Best Underfloor Heating: Complete guide to choosing the perfect system

Michael Tucker | September 25,

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Underfloor Heating Design Guide Introduction

When it comes to heating your home or building efficiently, underfloor heating—known as radiant heating in the USA—is a top contender.

This modern heating solution offers a range of benefits, from evenly distributing warmth across every room to reducing energy bills by operating at lower temperatures.

Whether you’re building new or retrofitting an existing space, a well-designed system can bring unparalleled comfort and energy savings.

In this guide, we’ll take you through the step-by-step process of designing an underfloor/radiant heating system.

Also, from calculating heat requirements and choosing the right pipe sizes to optimising flow rates and selecting the best manifold location, you’ll learn everything you need to know to ensure your system runs efficiently and effectively.

How To Design Underfloor/Radiant Heating

1. Manifold Placement and Setup

The heart of any underfloor (or radiant) heating system is the manifold.

Acting as the control centre, the manifold distributes heated water from the boiler or heat pump to the circuits under your floors.

Properly positioning and setting up the manifold is critical to ensuring the efficiency and performance of your system, whether you’re installing it in a small home or a large commercial space.

Suitable Placement for the Manifold:

• Centralised Location: Ideally placed centrally within the heated space to minimise the length of pipe runs and ensure even heat distribution.
• Accessible Area: Should be installed in a location that’s easily accessible for maintenance, such as a utility room, cupboard, or basement.
• Ventilated Space: Place the manifold in a well-ventilated area to prevent overheating and ensure the system operates efficiently.
• Avoid Damp Areas: Ensure the manifold is installed in a dry area, away from potential water damage.
• Height Considerations: For easier installation and maintenance, mount the manifold at a comfortable working height (typically between 1m to 1.5m from the ground).

Limitations to Consider:

• Distance from Heat Source: Avoid placing the manifold too far from the boiler or heat pump to minimise heat loss in the pipes.
• Pipe Length Restrictions: Limit the pipe runs from the manifold to prevent pressure drops and ensure consistent water flow; generally, pipe runs should not exceed 100m for a 16mm pipe.
• Space Constraints: The manifold requires sufficient space for installation, including enough clearance around the pipes, valves, and controls for easy access.
• Noise Sensitivity: In residential settings, avoid placing the manifold near bedrooms or living spaces, as it can produce operational noise.
• Multi-Story Buildings: In multi-story installations, consider separate manifolds for each floor to simplify the pipework and improve system control.

2. How to Calculate the kW/BTU for Underfloor/Radiant Heating

Before you can accurately size an underfloor (or radiant) heating system, it’s crucial to first understand the room’s heat loss.

The system’s heat output must match the heat loss to maintain a comfortable temperature.

Failing to account for the heat loss could result in a system that either underperforms, leaving rooms too cold, or wastes energy by oversizing the heating system.

Heat Loss:

Heat loss isn’t just about the size of the room; it also involves factors like insulation, window quality, and even heat lost through ventilation.

By calculating the total heat loss, you ensure that your underfloor heating system is appropriately sized to keep your space warm and efficient.

Additionally, factors contributing to heat loss include:

• Room Size and Surface Area: Larger rooms have more surface area for heat to escape, especially through walls, ceilings, and floors.
• Insulation Quality: Poorly insulated walls, floors, or windows increase heat loss significantly.
• Ventilation: Heat is also lost through ventilation, whether it’s natural airflow or mechanical systems like extractor fans.
• Desired Indoor Temperature: The greater the temperature difference between inside and outside, the higher the heat loss.

Heat Loss Formula:

Transmission losses

Each building component (walls, windows, roof, etc.) has its own U-value, measuring heat transfer, and requires separate calculations.

They are calculated using the following equation:

Metric:

Imperial:

Ventilation losses

This happens when hot indoor air is replaced by cooler outdoor air through ventilation or infiltration.

They can be calculated using the following equation:

Metric:

Imperial:

Where the Air Change Rate indicates how frequently the building’s air is completely replaced.

3. How to Calculate the Flow Rate for Underfloor/Radiant Heating

One of the key aspects of designing an efficient underfloor (radiant) heating system is calculating the correct flow rate.

The flow rate determines how much water needs to circulate through the pipes to deliver the required heat output.

Several factors determine this calculation, including the heat loss being overcome and the temperature difference between the supply and return water (ΔT).

What Is ΔT (Delta T)?

ΔT (Delta T) refers to the temperature difference between the water as it enters the underfloor heating system (flow temperature) and as it leaves (return temperature).

The value of ΔT influences the flow rate required to transfer the necessary heat to the room.

Typical values for ΔT:

UK Systems: Commonly designed with a ΔT of 5-10°C.

US Systems: Often use a ΔT of around 10-20°F.

Metric Flow Rate Formula:

The basic formula for calculating the flow rate is:

Flow Rate (L/s) = Heat Output (kW) / (ΔT × 4.18)

Where:

• Heat Output is the amount of heat required by the room, calculated in kW (kilowatts).
• ΔT is the temperature difference between the flow and return water (in °C).
• 4.18 is the specific heat capacity of water (in kJ/kg°C).

Imperial Flow Rate Formula:

Flow Rate (GPM) = Heat Output (BTU/h) / (ΔT × 500)

Where:

• Heat Output is the amount of heat required, calculated in BTU/h.
• ΔT is the temperature difference between the flow and return water (in °F).
• 500 is a constant based on the specific heat of water and conversion to GPM (gallons per minute).

How to Calculate Pipe Size for Underfloor/Radiant Heating

Selecting the correct pipe size is crucial to the efficiency and performance of your underfloor (or radiant) heating system.

The pipe size you choose affects the system’s overall energy consumption.

While the most commonly used pipe size is 16mm / 5/8″, variations in pipe diameter may be necessary depending on the specific requirements of the space you’re heating.

Simplify your heat load calculations in alignment with industry standards within h2x.

Common Pipe Sizes and Their Uses

• 12mm (3/8″) Pipes: Often used in retrofit projects with limited floor height. These pipes are smaller and easier to install in thin layers of screed but may require closer spacing and higher flow rates to achieve the same heat output.
• 16mm (5/8″) Pipes: The most common size for both residential and commercial underfloor heating installations. It offers a good balance between ease of installation, flow rates, and heat output. Suitable for most room sizes and floor types.
• 20mm (3/4″) Pipes: Typically used for larger or commercial spaces that require higher heat output over larger areas. These pipes allow more water flow, making them suitable for spaces with higher heat loss or larger floor areas.

Factors to Consider When Choosing Pipe Size

• Room Size and Heat Demand: Larger rooms with higher heat demands may benefit from larger pipes (e.g., 20mm or 3/4″) to ensure adequate heat output, while smaller rooms typically use 16mm or 12mm pipes.
• Pipe Length and Flow Rate: Longer pipe runs can create pressure drops and reduce system efficiency. The larger the pipe diameter, the lower the pressure drop, which can benefit large installations with long pipe runs. However, excessive pipe lengths should be avoided; pipe runs should typically not exceed 100m (328ft) for 16mm pipes.
• Pipe Spacing: Closer pipe spacing can compensate for smaller pipes, but wider pipe diameters typically allow for wider spacing without sacrificing heat output. For example:
• 100-150mm (4-6″) spacing is common for smaller pipes (12mm).
• 150-200mm (6-8″) spacing works well with standard 16mm pipes.
• 250mm (10″) or wider spacing may be used with larger pipes (20mm or more) in commercial applications.
• Floor Type: Certain floor finishes, like tiles or concrete, conduct heat more efficiently than wood or carpet, which may influence the pipe size and spacing needed. If a floor has poor thermal conductivity, you might opt for smaller pipes with closer spacing to ensure even heat distribution.
• Water Velocity: Keep the velocity of water within recommended limits—usually not exceeding 1 m/s (3 ft/s)—to avoid noise and excessive wear on the system.

How to Calculate Underfloor Heating Output

Determining the output of your underfloor (or radiant) heating system is essential to ensure that it can meet the heat demand of the room.

Several key factors, including pipe spacing, the type of floor finish, and the mean water temperature in the system, influence the heat output.

There are two primary methods to calculate the heat output of an underfloor heating system:

1. Manufacturer’s Data: Many manufacturers provide performance data for their specific underfloor heating products. This data often includes charts or tables that show heat output (in watts or BTU) for various pipe spacings, water temperatures, and floor finishes. Using this data allows you to design the system based on real-world testing of your components.
2. CIBSE (UK) or ASHRAE (US) Tables: For a more generalised approach, you can refer to industry-standard guidelines like the CIBSE tables (Chartered Institution of Building Services Engineers) in the UK or ASHRAE standards (American Society of Heating, Refrigerating and Air-Conditioning Engineers) in the USA. These tables offer guidance on typical heat output based on system design variables, such as pipe spacing and water temperature.

Key Factors Affecting Heat Output

• Pipe Spacing: The distance between the pipes in the system plays a significant role in the overall heat output. Closer pipe spacing increases the heat output, as more pipes distribute heat more evenly across the floor. Typical pipe spacings include:
• 100-150mm (4-6″): Provides higher heat output, ideal for spaces with higher heat demand or poor insulation.
• 200-300mm (8-12″): Suitable for well-insulated spaces with lower heat demand.
• Floor Finish: The type of floor covering dramatically affects the system’s ability to transfer heat. For example:
• Tiles and stone: Excellent heat conductors, allowing maximum heat transfer from the pipes to the room.
• Wood and laminate: Moderate conductors, often requiring slightly higher water temperatures or closer pipe spacing to achieve the desired heat output.
• Carpet: Acts as an insulator, reducing heat output. Systems designed for carpeted rooms may require smaller pipe spacing or higher water temperatures.
• Mean Water Temperature: The temperature of the water circulating through the pipes directly impacts the heat output. The higher the water temperature, the greater the heat output. However, the system should remain within recommended operational limits to avoid inefficiency or overheating. Typical flow temperatures range from:
• 35-55°C (95-131°F) for most residential systems.
• Higher temperatures may be necessary for poorly insulated spaces or those with less conductive flooring materials.

How to Work Out the Coil/Roll Length for Underfloor/Radiant Heating

Accurately calculating the coil or roll length required for your underfloor (or radiant) heating system is essential to ensure you have the right amount of pipe for your installation.

Factors like the total area being heated, the pipe spacing, and the system layout determine this calculation.

Having the correct coil length prevents delays, ensures optimal system performance, and avoids unnecessary wastage.

Key Factors to Consider

• Room Size: The total surface area of the heated space largely determines the overall pipe length.
• Pipe Spacing: The distance between the pipes significantly affects the required pipe length. Closer pipe spacing increases the total pipe length needed.
• Manifold Placement: The distance from the manifold to the heated area will also affect pipe length.
• Pipe Bends and Layout: Always account for extra pipe required for bends, connections, and any overlap between zones. It’s a good idea to add 5-10% to your calculated pipe length to cover these adjustments.
• Coil Sizes: Coils typically come in standardised lengths, such as 100m, 200m, or 500m. Be sure to choose the coil size that best fits your project to avoid wastage or having to join multiple coils.

Formula to Calculate Coil/Roll Length for Underfloor/Radiant Heating

Metric Formula:

Imperial Formula:

Where:

• Pipe Length: The total pipe length required for the room or zone.
• Pipe Spacing: The distance between each pipe loop (common values: 100mm – 300mm or 4” – 12”).
• Transit Pipe Length: The length of pipe required to connect the manifold to the heated area.
• 1.10: This multiplier adds 10% to account for extra pipe needed for bends and connections.

Underfloor/Radiant Heating Calculation Example

1. Heat Loss Calculation

First, calculate the room’s heat loss to determine how much heat is required.

And whilst this is a complex calculation, we will simplify it to keep this example concise.

Metric:

Imperial:

2. Flow Rate Calculation

At this point, once you know the heat loss / the desired heat output, you calculate the flow rate for the underfloor/radiant heating system.

Metric:

Flow Rate (L/s) = 2.4 kW / (10°C × 4.18) = 0.057 L/s

Imperial:

Flow Rate (GPM) = BTU/h / (18°F × 500) = 0.90 GPM

3. Pipe Size Calculation

For example, we will aim to use a 16mm (5/8″) pipe, which is commonly used in residential underfloor/radiant heating systems.

We need to verify the velocity is within the correct range, ideally less than 1m/s / 3 ft/s.

Metric:

Imperial:

You can also calculate fluid velocity flowing through pipes easily using our free online Pipe Velocity Calculator.

4. Heat Output Calculation

And then, the table below shows that a carpet floor finish, a room temperature of 20°C (68°F), and a mean water temperature of 40°C (104°F) require 200mm pipe spacing to achieve 60 watts per square metre.

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CIBSE Underfloor Heating Design Guide ().

This also means the floor temperature is: 25.7°C (78.26°F).

5. Coil Length Calculation

Finally, calculate the coil length required for the underfloor/radiant heating system, including extra length for bends and connections.

Metric:

Imperial:

Summary:

Floor Finish: Carpet

Floor Temperature: 25.7°C (78.26°F)0.

Total Heat Loss: 2.4 kW ( BTU/h)

Flow Rate: 3.42 L/min (0.90 GPM)

Pipe Diameter: 16mm (5/8″)

Velocity: 0.50 m/s (1.64 ft/s)

Pipe Spacing: 200mm (8″)

Mean Water Temperature: 40°C (104°F)

Final Pipe Length: 225.5 metres (723.6 feet)

How h2x Can Enhance Your Underfloor Heating Design Guide

h2x software can simplify the entire process for your radiant aka underfloor heating design, from planning to implementation.

The tool automates calculations for pipe spacing, loop length, and heat output, ensuring accuracy and optimal performance.

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It also allows you to input specific project parameters, such as floor construction and desired temperatures, and instantly generates tailored layouts.

With detailed reports and CAD/Revit export options, h2x helps engineers present professional, client-ready designs for both new builds and retrofit projects.

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Underfloor Heating Design Guide FAQs

What is radiant/underfloor heating, and how does it work?

Radiant heating, also known as underfloor heating, is a system that heats a space by circulating warm water or using electric heating elements beneath the floor surface.

This provides even, comfortable heat throughout the room without the need for traditional radiators.

What’s more, our underfloor heating design guide above details insights into how these systems function.

Is radiant/underfloor heating more efficient than traditional radiators?

Yes, radiant/underfloor heating systems typically operate at lower temperatures, making them more energy-efficient compared to radiators.

They provide consistent heat, which can reduce energy consumption and lower utility bills, especially when designed according to industry best practices outlined in this underfloor heating design guide.

How much does it cost to install radiant/underfloor heating?

Installation costs vary depending on the size of the room and labour rates.

On average, radiant/underfloor heating systems in the UK range from £75 to £100 per square metre, while in the USA, they typically cost between $8 and $12 per square foot.

What is the running cost of radiant/underfloor heating?

Running costs depend on factors like insulation, room size, and energy prices.

Generally, radiant/underfloor heating is cheaper to run than traditional heating systems.

Furthermore, our underfloor heating design guide can help you predict running costs more accurately by tailoring the system to your needs.

In the UK, running costs can range from £4 to £6 per day, while in the USA, it may be around $5 to $8 per day.

Can radiant/underfloor heating be installed in existing homes?

Yes, radiant/underfloor heating can be retrofitted into existing homes.

However, they may require raising the floor height or additional structural work.

Nevertheless, using our underfloor heating design guide will help to avoid common mistakes and ensure the system delivers maximum comfort and efficiency.

What type of flooring works best with radiant/underfloor heating?

Tile, stone, and concrete are the most effective floor coverings for radiant/underfloor heating because they conduct heat well.

Wood and laminate can also work but may require careful temperature control, while carpet can insulate and reduce heating efficiency.

How long does it take for radiant/underfloor heating to warm up?

Warm-up times vary based on the system and floor type.

In fact, wet radiant/underfloor systems with tiles or stone may take 30-60 minutes.

However, these systems are designed to maintain a consistent, comfortable temperature rather than quick bursts of heat.

What’s the difference between wet and electric radiant/underfloor heating systems?

Wet radiant/underfloor heating uses warm water circulated through pipes beneath the floor, while electric systems use heated cables or mats.

Wet systems are typically more efficient for larger spaces, while electric systems are easier and cheaper to install in small areas or retrofits.

Is radiant/underfloor heating safe to use with all floor types?

Most floor types are compatible with radiant/underfloor heating, but following the manufacturer’s guidelines is important.

Also, some natural woods and high-resistance carpets may need special consideration or temperature regulation.

How do I control a radiant/underfloor heating system?

A thermostat typically controls radiant/underfloor heating systems, allowing you to set the desired temperature and schedule.

You can also integrate smart thermostats to control zones and optimise energy use.

Does radiant/underfloor heating increase home value?

Yes, radiant/underfloor heating is considered a premium feature that can increase the value of a property.

It’s particularly appealing in modern or energy-efficient homes and adds to overall comfort.

Can radiant/underfloor heating be used as the sole heating system in a home?

In well-insulated homes, radiant/underfloor heating can often serve as the primary heating system.

However, supplementary heating may be required in larger homes or spaces with high heat loss.

Moreover, our underfloor heating design guide above provides more detail about using UFH as a home’s sole heating system.

How long does radiant/underfloor heating last?

Radiant/underfloor heating systems can last 50 years or more when properly installed.

Can radiant/underfloor heating be repaired?

Yes, you can make repairs, but accessing the pipes or electric mats may require lifting floor sections.

It’s essential to hire a professional for repairs to avoid further damage to the system.

What are the benefits of radiant/underfloor heating?

Some of the benefits are: distributes heat evenly, improves energy efficiency, eliminates visible radiators, enhances indoor air quality (since no dust is circulated), and increases comfort.

Does radiant/underfloor heating work with renewable energy sources?

Yes, radiant/underfloor heating is highly compatible with renewable energy sources like heat pumps and solar thermal systems.

Furthermore, it works efficiently with the lower temperatures these systems generate.

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