Ventilation systems and MVHR

Air conditioning and ventilation are different jobs

It is easy to assume that air conditioning brings fresh air into a room. Most split systems don’t. A wall-mounted or cassette unit takes in the air already in the room, cools or heats it, filters it and blows it back out. It changes the temperature and takes some moisture out, but it doesn’t replace stale air with fresh air from outside.

Ventilation does that job. It removes moist, stale air from kitchens, bathrooms and busy rooms and brings in fresh air to replace it. In a home or office where the windows stay shut, because the air conditioning is on, because of noise or security, or because the building is very airtight, ventilation needs planning in its own right. We look at both together so the two systems work with each other rather than against each other.

What MVHR does

Mechanical ventilation with heat recovery, usually shortened to MVHR, is a whole-building ventilation system. A central unit, often in a loft, cupboard or plant space, runs two sets of ducts. One extracts stale, moist air from wet rooms such as kitchens, bathrooms and utility rooms. The other supplies fresh, filtered air to living rooms, bedrooms and offices. Inside the unit, the two air streams pass through a heat exchanger without mixing, so much of the warmth in the outgoing air is passed to the incoming fresh air.

The result is a steady supply of fresh air without the heat loss of opening windows. MVHR is common in new and highly insulated homes and can be fitted in a refurbishment where there is room for the ductwork. How well it performs depends on the design, the duct routes, the balance between supply and extract and regular filter changes.

Where ventilation helps most

• Airtight new builds and deep retrofits, where there are few gaps for air to get in on its own.
• Homes with condensation and mould on cold walls and windows, especially bedrooms and bathrooms.
• Offices and meeting rooms where the windows are kept closed and the air feels stale by the afternoon.
• Loft conversions, extensions and garden rooms that have been sealed up well but have little natural ventilation.
• Shops, salons, surgeries and other businesses where people spend long periods indoors.

Building Regulations and design

In England, ventilation in new buildings and in many building works is covered by Part F of the Building Regulations, with guidance in Approved Document F, which has one volume for dwellings and one for other buildings (GOV.UK: Approved Document F). Whether your project has to meet it depends on the work involved, so we check that at the design stage and tell you if building control needs to be involved.

Good design starts with how the building is used: how many people, which rooms produce moisture, where the air can be supplied and extracted and how the ducts will run. We size the unit and the ducts, plan where the intake and exhaust terminals go on the outside wall or roof so they are well apart, and plan access so the filters can be changed easily. This page is general information, not a substitute for advice from building control.

Ventilation with air conditioning

Where we are fitting air conditioning in a building that also needs fresh air, we plan the two together. The ventilation brings in fresh air and removes stale air, and the air conditioning handles the heating and cooling. In larger buildings, the fresh air can be delivered close to the air conditioning units so the room stays comfortable. In homes, MVHR and a split system can sit side by side, with the MVHR running all the time and the air conditioning used when it is needed.

Getting this right at the design stage avoids rooms that are cool but stuffy, or fresh but too hot. It also avoids two contractors cutting holes in the same ceiling.

Commissioning and maintenance

A ventilation system needs setting up after it is installed. We measure the airflows at each supply and extract point and balance the system so each room gets the right amount of air and the supply and extract match. You get a record of the settings, the controls information and the manufacturer’s warranty details.

After that, the main job is changing or cleaning the filters on the schedule the manufacturer sets. Dirty filters cut the airflow and make the fan work harder. We can add your ventilation unit to the same visit as your air conditioning service, so both are looked after together.

What it costs

Every ventilation system is designed for the building, so we price after a survey. The size of the property, the number of rooms, the duct routes, the unit and the amount of building work needed to hide the ducts all affect the cost. You get a fixed price with your quote before any work starts.