In this paper, a co-simulation framework is presented to assess the impact on the distribution network of provision of support services (i.e. voltage support) by smart residential users. Such users are capable of providing flexibility by increasing/decreasing generation/consumption controlling the operation of an available set of flexible assets. The control of assets such as PV systems, electrical vehicles (EVs), heat storage and micro combined heat and power (mCHP) units is done by a Customer Energy Manager (CEM) after receiving flexibility requests as a result of an Aggregator-Distribution System Operator (DSO) interaction. In the presented framework, the distribution system is modeled in OpenDSS while the aggregator-flexible asset interaction, including the market-clearing procedure, is modeled using the Energy System Simulator (ESSIM). Results from several simulated scenarios are presented. According to the presented results, in summer, where over-voltage issues are expected due to the high PV penetration, a solution rate of 90% is estimated. For winter, in which under-voltage issues are more predominant, the solution rate is found to be around 70%.