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People often assume air conditioning will cure a stuffy room. It will make the room cooler, and usually drier, but a standard split system brings in no fresh air at all. It takes air from the room, conditions it and sends it back. So if a space feels close, smells stale or gets condensation, you'll probably need ventilation as well as cooling, and sometimes instead of it.
Below we explain the difference and the main types of ventilation used in UK homes and workplaces. We also cover how UK Aircon plans the two so they help each other out.
Is air conditioning the same as ventilation?
No. Air conditioning recirculates and conditions the air that's already in a room, while ventilation swaps indoor air for outdoor air. A wall-mounted split, a cassette and most ducted units pull air from the room, pass it over a coil to cool or heat it, filter it and blow it back out. The refrigerant pipes running to the outdoor unit carry heat. They don't carry air. Ventilation works differently. It gets rid of stale, damp or polluted air and replaces it with fresh air from outside, either naturally through windows and trickle vents or mechanically with fans.
| Air conditioning | Mechanical ventilation | |
|---|---|---|
| Main job | Controls temperature by cooling or heating the air | Replaces stale indoor air with fresh outdoor air |
| Fresh air supply | None in a standard split system | Yes, continuous or on demand |
| Humidity | Removes moisture while cooling | Removes moisture by extracting damp air |
| Carbon dioxide and odours | Not reduced | Diluted and removed |
| Filtration | Filters recirculated air | Can filter incoming air |
Heat recovery ventilation (MVHR)
Mechanical ventilation with heat recovery, usually called MVHR, supplies fresh air to living spaces and extracts stale air from wet rooms, passing both airstreams through a heat exchanger so most of the warmth in the outgoing air is transferred to the incoming air. The two airstreams never mix. You get a steady supply of filtered fresh air without throwing away heat you've already paid for.
MVHR works best in airtight buildings (new builds, deep retrofits, well-insulated extensions) where natural draughts no longer give enough air change. It runs all the time at low speed and boosts when you're cooking or showering. Many units also have a summer bypass, so cooler night air can come in without going through the heat exchanger.
Bear in mind MVHR ventilates. It doesn't cool. On a hot afternoon it will bring in warm outdoor air, which is why homes with a real overheating problem often pair it with air conditioning in the worst rooms.
Single-room heat recovery units
Ducting through the whole house isn't always practical. In that case a single-room heat recovery unit, fitted through an external wall, can give balanced ventilation to one space, for example a bedroom or home office that also has a split air conditioner.
Extract fans in kitchens, bathrooms and utility rooms
An extract fan deals with moisture and smells where they start, before they spread through the building. For new work, Building Regulations expect extract ventilation in kitchens, bathrooms and other wet rooms. You'll mostly come across two kinds: intermittent fans that run with the light or a humidity sensor, and continuous low-speed fans that boost when needed. In a commercial kitchen, the extract canopy does this job on a much larger scale, and the air conditioning must be designed so it does not pull against it.
Humidity, condensation and mould
Condensation forms when warm, moist air meets a cold surface, such as a window, an external wall or an uninsulated corner. Leave it and you end up with black mould, peeling paint and a musty smell. Cooking, baths and showers, drying washing indoors, even breathing, all put moisture into the air.
In summer air conditioning helps, because cooling pulls water out of the air as it goes, and that water is collected and drained away through the condensate pipe. Plenty of units also have a dry mode for muggy days, and our guide to domestic air conditioning for UK homes is a good starting point if you're choosing one. In winter it's a different story. When the system is heating or switched off, it does little to bring humidity down. Ventilation, particularly continuous extract or MVHR, controls moisture all year. Most people find indoor relative humidity of roughly 40 to 60 per cent comfortable, and keeping within that range discourages both mould and dust mites.
- Condensation on windows most mornings
- Mould in corners, behind furniture or around window reveals
- Musty smells in wardrobes or rarely used rooms
- Steam lingering in a bathroom long after a shower
If any of these sound familiar, sort out ventilation before you choose cooling equipment. You can find your nearest engineer through our air conditioning near me page if you'd like someone to take a look.
Filtration, allergies and indoor air quality
Every air conditioning indoor unit has washable filters that catch dust, fibres and pet hair from the recirculated air, and some models add finer filtration stages. Keep them clean. A clogged filter cuts airflow, makes the unit work harder and can start to smell. When the system is in heavy use, check the filters every few weeks.
Allergy sufferers usually get the best results from a mix of measures. That means filtered recirculation from the air conditioner and fresh filtered air from MVHR, with humidity kept in a sensible range and, if needed, a dedicated indoor air purifier. UK Aircon can supply and fit indoor air purifiers alongside air conditioning and ventilation equipment.
Particles are only part of the picture. With people in a room and the windows shut, carbon dioxide levels climb, and most of us notice that as drowsiness or a stuffy feel. Ventilation is the only thing that brings it down. Air conditioning on its own can't.
Designing air conditioning and ventilation to work together
Plan both at the same time and they'll cost less to run and be more comfortable to live or work with. We do it in four steps:
- Establish the problem. Is the room too hot, too humid, too stuffy, or all three? We look at heat gains, moisture sources, occupancy and existing vents.
- Provide fresh air first. Make sure the space has adequate ventilation, whether trickle vents, extract fans, single-room heat recovery or whole-house MVHR.
- Size the cooling for the remaining load. With ventilation accounted for, the air conditioning can be sized accurately rather than oversized to compensate.
- Coordinate controls and positions. Supply grilles and indoor units are positioned so airflows do not short-circuit, and settings are agreed so systems are not fighting each other.
Loft and attic ventilation
A hot loft radiates heat into the rooms below, and it can trap moisture that damages the timbers. Good attic ventilation through eaves, ridge or tile vents keeps the roof space closer to outdoor conditions. If you're converting a loft and want air conditioning in the new room, ventilating the rest of the roof void properly is part of the same job.
How we can help
UK Aircon installs air conditioning for homes and commercial premises, and we look at ventilation during the survey. Whatever we fit is designed so the two systems work together. To keep everything performing, see our maintenance and repair services or the blog guide to air conditioning maintenance. For an overview of everything we do, visit our services, or contact us to arrange a free survey.
Questions about ventilation and air conditioning
Does air conditioning bring fresh air into a room?
No. A standard split, cassette or ducted air conditioner recirculates and conditions the air already in the room. Fresh air must come from ventilation, such as windows, trickle vents, extract fans or a mechanical ventilation system.
What is the difference between MVHR and air conditioning?
MVHR supplies fresh filtered air and extracts stale air while recovering heat from the outgoing air. Air conditioning cools or heats the air inside a room. MVHR does not provide active cooling, so the two are often installed together.
Can air conditioning help with condensation and damp?
Air conditioning removes moisture while it is cooling and many units have a dry mode. However, it does little to control humidity in winter, so persistent condensation and mould usually need improved ventilation as well.
Do air conditioner filters help with allergies?
Air conditioner filters capture dust, fibres and pet hair from recirculated air, which can help allergy sufferers if the filters are cleaned regularly. Combining them with fresh filtered air from ventilation and a dedicated air purifier gives the best results.
Why does a room still feel stuffy with the air conditioning on?
Stuffiness is usually caused by rising carbon dioxide and odours from occupants, which air conditioning does not remove. The room needs more fresh air through ventilation, even if the temperature is comfortable.