A flexible demand-side can have a positive influence on electricity markets and the entire electricity infrastructure once the flexibility is properly harnessed. Such a goal is becoming attainable with emerging smart grid technologies which allow the controlling of consumption and its aggregation to electricity markets. In this paper, we focus on the aggregation of detached houses with direct electric space heating (DESH) in terms of two main targets. Firstly, a basic framework for the aggregation is proposed and secondly, the benefit of the heating load flexibility for the aggregator and the consumers is investigated in the Nordic day-ahead electricity market Elspot. The loads are controlled with a simple strategy based on a centrally transmitted thermostat set-point signal, and this strategy is benchmarked against a more complex direct load control approach. As there is great flexibility potential in the heating load, the aggregattor is assumed to act as a price-maker in the market, where it seeks to minimize its energy cost and schedule the flexibility. The loads participating in the control are provided with a bonus based either on the caused inconvenience or the provided flexibility. In simulations studies, we use a detailed Finnish detached house population model and hourly market data from Nord Pool's Elspot.