When to install a contactor snubber for electric underfloor heating

When it comes to electric underfloor heating, one thing that is often overlooked, is the electrical load of the system. This is only an issue when looking at larger areas such as a kitchen / diner, whereby the homeowner wants to use one thermostat to control the whole area. However, if the electrical load of your underfloor heating exceeds 16A you won’t be able to control it with a single thermostat unless a contactor snubber is installed.

In this short article, we will cover:

• What is a contactor snubber and when should you use one.

• Calculating the electrical load of the underfloor heating system.

• How to install a contactor snubber.

If you have a question either before, during or after reading this article, we recommend contact one of the team at ThermoSphere HQ. 

Please note that we recommend any electrical works undertaken must be carried out by a skilled and professional electrician.

What is a contactor snubber and when should you use one?

A contactor snubber is a high current switch device that can handle large electrical loads, but a simple contactor is not enough for electric underfloor heating circuits with large electrical loads.

Because contactors switch large currents on and off suddenly, there can be significant momentary rise in voltage, and this can cause electrical interference or damage components in the thermostat.

A snubber is used within electrical systems to limit (or snub!) the sudden changes in voltage and prevent damage or any strange behaviour from the thermostat. Without a snubber, these voltage surges can wipe a thermostats memory and cause irreparable damage.

In the instance of electric underfloor heating, ThermoSphere’s thermostats can switch electrical loads of up to 16A. If this is exceeded, then it can cause damage to the thermostat by melting the relay and circuit board inside which makes the thermostat as much use as a waterproof teabag. 

As a guide, we recommend installing a contactor snubber for the following room sizes as this is when the electrical load exceeds 16A (at 230 Volts).

Calculating the electrical load of the underfloor heating system

There are a couple of calculations you can use to identify whether you need a contactor snubber or not. These are:

Option 1
    

Option 2

We are going to be using the following example: A 6m² heated area using ThermoSphere Mesh with an output of 150W/m² and a total system output of 900W. Therefore, to work out if a contactor snubber is needed, we would run either of the following calculations:

Option 1
    

Option 2
  As the electrical load comes out at 3.91A this underfloor heating system can be switched by the 16A relay in the thermostat, and a contactor snubber is not required.

What size contactor snubber do you need?

Select a contactor snubber rated higher than the total current draw of your underfloor heating system. For example, a 52A system would need our 63A snubber. When selecting a ThermoSphere Contactor Snubber, you can choose from either a 25A, 40A or 63A.

How to install a contactor snubber

The diagrams below show how to wire up to 16A (without a contactor snubber) and larger areas where the electrical load exceeds 16A (with a contactor snubber) and the homeowner wants to control it from the same thermostat.

If you prefer video, watch ‘How to wire up a Contactor Snubber with electric underfloor heating’ on our YouTube channel.

Areas up to 16A  (Without snubber)

Areas larger than 16A (With snubber)

Questions? Contact the team 

If you have any questions about snubbers for your electric underfloor heating installation, please contact us via the website or give one of the team a call on 0800 019 5899.