Hot air or real solution? What role can decentralized ventilation systems play?

Panel: 8. Buildings: technologies and systems beyond energy efficiency

Authors:
Agneta Persson, eceee President, Sweden
Sanna Börjeson, Anthesis, Sweden

Abstract

Several studies have found that there is a large need for new energy- and cost-efficient heating and ventilation systems that are easy to install in Swedish existing single-family homes. The heating part of the equation is fairly easy to solve, but the ventilation is a larger challenge when aiming for supply and exhaust air with heat recovery (ESX). The main difficulties are related to installation of ducts and investment costs. A suitable solution may be decentralized ventilation systems (DVS). The units are ductless, which simplifies installation and maintenance. Several studies from other countries have confirmed that DVS led to a better air quality and provide increased thermal comfort. However, the systems' heat recovery rate, often stated to be up to 90 percent, has also been questioned.

There are two different kinds of decentralized ventilation systems. Regenerative heat exchangers with a built-in fan and a ceramic material accumulating energy from the extracted air. The air flow direction changes every minute, and the accumulated heat transfers to the supply air. These systems are usually installed in pairs. The other DVS principle has a recuperative cross counterflow heat exchanger transferring the heat between the air flows. Both types of DVS are installed directly in the building walls. In addition to the advantages of the systems being ductless the units have a low power demand resulting in low energy costs.

Despite the fact that there has been a major market development of decentralized units, there are only few studies on the systems’ potential, and there is no evidence-based monitoring on their performance in a Nordic climate. This study has analysed and proposed methods for testing, verifying, and developing DVS for the Nordic climate. The aim of the study was to contribute to an increased realization of energy-efficient measures that at the same time leads to improved ventilation in existing single-family homes.

The study shows that there is a gap between the development of the market and monitoring standards and legislation. There is currently no harmonised evaluation standard for these systems, which means that suppliers refer to different standards when reporting their system’s performance. Among Swedish suppliers of DVS there is a major lack of knowledge regarding how the systems have been tested and under what conditions the technical specifications have been developed. These uncertainties cause situations with incorrect information on energy performances.

Even in research these systems have been evaluated in different ways due to the lack of a common methodology. Three research studies on DVS have been reviewed. Regarding heat recovery research results between 63 and 80 % are reported, which is significantly lower than the manufacturers’ reports. Literature and contacts with suppliers also mention that a challenge for these systems is their noise levels, mainly due to the units being placed directly in the building walls. Usually, the noise level of the devices is moderate, varying from approx. 30 to 45 dB at a distance of one meter. One explanation why the noise level commonly is perceived as disturbing may be that, due to the ventilation changing the air flow direction in cycles, the noise profile continuously changes. A consequence of the noise level being perceived as disturbing may be that the users choose to reduce the air flow, which is counterproductive to the primary aim of installing the DVS, to achieve an improved air quality and provide increased thermal comfort.

In conclusion, the study confirms that a DVS can be an energy- and cost-efficient alternative when an installation of ESX with ducts isn’t technically possible, unaffordable, or not suitable for other reasons. However, to reach a broad homeowner acceptance of DVS there are several challenges requiring continued development. The most important are probably the systems' energy performance, air circulation, sound level and design.

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