Air-source heat pumps in older flats with radiators: actual flow temperature, energy consumption and solutions

Air-source heat pumps are transforming heating and cooling in Spain, but when it comes to older flats with traditional radiators, many questions arise: Will a heat pump work? What flow temperature do I need? How much electricity will it actually use? Do all the radiators need replacing? In this article, we answer these questions with a practical, realistic approach for anyone who wants to upgrade their heating without undertaking a full renovation.

If you’re looking for an overview of combining air-source heat pumps and radiators, we recommend reading our article first: Air-Source Heat Pumps with Radiators: How They Work, Energy Consumption and Cost. Here, we focus on the specific challenges and solutions for older flats.

1. Assessing the existing radiator system

Before considering the installation of an air-source heat pump, it is essential to assess the flat’s radiator system:

• Radiator type and size: Older radiators are usually made of cast iron or aluminium, have high thermal inertia and are designed to operate at high flow temperatures (70–80 °C).
• Heat exchange surface area: The greater the radiators’ surface area, the easier it is to provide enough heat at lower temperatures.
• Condition of the system: It is important to check the pipe insulation, whether thermostatic valves are fitted and the overall condition of the circuit.

Conclusion: Replacing all the radiators is not always necessary, but it is worth assessing whether their size and placement allow the system to operate at lower flow temperatures (45–55 °C).

2. Weather compensation curves and actual flow temperature

The key to making an air-source heat pump work well with radiators in older flats is to adjust the flow temperature according to actual heating demand and the outdoor temperature. This is achieved with a weather compensation curve:

What is a weather compensation curve?

It is a control setting that adjusts the temperature of the water supplied to the heating system according to the outdoor temperature. For example, when it is 0 °C outside, the flow temperature might be 55 °C; at 10 °C, it might fall to 40–45 °C.

Why is it important?

The lower the flow temperature, the higher the heat pump’s efficiency (SCOP) and the lower its electricity consumption.

What temperature do I actually need?

In most older flats that have been properly assessed, the system can operate at between 50 and 60 °C on the coldest days, and at much lower temperatures for the rest of the year. A temperature of 65 °C may be needed only during extreme cold spells, and then only occasionally.

3. Adaptation options: radiators, fan coils and electric backup

a) Low-temperature (LT) radiators:
If the existing radiators are large or can be expanded, they can be retained and operated at 45–55 °C. If they are small, they can be replaced with low-temperature models, which offer a larger heat exchange surface area.

b) Fan coils:
These heat emitters work very well with water at 35–45 °C and can also provide cooling in summer. They are an attractive option if you want to use the heat pump for cooling as well.

c) Occasional electric backup:
On the coldest days, an electric backup heater can be programmed to cover peaks in demand without oversizing the heat pump. The extra electricity use is occasional and does not undermine annual savings.

4. Energy consumption and SCOP estimates by climate zone

The efficiency of an air-source heat pump (SCOP) depends on the flow temperature and the local climate. Here is an illustrative estimate for an older 90 m² flat in three Spanish climate zones:

Illustrative figures for a well-insulated home with adapted radiators and weather compensation. Actual savings depend on electricity and gas prices.

5. Real-world examples and noise management

• Example 1: An 80 m² flat in Madrid with cast-iron radiators, a maximum flow temperature of 55 °C, an actual SCOP of 3,4 and annual savings of 45% compared with natural gas.
• Example 2: A 100 m² flat in Valencia with expanded aluminium radiators, an average flow temperature of 48 °C, a SCOP of 4,0 and savings of 60%.
• Example 3: An older flat in Burgos with small radiators, replaced by fan coils in the living room and bedrooms, a flow temperature of 40–45 °C, a SCOP of 4,2 and comfort in both winter and summer.

What about noise?
Modern heat pumps (such as Ecoforest’s ecoAIR+) are very quiet. Outdoor noise is comparable to that of an inverter air conditioner, while indoors, a properly installed hydraulic system should not cause any disturbance.

6. Conclusions and recommendations

• Air-source heat pumps are entirely viable in older flats with radiators, provided a professional assessment is carried out and the system is adapted as needed.
• The key is to operate at the lowest possible flow temperatures, using weather compensation and, if necessary, adding radiator capacity or combining the system with fan coils.
• Energy consumption and savings depend on the climate, insulation and how well the system is fine-tuned.
• Always consult an approved installer and choose efficient, modulating equipment.

Want to know whether your flat is suitable for an air-source heat pump and how much you could save? Contact Ecoforest for a personalised assessment and discover how to make the switch to efficient, renewable heating and cooling.

Frequently asked questions

Do I have to replace all the radiators?

Not always. If they are large and the flat is well insulated, they may be suitable. If not, you can add radiator capacity or replace them with low-temperature models.

What happens on the coldest days?

You can programme occasional electric backup or temporarily raise the flow temperature without undermining annual savings.

Can I use the heat pump for cooling?

Yes, if you install fan coils or underfloor heating and cooling.

Would you like a personalised quote for your older flat? Our team can advise you with no obligation.