Air-to-water heat pump: is it the best choice for a renovation?

Renovation and heating: why is an air-to-water heat pump the obvious choice today?

Replacing a boiler during a renovation is the perfect time to do it!

During renovation work, walls, floors and pipework are all affected. This is precisely the context in which installing an air-to-water heat pump makes the most sense: the work is already underway, access is easier, and the indoor unit can be connected to the existing heating system without the need for additional work. Missing this opportunity often means having to return later to a refurbished house to carry out work that would have cost less if it had been incorporated from the outset.

The air-to-water heat pump: the only solution that truly replaces a boiler

Of all the heat pumps, the air-to-water heat pump is the only one that handles both heating via a hydraulic circuit and the production of domestic hot water,  just as the old boiler did. An air-to-air heat pump heats the ambient air but does not produce hot water and cannot be integrated with existing radiators. This is why, in a renovation project aimed at replacing the entire heating system, the air-to-water heat pump is the only type of heat pump that is truly suitable.

Your existing heating system: what to check first

Old oil or gas boiler: what changes with a heat pump

An oil- or gas-fired boiler heats the water in the circuit to 70 or 80 °C. An air-to-water heat pump, unless it is a high-temperature model, operates at much lower water temperatures. If your central heating system has been designed to operate at high temperatures, the heat pump will either need to be adapted to your existing radiators using a high-temperature model, or you will need to replace your radiators. This factor determines the entire rest of the installation and must be assessed before you even choose a heat pump model.

The volume of water in the system: a common problem in renovation work

A heat pump requires a minimum volume of water in the circuit to operate correctly. However, systems with radiators often contain far less water than underfloor heating. When the volume is insufficient, the heat pump produces too much heat too quickly, causing the compressor to shut down prematurely and restart constantly: this is known as ‘short cycling’, which wears out the unit and drives up the bill. The almost universal solution in renovation projects is to install a buffer tank, which acts as a hydraulic flywheel and smooths out the heat output from the heat pump.

Existing radiators: can we really keep them?

Old cast-iron radiators and heat pumps: what this means in practice

Old cast-iron radiators were designed for water at 70–80 °C. If you install a low-temperature heat pump that produces water at 35–45 °C, these radiators will not give off enough heat to compensate. In this case, you have two options: opt for a high-temperature heat pump, capable of producing water up to 65 °C and therefore compatible with the cast-iron radiators without needing to replace them, or replace the radiators with low-temperature models, which fits naturally into a more extensive renovation.

When can existing radiators be kept?

If your home has relatively new steel or aluminium radiators that are correctly sized for the rooms they serve, a low-temperature heat pump can be installed without needing to replace them. The condition is that the home must be sufficiently insulated to reduce heating requirements, which will allow the existing radiators to operate efficiently even at a water temperature of 45 to 50 °C. In over 70 per cent of renovation projects, radiators can be retained provided the heat pump is correctly sized and, if necessary, only those that are undersized in large rooms are replaced.

Insulation and air-to-water heat pumps: in what order should the work be carried out?

Insulation has a direct impact on the heat pump’s efficiency

An air-to-water heat pump is all the more efficient the better the home retains heat. In a poorly insulated home, the heat pump has to heat the water to higher temperatures to compensate for heat loss, which reduces its COP – the ratio of electrical energy consumed to heat produced. The lower the COP, the higher the bill. Conversely, every improvement to insulation – in the loft, walls or windows – allows the heat pump to operate at a lower temperature, thereby consuming less energy whilst maintaining the same level of comfort.

For partial refurbishments, a high-temperature heat pump is often the best place to start

Not all renovations are carried out all at once. When the insulation has not yet been replaced, or has only been partially replaced, a high-temperature air-to-water heat pump remains a viable option: it adapts to the building’s current heat loss, supplies the existing radiators without replacing them, and can become more efficient as the insulation work progresses. This is a phased approach to renovation, which avoids stalling the project whilst waiting for all conditions to be perfect.

Domestic hot water in a renovation project using an air-to-water heat pump

How does the CAP deal with domestic hot water?

When renovating and replacing an old boiler, the issue of domestic hot water immediately arises. An air-to-water heat pump can provide this via a storage tank connected to the indoor unit. The heat pump heats the water in the tank, preferably during off-peak hours to keep electricity costs down. The size of the tank depends on the number of occupants and the household’s hot water usage patterns.

What has changed compared with the old boiler

With a gas or oil-fired boiler, domestic hot water is produced on demand or via an integrated storage tank. With an air-to-water heat pump, production is decoupled: the heat pump heats the storage tank at specific times, and hot water is drawn from this supply. This operating method requires a storage tank that is correctly sized and well positioned within the system. In some cases, if a large volume of domestic hot water needs to be produced, the installer may recommend a separate thermodynamic tank, which itself captures heat from the ambient air in a utility room to heat the water, thereby reducing the load on the main heat pump.

What you need to prepare before installing an air-to-water heat pump

Flushing the circuit: a mandatory step

A heating circuit that has been in operation for 10, 15 or 20 years with a boiler contains sludge deposits: mainly magnetite, an iron oxide produced by the corrosion of pipes and radiators. If an air-to-water heat pump is connected to this system without cleaning it, this debris will migrate to the heat pump’s heat exchanger, clogging it up and causing a drop in performance. In the most serious cases, it can lead to premature failure and invalidate the manufacturer’s warranty. Flushing the heating circuit before installing the heat pump is not optional: it is a standard procedure that any reputable installer should carry out.

The power supply to be checked

An air-to-water heat pump consumes electricity, and its power output may exceed 12 kW depending on the size of the property and the type of installation. Above this threshold, a three-phase power supply is required. If your electrical installation is old or single-phase, you should check with your installer whether the fuse box needs upgrading before installing the heat pump. This is a technical issue that is often overlooked in renovation quotes, but which can cause delays if the application for connection to the electricity supplier is not made in good time.

Installing a hybrid heat pump during a renovation: when is this the best option?

When the property is not yet ready for a heat pump alone

A hybrid heat pump combines an air-to-water heat pump with the existing boiler. The heat pump provides heating in mild weather, whilst the boiler takes over during periods of extreme cold or when the outside temperature drops sharply. This setup is particularly well-suited to phased renovations, where the insulation is not yet sufficient to allow a heat pump alone to meet all heating requirements without excessive energy consumption. It also allows the existing boiler to be kept in good working order without having to scrap it straight away.

The limitations of the hybrid CAP that you need to be aware of

The hybrid heat pump is an effective transitional solution, but it involves maintaining two heating systems: two maintenance contracts, two energy sources, and two units to manage. In the long term, once the energy-efficiency renovation is complete, many homeowners eventually switch to an air-to-water heat pump alone to simplify the setup and maximise savings on their heating bills. The hybrid heat pump is therefore often a step in a renovation project, not an end in itself.

The installer’s role in the success of a renovation involving an air-to-water heat pump

A thermal assessment before any quote is provided

When renovating, you cannot simply choose a heat pump from a catalogue. The installer must first carry out a thermal assessment of the property: calculate heat loss room by room, assess the condition of the radiators and their ability to operate at lower temperatures, check the volume of the hydraulic circuit, and determine the heating capacity required based on the size of the property and its current level of insulation. Incorrect sizing (a heat pump that is too small or too powerful) will have an impact on the unit’s lifespan and on your energy bills.

The Water Act: the regulation that changes everything

Once an air-to-water heat pump has been installed as part of a renovation, setting the water temperature control curve is crucial. This is the heating curve that automatically adjusts the temperature of the hot water produced according to the outside temperature. An incorrectly set water temperature curve – one that is set too high – leads to unnecessary energy consumption. Lowering the water temperature by 10 °C can improve the Coefficient of Performance (COP) by around one point – which makes a noticeable difference to the annual bill. This fine-tuning must be carried out by the installer during commissioning, and then checked during the first few seasons of the heat pump’s operation.

An air-to-water heat pump is the right choice for the vast majority of renovation projects — provided the installation is approached methodically. The existing heating circuit must be cleaned, the power supply checked, the buffer tank correctly sized, the radiators assessed and the water flow control system properly configured. This is not simply a matter of hastily replacing a boiler: it is a complete heating installation, requiring a meticulous installer and a well-planned renovation project. Those who carry it out to the highest standards end up with a heating system that is efficient, durable and significantly cheaper to run than an old oil or gas boiler.