EU Research Project: A Whole House Low Energy Ventilation System Mike McEvoy Ryan Southall The Martin Centre for Architectural and Urban Studies Department of Architecture Cambridge University
The background to this project is the uncertain direction to which building legislation is leading with regard to health, energy and housing design. • Insulation requirements within the building regulations have progressively increased since the 1970s. The current changes to part L being the latest step towards Scandinavian levels of insulation. • Background ventilation in winter used to be largely due to infiltration through cracks in the external fabric. In order to save energy better sealed construction is also becoming a standard. • There are however growing concerns about air quality in houses and its implications for human health. In the UK this is largely a problem of humidity and condensation, mould and the growing incidence of asthma in the population. • Other pollutants such as volatile organic compounds (VOCs) emitted from contemporary building materials, particulates and in some places radon, makes indoor air quality a major issue for human health.
There have been a number of technical responses to these issues in recent years: • Trickle vents within windows and extract fans in kitchens and bathrooms are the most familiar solution. This method has come close to the limits of its capacity since the doubling of air flow requirements required by the 1995 changes to the regulations. Yet larger vents will result in draughts and excessive heat loss. • A variety of mechanical systems are being widely promoted. The usual technology in Scandinavia is ‘Mechanical Ventilation Heat Reclaim (MVHR). This entails long lengths of potentially unhygienic ductwork and a constantly running fan. In our climate the energy efficiency of MVHR is dubious. • Our research project uses ‘supply air’ windows as a heat reclaim device, in combination with passive stack vents, to form a passive whole house ventilation system.
The ‘Supply Air Window’: An extra pane of glass is spaced away from the front of a conventional window. Air is drawn through vents at the bottom of the outer pane, it is warmed as it rises within the cavity, by solar gain but also by heat reclaim from the room, so it enters the room pre-warmed through vents at the top of the inner window. • Their best performance relies on controlled air flow. Air is pulled through the window by the negative pressure induced by the passive stack vents. The window has a simple non-return valve at the top to prevent back flow of air into the window which would cause condensation within the window. • The optimum energy efficiency of the window depends on the air flow being smooth and laminar which is a function of the gap width. The flow can be disrupted by the excessive air speeds resulting from wind pressure. We have been working with Titon of Colchester who make pressure regulating vents. • The idea has been tried before particularly in the cold climates of Canada and Finland. For a while the windows were marketed in Finland but given the outside temperatures, the air entering the rooms was still very cold and the windows became overly complicated. The approach is better suited to our temperate climate.
In the first instance the project was supported by a grant from the Engineering and Physical Sciences Research Council. • We built a laboratory test rig to establish the gap width which achieves laminar rather than turbulent flow. One of the panes was heated electrically and the air flow was monitored for speed and temperature. • The later tests were done in test cells. A test cell is a highly insulated box that incorporates monitoring devices to measure the relative performance of passive solar components. • One of the test cell studies was to check that the same performance could be achieved across the range of manufactured window sizes whether short and wide or tall and narrow. • This shows how the effective U-value alters throughout the day. As solar energy enters the room the U-value becomes negative. In highly insulated buildings direct solar gain is less useful, even in winter it can quite readily lead to localised over-heating. The supply air window however uses a high proportion of available solar energy to pre-heat ventilation air thus avoiding draughts.
At night time the U-values are positive but never rise above around .6 W/m2K. This very good performance is achieved by locating a low-E coating on the side of the inner pane facing into the air flow. • The final test cell study was carried out at the BRE in Scotland. A passive stack vent was constructed from the back of the test cell. The air flow through the window was very encouraging. • We have since obtained further funding through the EU 5th Framework Programme to install the system in dwellings in Poland, Denmark and here in Armagh. • The project in Poland is a demonstration of the concept in adjoining flats, one having conventional windows and the other ‘supply air’ windows. The flats are being monitored to check relative comfort conditions and energy consumption. • In the houses in Armagh and Denmark an advanced type of vent is to be tested and an innovative method of reducing sound transfer through the windows. • The window specification depends on the climate, in Poland and Denmark the windows are to be tripled glazed, in Armagh double glazed. One of the aims of the project is to establish the range of climates to which the approach is appropriate.
The windows are being made in Poland but the prototype has been designed by Whitaker and Co who are timber window manufacturers in Bradford. In essence the system is a form of secondary glazing and may in that form be applicable to the retrofit of existing buildings as well as new build.
The windows will be installed in flats in Poland and houses in Denmark and Ireland. In Denmark the houses are in Jutland on the western peninsula near the North Sea. In Poland they are in the South-western city of Wroclaw and in Armagh in N. Ireland. • These locations were chosen for the variation in climates and their geographical spread over Europe. Ireland possesses a mild winter/summer overcast climate, Poland represents a dry inland continental European climate with harsh winters and warm summers whilst Denmark has cold wet winters and cool summers. • For Poland and Denmark the minimum temperature for a representative January is -15°C, and this has therefore been taken as the base case for the simulations.
The building regulations that pertain to this work for the three countries are as follows: • For Denmark these are; • · A window U-value of no more than 1.8. • · Internal air speeds of no more than 0.15 ms-1. • · Each room in the house should receive a ventilation rate of at least 0.5 ach/hour or 0.32 l/s per m2 of room area. • · Habitable rooms should have at least 60cm2 openable area to the outside per 25 m2 floor area when naturally ventilated. • · Kitchen should have at least a 30 cm2 (or 100cm2 from access room) opening to the outside for supply. • · Bathroom and laundry should have at least 100cm2 opening to an access room for supply. • · Kitchen, bathroom and laundry should have a natural exhaust draught with a duct area of at least 200cm2. If mechanically ventilated exhaust rates should be 20, 15 and 10 l/s respectively. • For Poland; • · A window U-Value of no more than 2.6 or 2 depending on climate zone. • · Kitchen living room and bathroom should be at at least 20C. Hall should be at least 16C. • · Ventilation rates in the kitchen should be at least 70m3/hr (21l/s) if there is a gas cooker and 30 m3/hr (8.3 l/s) if electric (Electric in this case). • · For the bathroom 50 m3/hr (14 l/s) should be provided.
Polish flats are new build and are on the third floor of a large complex with central courtyard area. Flats consist of a main kitchen/bedroom, bathroom and hall. Two easterly facing windows are placed within the kitchen/bedroom 1.5 x 1.5 m in size. The windows in one of these flats will be replaced by Supply Air windows incorporating pressure controlled vents whilst the other will be left as it is as a control. • The flats are identical mirror images apart from the positioning of radiators within the main room. One flat has two radiators underneath each window and the other has one higher powered radiator beneath only one window. • Room areas are 20.5 m2, 5.5m2 and 6.6m2 for the kitchen/bedroom bathroom and hall respectively. • Ventilation is provided through the use of a central Passive Stack system that runs all the way up the centre of the building. Exits from each flat are provided in the kitchen near the cooker, and in the bathroom.
The Danish properties are new build single storey family houses forming part of a semi-urban development. Both houses are situated within a row of similar properties and are identical mirror images. • Each consists of a kitchen/living room, two bedrooms, bathroom laundry room and hall. Three windows will be placed within the house, one in each bedroom and one in the living area. • The living room window and one of the bedroom windows face approximately South, whilst the other bedroom window faces North. • Window sizes are 1.81 x 1.21 m for the living room and bedroom and 1.21 x 1.21 m for the other bedroom.
The ventilation within test houses will be provided by Passive Stack Ventilation. In the case of Poland the existing ventilation system is passive and will be retained for both the test house and control. In Denmark a PSV system will be installed drawing air from the kitchen, bathroom and laundry, through the attic space and roof in compliance with Danish building codes. • The control house will have a highly efficient mechanically ventilated heat reclaim system installed which are becoming the standard for Scandinavian houses. With this system air is extracted from the kitchen and bathroom and used to pre-heat incoming air, which is supplied to the bedrooms and living room. The system we will install has a heat exchange efficiency of over 80%.
Simulations were carried out using advanced Computational Fluid Dynamics (CFD) techniques. • CFD is the most advanced computational method for modelling fluid flow and heat transfer. • Has the fundamental laws of conservation as it’s foundation. • Model is split into numerous small volumes over which the conservation laws are applied. Physical values of speed, temperature etc can then be calculated and displayed for each of these volumes. • CFD software FLOVENT which is a building orientated code. Includes a cad modeller for the building of the case geometry. • Geometry is then attributed for conductivity, density, transparency etc. • Boundary conditions i.e. inlet/outlet speeds and temperatures and external conditions are supplied. • Problem is solved by iteration with solution occurring when conservation laws satisfied. • However the technique is very computationally expensive and has been used here only for steady state analysis. • Parameters predicted include window U-Value, room thermal comfort and window air exit temperatures.
Dynamic analysis of the building behaviour was also carried out using a well validated and freely available zone network model called ESP-r. • In this case each room is a single zone and the conservation of heat and mass is applied to these zones through specified paths. • To model the window cavity 8 zones are required to account for thermal stratification. • The relative simplicity of the model compared to CFD allows dynamic analysis to be undertaken with 6 minute time steps over the heating season. • Climate data from the sites can be included to give site representative results. • Includes a simple cad modeller for geometry building. • Geometry is attributed in a similar way to the CFD technique. • Most boundary conditions taken from weather data except for air flow rates which specified. • Parameters predicted include PSV performance, heating loads and influence of environmental factors on building behaviour.
CFD simulations of U-Value show that a 30mm cavity width is the best compromise between size/weight of window and performance. • Also, the larger the window the higher the U-Value with the smaller Danish window (1.2 x 1.2) achieving 0.41 whilst the larger achieves 0.59. • The position of the double glazing unit (internally or externally) has only a slight effect on the U-Value of the window, and thermal comfort will therefore be the primary factor behind this design decision. • The U-Value of the Polish windows can reach 0.8. This is due to the the large size of the windows and the position of a high powered radiator beneath one of the windows in one of the flats which causes a rising thermal plume and heats the internal pane of glass.
The pre-heating of the incoming ventilation stream reduces the ventilation heat load of the building. • Night-time ventilation pre-heat solely due to warmth from the room is dependant on cavity width in a similar way to U-Value. Also heavily dependant on glazing configuration. • Daytime solar pre-heat is heavily dependant on the incident solar irradiation. Only the Danish windows face a southerly direction and they show the highest levels of pre-heat. • Polish windows show best levels of pre-heat in the morning.
Thermal comfort within the zone is determined within the room using Flovent’s inbuilt Percentage Proportion Dissatisfied (PPD) calculator. • We consider a PPD of less than 10 to be acceptable. • For the Polish and Danish homes we take a base case of -15°C • Thermal comfort is improved by placing the double glazing unit on the inside. • PPD is below 10 except for small areas around the radiators and below the unheated window. • In general thermal comfort is very good.
Simulations are used to check the effectiveness of the PSV system at delivering the required ventilation rates • Danish PSV system must provide 24l/s for the flat. This is achieved at 11°C or below with no wind. If there is a wind speed above 3m/s outside then this condition is met at all times. • The Polish PSV system, due to its large size provides in excess of the 80 m3/hr required
Dynamic simulations are used specifically to look at dynamic heating patterns, ventilation pre-heat, whole zone thermal comfort.
In conclusion; • A cavity width of 30 mm has been shown to be the most effective at reducing the window U-Value, whilst keeping to a minimum the weight/cost of the window. • Window U-Values are significantly below that required by regulation. The values range from 0.41 for the smaller Danish window to 0.8 for the larger Polish ones. • Night-time ventilation pre-heat ranges from 25 to 35% of the ventilation heat load. On clear days this can reach 55% in the Danish case and 45% in the Polish. • Thermal comfort conditions are very good within the buildings especially where there is a heater underneath the window. In the Polish flat, where this is not the case for one of the windows, a small area of discomfort is created near the floor and wall but does not greatly infringe on the habitable zone. • Thermal comfort is improved with the double glazed unit on the inside of the window unit and this will form the basis of the windows to be installed. • Ventilation rates provided by the PSV achieve the recommended standards except in Denmark on days when the ambient temperature is above 11°C and there is little wind. It may therefore be necessary to include a low power back-up fan for these situations.