How and to what extent, domestic heating systems can be helpful in regaining power balance in a smart grid, is the question to be answered in this paper. Our case study is an under-floor heating system supplied with a geothermal heat pump which is driven by electrical power from the grid. The idea is to deviate power consumption of the heat pump from its optimal value, in order to compensate power imbalances in the grid. Heating systems could be forced to consume energy, i.e. storing it in heat buffers when there is a power surplus in the grid; and be prevented from using power, in case of power shortage. We have investigated how much power imbalance could be compensated, provided that a certain, yet user adjustable, level of residents' thermal comfort is satisfied. It is shown that the large heat capacity of the concrete floor alleviates undesired temperature fluctuations. Therefore, incorporating it as an efficient heat buffer is a viable remedy for smart grid temporary imbalances.