How to Install a Honeywell RTH221 on a Two-Wire Boiler System


A boiler system is a heating system that consists of a boiler, a pump and baseboards connected by water piping. It uses a boiler to heat and circulate water, which distributes heat throughout a home. The Honeywell RTH221B thermostat maintains the temperature of the home. It regulates the warmth of the house as the water has a varying temperature between 120 and 210 degrees Fahrenheit. Installing a new Honeywell RTH221B thermostat for a two-wire, low-voltage application is better than using a mechanical thermostat because it is digitally regulated, is more accurate and is not affected by the level/mounting like the traditional mechanical thermostat. It takes about three to four hours to install.

Things You'll Need

  • 6-in-1 screw driver
  • Staples or plastic fasteners
  • 18-gauge 2-wire cable
  • 2 AAA Batteries
  • Read the manual of the boiler system and locate the terminals, junction box or leads for an external thermostat. Loosen the locking screw at the bottom of the thermostat and take it apart by lifting the face from the bottom, removing it from the wall plate (base). The thermostat has five terminals, which are labeled Y, G, Rh, Rc and W. Locate the W and Rh terminals, as these will be the two that will be used. A jumper ties Rh and Rc together; keep the jumper connected.

  • Choose the location for the Honeywell RTH221B thermostat. Ensure it is away from the heat source such as across the room on an internal wall. It should be in an area where wires/cables can be installed from the thermostat to the boiler. Measure the distance from thermostat to the boiler and add about two feet to determine the length of the wire you will need to install the thermostat.

  • Turn off electric power to the boiler by switching off the associated circuit breaker. Check by turning on the switch located on the side of the boiler and if fires, power is still on. Find the correct breaker.

  • Run the 18-gauge, 2-wire cable from the boiler to the thermostat. Do not cut off the excess wire, and leave it loose in both ends. This will be useful for future trouble shooting. Ensure that no wires are hanging or loose. Use staples or fasteners to attach it to walls, ceiling etc. If it is too tight it can damage the wires.

  • Strip off 1/4 inch of the wire insulation and connect the two wires to the terminals for the thermostat in the boiler. Every boiler is different, so it is important to follow the instructions from the boiler manual in connecting the wires.

  • Mount the Honeywell RTH221B thermostat to the wall with the screws provided. Strip off 1/4 inch of the wire insulation. Loosen the screw of the W terminal and insert the first wire. Tighten the screw and the wire is clamped. Then, insert the second wire to the Rh terminal following same procedure as the first wire.

  • Insert two AAA batteries in the thermostat, and mount the thermostat's face to the wall plate. Tighten the screw at the bottom of the thermostat, and turn on the electric power to the boiler.

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