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Indoor Conditions

Energy designer needs to be convinced over the properties of a building for not to over-dimensions its energy systems (
A passive house has more constant room temperatures than a typical house (VTT).
Ventilation unit and supply air terminal with heating coil (VTT).
Passive cooling by exterior shading is an efficient method for comfort in summer. Shading provides 50 – 60 % reduction in a buildings cooling energy demand even in warm Mediterranean countries. (VTT)

Design aim
A passive house aims at very low energy consumption and low heating power demand compared to contemporary national standards. Thus a Passive House’s CO2 emissions are also low. In a Passive House, energy-efficiency is a route to good indoor conditions. The concept bases on low heat losses of a building, thus enabling simple building services’ systems. For energy design, the basic requirement is adequate dimensioning of the energy systems supplying for the energy demand.

A passive house has a high level of insulation with minimal thermal bridges, low infiltration, and it utilizes passive solar gains and heat recovery to accomplish these performance requirements. Renewable energy sources can be used to meet the resulting energy demand.

A Passive House’s envelope structures are highly insulated and airtight. The house’s heat losses are low. Therefore it does not need traditional heating systems like radiators or floor heating. Due to low heating energy demand, ventilation heating can be the primary heating system. Energy designer’s main task is to set requirements for the building as a whole so that the heating system can operate efficiently in all climate conditions, and serve for good, comfortable, and draught-free indoor conditions.

Air quality
An airtight house requires a well-designed and dimensioned ventilation system for good indoor air quality, and vice versa: any ventilation system (natural, mechanical or hybrid) requires good airtightness to make ventilation flows controllable. Energy-efficiency can not be considered as a ground to reduce the ventilation rate. Building code requirement should be considered as minimum acceptable ventilation.

Different rooms may have different temperatures due to solar gains, occupation, and internal heat loads. Room based control or passive measures for temperature differentiation may be necessary if high comfort requirements are set for individual rooms. In centralized ventilation heating system supply air temperature is the same for the whole supply. In a room based control system the supply air can be heated at or close to air inlet terminals. The system allows for a wide range of fast and accurate control of the inlet air temperature.

Over-heating is possible due to solar gains in summer. Energy designer needs to make early-on scenarios on the over-heating probability, and suggest ways and means to avoid it. Passive ways and means such as solar shades, night cooling, and increased ventilation are preferred. Mechanical cooling systems should be avoided. Ground air duct or water systems for air pre-heating or cooling can be used.

Air supply
Supply air can be delivered through air inlet terminal on inner walls. Supply through roof inlets close to the exterior wall may give a distribution to the whole zone, but this is a question of dimensioning and quality of inlet terminals. The advantage of placing inlets on internal walls is the lower need for suspended ceilings or embedding of ducts into structures. That is an architectural advantage and at the same visual comfort in the house.

A Passive House utilizes free energy such as passive solar energy and heat from the occupants and equipment. Adequate thermal mass is recommended for efficient utilization these energy sources. A massive exposed floor or ceiling is enough to provide adequate mass in Central and Northern Europe. Also night ventilation with cool air might also be necessary. On the system design point of view the air temperature needs to have gliding set points, between which the room temperature may fluctuate. High thermal insulation level together with adequate mass helps for reducing cooling in summer.

Thermal comfort
In ventilation heating system supply air is distributed to all rooms, hall or vestibule, and separate kitchens. However, floor heating is in cold climate typical for comfort purposes in wet rooms for comfort and moisture control. The floor temperature should be designed to be lower than in conventional floor heating systems. Suitable level is, e.g., 2 - 4oC above room temperature. Higher temperature difference may cause over-heating.

Thermal comfort and indoor air quality require good mixing of supply air into the room air. The mixing minimizes temperature gradient in the room. Good air mixing is especially challenging in winter as the supply air is warm (near 50oC). If the inlet terminals are placed high, air velocity needs to be high enough to achieve good mixing rate. On the other hand the velocity should not exceed 0.15-0.20 m/s in the occupied zone since high velocities may cause comfort problems.

Vertical temperature difference should not exceed 2oC between heights of 0.1 m and 1.1 m, referring to ankle and neck level of a person sitting. Thermal comfort may become a problem in high rooms (2-storey high) due to poor air mixing if the inlet terminals are placed high. In a Passive House the building envelope and windows have such a high thermal quality that enable to maintain low vertical temperature differences. The air distribution from floors underneath windows gives a rather good air mixture, but dust from floors might be carried by the air flow.