The created control strategy permits acting directly on the flow rate by means of an ON/OFF signal, which operates directly on the pumps in order to avoid overheating conditions in winter and excessive cooling in summer. For the internal air temperature control a band of variability of 19^21°C was assumed in winter and of 25^27°C during summer.

The adopted control strategy also intervenes on the supply temperature of the radiant ceilings. It is in fact necessary to supply such terminals in a way that the surface temperatures are not too high in winter, in order not to generate conditions of thermal discomfort, nor too low in summer in order to avoid water vapour surface condensation phenomena [9].

During winter the control system determines the required inlet temperature as the sum of a first stationary term corresponding to the regime and of a second variable term [6,10]. The inlet temperature of the radiant ceiling (tiC) in stationary conditions it is the function of the external air temperature (tOA), seen as the sole constraint on the system during winter. This bond has good linear approximisation with the incline of the line which unequivocally depends upon the dispersive properties of the building shell elements [1,6]. The stationary term was determined with a prelimiary simulation campaign conducted in the absence of solar radiation and the relation obtained is the following:

tIC (steady) = -0.4726 • tOA + 29.452 (1)

Exactly because the effect of solar radiation is not taken into consideration, it is necessary to join the ON/OFF control on the feed pump to interrupt the flow rate in order to avoid overheating of the environments. The value calculated with the Equation (1) permits the regulation, in an efficient manner, of the internal air temperature when this is around 20°C, but if it moved away from such a value, for example upon morning starting of the plant, such a temperature is not sufficient in that it requires some time to work properly. In such a case, to accelerate the response, the inlet temperature value determined by the Equation (1) is correct with a second transitory term which is made to depend upon the internal air temperature (tIA) and external air [6,10]. The relations obtained are the following:

tCC (transient) = K1 • (20 — tIA) (2)

with Ki linear function of the external air temperature:

к = 0.0473 • tOA +1.019 (3)

The function of the K1 parameter is that to produce an increase in the inlet temperature of the plant decreasing with external air temperature. The transitory term gradually decreases as the internal air temperature increases, until it cancels itself for tIA=20°C. The inlet temperature of the radiant ceiling in winter is determined by the relation:

tIC =(- 0.4726- tOA + 29.452)+ [(0.0473- tOA +1.019)0 — tIA)] (4)

In summer, regulation of the inlet temperature is obtained by means of a control system which determines, hour by hour, the air dew-point temperature (tdp) within the environment. The inlet temperature is then set equal to the dew-point temperature, since the thermal resistances between the water circulating in the pipes and the ceiling surface ensures a ceiling surface temperature that is always greater than that of the dew-point [11]. A defect of this control strategy consists of the necessity of maintaining the value of relative humidity within the environment at acceptable levels. In fact, too high relative humidity leads to high dew-point temperatures, and therefore to surface ceiling temperatures that are insufficient for the complete removal of sensible loads. Preliminary simulations have demonstrated that the external air exchange, set at equal to 1.2 Volh-1, is sufficient in order to maintain, during the plant functioning times, relative humidity between 40% and 60%. Furthermore, the thermic resistance between the fluid circulating in the pipes and the ceiling surface leads to a AT« 4°C, therefore in the most critical moments this safety margin is exploited to further lower the inlet temperature and to guarantee the complete removal of sensible loads. The relation obtained for the inlet temperature of the radiant ceiling which ensures the variability of the internal air temperature between 25°C and 27 °C during summer is the following:

tIC(summer) = tdp — 2 • (tIA — 25) (5)