstances from a room which contains a wood burning stove or open fire as it could lead to spillage of combustion products into the room. But it is quite acceptable to supply air to such a room.
The loss of heat can be avoided by using Heat Recovery Ventilation which works in the same manner as Central Extract Ventilation but additionally supplies fresh air from the exterior via a second fan instead of through trickle vents.