Monthly Archives: January 2026

When researching underfloor heating for an existing home, many people come across two main installation approaches: traditional overlay underfloor heating systems and modern milled underfloor heating installed directly into concrete floors.

Because overlay systems have been widely used for many years, it is understandable that homeowners often assume an overlay or screed build-up is required for underfloor heating to work effectively. In reality, advances in installation techniques now allow water underfloor heating to be retrofitted into existing concrete or screed floors without an overlay, using precision milling.

Both methods can deliver excellent comfort when properly designed. The difference lies in how the heat is transferred and how the system integrates with the existing structure of the building.

What is an overlay underfloor heating system?

An overlay system involves installing insulation panels or low-profile boards on top of the existing floor, laying the underfloor heating pipework within them, and then covering the system with a new screed or floating floor construction.

This approach is commonly used in new builds and some renovations because it creates a complete heating layer above the structural slab. However, in retrofit projects it also introduces:

• An increase in finished floor height
• Longer warm-up times due to thicker build-up
• The need to modify doors, skirting and thresholds
• Additional drying time when new screed is used

What is underfloor heating without an overlay?

With a milled underfloor heating system, the heating pipes are installed directly into channels cut into the existing concrete or screed slab. No insulation boards or additional screed layers are added on top.

This creates a true low-profile underfloor heating system where:

• The floor height remains unchanged
• The pipework sits closer to the finished surface
• The existing concrete acts as a stable thermal mass
• The system can be installed quickly, often within 1–2 days

This method is particularly suited to retrofit underfloor heating in existing concrete floors, where maintaining levels and minimising disruption are important.

How heat transfer differs between overlay and milled systems?

Overlay underfloor heating

In an overlay system, heat travels through multiple layers:

Pipe → overlay board → screed or levelling compound → floor finish → room

This creates a larger thermal mass above the pipes, which can provide stability but often results in:

• Slower heat-up times
• Higher flow temperatures
• Reduced responsiveness to thermostat changes

Milled underfloor heating in concrete floors

With a milled system, the heat path is much shorter:

Pipe → existing concrete slab → floor finish → room

Because the pipes are closer to the surface:

• Heat reaches the room more quickly
• Lower water temperatures can be used
• The system responds faster
• Efficiency improves, particularly with heat pumps

This is why milled water underfloor heating is often described as a highly efficient low-temperature heating system for retrofit projects.

Why overlay systems became standard

Overlay systems were developed to standardise construction build-ups and simplify installation on site, especially in new developments. They remain a valid solution in many situations, particularly where insulation upgrades and full floor reconstructions are already planned.

The perception that underfloor heating must sit in an overlay is largely due to familiarity rather than performance limitations.

Why underfloor heating without screed is increasingly chosen for retrofit projects

For existing properties, particularly those with solid concrete floors, installing underfloor heating without an overlay offers several practical advantages:

• No increase in floor height
• No need to remove the existing slab
• No long screed drying times
• Faster installation programmes
• Excellent compatibility with heat pumps
• Even heat distribution using the concrete as a thermal store

Rather than adding layers, this approach integrates the heating directly into the building fabric, making it one of the most effective solutions for retrofit underfloor heating on existing concrete slabs.

Choosing the right underfloor heating system for your home

Both overlay and milled systems can provide comfortable, efficient heating when correctly designed. The most suitable option depends on:

• Floor construction and available depth
• Whether insulation upgrades are planned
• Ceiling height limitations
• The heat source (boiler or heat pump)
• Desired response time and control
• Installation timeframe and disruption tolerance

In many renovation projects where maintaining levels and achieving fast, efficient heating is a priority, underfloor heating installed directly into the concrete floor using milling offers a technically elegant and future-proof solution.

So is there a definitive answer?

The idea that underfloor heating must be installed with an overlay or thick screed comes from traditional construction practice rather than a technical requirement. Modern milling techniques allow water underfloor heating to be installed without an overlay, often delivering faster response times, lower running temperatures and excellent compatibility with low-carbon heating systems.

Understanding both approaches allows homeowners to make informed decisions based on performance, efficiency and practicality, rather than assumption.

Book a free underfloor heating assessment

If you’re considering underfloor heating in an existing property and want to understand whether a milled, no-overlay system is suitable for your concrete or screed floor, a professional assessment is the best place to start.

We’ll review your floor construction, heating system and renovation plans, and provide clear advice on the most efficient and practical solution for your home.

Book your free, no-obligation underfloor heating assessment today.

Retrofit underfloor heating requires a very different approach from traditional new-build installations. Existing floor structures, heating systems and finished levels all need to be carefully considered to ensure the system performs efficiently without disruption.

At Channel Heat Systems, we specialise in installing water underfloor heating into existing concrete and screed floors using precision milling techniques. This allows us to integrate a low-profile heating system directly into the existing slab without raising floor levels, removing the floor, or extending renovation timelines.

This guide explains the professional retrofit underfloor heating installation process, step by step, so you can understand how modern systems are designed, installed and commissioned for long-term comfort and efficiency.

Step 1: Initial assessment and system design

Every retrofit project starts with a professional assessment. At this stage, we review:

• The existing floor construction (concrete or screed)
• Floor depth and structural condition
• Current heating system (boiler or heat pump)
• Room layout and heat-loss requirements
• Renovation plans and floor finishes

This information allows us to design a system that delivers the correct heat output, flow temperatures and zoning ensuring efficient performance from day one.

A proper design is essential, particularly in retrofit projects, where accuracy matters more than adding build-up.

Step 2: Preparing the existing concrete floor

Once the system design is finalised, the existing floor is prepared for installation.

Unlike traditional retrofit methods that involve removing floors or adding insulation boards and screed, our approach keeps the original slab in place. This avoids unnecessary demolition and helps maintain existing floor heights.

The surface is checked, marked out and prepared for precision milling.

Step 3: Precision milling of heating channels

This is the key step that makes retrofit underfloor heating possible.

Using specialist milling equipment, we cut accurately spaced, depth-controlled channels directly into the existing concrete or screed floor. These channels follow the exact pipe layout defined during the design stage.

Because the channels are milled into the slab itself:

• There is no increase in floor height
• No need for new screed layers
• Minimal dust and disruption
• Faster heat transfer once the system is operating

This method allows underfloor heating to be installed cleanly and efficiently, even in lived-in homes.

Step 4: Installing the underfloor heating pipework

Once the channels are milled, the underfloor heating pipes are laid neatly into the grooves and secured in place.

The pipes sit close to the finished floor surface, which is one of the reasons milled systems offer:

• Faster heat-up times
• Improved responsiveness
• Excellent performance with low-temperature heat sources such as heat pumps

Careful installation at this stage ensures even heat distribution and long-term reliability.

Step 5: Pressure testing and system checks

Before the floor is sealed, the entire system is pressure-tested to confirm there are no leaks and that the pipework is performing exactly as intended.

This step is critical in any water underfloor heating installation and ensures the system is safe, durable and ready to be commissioned.

Step 6: Sealing the channels and preparing for the final floor finish

After testing, the channels are sealed and the surface is made ready for the final floor covering.

Because there is no screed to dry, there is no waiting period. The floor can typically be tiled, finished with engineered wood, vinyl or other suitable coverings almost immediately.

This is one of the reasons retrofit installations can often be completed in just 1–2 days.

Step 7: Connecting to the heating system and commissioning

The underfloor heating system is then connected to the manifold and integrated with the existing heating system – whether that’s a modern boiler or a heat pump.

Controls are configured, zones are set up, and the system is fully commissioned to ensure:

• Correct flow temperatures
• Balanced heat distribution
• Efficient operation alongside any existing radiators

The result is a system that delivers steady, even warmth throughout the space!

Why this retrofit underfloot heating installation method works so well

By installing underfloor heating directly into the existing concrete floor, this approach avoids the most common issues associated with retrofitting:

• No raised floors
• No door trimming or threshold changes
• No long drying times
• Bo major building work

At the same time, it delivers the comfort and efficiency benefits underfloor heating is known for, making it a practical upgrade for many homes that were previously considered unsuitable.

Is retrofit underfloor heating right for your home?

Most properties with concrete or screed floors are suitable, but every home is different. Floor depth, condition and existing heating systems all need to be assessed properly before installation.

That’s why a professional assessment is the best way to understand what’s possible.

Book a free retrofit underfloor heating assessment

If you’re planning a renovation or want to upgrade how your home is heated, our team can help you explore whether retrofit underfloor heating is right for your property. Request a free consultation today!