MVHR and Cooling: What the System Can and Can't Do

Every summer, without fail, we get a wave of enquiries asking some version of the same question: can my MVHR system cool the house? It's a reasonable thing to ask, particularly after a hot July. The honest answer is: partly, in the right conditions, but probably not as much as you're hoping for. Here's why, and what to do about it.

^{Solar shading (external sliding blinds) on a retrofit project in Queen's Park, NW London}

What MVHR Actually Does

Mechanical Ventilation with Heat Recovery is, at its core, a ventilation system with a useful thermal side-effect. It continuously extracts stale air from wet rooms and kitchens, passes it through a heat exchanger, and uses the recovered heat to warm incoming fresh air from outside. In winter, this means you're not losing the heat you've paid to generate every time the building breathes. The efficiency of a good heat exchanger is typically 85 to 95%, which is significant.

In summer the process can work in reverse to a limited extent. If the indoor air is cooler than the air outside, the system can pre-cool incoming air by passing it through the same exchanger. But this only works when the indoor temperature is lower than outdoor, and it only works within the constraints of what air can physically carry.

That constraint is the critical one. Air has a low thermal mass. It doesn't hold much heat or coolth relative to its volume. For effective cooling you need to move a lot of air, typically around 2 air changes per hour. MVHR systems are designed to operate at around 0.5 air changes per hour at boost, primarily to maintain air quality rather than to shift heat. The gap between those two figures tells you most of what you need to know about the system's cooling limitations.

MVHR is also a single zone system. It supplies and extracts air across the whole building without the ability to target specific rooms. A south-facing bedroom that overheats on summer evenings will get the same treatment as a north-facing study that's perfectly comfortable. That bluntness limits its usefulness as a cooling tool in buildings with varied solar exposure.

Designing Out Overheating From the Start

The most cost-effective cooling strategy is the one you build into the design before anything is constructed. Retrofitting solutions to an overheating building is always more expensive and often more compromised than addressing the root cause at design stage.

Overheating in UK buildings is overwhelmingly driven by solar gain, primarily through glazing. Large south and west-facing windows, without adequate shading, will overheat a well-insulated building in summer regardless of how sophisticated the ventilation system is. The insulation that keeps the building warm in winter also keeps the heat in once it's entered.

The Passivhaus Planning Package, PHPP, is the modelling tool we use to assess overheating risk. It allows us to test different orientations, window sizes, glazing specifications, and shading strategies before anything is built, and to understand the interaction between them. A 15-degree rotation of a building on its site, a deeper overhang over south-facing glazing, or a reduction in west-facing window area can each have a material impact on summer comfort that no ventilation system can replicate.

The key shading strategies we return to most often are:

External overhangs and eaves sized to block high summer sun while admitting low winter sun. This is the most passive and maintenance-free solution when the geometry works.

Adjustable external louvres or blinds for facades where a fixed overhang can't do the full job. The Herbert Paradise project uses external sliding blinds on the south and west elevations, which the occupants can adjust through the day. They work.

Brise soleil elements on facades with significant glazing, particularly on commercial or larger residential projects where the architectural language allows for it.

Vegetation, particularly deciduous trees on southern boundaries, which provide shade in summer and allow solar gain in winter. This is underused as a strategy, possibly because its benefits are less immediately legible than built shading devices.

Thermal mass in the floor slab and internal walls absorbs heat during the day and releases it slowly, moderating temperature peaks. It works best when combined with night purge ventilation, opening windows in the evening to flush warm air and pre-cool the mass overnight.

^{Team discussion at the ‘drawing board’, we use 3D drawing software to accurately map out the location of the duct routes and position of the MVHR unit}

If the Building Is Already Built

Not every overheating problem can be solved at design stage, because not every project starts from scratch. For buildings that are already overheating, the options break down into passive interventions and active cooling systems.

On the passive side, external shading can often be added retrospectively. External blinds, awnings, or pergola structures can be attached to most buildings without structural intervention. Internal blinds are considerably less effective than external shading, because the heat has already entered the building by the time it hits an internal blind, but they're better than nothing.

High-performance glazing upgrades, replacing single or poor double glazing with solar control triple glazing, can reduce solar gain meaningfully. The embodied carbon and cost of glazing replacement needs to be weighed against the benefit, but for a building with severe overheating through large areas of glazing it can be justified.

Natural night-time ventilation is free and effective if the building layout allows it. Opening windows on opposite sides of the building in the evening creates cross-ventilation that flushes warm air and cools the thermal mass overnight. Security concerns in urban locations are a real constraint on this, but it's worth designing for where possible.

Internal heat gains deserve more attention than they usually get. In a well-insulated building, the heat from occupants, cooking, lighting, and appliances compounds with solar gain. LED lighting and efficient appliances reduce this significantly. Cooking early in the morning or late in the evening rather than at the hottest part of the day makes a measurable difference. These sound like small things but they add up in a tight building.

When Active Cooling Is Necessary

There are buildings and situations where passive strategies aren't sufficient and active cooling is the right answer. This is particularly true for buildings with high internal heat loads, large west-facing glazing that can't be shaded effectively, or in locations where urban heat island effects raise ambient temperatures significantly above rural equivalents.

The main options worth knowing about:

Split-system air conditioners are the most common active cooling solution for residential buildings. An outdoor condenser unit and an indoor evaporator, installed in the rooms that need it. They're efficient when sized correctly, relatively simple to install, and can cool a room quickly. The environmental case depends heavily on the electricity supply: running a heat pump on renewable electricity is a very different proposition to running one on grid electricity at peak times.

Fan coils circulate air over a chilled water coil and return cooled air to the room. They can be ceiling or wall mounted, provide targeted cooling to specific rooms, and can be connected to a central chiller or to a heat pump. They're particularly useful for rooms with high occupancy or high appliance loads where cooling demand is localised.

Chilled beams use chilled water circulated through ceiling-mounted elements, cooling through natural convection. They're quiet, efficient, and architecturally clean, but they require a chilled water circuit and are more commonly used in commercial buildings than residential.

Geothermal cooling uses the stable temperature of the ground as a heat sink, circulating fluid through buried pipes to transfer heat from the building to the earth. It's highly efficient and genuinely sustainable, but the upfront cost and site requirements limit its applicability.

One important point: active cooling systems can work alongside MVHR without compromising the ventilation system's performance. They're separate functions. The MVHR continues to manage air quality and heat recovery; the cooling system manages temperature. They don't need to be integrated and are generally better kept separate.

Putting It Together

The hierarchy for managing summer overheating is straightforward, even if the execution requires careful judgement:

First, design out the problem. Orientation, window sizing, shading, and thermal mass are the primary tools. Get these right and you may not need anything else.

Second, use MVHR intelligently. Night purge ventilation through the MVHR system, or through openable windows where security allows, pre-cools the building overnight. The system maintains air quality throughout and provides modest passive cooling when indoor temperatures are lower than outdoor.

Third, add targeted active cooling where the passive strategy isn't sufficient. Sized appropriately to the actual cooling load, not oversized, and ideally connected to a renewable electricity source.

At RISE we model overheating risk using PHPP on every project where there's meaningful glazing. The results consistently show that the right design decisions made early are worth more than any technology applied afterwards. That's true of energy performance generally, but it's particularly true of cooling, where the problem is almost always easier to prevent than to cure.

If you're concerned about overheating in an existing building or planning a project where summer comfort is a priority, we're glad to talk it through.

→ Email us at architects@risedesignstudio.co.uk
→ Or call the studio on 020 3947 5886

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