What to consider when choosing a hybrid system with heat pump and boiler

05/02/2022 – A hybrid system is the result of combining two generators powered by different energy sources (fuel and renewable source), therefore also called bivalent system:
a condensing boiler powered by LPG or methane, called an indoor unit because it is installed inside;
a heat pump (HP)air-to-air or air-to-water, called an outdoor unit because it is installed outdoors.

In the first part of the focus we interviewed eng. Filippo Busatopresident AiCARR, who provided us with explanations on the general aspects of the technology and explained when this system actually represents a valid alternative to the more traditional ones, such as boilers, or to the more popular ones, such as heat pumps.

In this second part we interviewed RIELLO that last year, in the 2021 edition of ADA – archiproducts design awardswas awarded in the construction section with his hybrid wall system pro; we will go down together with Simone Martinelli (Pre Sales Manager – RIELLO) more in detail of the operation of a hybrid heat pump system, trying to dispel the various preconceptions and discover the real advantages and potential of this system.

In a hybrid system, why does the boiler and not the heat pump come into operation when it is very cold?

In conditions of temperatures close to freezing and with more or less high relative humidity, heat pumps “suffer”. I sistemi ibridi factory made can solve these critical issues in the operation of heat pumps.

In fact, in such conditions, in order to capture energy from the outside air, the heat pumps must work with one evaporation temperature coil side below 0 ° C, with consequent ice formation and reduction of heat exchange. Plus, the more it is high relative humidity the faster this phenomenon will be.

This situation determines the COP reduction (Coefficient of Performance) up to 30% and the need for periodicals defrost cycles even close together with a consequent reduction in the power delivered. In such conditions it is often necessary to turn on the boiler to give continuity to the system and improve the overall performance. “

COP and Temperature Ratio © RIELLO


Couldn’t the problem be solved by using an oversized HP?

“Heat pumps reduce the power delivered according to the external temperatureit could therefore be thought that it is correct to oversize the heat pump in order to satisfy the system load at the design temperature.

However, this statement is wrong from the point of view of costs (adding KW of power is very expensive) and energy benefits; it is well known, in fact, that in a residential building, 80% of energy consumption occurs when the load factor of the system is between 20 and 60%.

The choice of a larger heat pump size it would mean a high initial investment to satisfy a few hours of heating with unsuitable external conditions (below 2 ° C) in which the COP of the heat pump begins to degrade significantly. “

“Defrosting cycles reduce efficiency” was previously stated, translated into other terms it means increased operating costs and reduced HP power. How is all this handled in a hybrid system?

“I defrost cycles are to be avoided in heat pumps because they reduce the efficiency as well as the power supplied to the system. In hybrid systems the associated system intelligence to a careful choice of bivalence temperature can limit and / or avoid this phenomenon.

If the heat pump is in “crisis” during operation despite being above the bivalence temperature, the system switcherebbe automatically to the condensing boiler, if the system temperature set was not reached on schedule.

This type of approach is even more evident in Hybrid systems with heat pump and condensing boiler in series such as our light commercial Hybrid system, because the heat pump operating with the return water will be in more favorable conditions for its operation. “

Concepts such as Bi-valent temperature o Balance Point BT or again system temperature set, are central to a hybrid system. What do these terms represent?

“To meet the heat requirement of the building, the generator is sized considering the maximum power required at the design temperature. In the case of a system with a heat pump only, this would have a rather high power only to satisfy the 15-20% of annual consumption that occurs in the coldest heating period with greater load.

In such conditions, heat pumps are not very efficient because they are located at operate in non-congenial conditions; ie with maximum system delivery temperature and minimum external temperature. Hybrid systems, which are bivalent, solve this criticality. The heat pump can be sized up to 60% of the system load, and the boiler will manage the remainder of the load.

The bivalent temperature (θbiv) is the external air temperature for which the HP is able to satisfy the thermal load; it must be evaluated taking into consideration the climatic zone, the characteristics of the building, the renewable quota required and the system delivery temperature.

From LV to higher temperatures, the HP will be suitable for heating, while from outside temperatures lower than LV the boiler will operate.

In the graph below it can be seen how the bivalent temperature (θbiv) has been chosen is in a range in which the heat pump still offers high COP, in this way it is possible to optimize the costs / benefits of the system. “

What to consider when choosing a hybrid system with heat pump and boilerThermal load sizing © RIELLO

The sizing of a Hybrid Pd

“A hybrid system must be sized with the aim of making the most of the heat pump when it is in the working range of maximum efficiency and / or when the energy conditions allow it,” said RIELLO.

For the sizing of a hybrid system, you can decide to set only one temperaturethe Bivalent temperature (BT) o due temperaturethat is the bivalent and the temperature of cut-off, the outside air temperature below which the HP will switch off and only the boiler will work.


By setting solo the bivalent temperature we will have a operation of the alternative bivalent type plantonly the boiler or only the HP will work:
– Texternal bivalent, only the boiler comes into operation;
– Texternal > Tbivalent, only the HP comes into operation;

By setting both the BT and the cut-off we will have a operation of the bivalent type plant parallelwhich offers the advantage of exploiting the coexistence of the two generators:
– Texternal cut-off, the HP must switch off and only the boiler will be able to function;
– T cut-off external bivalent, coexistence of the HP and the boiler;
– Texternal > Tbivalent, the boiler will switch off and only the HP will work.

Finally, according to Simone Martinelli, “the ideal cut-off temperature must allow the heat pump to cover 70-80% of the energy consumed by the system and a load factor of up to 60%, the remaining part will be managed by water heater. Furthermore, in the presence a photovoltaic system (FV) the cut-off temperature can be lowered, because it will be possible to exploit the electricity produced “.


What to consider when choosing a hybrid system with heat pump and boiler Bivalent heating system with heat pump and boiler

What to consider when choosing a hybrid system with heat pump and boiler Bivalent heating and DHW production system combined with heat pump, boiler and single-coil boiler


Source: Le ultime news dal mondo dell'edilizia by www.edilportale.com.

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