How does indoor temperature affect air flow in passive stack ventilation?

Indoor temperature significantly affects air flow in passive stack ventilation. This relationship hinges on the stack effect, a natural phenomenon where warmer, less dense indoor air rises through vertical ducts, creating a pressure differential that draws cooler air in through lower-level vents. The greater the temperature difference between inside and outside, the stronger this effect and the resulting air flow.

The Fundamental Science: Understanding the Stack Effect

Passive stack ventilation (PSV) is a beautifully simple concept that relies entirely on natural physics rather than mechanical force. At its heart is the stack effect, sometimes called the chimney effect. Imagine a hot air balloon. The burner heats the air inside the balloon, making it lighter than the cooler air outside, which causes the balloon to rise. A PSV system operates on precisely the same principle within the structure of your home.

Warmer air inside your property, generated by occupants, cooking, bathing, and heating systems, becomes less dense. This buoyant air naturally seeks to rise upwards. PSV systems capitalise on this by providing a dedicated pathway: vertical ducts that run from high-moisture areas like kitchens and bathrooms to terminals on the roof. As this warm air ascends, it creates a area of slightly lower pressure at the bottom of the duct. This negative pressure actively pulls cooler, fresher air into the building through purposely placed lower-level inlets, such as trickle vents in windows or wall vents. This creates a continuous, silent cycle of air exchange without a single watt of electricity being used.

The UK Climate: A Seasonal Rollercoaster for PSV

The performance of a PSV system is not constant; it fluctuates dramatically with the UK’s distinct seasons because its efficiency is directly proportional to the temperature differential (ΔT) between the indoor and outdoor environments.

Winter (High ΔT): This is where PSV systems are theoretically at their most effective. During a typical British winter, indoor temperatures are often maintained at a comfortable 18-21°C, while outdoor temperatures can frequently hover near freezing (0°C) or even lower. This significant difference of 18-21 degrees creates a powerful stack effect, driving strong air flow and providing decent ventilation for extracting moisture and pollutants.
Summer (Low ΔT): Conversely, during a warm British summer, the system’s performance can plummet. On a day where it’s 25°C outside and you might have your windows open, the indoor temperature could be a similar 24-26°C. With a ΔT of perhaps just 1 or 2 degrees, the driving force for the stack effect virtually disappears. Air flow through the stacks becomes negligible, stagnant, or can even reverse, drawing warm air downwards. This is why homes relying solely on PSV often suffer from poor indoor air quality, stuffiness, and lingering humidity during warmer periods.

The Inherent Limitations of Passive Systems

While elegant in their simplicity, PSV systems come with a set of inherent challenges that are crucial to understand. They are, by their very nature, uncontrolled. Their performance is at the mercy of the weather, leading to significant inconsistency.

Unpredictable Performance: You cannot dictate the rate of air change. It might be excellent on a cold, windy night but wholly inadequate on a still, warm day. This inconsistency makes it difficult to guarantee stable humidity levels and air quality year-round.
Dependence on Airtightness: Modern UK building regulations demand increasingly airtight homes to improve energy efficiency. Ironically, this can severely hamper a PSV system, which relies on passive infiltration through vents for its air supply. If a home is too airtight, the necessary replacement air cannot enter, stifling the stack effect and rendering the extract vents ineffective.
Risk of Downdraught: In certain wind conditions, particularly with certain roof terminal designs, wind can force its way down the stack, pushing air back into the home instead of extracting it.
No Filtration: PSV systems bring in fresh air through uncontrolled vents. This air is unfiltered, meaning pollen, dust, pollution, and other allergens from outside are drawn directly into your living space.

Taking Control of Your Air

At VENTI, our core purpose is to empower you to breathe freely, and a key part of that is moving from passive hope to active control. Relying solely on the whims of the weather for your ventilation is a gamble with your health and your property. Damp, mould, and condensation are not just inconveniences; they are indicators of a poor indoor environment that can impact respiratory health and the very fabric of your home.

This is where modern Mechanical Ventilation with Heat Recovery (MVHR) systems, like our whole-house RESPIRO or single-room FLUXO and AUREN units, present a profound upgrade. Think of it as the evolution from a sailboat (PSV), which depends on the wind, to a motorboat (MVHR), which has its own reliable engine.

These systems use highly efficient fans to provide a constant, measurable, and controlled supply of fresh, filtered air and extraction of stale air. Crucially, they incorporate a heat exchanger that captures the thermal energy from the outgoing stale air and uses it to warm the incoming fresh air. This means you achieve superb ventilation without the colossal heat loss associated with opening windows or the unpredictable nature of PSV. You get clean air and energy efficiency, regardless of the indoor or outdoor temperature.

For truly consistent, filtered, and energy-efficient air quality that is unaffected by indoor temperature fluctuations, explore our range of intelligent mechanical ventilation solutions designed for UK homes.