SUPPLY AND EXHAUST VENTILATION SYSTEM COMPARISON

SUPPLY AND EXHAUST VENTILATION SYSTEM WITH HEAT RECOVERY IN COMPARISON TO A DEMAND-BASED (MOISTURE-CONTROLLED) EXHAUST VENTILATION SYSTEM

M. Krus, D. Rösler, A. Holm

Fraunhofer-Institut for Building Physics, 83626 Valley, Germany

As a countermeasure to global warming, the energy demand of buildings is to be reduced by specific measures, for example thermal insulation or intelligent ventilation systems. A demand-based (moisture-controlled) exhaust ventilation system is assessed in comparison to a supply and exhaust ventilation system with heat recovery by means of computational investigations. This assessment of different ventilation systems is performed by means of the newly developed hygrothermal indoor climate simulation model WUFI®-Plus. By implementing the individual ventilation systems the energy demand, especially the primary energy consumption on the basis of applying various fuels, as well as the effects on the indoor climate and the C02 content of the indoor air are calculated and compared. Moreover, air change rates are investigated resulting from the use of a demand-based exhaust ventilation system. The calculations are based on a model apartment with a ground floor of 75 m2 and an assumed 3-person household. These investigations comprise 3 different climates in Germany (cold, medium and hot climate).

Despite the high heat recovery coefficient of the supply and exhaust ventilation system an only slightly higher energy use occurred for the demand-based exhaust ventilation system. If regenerative energy sources such as wood are used, primary energy consumption of the demand-based exhaust ventilation system is even lower in comparison to the supply and exhaust ventilation system with heat recovery. With demand-based exhaust ventilation system, the C02 concentration of the indoor air remains permanently below 1200 ppm.

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Health risk associated with passive houses: An exploration

Evert Hasselaar*
Delft University of Technology, The Netherlands
*Corresponding email: e.hasselaar@tudelft.nl

SUMMARY
The passive house standard of northern European countries functions as an inspiration for home owners and project developers for building or retrofitting with high energy ambitions.
Passive houses typically involve high insulation levels and heat recovery ventilation. Residual heating is based on heating of the inlet airflow, but other solutions (stove etc.) are applied as well. The development of energy-efficient building is technology driven. The feedback from the consumers is low and there have been complaints by occupants about perceived health effects of heat recovery ventilation. Examples of passive houses are analyzed to find
indicators of “emerging” health risks. Potential problems are overheating, noise from installations, legionella contamination of domestic water buffers, low ventilation volumes, complex control mechanisms and lack of flexibility of ventilation services.

Recommendations
are given for the improvement of the user friendliness of indoor climate systems for passive houses.

KEYWORDS
Energy performance, Passive house, Health, User influence

INTRODUCTION
In the Netherlands the energy use of new buildings is subject to performance based legislation. The energy consumption is calculated on the basis of a physical model and results in a dimensionless Energy Performance Coefficient (EPC). Since its introduction in 1996 the EPC-value has become stricter and was reduced from 1.4 in 1996 to 0.8 in 2006, which is an improvement of 43%. Each step supported the market growth of certain products or systems.
Since 1998 the application of heat recovery balanced ventilation systems in newly constructed dwellings grew from 5% to about 40% (Hasselaar, 2006). Heat recovery ventilation represents a cost-effective way to reduce the EPC-value, because of the highly efficient heat exchanger (>80% thermal efficiency) and improved motorized fans that save approximately 50%
electrical energy compared to conventional fans.
The ambition to reach high energy performance has led to the development of the Passive house standard, where the strategy to reduce the energy demand is followed to such extremes, that a central heating system is not required. The energy demand for space heating and cooling of passive houses is limited to maximum 15 kWh/m2y of treated floor area (in Western European climate regions). The primary energy use of all appliances including domestic hot water, space heating and cooling, lighting and domestic appliances must not exceed 120 kWh/m2y. Many passive houses have been built in Austria and Germany and other countries follow: Sweden, Belgium, while the interest in passive houses and passive renovation is increasing in the Netherlands as well (Mlecnik, 2005). Because innovative concepts are often driven by top-down energy policy and the most economic way to meet regulations, the building sector tends to focus on the reduction of the Energy Performance Indoor Air 2008, 17-22 August 2008, Copenhagen, Denmark – Paper ID: 689 Coefficient rather than the actual energy performance. The technology driven approach creates the risk of poor user orientation or conflicts with indoor environmental quality.
In energy efficient designs, a three step strategy is followed: reduce the demand for energy, supply energy through sustainable sources and finally apply systems with high energy efficiency. The leading paradigm of achieving energy quality is to apply improved technology without change of behavior required, but occupants of collectively self-built projects have the opinion that behavior does matter in reaching the maximum energy effect (Ornetzeder, 2001).
User friendly technological solutions will provide control functions to make performance behavior dependent and with flexibility to allow individual adaptations. This requirement turns the “trias energetica” into a four step strategy, the fourth step being: provide control systems in support of energy conscious handling of processes by the users. Direct involvement of occupants in the design process of dwellings and climate systems is one way of promoting the user friendliness, allowing occupants to “learn” behavior that is more adapted to the needs of sustainable housing (Ornetzeder, 2001). The question is how to organize participation in the design of new buildings and of renovation projects.
Passive house designs typically involve very high insulation levels, triple glazed windows in frames with thermal barrier and perfect sealing. The envelope is without thermal bridges. Heat recovery ventilation is standard. Often, solar thermal and photovoltaic systems are applied.
Because of overheating risk in the summer, services for night time cooling (ventilation) are provided, in certain cases by applying ground-to-air heat exchangers. Residual heating can be a simple electrical heat resistance radiator or wood burning stove. Often, however, floor or wall heating are applied, which are systems based on low temperatures and large surfaces.
Low temperature systems increase the efficiency of solar systems and heat pumps. Passive houses require space for a thick layer of insulation materials in the envelope and space for equipment such as a buffer for the solar domestic system, ducts and unit for HRV or a heater/hot water back-up for the solar system. Sun shading is provided in most houses (Strom, 2005).

METHOD
The relationship between occupant behaviour and technical services in dwellings represents an interdisciplinary research field that links the social en technical sciences. Work in this field started with involvement in problem analysis and technical trouble shooting and expanded towards studies of user complaints about environmental health. The home visits (500 dwellings) provide data on technical performance, perception and behaviour.
The paper presents an exploration into the indoor environment of passive houses, based on dscribed cases (Daniels, 2007; Greml, 2004; Mlecnik, 2005-2008; Castagna, 2008; Römer, 2005; Strom, 2005) and participating in international discussions on passive houses (Demohouse, Green Solar Cities, Passive House expert meetings). Field data are collected in two passive houses, in minimum energy houses and standard houses, that include installations or design features that are common in passive houses. Therefore the focus is on discussion rather than the presentation of results.
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come progettare una casa passiva in Italia – arch. Bart Conterio

www.0-co2.it

Allo stato attuale della tecnica le costruzioni ad alta efficienza energetica sono rappresentate dalle “case passive” che, applicando, ad esempio, lo standard di origine tedesca “passivhaus”, sono edifici che hanno un fabbisogno energetico del riscaldamento non superiore a 15 kWh/mq/anno ( lo stesso dicasi per il raffrescamento estivo): per comprendere meglio l’alto standard di efficienza energetica richiesto, si può prendere come riferimento la situazione italiana, in cui, in media, un abitazione consuma per il riscaldamento 106 kW/mq/anno  e 160 kW/mq/anno per l’insieme dei consumi domestici.

Tuttavia i  criteri progettuali di un edificio ad alta efficienza energetica per lo più sviluppati, sperimentati e messi a punto nei paesi dell’Europa centrale e settentrionale (in cui la priorità è costituita dal contenimento delle dispersione del calore nella stagione invernale), devono essere adeguatamente ponderati, rivisitati ed adattati al clima temperato-mediterraneo, poiché, alle nostre latitudini,  è fondamentale risolvere il problema del surriscaldamento estivo e del conseguente contenimento energetico delle spese di condizionamento, così come annunciato dalla direttiva  2010/31/UE. Infatti in tale area climatica l’involucro edilizio di una costruzione ad alte prestazioni energetiche, dovrà non solo garantire la riduzione delle perdite di calore verso l’esterno e lo sfruttamento dei guadagni di energia solare in inverno, ma dovrà anche assicurare la protezione dagli apporti solari estivi e, soprattutto, il controllo e lo smaltimento adeguato degli apporti di calore gratuiti interni.

Più nel dettaglio,  le case passive costruite in Europa adottano prevalentemente  la tecnologia delle pareti multistrato leggere (pareti stratificate a secco con la tecnologia S/R, pareti in legno, etc)  con un pacchetto costituito, quasi totalmente, da isolanti termici ad elevato  spessore (anche 20-30 cm), a basso peso specifico e quindi a bassa massa di accumulo, al fine di ottenere valori di trasmittanza termica molto bassi (inferiori comunque a 0,15 W/mqK). E’ comunque da considerare che tali tecniche di super-isolamento, trovano indicazione soprattutto in zone a carattere continentale dove i consumi per il riscaldamento invernale prevalgono nettamente su quelli per il raffrescamento estivo. Inoltre, mentre nel periodo invernale il requisito principale è la protezione del trasferimento del calore dagli ambienti interni all’esterno, durante il periodo estivo, uno dei requisiti è quello dello smaltimento, di notte, del sovraccarico termico accumulato durante il giorno: purtroppo, questa tipologia di involucro “iperisolata”, essendo caratterizzata da una bassa massa termica e quindi da una limitata inerzia termica, non pemette di “scaricare” adeguatamente nelle ore notturne, il calore accumulato durante il giorno innescando, così, un  processo di surriscaldamento. In area climatica mediterranea tale fenomeno di sovraccarico termico risulta molto spesso irreversibile se non vi è,  nella costruzione,  un perfetto controllo delle fonti di irraggiamento solare (effetto serra) ed una adeguata gestione degli apporti gratuiti di calore all’interno dell’edificio. (persone, elettrodomestici ed apparecchiature elettriche, illuminazione artificiale, etc). Oltretutto, con questa tipologia di involucro non è possibile sfruttare i benefici dei sistemi passivi di riscaldamento, vista la limitatezza e, alcuni casi, la totale mancanza, di superfici dotate di massa di accumulo termico in grado, quindi, di accumulare il calore quando necessario, per poi  distribuirlo agli spazi interni quando l’effetto del guadagno solare cessa. Anche per quanto riguarda il raffrescamento passivo, la massa di accumulo termico appare necessaria in quanto potrebbe essere sfruttata come vero e proprio pozzo termico.    

A questi inconvenienti si è cercato di porvi rimedio mediante l’adozione elementi strutturali dotati di massa di accumulo termico (come ad esempio solai, pavimenti, corpi scala in cemento armato, etc)  e/o l’impiego nella stratificazione delle tamponature esterne, di  materiali dotati di una maggiore densità e/o calore specifico (ad esempio  pannelli in legno massiccio tipo X-LAM, lana di legno e fibra di legno ad alta densità, fibra di legno mineralizzata, fibre di cellulosa o canapa, etc): Ma il ricorso a tali soluzioni, anche se in alcuni casi consente di raggiungere degli ottimali valori di trasmittanza termica periodica  e dei valori di sfasamento ed attenuazione più che accettabilinon permette, comunque, di raggiungere degli adeguati valori di massa termica (=>330 kg/mq),  di capacità termica areica interna periodica e di ammettenza interna estiva:  infatti un involucro edilizio caratterizzato da una scarsa ammettenza interna e da una insufficiente capacità termica areica interna periodica, (che in parole povere rappresenta la capacità di un componente edilizio di accumulare i carichi termici provenienti dall’interno) può innescare, all’interno dell’edificio, dei fenomeni di surriscaldamento sia nella stagioni estive che nelle stagioni intermedie, (soprattutto  in ambienti con  alto indice di affollamento) e, quindi, delle condizioni di discomfort termico….. leggi l’articolo completo