Two heat pumps and an existing boiler: how to build a cascade without unnecessary equipment

The building already has a working boiler. The owner wants to reduce costs and add heat pumps. The boiler does not necessarily need to be removed straight away: it can remain as a peak-load heat source or backup.

But installing two outdoor units side by side does not in itself create a cascade. It requires shared control logic, coordinated hydraulics and clearly defined load allocation. These solutions also appear in manufacturers’ system diagrams: NIBE describes control of multiple subordinate heat pumps, while Caleffi covers combining heat pumps with boiler equipment. (NIBE — S1155 system solutions: cascades and domestic hot water) (Caleffi — hybrid system with heat pumps and a boiler)

Calculate capacity at the required operating conditions

An illustrative office requires 30 kW at the design winter temperature. Two units rated at 16 kW each have been selected, but at the actual outdoor and flow temperatures in our example, we assume an output of 14 kW each.

Total:

Q₍HP₎=14+14=28 kW

There is a 2 kW shortfall. The boiler can provide this if the system is designed for such combined operation. This is a valid, deliberate design approach, not necessarily a sizing error.

However, if one heat pump fails, only 14 kW remains — approximately 47% of the required capacity. Two units do not automatically provide full redundancy. To establish that, all available heat sources and their ability to operate after a failure must be checked.

Supplementary heating and emergency backup are different roles

A boiler that only needs to add 2 kW during the coldest hours may face a very different requirement while both heat pumps are being repaired.

I would therefore specify separately in the technical brief how much heating must remain available if one unit fails, whether full comfort must be maintained during a breakdown, and how long reduced operation is acceptable.

We also check common points of failure: the power supply, circulation, controller and main pipework. This is a project-specific assessment, not a property conferred by the word ‘cascade’.

When is it more economical to bring the boiler on?

There is a thermal reason: the heat pumps cannot cover the load. There is also an economic reason: the variable cost of heat from the boiler becomes lower.

For a calculation excluding auxiliary energy consumption:

c₍HP₎=p_e/COP; c₍boiler₎=p_f/\eta

For illustration, assume electricity at 0,24 €/kWh, fuel at 0,08 €/kWh of fuel energy and boiler efficiency of 0,92. These are example prices, not current tariffs.

Break-even point:

COP₍threshold₎=(0,24×0,92) / (0,08)=2,76

At a COP of 3,2, heat from the heat pump costs 0,075 €/kWh, compared with approximately 0,087 from the boiler. At a COP of 2,2, heat from the heat pump costs around 0,109 €/kWh.

For a real-world comparison, we include auxiliary costs and use the same accounting boundary. The economic changeover point does not necessarily coincide with the temperature at which heat pump capacity becomes insufficient.

The hydraulics must support the chosen control logic

The required flow through active units must be maintained, and the boiler must not force the heat pump to operate at an unacceptable return temperature. Manufacturers’ hybrid system designs provide coordinated circuits and controls for this purpose. (Caleffi — hybrid system with heat pumps and a boiler)

For the shoulder seasons, we would also check minimum output: whether it is better to run one heat pump, run two at part load or change the operating sequence. The characteristics of the specific units determine the answer.

A good cascade is not about having as many heat sources as possible. It provides a clear answer to which source is running, why it is running and what will remain available if a failure occurs.