We have been thinking about steps to make our house more sustainable. We live with 4 people on 133 m2, and as the house was built in 2002 it is fairly well insulated. It has mechanical ventilation and the roof potential for solar PV has been fully utilized with 16 solar panels (270 Wp) producing an estimated 3672 kWh/yr. Energy used for heating and hot water is supplied by a district heating system and below average (around 28 GJ/yr). We are unhopeful about the prospects and speed for a more sustainable district heating system in the neighbourhood.
In 2007 the living room on the first floor was extended into the garden, which is on the south side. The extension has a flat roof. The rear façade has large windows, particularly on the first floor (estimated 8m2). As a result, the living room can become very warm during summer, and this will likely become worse in the future.
There are several measures left for making the house more sustainable.
- Energy savings – The house has fairly good thermal insulation, and we assume that we can save another 5% on electricity for lighting and appliances (down from 3400 kWh today). Furthermore, energy could be reduced by closing cracks and a better insulated front door (estimated reduction 10%), triple glazing (10 – 20%), infrared instead of regular heating in the bathroom (0 – 5%)
- Heat recovery – Recovering heat from the ventilation system would imply replacing our system C ventilation with a system D. This could potentially reduce heat demand by 90%. However, it would be a costly system (investment 3-6kE), as it would require a tube network throughout the house, and therefore unattractive. A more interesting option would be to recover shower heat and reduce the demand for hot water by an estimated 10%. This would be definitely worth the investment when renovating the bathroom.
- Heat pump – In particular air-water heat pumps have become very popular in recent years. Yet a borehole thermal energy system would bring advantage of lower noise levels and some cooling during hot summers. Inner unit could be placed in the attic, above the stairs (see figure). Adjustments will be needed to fit in the tubes alongside the stairs. Ideally, boreholes would be placed in the front yard (feasibility to be assessed). Installing such a system would cancel our heat demand from the district heating system (currently 1600 euro/yr for 28 GJ but likely increasing in the future). However, it would require an additional electricity consumption of 4560 kWh/yr and an annual electricity bill of around 1000 euros (see table). The heat pump would thus imply a saving of around 600 euro per year, which is modest compared to the 15 kE investment for a borehole system.
It seems however that this year (2021) a larger ventilation pump will become available on the market, which would recover heat from the mechanical ventilation and should be particularly suited for our house. Regrettably, this would not offer the opportunity to cool our house some degrees during summer, but it will be worth finding out more.
Table 1. Calculation electricity use heat pump for a 2002 house without recovery ventilation and 100% coverage of heat demand. Input data have been taken from www.warmtepomp-weetjes.nl.