Numerous studies have investigated the application of multi-zone demand-controlled ventilation for office buildings. However, although Swedish regulations allow ventilation rates in residential buildings to be decreased by 70 % during non-occupancy, this system is not very common in the sector. The main focus of the present study was to experimentally investigate the indoor air quality and energy consumption when using multi-zone demand-controlled ventilation in a residential building. The building studied was located in Borlänge, Sweden. This building was recently renovated with better windows with low U values, together with internally-added insulation materials. The building had natural ventilation, which decreased significantly after retrofitting and resulted in poor indoor air quality. Therefore, a controllable mechanical ventilation system was installed. The ventilation rate was controlled according to the demand in each zone of the building by CO2 concentration as an indicator of indoor air quality in habitable spaces and relative humidity and VOC level in the toilet and bathroom. The study showed that multi-zone demand-controlled ventilation significantly reduced the CO2 concentration leading to improvement in indoor air quality. However, building with demand-controlled ventilation consumed more energy than natural ventilation as it increases the ventilation loss by forcing more air into the building. Nevertheless, in the demand-controlled ventilation system, the energy consumption for the ventilation fan and ventilation loss was almost half of the constant high rate ventilation flow.
Rising energy prices have contributed to the development of heat pump-based heating systems in Sweden. Low flow temperature in the secondary heat distribution system to rooms is a requirement for energy-efficient systems. This increases the thermal efficiency of the heat pump and decreases thermal losses in the distribution system. Flow temperatures in water-based systems for heat distribution in buildings have been decreased from 55°C to temperatures around 30°C. This is to maximize the efficiency of heating systems that are based on heat pump technology. Different technical solutions have been suggested to guarantee space-heating requirements with low temperature difference between heating units and ambient air. Floor heating has in many cases been considered a good option, and the popularity of such systems has dramatically increased. Complicated installation work, moisture problems and slow thermal control with floor heating are reasons enough to find alternative low-temperature units for heat distribution in rooms. This may result in a combined heating and ventilation system that operates with forced convection.
Energy consumption for heating and ventilation of buildings is still in 2011considered far too high, but there are many ways to save energy and construct lowenergy buildings that have not been fully utilised. This doctoral thesis has focused onone of these - low temperature heating systems. Particular attention has been given tothe ventilation radiator adapted for exhaust-ventilated buildings because of itspotential as a low energy consuming, easily-operated, environmentally-friendlysystem that might also ensure occupant health and well-being.
Investigations were based on Computational Fluid Dynamics (CFD) simulations andanalytical calculations, with laboratory experiments used for validation.
Main conclusions:
This work demonstrated that increased use of well-designed ventilation radiatorarrangements can help to meet regulations issued in 2008 by the Swedish Departmentof Housing (Boverket BBR 16) and goals set in the Energy Performance of BuildingsDirective (EPBD) in the same year.
Performance of heat emitters in a room is affected by their interaction with the ventilation system. A radiator gives more heat output with increased air flow along its heat transferring surface, and with increased thermal difference to surrounding air. Radiator heat output and comfort temperatures in a small one-person office were Studied using different positions for the ventilation air inlet. In two of the four test cases the air inlet was placed between radiator panels to form ventilation-radiator systems. Investigations were made by CFD (Computational Fluid Dynamics) simulations, and included visualisation of thermal comfort conditions, as well as radiator heat output comparisons. The room model was exhaust-ventilated, with an air exchange rate equal to what is recommended for Swedish offices (71 s(-1) per person) and cold infiltration air (-5 degrees C) typical of a winter day in Stockholm. Results showed that under these conditions ventilation-radiators were able to create a more stable thermal climate than the traditional radiator ventilation arrangements. In addition, when using ventilation-radiators the desired thermal climate could be achieved with a radiator surface temperature as Much as 7.8 degrees C lower. It was concluded that in exhaust-ventilated office rooms, ventilation-radiators can provide energy and environmental savings.
Studies indicate that a high ventilation rate with fresh air supply directly from outdoors gives better thermal comfort conditions, less SBS (Sick Building Syndrome) symptoms and increased work productivity. The drawbacks with a high ventilation rate in natural or exhaust ventilated buildings are normally increased energy use for heating and cold air draught. Such problems may be minimized with ventilation radiators, radiators where cold ventilation air is brought directly from outdoors through a wall channel into the radiator where it is heated before entering the room.
This paper discusses advantages with ventilation radiators in comparison to those of traditional heating systems. Focus has been on energy aspects and thermal comfort. The main conclusions are that ventilation radiators may give a stable and uniform thermal indoor climate. The high thermal gradient between cold ventilation air and the radiator surface inside the ventilation channel also makes the ventilation radiator more efficient than other systems. A method to vary indoor climate on a daily basis according to where people stay is proposed for additional energy savings with ventilation radiators. The deductions were based on results from CFD simulations in a well validated office model.
Thermal comfort aspects in a room vary with different space heating methods. The main focus in this study was how different heating systems and their position affect the indoor climate in an exhaust-ventilated office under Swedish winter conditions. The heat emitters used were a high and a medium-high temperature radiator, a floor heating system and large wall heating surfaces at low temperature. Computational fluid dynamics (CFD) simulations were used to investigate possible cold draught problems, differences in vertical temperature gradients, air speed levels and energy consumption. Two office rooms with different ventilation systems and heating needs were evaluated. Both systems had high air exchange rates and cold infiltration air.
The general conclusions from this study were that low temperature heating systems may improve indoor climate, giving lower air speeds and lower temperature differences in the room than a conventional high temperature radiator system. The disadvantage with low temperature systems is a weakness in counteracting cold down-flow from ventilation supply units. For that reason the location of heat emitters and the design of ventilation systems proved to be of particular importance. Measurements performed in a test chamber were used to validate the results from the CFD simulations.
This paper deals with heat output optimization of a ventilation radiator by varying the distribution of vertical longitudinal convection fins. A ventilation radiator, which combines ventilation air supply and heat emission to the room, has a higher driving force on air in between the radiator panels compared to traditional radiators and can for this reason have more heat transferring surfaces to improve thermal efficiency. Improving the thermal efficiency means a lower water temperature is required for heating and energy can be saved in production and distribution of heat in systems with heat pumps, district heating or similar. The investigation was made using Computational Fluid Dynamics (CFD) simulations while analytical calculations were used for verification of different flow and heat transfer mechanisms. Results showed that heat transfer can be increased in the section where ventilation air is brought into the room by slightly changing the geometry of the fins, decreasing the fin to fin distance and cutting off a middle section of the fin array. This change in internal design could mean considerable increase in thermal efficiency for the ventilation radiator as a whole.
Ventilation radiators, heat emitters where cold ventilation air is brought directly from outdoors into the room via heated radiator surfaces, are becoming more and more common in Scandinavia. Because these systems combine both heating and ventilation several interesting aspects arise that may be used to save energy and improve indoor thermal climate. The heating aspects in wintertime have been discussed in previous papers from KTH STH. This study investigates whether ventilation radiators may be used for cooling in summertime. Results from the study show that condensation of water is the main problem to tackle when ventilation radiators are used for cooling purposes. It is difficult to avoid condensation, especially inside the ventilation channel where incoming ventilation air comes into contact with chilled radiator surfaces. The problem increases with increased temperature difference between radiator surface and ventilation air. This is why ventilation radiators seem unsuitable for cooling in summertime without risking condensation of water. However, if condensation of water is allowed in the ventilation channel only, ventilation radiators may be functional for cooling. The trick is to find a way to drain water from the ventilation channel to avoid hygiene problems.