How to work out the loading of underfloor heating.

What is the loading of my underfloor heating?

The main area of technical support is usually with underfloor heating and flooring and I have been asked many, many questions from electricians and installers.

Some of the most common questions I have been asked are;

• What is the loading of the under floor heating?
• Can I run it off a fused spur?
• What size feed should I allow for the under floor heating?
• Can I use my solar PV panels to run my under floor heating?

To cover the main topic which in turn will answer the first 3 questions here is a very simple way to calculate your loadings, one of the first lessons in Electrician School.

Firstly it is important to understand that the heat out put from each mat will vary dependent on voltage fed into it. The heating mats are basically made up of a resistive element, the higher the resistance the lower the heat output and in turn the lower the draw of current (amps).

All the heating elements are rated at 220-250 volts AC as a standard most electricity supplies in the UK average approx. 240v so for this example I will use Standard Test Conditions (STC) voltage and average UK voltage so you can see the difference.

The Formula

V2 / Resistance = Watts

The Maths (STC) based on 0.5m x 1.2m heating mat with a resistance of 600 Ohms

230 x 230 = 52900

52900 / 600 = 88.17w this is the wattage based on 230v

The Amps

88.17 / 230 = 0.38A This is your actual loading for the heating mat based on 230v

The Maths at 240v based on 0.5m x 1.2m heating mat with a resistance of 600 Ohms

240 x 240 = 57600

57600 / 600 = 96w This is the wattage based on 230v

The Amps

96/240 = 0.4 Amps This is your actual loading for the heating mat based on 240v

Now if you measure the resistance of each mat you will be able to work out your exact loadings for your installation, please remember that voltage can fluctuate within a household and allowance should be made for that.

Now this leads to other questions from my experience and the most common being...

If my voltage is higher is it going to be more expensive to run?

The answer is simply no it will cost the same to run at 230v as it would at 250v, although the load is greater the heating will be on for a shorter amount of time due to the quicker distribution of heat.