How to Wire a Thermostat for Your Boiler

person turning boiler thermostat dial
What You'll Need
Wire cutters
Cordless drill
24-volt wiring
Wire snake

The boiler thermostat is one of the most important devices in the home, yet it is often overlooked. This relatively small device is responsible for controlling the level of heat in your home during the cooler months. In the past, boiler-thermostat wiring used to be rated for 110 volts, but now modern systems are rated for 24 volts. Modern thermostats are capable of controlling both heating and air conditioning, and they can be programmed based on the energy needs of the home. By learning the basics of this system, you can replace or install a thermostat at a fraction of the cost of hiring a contractor.

Step 1 – Run the Thermostat Wiring

A thermostat works by reading the ambient air temperature, so you’ll want to place it in a large, open room or common area, such as a hallway. Once you’ve determined the location, you can begin mapping out the route back to the boiler.

Use the wire snake to pull the wire through closed walls and into your attic (or basement depending on the type of home you are working in). From there, you should have a clean run through the attic, then down the boiler room wall. Most boiler rooms have open framing, so you shouldn’t need to snake the wire down this wall.

Step 2 – Install the Thermostat

Now that you’ve run the wiring, you can mount the back plate of the thermostat to the wall. Take extra care to ensure that you mount the plate level. This is crucial to making sure the mercury switch inside the thermostat functions properly. Use the cordless drill to secure the back plate with drywall anchors and screws. This hardware is usually included with the boiler thermostat.

Identify which terminals are the “R,” “W,” and “Y,” and loosen each one with the screwdriver. The two main types of 24-volt thermostat wiring are two-wire (red and white) and three-wire (red, white, and blue), but you can find up to eight different wires. If this is the case, you should check with your specific manufacturer's instructions before you continue.

Two-Wire Installation

For a two-wire installation, connect the red wire to the “R” terminal, and connect the white wire to the “W” terminal.

Three-Wire Installation

In a three-wire installation, connect the red wire to the “R” terminal, the white wire to the “Y” terminal, and the blue wire to the “W” terminal. Use the wire cutters to strip each wire so that there is approximately 1 inch of bare copper showing. Now, loop each copper end around each terminal, and tighten the screw on the terminal.

Step 3 – Connect the Wiring to the Zone Valve

The zone valve acts as the relay between the thermostat and the boiler by working in conjunction with a transformer. The transformer converts the 110-volt feed from the panel to a lower voltage that matches the thermostat wiring.

Take the cover off the transformer and install the boiler thermostat wiring to the corresponding terminals in the transformer. The 100 volts powers the circulating pump and blower unit so that the transformer acts as a relay between the individual zone valves and circulating pumps.