When you have to replace a thermostat in your house the wiring might become a problem. You have to decide whether you are going to splice the thermostat wire or run only new wires without any splicing. 

There are two aspects you might have questions about. Firstly, you want the assurance that the new thermostat will work effectively with spliced wires, and secondly, you might wonder if it is within the local authority’s building and safety regulations to use spliced wires for your thermostat.

If you have to, you can splice the thermostat wire and generally, if the splicing has been done correctly, the thermostat’s function will not be affected. And as thermostats normally only need low-voltage power there are no safety regulations that prohibit the use of spliced thermostat wire.  Thus, although it is always better to have conductors that have not been spliced, a spliced thermostat wire can be just as good if correctly spliced. 

In this article, we’ll look at three different methods to splice electrical wires, including thermostat wires. Then we’ll give the basic steps to follow when splicing your thermostat wire, and end off with some general ideas and tips regarding things you have to avoid when splicing wires and the possible problems you can encounter with spliced electrical wires.

Methods to splice wires

Before we look at ways to splice wires, let’s have a look at the definition of “splice.” According to definitions given in dictionaries, a splice is when two or more conductors are joined together in such a way that a permanent electrical and mechanical bond is formed. 

Many electricians believe that the best way of joining conductors together is with either a crimp or solder process. When none of these two options is possible, they recommend that you use a twist-on wire connector.

1. The crimp process

With the crimping process, you join wire to wire or wire to a terminal. The bonding between the two wires in the case of a thermostat wire occurs when one stripped wire is deformed and compressed tightly to the other stripped wire with the correct tool. Crimping can only be done correctly with the right crimping tool. Don’t just use pliers to try and crimp the two wires. 

Because it will be low-voltage power flowing through the thermostat wires, you can insulate the joint with electrician’s tape (insolation tape).  But remember, insolation tape is to insulate, not to keep the joined wires in place. Affix the wire to a solid surface to ensure that it can’t bend or be torn apart. 

2. Solder process

If you want to combine 2 wires like thermostat wires, you can also use solder to make a connection. Solder connections normally last for a long time. 

Start the soldering process, just as in the case of the crimping process, by stripping the isolation material from both wires for about an inch. Then wrap the two wires around one another. The last step is to melt the solder directly onto the wires. When cooled off, the wires will be securely connected.

As with the crimping process, you can now insulate the joint with electrician’s tape (insolation tape).   

3. Use a twist-on wire connector

If you want to use a twist-on wire connector to splice your thermostat wire, you also start by stripping the isolation material from the two wires. You must strip it in such a way that the stripped part is the same length as the connector you want to use. 

Then you twist the ends of the wires together.  Twist the electrical wires always in a clockwise direction. By doing that, you ensure that the wires are not un-twisted when you screw on the twist-on connector.

After you’ve twisted the two wires together, the next step is to twist the connector onto the stripped and twisted wires. The connector always screws on clockwise. 

As the wires are drawn into the connector when you are screwing the connector on, they are squeezed firmly together inside it.

Basic steps when splicing a wire

When you’ve decided which method of splicing you are going to use, get everything you need and start working by following the basic steps:

  • Before doing any work on electrical wires whether it is the wiring of high-voltage or low-voltage power, the power source must be turned off. Put the applicable circuit breaker off at the breaker panel. 
  • Ensure that the thermostat wires match in terms of wire gauge. Nowadays most wiring has the gauge printed on the outer sheathing of the wire. 
  • Remove the isolation material around the wire with a wire ripper. 
  • Use the method you’ve decided on to splice the thermostat wire and complete the process. 
  • Put isolation material like electrician’s tape (insulation tape) over the joined wires.  Just a reminder again – electrician’s tape is not designed to hold spliced wires together – it is only isolation material.

Things to keep in mind when splicing wires

Thermostat wires carry low-voltage current

 Thermostats in household appliances and systems normally use a low-voltage current to function. Thus, the thermostat wires carry low-voltage power. This excludes thermostat wire splicing from certain safety regulations that are applicable for high-voltage conductors.   It is for instance, not compulsory to use a junction box when splicing thermostat wire. 

Use same gauge wires

When you want to join two thermostat wires together, always ensure that both have the same gauge. The most common thermostat cabling is 18 gauge. 

Possible problems with spliced wires

There is always the possibility that the wire can break or get loose where it has been spliced. Spliced wires also may lead to resistive failures.  


Although it is always better not to splice electrical wires, even if they are low-voltage wires like thermostat wires, you can splice thermostat wires if you have to.   

The problem with a spliced wire is that if it hasn’t been done well it can easily break. And when your connection to your thermostat is interrupted, your whole HVAC system will not work. As the thermostat is probably the most important element in your HVAC system, it is crucial that it doesn’t stop working because of a broken wire. This is one of the main reasons why some electricians will never splice a thermostat wire. 

It is also not against any building or safety regulation to use spiced thermostat wires. The important thing is that you have to make the splicing as secure and stable as possible. The three splicing options discussed in this article will normally not give you problems later on. 

So, you can use any of the three methods, namely the crimp process, soldering or the twist-on wire connector for the best results.  


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