Ventilation and heating in passive houses: Comparison of variants in terms of thermal comfort and sustainability
The Passive House concept is based on the idea to keep the heat losses so low that comfortable room temperatures can be maintained solely by heating the hygienically required supply air. But especially in multi-family housing this type of air heating is rarely being implemented in Passive House projects in Austria. The potential savings in building services are not fully exploited.
Contents and Objectives
Within a survey the predominant opinion of the stakeholders related to air heating for residential housing in Passive House standard was collected. The measurement data from completed Passive House projects were evaluated especially in view of possible differences in thermal comfort between conventional and air heating system. Additionally, dynamic building simulations were performed to systematically analyze and quantify possible differences in terms of comfort and controllability. The cost differences of realized Passive House projects were evaluated within a life cycle analysis. The aim of this study was to provide quantifiable facts for the up to date ongoing discussions about the pros and cons of air and hydronic heating systems.
Telephone interviews were conducted with five different target groups (energy consultancy institutes, technicians of the housing subsidy administration, housing developers, planners, facility managers). The questionnaires were centred around the following topics: General opinion on air heating in Passive Houses, specific arguments when recommending/deciding for or against air heating, cost aspects of air heating systems and general experiences with Passive Houses. For the measurement data analysis 20 dwelling units with air heating and 22 units with hydronic heating system (radiators or underfloor heating) were evaluated.
The dynamic building simulations were based on a real floor plan with an area of 76 m². The living room, bedroom and children's room was simulated either with air heating (LH), radiator heating (HK) or floor heating (FH). To depict the current building practices the air heating and the floor heating were controlled by a PI controller based on the operative temperature of the corridor. The radiators were modelled with thermostatic valves for each room. The corridor and entrance area remained (apart from indirect heat dissipation) unheated, while the bathroom was modelled with an ideal heating. The reference model assumed a desired comfort temperature by the user of 22°C. To reflect user intervention based on thermal comfort sensation, a regular (weekly 1x) "re-adjustment" of the temperature setting of the controller was implemented. Additionally, computational fluid dynamics (CFD) simulation of the living room were performed to consider the influence of the air flow conditions for the evaluation of thermal comfort.
The investment costs of the three different heating distribution systems (air, radiator, floor) were determined using the cost information of realized housing projects. In a live cycle analysis the costs and sustainability parameters (CO2, primary energy) were compared and evaluated.
This study (focusing on multi-family homes) shows that in Passive Houses the requirements of the strictest comfort class (acc. to ISO 7730) can be met by with an air heating system as well as with a hydronic system. This always applies to the living room (or the room where the temperature sensor is positioned). If the bedrooms should also meet this strict comfort class at all times, a room-wise temperature control is required. With a flat-wise control (as commonly implemented for air and floor heating), undesirable temperature differences between rooms can occur depending on boundary conditions and user behaviour. A careful planning might be required in this case.
The cost analysis of the current building practice shows that an air heating system (with flat wise regulation) can be expected to save around 800 € or 1100 € compared to the radiator heating or floor heating, respectively. Possible Passive House related savings (cost-optimized positioning and smaller radiators) could still reduce these differences. An air heating with room-wise control (one heating coil per room) is estimated to have similar costs as the radiator solution (with thermostatic valve).
Prospects / Suggestions for future research
As this study shows, the classic radiator solution using thermostatic valve is (due to its high production volume) only slightly more expensive than air heating while providing a room-wise temperature control. Compared to the usual installation below the window, a Passive House allows a simplified distribution network and therefore bears the potential of cost reduction not only with air but also with radiator heating. For refurbishment projects where a ventilation system has to be installed with little or no available space for ducting, a ductless ventilation concept like the so-called active overflow principle offers an attractive solution. There herein presented air heating based approach of equipping the active overflow element with a heating device could provide an interesting possibility of combining heating and ventilation in a minimal invasive manner. Further research is needed for a systematic evaluation of the potential and the limits of this approach.
University of Innsbruck - Institute for Construction and Materials Science, Unit energy Efficient Buildings