HVAC designers' guide

Heat loss and heating power needs are small in a passive house and, as a result, the house does not require traditional heat distribution systems, such as radiators or floor heating. Ventilation heating is a sufficient heat distribution method (Figure). The objective of the energy design of a passive house is to present the requirements for the house’s heat-technical functionality as a whole so that the heat need can be covered efficiently under all conditions. The objective of design is to achieve a good, comfortable and draught-free microclimate.

Ventilation heating has two principle alternatives. Supply air can be heated in a centralised manner, immediately after the ventilation machine or separately for each room in ventilation terminals. The first alternative produces air of equal temperature in each room. Room-specific temperature control requires that supply air is heated in terminals or ducts before the terminals. In this case, the air temperature can be adjusted as desired.

A good annual utilisation rate of heat intake of ventilation can be used to reduce the heating need and the temperature level of supply air. The temperature of supply air must not exceed 50°C. Fresh air can be preheated before heat intake in order to prevent the heat exchanger from freezing. Fresh air can be conducted to the ventilation machine through a ground pipeline where it either cools down or heats up. However, the impact of this type of a system on hygiene or health has not been identified (condensed moisture in the pipeline, mould, dirty pipes, etc.).

Ground heat or cold can also be taken advantage of using liquid circulating in the ground pipeline, in which case the system contains a heat exchanger, pump and a bore well or a ground pipeline. The preheating or cooling power required determines the length of the pipeline or the depth of the well. An underground horizontal pipeline produces 10-20 W/m of heating power.



Figure: A basic chart of a ventilation heating system. Using a ground heat exchanger, the temperature of fresh air can be increased and the risk of frozen ventilation heat intake can be removed.

In ventilation heating supply air is divided into all rooms. Floor heating is a useful system in wet areas for drying the floor quicker. The floor temperature must be measured to be lower than in normal floor heating so that the temperature is 1-3°C higher than the air temperature. A higher temperature level may cause overheating.

Supply air terminals can be located in partitions. Terminals placed in the ceiling can serve a full area better than wall installations, but the primary question deals with the selection and measurement of the terminal. Because ventilation ducts in the ceiling must be located below the air seal, a sufficient space for the ducts must be reserved in roof installations.

Thermal habitability and indoor air quality require that the supply air mixes well with the indoor air. Mixing reduces vertical temperature differences in rooms. Mixing is particularly important when it is extremely cold during the winter. The speed of air supplied from terminals located high up must be sufficiently high so that mixing is efficient. The air speed must be low in living areas, a maximum of 0.15-0.20 m/s, so thatthe air flow does not reduce habitability.

Vertical temperature difference in rooms must be less than 2°C between 0.1 m and 1.1 m, i.e. between a sitting person’s ankle and neck. Good thermal habitability is harder to achieve in higher rooms (two-storey rooms). The heat-technical properties of a passive house’s outer shell are good, because of which it is easy to maintain small temperature differences. Conducting air into rooms through terminals located in floors helps to achieve good mixing but air blown from below may raise dust.

Passive solar heating is part of the heating system in a passive house. The temperature of different rooms may vary according to the solar load, use of the rooms and other internal loads. Room-specific temperature control is recommended for the temperature objectives set for different rooms. The heating period of a passive house is short compared to a normal house. The sun’s heat load may cause overheating in a passive house as early as at the early stages of spring. As a result, the bypass of heat intake may be useful in order to avoid the cooling need. The design should create the cooling solution from passive means. These include shading of windows, overnight cooling using ventilation and efficient ventilation over daytime.

The design should create the cooling solution from passive means. These include shading of windows, overnight cooling using ventilation and efficient ventilation over daytime (Figure). Replacement air for ventilation can be brought in from the north side of the house. It is possible to use ground heat for preheating fresh air in winter and cooling it in summer. Preheating fresh air in winter will reduce the cooling risk related to ventilation heat intake and improve the utilisation rate.

 
Figure:A building’s total design has an impact on the microclimate control in a passive house. A reduction in cooling need should be started using passive means. Sunshade windows and shades below windows are the most efficient passive methods.

A building’s total design has an impact on the microclimate control in a passive house. A reduction in cooling need should be started using passive means. Sunshade windows and shades below windows are the most efficient passive methods. A passive house utilises free energy, that is, heat released from people and equipment. The level of utilisation is affected by the amount of thermal mass, the area, heat transfer coefficients on the surface of mass, heat conductivity of mass and its covering surfaces, carpets, wall fittings, etc. The utilisation of mass is the most efficient when it is located inside the heat insulation. The amount of thermal mass required is not great. For example, a massive floor in a light-structure house is sufficient. The good level of heat insulation in a passive house supports the utilisation of mass.

The benefits of thermal mass can be used when the heating control is sliding. The indoor temperature may vary within the slider limits. In this case the structures can bind and release heat on the basis of the temperature differences. For the system’s operations, the temperatures must be allowed to vary so that thermal mass can reserve and release heat effectively. A good level of heat insulation and thermal mass will also reduce the cooling need in summer.

The need for heating power in a passive house is small in measurement conditions. For example, it is less than 3 kW in a 150 m2 detached house. As a result, room-specific power need calculations must be accurate. Excess power can make temperature control more difficult and increase energy consumption. At the same time, over-measurement will increase system costs.

An incorrectly-selected fireplace may cause excess heat and reduce thermal habitability in a well-insulated house. As a result, sufficient attention should be paid to the massiveness of fireplaces in their selection. Because the heating power need of a passive house is small, the heat release power of a fireplace should also be small. The heat reserve capacity and heat release power of a fireplace are directly comparable to the mass of the fireplace.

Light fireplaces release the thermal energy contained by burned wood rather quickly and at a great instantaneous power into the area to be heated. Massive fireplaces release the thermal energy bound in wood at a smaller instantaneous power than light fireplaces but over a longer period of time. Figure presents a basic chart of the changes in thermal powers of different fireplaces after igniting the charge. The average period is the time representing the massiveness of a fireplace, during which the fireplace has released half of the energy of the charge burned in the fireplace into the surroundings. The massiveness of the fireplace selected has a clear connection to the need of heating power and the use of the fireplace. However, the suitability of a fireplace in a passive house depends on the actions and habits of the user.

Design cooperation between the architect, HVAC designer and structural designer is necessary. This will prevent solutions that increase the energy need or reduce habitability. The architecture of a passive house is the developer’s wish but good design cooperation will also guarantee the house’s value, long-term durability and easy maintenance.