The building examined is a small house, as shown in fig.1. It is made up of nine rooms dislocated on two floors with a heated area of 163 m2. The first floor is over a not heated basament, while the second floor is located beneath a not heated attic.

Radiant floor has been made by drowning a copper pipe with inner diameter of 12 mm in the slab, using a step of 15 cm. The heat transfer fluid, at low temperature and with a 0.112 kg/s flow rate, flows inside. Three localities of Italian territory have been considered: Cosenza, Rome and Milan. Table I shows for each city latitude, period of heating and outside mean air temperature in such period.

Table I — Latitude, days of house heating and outside mean temperature in the period of heating for the three cities.

The thermal calculation to respect the Law 10/91 has determined thickness of insulator (k = 0.045 W/mK) in dispersing external walls equal to 2 cm for Cosenza, 4 cm for Rome and 6 cm for Milan. The values of solar radiation and outside air temperature used in simulations, have been generated by the specific TRNSYS type 54 supplying hour values for each locality, starting from their correspondents monthly average data, so as to model a typical meteorological year (TMY) [4,5]. The values of beam and diffuse radiation on the horizontal plan have been obtained by the Reindl’s relations [6]. They use solar altitude angle and hourly clearness index as calculation parameters. The projection on tilted surface has been carried out by employing isotropic sky model [7]. Radiant floor has been simulated by a finite difference code [8,9] developed in TRNSYS environment dividing the floor into adiabatic segments, imposing inlet temperature to every segment equal to the outlet temperature from the previous segment. Such a code, linked to the Type 56 [10], describing the heated environment with its walls by separating valuation of inside and outside radiative and convective heat exchange, can analyse in detail the interaction between radiant floor and the heated environment. The type of control plays an important role in such heating systems characterized by great thermic inertia [11]. A control strategy has been chosen expecting inlet temperature to be variable in function of outside air temperature associated to a ON/OFF control on inside temperature in the dead band 19-21°C. Inlet temperature, which is generally different in each room, assures in absence of solar radiation an inside air temperature of 20 °C when in the adiacent rooms there is the same temperature. Inlet temperature assumes the following mathematical expression:

tiniet = k(20 — tae)+ 20 (1)

where the “k constant factor only depends on the mean heat loss coefficient of the room dispersing walls. To estimate domestic hot water requirement we have considered that each person consumes 50 liters of water at the temperature of 50 °C. We have considered the presence of 4 people and the temperature of water coming from the adduction circuit

of 10 °C, and we have determined in these conditions a 33.512 MJ daily energy requirement and a annual energy requrement of 12232 MJ. Tables II and III show, for the three localities considered, in the case of combined control, monthly and seasonal thermal energy requirement, monthly and seasonal mean inlet temperatures, calculated when the pump is on. Inlet temperature, between 23 °C and 31°C, shows that combined control in the radiant floor allows to use a low temperature fluid; moreover low temperatures supplying radiant floor permits to obtain, under the same conditions of tank’s storage volume, a greater usable stored energy.