In this article possibilities and constraints of grid flexible control of heat pumps for domestic heating are investigated. By creating dynamic coalitions of heat pumps and control their power consumption, demand response can be offered to the power grid. For a functional power grid, the heat pumps electrical power consumption needs to follow the electrical grids needs accurately. Possibilities to externally control the power consumption of a ground source heat pump has been investigated. Both direct control, where the compressor speed is set directly, and indirect control, achieved with outdoor temperature sensor override, has been evaluated. For the evaluation a test cycle for laboratory testing of a heat pumps grid flexibility has been developed. Based on the test cycle, the heat pumps possibilities to follow the load profile using both direct and indirect control was tested in laboratory. Both control methods were possible to use, with the direct control being significantly more accurate. Using direct control, the power consumption of the compressor managed to be within ±10% of the stated power consumption for 97% of the time.