HVAC in well-insulated buildings

In an energy-efficient building, the space heating need is low and therefore can be satisfied by ventilation heating. For example, in a passive building, the maximum power needed for space heating is 10 – 15 W/m2 , which amounts to 1.0 – 1.5 kW of total power needed per 100 m2 of treated area. A traditional heating system based on radiators or floor heating is no longer necessary. Heat can be provided by means of ventilation.

There are two principle alternatives: 
  • supply air can be heated centralised immediately after leaving the ventilation machine or
  • separately for each room, in ventilation terminals.

The first alternative produces air of equal temperature for each room. In the second case, the air temperature can be adjusted for each room. 

In an energy-efficient building which has a good level of thermal insulation in the building envelope, good thermal comfort can be achieved using lower room temperature levels. The room temperature objective in the design process is generally 20 – 21°C.

Floor heating is useful in bathroom areas, since it provides comfort and permits quick drying of the floor. However, the floor temperature must be kept lower than in normal floor heating, in order to avoid overheating. The floor temperature should be only 1 - 3°C higher than the air temperature. In other rooms, large floor heating areas should be avoided.

A 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.

Passive solar heating is part of the heating system in a passive house. The temperature of different rooms varies due to the solar load and internal loads and therefore room-specific temperature control is recommended. The heating period of a passive house is short compared to a normal house. The solar heat load may cause overheating as early as 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 windows, overnight cooling using ventilation and efficient ventilation over daytime (see the 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 the winter reduces the cooling risk related to ventilation heat intake and improves the utilisation rate.

Sunshade windows and shades below windows are the most efficient passive methods.

Reduction in cooling

Attention should be paid to the size 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.

Ventilation rates and heat recovery
Building regulations usually require a minimum ventilation rate in the range of 10 – 15 l/s per person, which is approximately 1 l/s per m2 in office buildings with normal occupant density, and 0.5 air changes per hour in living spaces of the residential buildings.

An example of ventilation rates for offices depending on the pollution load in three categories (CEN 1752)

Category  Occupants only  Low polluting materials  High polluting materials 
   l/sm2  l/sm2 l/sm2 
 A 1.0 2.0  3.0 
 B 0.7  1.4  2.1 
 C 0.4  0.8  1.2 

When designing the rate for ventilation, carbon dioxide concentration can be used as a surrogate for ventilation rates, but its use to measure ventilation is uncertain as its concentration in buildings seldom reaches a steady state due to variations in occupancy, ventilation rates and outdoor air concentration. Steady-state values of carbon dioxide concentration can be calculated from CO2 generation of 0.00567 l/s per occupant in office buildings.

Energy density in the exhaust airflow is high, and heat recovery is an economic way of reducing the energy and operation cost of ventilation. Heat recovery becomes more feasible with high air flows and low outdoor temperatures. Limit values can be set for minimum efficiency of heat recovery and size of the air handling system where the heat recovery is applied. Today, building codes require annual efficiency rates in the range of 30 – 40 %. For a passive building, an annual efficiency rate of a minimum of 75 % is required. Modern heat exchangers can achieve recovery rates of up to 90 % with respect to these heat losses. However, in cold climate conditions the efficiency rate is lowered due to the need for defrosting ice from the heat exchange unit. 

Fresh intake air can be preheated prior to entering the heat intake in order to prevent the heat exchanger from freezing. A ground heat exchanger for preheating the supply air reduces or even eliminates the defrost demand. A subsoil air heat exchanger is not recommended in cold climate conditions because of possible moisture condensation and hygienic problems. A ground loop system with a heat exchanger for pre-heating fresh air was successfully tested in a Paroc passive pilot building.

Ground heat or cold can 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.

Insulation of HVAC systems

In today’s airtight, low energy buildings, Heating Ventilation and Air Condition (HVAC) systems are becoming increasingly important. The heated and cooled air and water must keep the temperature until it reaches the intended destination; any unwanted heat losses must be ventilated, which generates additional energy consumption.

It is therefore important to look not only at the end temperature, but also at the heat loss. Even though there is a very small change in the end temperature, the heat loss is sill significant. 

Calculation example: 
Temperature and heat loss in a ventilation duct

Dimension:  315 mm 
Length:  30 mm 
Air temperature:  20°C 
Airspeed:  3 m/s
Surrounding temperature 6°C 

Insulation Heat loss, W  End temperature, °C 
Uninsulated 2607  12.9 
80 mm 226  19.3 
150 mm  143  19.5 

Pipes are integral parts of HVAC systems and must be insulated to reduce energy consumption and operating costs. Thermal insulation is needed to maintain the temperature of the water in the pipes within the correct margins. 

Calculation example: Heat loss in a warm water pipe

Pipes are integral parts of HVAC systems and must be insulated to reduce energy consumption and operating costs. Thermal insulation is needed to maintain the temperature of the water in the pipes within the correct margins. 

Dimension:  22 mm 
Air temperature:  55°C 
Surrounding temperature 20°C 

Insulation λ value
Insulation thickness Heat loss
Heat loss,
kWh /m, year 
Uninsulated  - 0 mm  40 350 
PAROC Hvac Section  0.035 20 mm   6.0 52
PAROC Hvac Section  0.035 40 mm  4.5 39
PAROC Hvac Section  0.035 60 mm  3.8 33

Also, cold installations need sufficient insulation, both to prevent condensation and to reduce cost. In general, it is three times more expensive to lower the temperature by one degree compared with increasing the temperature by one degree.

There is also a health aspect to keeping the temperature at the right level. If the warm water temperature drops too much, there is an increased risk of spreading diseases (e.g. Pontiac fever or Legionnaires' disease) via warm water. Bacteria thrive in temperatures between 25 – 45 °C, optimally at 35 °C.

Use Paroc’s calculation program to see what insulation your project needs.