There's a delicate balance between too much and too little ventilation.
Ventilation is important: it prevents rot occurring in roof-spaces, under-floor voids and other unseen building cavities; it lets in oxygen, which you need for solid fuel, gas and bottled gas fires to use in the burning process; and it also lets out carbon dioxide, water vapour and fumes which you don't want in your home.
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Choosing the right amount of ventilation will ensure you don't pay more than you need to for your energy bills, as too much ventilation can lead to unnecessary heat loss.
When to ventilate
In most parts of the home, if you have your heating on then your windows should be closed.
The vast majority of homes get enough ventilation through natural air leakage. This low level of ventilation takes away carbon dioxide and water vapour caused by people breathing and from activities such as drying clothes, and replaces it with air that has higher oxygen content.
Ventilation or air leakage will also take away odours, help to remove excess moisture in the air and reduce the likelihood of condensation on surfaces.
Generally speaking, if you need to leave your windows open -- other than in rooms where there is a likelihood of damp air, such as kitchens and bathrooms -- it's a sign that the heating system is producing too much heat.
With central heating, this may be because the thermostat is set too high or a thermostatic radiator valve is set too high in a particular room. There could also be a fault with the heating system, such as a broken thermostat.
In kitchens and bathrooms, you should only open the windows for short periods, as too much ventilation will make the room colder.
Warmer air can hold more moisture and warmer surfaces are less likely to get condensation on them. Therefore, the need to ventilate away damp air needs to be balanced with allowing too much warm air out and too much cold air in.