November 26, 2013

Polish seminars – Part 3: Per Kempe´s practical experience with energy-efficient multi- and single family buildings in Sweden

According Per Kempe these energy-efficient multi- and single family buildings have an energy performance better than 60% of building code requirements, BBR (total of 55 kWh/(m2.a): heating 20-25 kWh/(m2.a) and DHW 25 kWh/(m2.a), and electricity 5-10 kWh/(m2.a)).

In order to achieve an energy-efficient residential building there is a great need to have good airflow balance (supply/exhaust flow), balanced supply air system with cooking hood, and good DHW circulation losses.

The form factor influences the performance of a building, i.e. form factor is based on sum of outer façade surfaces and Atemp area. The example for a terraced house with form factor 3 and an apartment in a multi-family house shows that different form factors have large influence on different energy behaviour. The air tightness and air leakage factors are important in wind-driven situation, i.e. the amount of under pressure which can get in the apartment if the supply air in the apartment is too low.

“The terraced house has a large form factor (Acs/Atemp= 3) and would not get too large under pressure when the exhaust airflow (40 l/s) is much larger than the supply airflow (28 l/s).
The under pressure becomes around 14 Pa and depending on the fan curves it might have a small influence on the air flows. Mainly the difference in air flows will leak through the climate shield and create a large increase in heat power demand and heat energy use. In one example for an energy-efficient terraced house with air flows as above the heat power increased with about 40%. In a building according to the building code it would have become 10-15% increase.

To get low noise level, one must choose low noise system and needs to have the fans running smoothly, with air speed below 2 m/s in duct systems, to avoid sharp edges and screws heads in the supply duct system.

There is an example with a screw giving a noise problems in a bedroom (permitted noise according BBR is maximum of 26 dB(A)). The contractor forgot the reinforcement in the wall which was necessary in order to mount the supply air device. To fix this problems, installers put extra screw inside the inlet air hole to securely mount the device in the wall and this increased the noise level by 9 dB(A). All this was revealed by on-site measurements after complaining from occupants.

Another example was with testing of a cooking hood with/without a fan. With a higher speed one can get 150 l/s with under pressure with 85 Pa (compared to exhaust of 75 Pa) which makes the air to go back to the apartment. One needs to think how to get the extra air (flow) in an apartment when using a cooking hood; otherwise this situation can influence the ventilation balance in the apartment.

In Sweden the maximum waiting time for hot water in residential buildings is 10 seconds. This means that one need to have a shaft close to kitchen/bathroom, otherwise there would be a need for extra circulation network in the apartment itself.