Guide to Mechanical Ventilation & Heat Recovery

Offering year-round fresh ventilation and a welcome boost of low-cost warmth during the colder months, this system is becoming a must-have for many self-build projects
by Build It
18th November 2015

Increasingly popular for efficient custom homes, this technology builds on the principal of mechanical ventilation – whereby stale air is automatically removed from a home and replaced with a fresh incoming provision.

In addition, MVHR extracts and recycles any warmth in the outgoing air by transferring it into the new supply – hence the ‘heat recovery’ part of the name. The result is a two-in-one product that can help optimise the climate in your home.

What is MVHR?

MVHR replaces trickle vents and the use of individual extract fans with a single, whole-house solution. It works most effectively in new homes built to modern standards of energy efficiency. “The better the air tightness, in particular, the more heat you can recover as you won’t be losing warmth through weak points in the structure,” says Clarissa Youden, associate director at Total Home Environment.

Retrofitting is possible but rare, as installation can be disruptive and the fabric of the building will need to be significantly upgraded.

It’s usually specified alongside your primary heating system (such as a gas boiler or heat pump) to help optimise energy usage. “It’s quite common to save around a third on your bills,” says Clarissa.

“You also get the benefit of constant fresh air – and when used in conjunction with a class F7 filter, you’ll enjoy an internal climate that’s cleaned of most pollen and allergens, which can be a boon for those with allergies or respiratory conditions.”

An MVHR unit will also keep a lid on humidity levels, which reduces the risk of condensation and mould growth. It will provide constant odour removal from kitchens and bathrooms, too, with boost options available if required.

The main unit runs on electricity – although it only requires as much energy as a typical low-energy light bulb. According to the Green Building Store, that equates to around £73 per year – which is likely to be far outweighed by savings on your heating bills.

The Hills’ Passivhaus MVHR by Paul Heat Recovery

passivhaus bungalow

When Tony and Jean Hills decided to build a prefab bungalow, they knew from an early stage that they wanted to achieve Passivhaus standards to keep running costs down. An essential ingredient of this energy-saving design approach is to establish a highly airtight living environment with minimal heating requirements; exactly the kind of scenario where it makes sense to install MVHR.

“Our north-facing home is 100m up on an exposed site in Inverness – further north than Moscow,” says Tony. “After three months of complex calculations, the Passivhaus Trust insisted that we fit a Paul Novus 300 MVHR system. It’s able to recover 94.4% of heat from moisture-laden rooms and add it to fresh, purified air that’s quietly pumped into the bedrooms and living room.”

Working in conjunction with a 4m2 array of solar thermal panels, an Ecotec boiler from Vaillant and a 300 litre storage tank, the MVHR system makes a big contribution to the Hills’ minuscule heating bills, which are expected to amount to just £140 per year.

Passivhaus Bungalow

Installating MVHR

The heart of the system is the heat recovery unit, which is usually fitted in the loft – either on a bed of rigid insulation or hanging from the rafters to reduce vibration. You’ll need to engage a specialist engineer to determine the correct output, ducting runs and terminal locations.

“The ducts are the veins of the system and should be configured in the straightest, shortest runs possible with the least amount of bends to help maintain good airflow,” says Clarissa. “3D computer-aided design helps with this.”

If the main appliance is in the loft, dropping into the rooms below should be easy. Some ducts may need to run through the floor structure, so consider using metal-web joists to allow your installer easy access.

Some boxing out may be required to disguise the pipes. As a guide, for a typical three-bedroom new build home this part of the process should take around three days – plus an extra day for commissioning. The system will run constantly in the background, providing the whole-house air changes necessary to ventilate a modern home.

The only maintenance is likely to be changing the filters two or three times per year. Good controls are a must – some can be trained to your home’s exact volume and space heating demands. Look for features such as an automatic summer bypass, too. This disables the heat recovery function in balmy weather.

What does mechnical ventilation and heat recovery cost?

MVHR systems are always individually-tailored to suit the property, so there are no hard-and-fast rules on price. As a general guide, a complete system for a 150m2 new build home, insulated to a good specification, is likely to cost around £4,000 including design and installation.

Photo: Total Home Environment’s Genvex Premium MVHR unit

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