When it comes to heating your home or building efficiently, underfloor heating—known as radiant heating in the USA—is a top contender.
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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.
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.
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 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:
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:
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:
Where the Air Change Rate indicates how frequently the building’s air is completely replaced.
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).
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.
The basic formula for calculating the flow rate is:
Flow Rate (L/s) = Heat Output (kW) / (ΔT × 4.18)
Where:
Flow Rate (GPM) = Heat Output (BTU/h) / (ΔT × 500)
Where:
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.
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:
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.
Where:
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.
At this point, once you know the heat loss / the desired heat output, you calculate the flow rate for the underfloor/radiant heating system.
Flow Rate (L/s) = 2.4 kW / (10°C × 4.18) = 0.057 L/s
Flow Rate (GPM) = BTU/h / (18°F × 500) = 0.90 GPM
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.
You can also calculate fluid velocity flowing through pipes easily using our free online Pipe Velocity Calculator.
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.
CIBSE Underfloor Heating Design Guide ().
This also means the floor temperature is: 25.7°C (78.26°F).
Finally, calculate the coil length required for the underfloor/radiant heating system, including extra length for bends and connections.
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)
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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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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Radiant/underfloor heating systems can last 50 years or more when properly installed.
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.
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.
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.
Underfloor heating is a great and increasingly popular way to keep your home warm. However, despite its many benefits (it is comfortable to walk, it distributes heat evenly around every corner of the room and allows you to place shelves or paintings at a potential radiator location), it can be expensive to install such heating, especially in older homes, so it is advisable to think carefully before making a decision.
This guide explains everything you need to know before you decide to install underfloor heating.
What is underfloor heating anyway?
How much do we even know about underfloor heating? We know it's a system that virtually converts the floor into a heater, into a radiator and prevents leaks in your home. Although it is best to install underfloor heating in a new building, it is possible to retrofit underfloor heating in your existing home by choosing the low profile option.
Underfloor heating involves installing piping in the floor so that all rooms are heated from the ground up. There are two main types:
• ’hot water’ or wet underfloor heating
• electric underfloor heating.
You are probably wondering what the differences are between the two heating systems and which one is more suitable for your household. We will try to explain some of the advantages and disadvantages of both these modes of underfloor heating:
• Electric underfloor heating is a network of wires under your floor, while wet underfloor heating is a series of pipes linked to a boiler or solar panels that pump hot water under the floor.
• Electric underfloor heating is easy to install and you can do it yourself, while a wet underfloor heating system will require a professional.
• Electric underfloor heating is cheaper.
• Electric underfloor heating runner costs are higher than water-based underfloor systems.
How much does it cost to install a floor heating system?
Installation costs will vary depending on the size of the room being heated, the amount of heating the building needs and whether it is a new building, renovation or conversion.
Prices for floor mats for electric underfloor heating start at around £18 / m2. Besides, you have to count on the cost of insulation board, screed and heating controls. You will also need to pay a qualified electrician to connect it to your electricity supply.
The cost of installing a wet system may vary depending on factors such as proximity to the boiler room. However, it will certainly be more expensive than installing electrical floor mats, especially if it is necessary to raise existing wooden floors or excavate concrete floors. It can cost from around £ 20 / m2 to £ 40 / m2 installed.
Underfloor heating is more expensive to install than a comparable radiator system, 20% -50% more.
And keep in mind: underfloor heating takes a while to get started, so you'll need to adjust the timer to start it at the right time before you can enjoy its warmth.
Which flooring works well for underfloor heating?
Materials that absorb heat rather than insulate are used as the optimum substrate, allowing heat to radiate into the room, ie tiles or stone. Wood floors can tend to insulate and reduce efficiency, but thinner wood flooring can play a conducive role.
How is underfloor heating controlled?
The controls of the underfloor heating system have two main parts:
a digital zone controller to be used to determine the temperature of each room/area, and the manifolds and valves operated by the zone control. It will be positioned where the homeowner can make adjustments, while manifolds should be placed where they will not be conspicuous - under the stairs or in the closet.
There is no regular maintenance mode associated with underfloor heating, but if things go wrong, it will usually be with the control systems, not the system itself.
Choosing the right floor heating supplier
As with everything you invest in your home, you will need a reliable supplier with proven quality and design for underfloor heating. There are special floor heating companies that are often in the best position to achieve this.
Finally, if you decide to pair underfloor heating with heat pumps, it is a good idea to choose a supply and installation company. Both of these systems are complex, and the whole heating system to work in perfect harmony can be difficult.
Professionals should also be hired to determine the right thickness of screed because thick screed will give a longer reaction time (the time taken to warm up and cool down), while thin screed will have a quicker reaction time.
For more information, please visit 3D Underfloor Heating Module.