Air and climate in an underground car gallery

Why air must be pushed in, not just pulled out

A room above ground breathes through windows and small gaps. A room deep below ground has neither. If a system only extracts, air is dragged in by the easiest path instead. That path may be the lift shaft, the house stair or a joint in the walls, and it brings dust, fumes, damp and radon with it.

So a BELOW gallery has two halves. A supply side pushes in clean, dried air. An extract side pulls stale air and engine fumes out. The balance between them is what keeps the gallery clean.

Two needs that pull in opposite directions

When an engine runs, the car zone needs a lot of air for a short time. The rest of the time, the collection needs a little air that is very clean and very dry.

The system therefore runs in modes. A low, filtered, dried flow is the normal state. A strong purge is the event. Approved Document F Volume 2 sets the carbon monoxide limits for car parks below ground: 30 ppm averaged over 8 hours, and 90 ppm as a 15-minute peak at ramps and exits.

Filtration in stages

Outdoor air is cleaned in stages before it reaches the cars. Filters are graded under ISO 16890 by the size of particle they catch.

1. Pre-filter. Catches leaves, pollen and coarse dust, and protects the finer stages.
2. Carbon filter. Activated carbon takes out traffic gases such as nitrogen dioxide, plus ozone and other vapours. Ozone cracks rubber, so nothing that makes ozone is ever fitted.
3. Fine-dust filter. A final ePM1-class filter removes fine soot before the air reaches the paintwork.

Approved Document F also allows intake to be reduced for short periods when local traffic peaks, with expert advice. A gallery at rest needs little outdoor air, so this suits it well.

Dehumidification for the collection

Hagerty UK singles out moisture as a major threat to stored classic cars. Below ground, moisture comes from new concrete drying out, wet cars, people at events and humid summer air.

All outdoor air is dried before it enters. Separate units then hold the room between purges, with a spare unit so one failure does not let the air turn damp. Humidity is held within a steady band agreed for the collection, and kept stable through the seasons. Stability matters as much as the level itself.

The pressure cascade

Air moves from higher pressure to lower pressure. A cascade uses that to keep each space clean.

• The house and the lobby sit at positive pressure, so fumes and radon never rise into the house. The lobby also works as an air lock.
• The gallery sits slightly positive, so clean, dry air leaks outward rather than dust and fumes leaking in.
• The car zone, where engines start, sits negative. Fumes stay there and are extracted there.

Pressure sensors across each boundary report to the building controls, and raise an alarm if the cascade reverses. In a fire, the fire strategy takes over and sets the pressures instead.

A purge before the engine starts

The system is demand-controlled. When the lift is called, or the ramp door opens, extract in the car zone steps up first, before any engine turns over. Sensors for carbon monoxide, nitrogen dioxide and fuel vapour then hold the purge until the air is clear again.

Fuel vapour is heavier than air, so its sensors sit low, near the floor and in drainage channels.

Smoke clearance in two independent halves

Approved Document B Volume 2 sets out how an enclosed car park clears smoke. The mechanical system runs in two parts, each able to work alone and each on its own independent power supply. Half the outlets sit high and half sit low.

The same extract fans can serve daily purge and fire mode. In a fire, the conditioning unit shuts off, so smoke is never drawn into the filters and dehumidifiers. Replacement air comes in by the ramp or a protected shaft, never the car lift shaft.

Discreet garden intake and exhaust

Approved Document F asks that intakes sit away from pollution sources, including underground car parks. Exhausts should sit downwind of intakes, and must not discharge into courtyards, enclosures or architectural screens.

In practice that means a clean intake in a planted plinth or a pavilion wall on one side of the garden. The exhaust rises high and vertical on the other side, through a pavilion roof or a pergola column. Councils also care how vents and plant look, in a National Landscape, a conservation area or the setting of a listed house. So both are agreed at pre-application stage. Our planning permission guide covers that process.

Sensors, including radon

A gallery carries a network of sensors: carbon monoxide, nitrogen dioxide, fuel vapour at low level, carbon dioxide where people gather, temperature and humidity at car height, pressure, fine dust and leaks.

Radon is a natural gas from the ground, and a basement is where it collects. The HSE says an occupied workplace below ground needs radon measured anywhere in the UK, and sets a workplace reference level of 300 becquerels per cubic metre as an annual average. The UK target level for radon in homes is 100. A slightly positive gallery helps keep radon out, and a continuous monitor raises the fresh-air rate if levels rise.

The UK government's 2023 guidance on electric vehicles in covered car parks notes that off-gas detection for EV batteries is coming into the market. We weigh it for each collection.

Power that never fails

A gallery deep below ground depends on electricity for its air, its pumps and its security. So power is built in layers, and no single failure leaves the collection exposed.

Life safety on its own supplies

Life-safety systems carry their own protected supplies. Under BS 5839-1:2025, fire alarm batteries are sized for 24 hours on standby plus 30 minutes in alarm. BS 5266-1 recommends three hours of emergency lighting where a building may be reoccupied before its batteries recharge. The smoke fans run in two halves, each on an independent supply, as Approved Document B sets out.

Protecting the collection

A flood during a power cut is the classic basement failure, because storms bring both at once. So at least one sump pump has its own battery backup, on top of the generator. Security and monitoring stay powered, because a power cut is a known window for attack. At least one dehumidifier stays on too, since humidity creeps up in a sealed room within days.

Bridging the gap

An uninterruptible power supply keeps the controls, the security panel, the cameras and the network running without a flicker. Within seconds a standby generator starts and takes over automatically. Loads then restart in order: pumps first, then fans, dehumidifiers and the lifts.

The generator, out of sight

The generator sits outside, in a discreet acoustic enclosure in the garden or within a garden building. Where possible it runs on HVO, a renewable fuel that engine makers approve as a direct replacement for diesel in standby sets. Its fuel is held in a bunded tank, so any leak is contained.

Battery storage, if used, stays out of the gallery. PAS 63100:2024 rules out home battery storage in a basement without access to the outside, so it goes in an outbuilding or the garden plant enclosure.

The lifts in a power cut

The car lift is specified to lower safely on battery to the lowest level and open its doors. The owner's passenger lift returns to a safe floor, opens and waits. Both keep an alarm that works with the power off.

Tested every month

As part of aftercare, the generator runs on load every month. Once a year we switch off the mains and test the whole system together: fans, pumps, lifts, alarms and monitoring. Every result is logged and reported.

Remote monitoring

Every reading reaches a dashboard the owner and the collection manager can see from anywhere. Alarms go to phones in two levels: advisory to the maintenance team, urgent to the owner as well.

Logged data is kept, because it is the evidence if a car is ever damaged. The building controls sit on their own network, separate from the house, and remote access runs through a managed, secure connection. Our car gallery security guide explains why.

Clean, managed air, humidity held for the collection, and power that never fails are three of the twelve principles of the BELOW Standard.