Heat pump and radiant systems: “cost-optimal” solution for the energy requalification

20/09/2022 | small residential buildings | radiant floor systems

Floor radiant system

Andrea Bighinzoli - Giacomini S.p.A., Stefano P. Corgnati, Carola Lingua, Maria Ferrara - Dipartimento di Energia, Politecnico di Torino


Nowadays, the ambitious targets set by the European Commission for decarbonization of the construction sector can be achieved through the large-scale deployment of Nearly Zero Energy Buildings (NZEB). In NZEB buildings, the goal of reducing energy consumption and the related CO2 emissions can be easily achieved right from the concept design stage, through the coupling between building envelopes with adequate performance and efficient HVAC (heating, ventilation and air conditioning) systems integrated with renewable energy production technologies. These concepts are the basis of Directive 2010/31 / EU [1], also known as the Recast of the Energy Performance of Buildings Directive, which introduced not only the NZEB concept for the first time, but also a comparative methodology framework to guide Member States in defining the minimum energy performance of NZEBs with a view to achieving cost-optimal levels. This methodology allows evaluating different energy efficiency solutions taking into consideration not only the technical variables (from architecture and energy points of view) but also the economic-financial ones, in terms of investment, maintenance, management and disposal costs.

This article analyzes the impact of using different HVAC system alternatives combined with different level of thermal insulation on the performance of a single-family house. The selected building is a representative case study already presented in the literature [2], for which 9 design alternatives determined by the combination of 3 increasing levels of insulation of the building envelope and 3 different energy system architectures were examined.

In particular, the 9 "envelope + system" alternative packages that are generated are characterized by an energy performance value (expressed in terms of primary energy requirement [kWh/m2] net of the contribution of renewable energy sources) and by a corresponding global cost value, expressed in terms of €/m2, determined using the methodology illustrated in the following chapter.

The objective of the study thus concerns the implementation of the "cost-optimal" methodology to evaluate and compare alternatives of energy retrofit interventions, in order to identify the alternatives leading to reducing costs in the life cycle of the building while reaching a good energy performance level.

The cost-optimal methodology

The cost-optimal methodology was developed with the aim of identifying the energy design configurations for reaching a “cost-optimal” NZEB. In fact, the methodology can be generally adopted as a decision support tool, which is able to guide the choices of the design team and/or the client throughout the design process. The cost-optimal analysis allows comparing the energy (kWh/m2) and economic (€/m2) performance of different design configurations and identifying one or a set of solutions that fall within the so-called cost-optimality area. Such area is therefore populated by design configurations characterized by a “level of energy performance which leads to the lowest cost during the estimated economic life cycle of the building ", according to the definition provided by the European Commission.

In the present study, the energy performance assessment was performed with the Energy Plus dynamic simulation software [3], while the cost assessment was performed according to the global cost method detailed in EN 15459: 2007 [4].

In detail, the global cost indicator was determined for each different energy design configuration. Such global cost derives from the estimation of the net present value of all costs incurred over a defined calculation period, considering the residual values ​​of the components characterized by a lifespan longer than the defined calculation period. The global cost is therefore determined by summing up all the discounted costs, considering an appropriate discount rate depending on the incurrence time of the cost, including the initial investment costs, the periodic replacement costs, the annual maintenance costs and the annual energy costs and subtracting them from the final value, as shown in the equation (1) below:

Equazione calcolo costo globale


where Cg(τ) represents the global cost referred to the starting year τ0, CI is the initial investment cost, Ca,i (j) is the annual cost for component j at year i (including management, periodic and replacement costs), Rd (i) is the discount rate for year i, Vf,τ (j) is the final value of component j at the end of the calculation period (referred to the starting year τ0).

The case study: the single-family house in Piedmont, Italy

The building used as a reference for the development of the analyses is a case study of NZEB, which has already been analysed in the literature in its design and performance and can be considered as representative of NZEB buildings in the Piedmont Italian region [5,6]. It is a single-family house with a useful floor area of ​​about 140 m2, located in climatic zone E (according to the Italian climate zone classification).

  • Level 1 or baseline, corresponding to the maximum values ​​of thermal transmittance imposed by law for the specific climatic zone;
  • level 2, corresponding to the thermal transmittance values ​​suggested by the energy regulation of the Municipality of Turin;
  • level 3, corresponding to the thermal transmittance values referenced in the PassiveHouse protocol.

Instead, with regard to the energy system configurations, 3 alternatives have been defined as follows:

  • system type A (baseline): Gas condensing boiler with radiators for heating, multi-split system for cooling, a 3 kWp of photovoltaic system and 60% of required domestic hot water produced by a solar thermal system;
  • system type B, Water-to-water reversible heat pump with fan-coils for heating and cooling, mechanical ventilation with heat-recovery, a 3 kWp of photovoltaic and 60% of required domestic hot water produced by a solar thermal system;
  • system type C, Water-to-water reversible heat pump with radiant floor for heating and cooling, mechanical ventilation with heat-recovery and dehumidification, a 6 kWp of photovoltaic system and 60% of the required domestic hot water produced by a solar thermal system.

The following table represents the coupling matrix between insulation levels and energy system architectures, with indication of the codes denoting the nine examined design configuration alternatives. In particular, the 1A case represents the baseline (also called reference building, RB).

Table 1 – Codes denoting the 9 analysed design configuration alternatives


The following figure shows the so-called “cost-optimal” diagram for the examined solutions. Each dot in the diagram represents one of the previously defined design configuration alternatives. The line interpolating the solutions from 1A to 3C is the so-called “global cost curve”, of which its minimum point is the so-called cost-optimal point and corresponds to the alternative 2C.

Analyzing the figure, it emerges that the configurations where system type C is used (reversible heat pump with radiant floor) are those laying on the cost-optimality curve with higher energy performance values. Furthermore, the 2C optimum point is characterized by an intermediate level of thermal insulation and it is shown that a further increase in the insulation level, see the case 3C, leads to exit the cost optimality area with a global cost increase that is even higher than case 1C (basic insulation level).

The tendencies shown in terms of “cost-optimal” curve can be obtained also by the analysis of different radiant panel solutions, considering false radiant ceilings too: the application of this solution shows a growing interest from the market, not only of the tertiary sector but also for the residential one. Obviously, the couple of value (energy performance; global cost) characterizing each specific solution depends on the climatic zone of the building, with the corresponding heating and cooling needs to be matched with the occupants’ expectations: solutions can move from “total radiant” for heating/cooling with dehumidification, to “hybrid” configurations with radiant floor for heating and fan coils for cooling, with heavy summer seasons.


The performed analysis has demonstrated that the cost-optimal analysis is an effective tool for comparing different energy design alternatives for buildings, evaluating both an energy (in primary energy terms) and an economic-financial indicator (the global cost).

Further, the investigations conducted on a representative case study for a single-family house has shown how the design alternatives where system type is "heat pump & radiant floor" are "cost-optimal" solutions and that the optimal design configuration is characterized by an adequate and balanced value of thermal insulation, thus contrasting the super-insulation tendency.

Similar results can be obtained analyzing different configurations of radiant panels, applying both floor and false ceiling solutions.



  1. Direttiva 2010/31/UE del Parlamento Europeo e del Consiglio del 19 maggio 2010 sulla prestazione energetica nell’edilizia (Recast), Gazzetta ufficiale dell’Unione Europea (2010).
  2. Barthelmes V.M., Becchio C., Corgnati S.P., Guala C. 2015. Design and construction of an nZEB in Piedmont Region, North Italy. Energy Procedia, 78, 1925-1930.
  3. EnergyPlus Energy Simulation Software Web Site. Disponibile online: http://apps1.eere.energy.gov.
  4. UNI EN 15459:2008 “Prestazione energetica degli edifici - Procedura di valutazione economica dei sistemi energetici degli edifici” (2008).
  5. Barthelmes V.M., Becchio C., Bottero M.C., Corgnati S.P. 2014. The Influence of Energy Targets and Economic Concerns in Design Strategies for a Residential Nearly-Zero Energy Building. Buildings 4, 937-962.
  6. I. Abbà, G. Crespi, C. Lingua, C. Becchio, S.P. Corgnati, Theoretical and actual energy behaviour of a cost-optimal based NZEB, REHVA Journal, 1/2021, 22-27