1. Will it work with your system?
The RTH2300 (sold as RTH2300B and similar retail codes) is a basic battery-powered programmable thermostat. Honeywell’s RTH2300/RTH221 Quick Installation Guide (69-2448EF-03) now carries a “legacy product” banner, but it is still the official wiring reference, and the RTH221 shares the same wiring. The guide lists these compatible systems:
- Gas, oil or electric furnace
- Central air conditioner, or central heating and cooling
- Hot water system, with or without a pump
- Millivolt system (for example a gas wall heater powered by its pilot)
- Heat pump without auxiliary or backup heat
It is not for heat pumps with auxiliary or backup heat, or for any multistage system (Y2 or W2 wires). It is also a 24 V and millivolt thermostat, not a line voltage one: if your heater is wired with thick wires and wire nuts, read our line voltage thermostat guide first. If your old thermostat had wires on Y2, W2, AUX or E, the RTH2300 is the wrong model; our heat pump wiring guide explains what those systems need.
2. The six terminals
The wallplate has screw terminals in this order: O/B, Y, RC, R, G, W, with a factory jumper joining RC and R. What each one does:
| Terminal | What it connects to | Notes from the guide |
|---|---|---|
| R | 24 V power from the transformer (heating transformer if there are two) | Joined to RC by the factory jumper |
| RC | 24 V from the cooling transformer | Remove the jumper only if you have both R and RC wires |
| W | Heat call | Jumpered to Y on a heat pump without backup heat |
| Y | Compressor (cooling, or heating and cooling on a heat pump) | |
| G | Fan | If used, set JP1 for your heat type |
| O/B | Heat pump reversing valve | Set JP2 to O or B |
There is no C terminal. The guide is direct: wires on C or C1 “will not be connected to your new thermostat.” It also says to ignore wire colours and identify wires by the terminal letter they came from; our wire colour guide shows why. For any letter you do not recognize, our terminal letter index explains it.
3. When the old labels don’t match
Older thermostats use other letters for the same jobs. The guide’s alternate wiring chart maps them:
- Y1 or M go on Y.
- F goes on G.
- W1 or H go on W.
- RH, 4 or V are heating power and go on R; remove the R–RC jumper if a second power wire is also on RC.
- C, C1 and X are not used. Tape the bare end and push the wire back.
- B is not used if you already have an O wire.
That last rule matters because B means different things on different brands. On many heat pumps B is the reversing valve energized in heat, but on some older systems it was the 24 V common. If you find both O and B, the B is likely a common, which is why the guide leaves it off. Our blue B wire as common page explains how to check.
4. Wiring steps
- Turn off power to the heating and cooling system.
- Remove the old thermostat but leave its wallplate and wires attached for now. If it contains a sealed mercury tube, do not put it in the trash; the guide says to ask your local waste authority about recycling.
- Label every wire with the letter of the terminal it was on. Tape off any wire that was not connected, or that was on C or C1.
- Remove the old wallplate only after all wires are labelled, and wrap the wires around a pencil so they cannot fall into the wall.
- Mount the new wallplate: loosen the captive screw at the bottom, separate the thermostat, feed the wires through the centre opening and screw the plate to the wall with the anchors supplied. Levelling is cosmetic only.
- Connect each wire to the terminal with the same letter, using the chart above for odd labels, and tighten the screws. Remove the R–RC jumper only with two power wires.
- Set JP1 and JP2 on the back of the thermostat (next section).
- Install 2 AAA batteries, hook the thermostat onto the top of the plate, push it on, tighten the bottom screw and turn the power back on.
If you are not sure which wire carries power, our multimeter guide shows the 24 V test, and our general how to replace a thermostat guide covers the same job for other models.
5. Jumpers JP1 and JP2
Two small jumpers on the back of the thermostat tell it what kind of system it is driving. Both are easy to miss, and both cause symptoms that look like wiring faults.
JP1, heating fan control. Set it if you connected a wire to G.
- HG (factory position): gas or oil furnace. The furnace runs its own fan once the heat exchanger is warm.
- HE: electric furnace. The thermostat turns on the fan with the heat call.
Honeywell’s sign of a wrong setting on a gas or oil furnace: cold air from the vents when the heat starts, because the fan runs before the furnace has warmed up.
JP2, heat pump reversing valve. Set it only if you have a heat pump.
- O (factory position): an O wire is on the O/B terminal.
- B: a B wire is on the O/B terminal.
The guide warns that a wrong JP2 setting reverses the heat pump: it cools in Heat mode and heats in Cool mode. Our O/B terminal page explains which heat pumps use O and which use B.
6. Heat pump wiring
For a single-stage heat pump with no strip or backup heat, a typical old thermostat has R, Y, G and O (or B), and sometimes a C wire. On the RTH2300:
- R on R (jumper to RC left in place).
- Y on Y, the compressor.
- G on G, the fan, with JP1 set as described above.
- O or B on O/B, and JP2 set to match.
- A short piece of thermostat wire from Y to W. The guide requires this jumper “if you are using a heat pump without auxiliary/backup heat,” so a heat call runs the compressor.
- C, if present, taped off.
If your old thermostat had a wire on W2, AUX, E or W with a heat pump, your system has backup heat and the RTH2300 cannot control it. Rheem and Ruud heat pumps often use B; our Rheem and Ruud B wire page covers that case.
7. Setup menu after wiring
Hold the up and down buttons together for three seconds to open the installer menu. Change a value with up or down, and press both together for one second to move on. The menu saves and exits after 60 seconds without a press, or when you press Run. The guide lists four functions:
- 1, temperature display: 0 for °F, 1 for °C.
- 2, time format: 0 for 12-hour, 1 for 24-hour.
- 3, heating cycles per hour (set with the system switch on Heat): 5 for a standard gas or oil furnace under 90% efficiency, 3 for hot water or a high-efficiency furnace over 90%, 2 for steam or gravity heat, 6 for any electric heat. 4 is a second option for standard gas or oil.
- 4, compressor protection: 1 on, 0 off. When on, the compressor waits 5 minutes before restarting.
If you are moving from this thermostat to a Wi-Fi model, our Honeywell RTH6580WF wiring guide covers the next step up, which does need a C wire or Honeywell’s wire adapter.
8. If something runs wrong
- “Cool On” or “Heat On” flashing: compressor protection is counting down its 5-minute wait. This is normal.
- Heat pump heats in Cool and cools in Heat: JP2 is on the wrong setting.
- Cold air when heat starts: JP1 is on HE with a gas or oil furnace.
- Heat pump never heats: check the Y–W jumper is in place and tight.
- Nothing happens at all: check the batteries, then the breaker, the furnace switch and the furnace door switch. Our thermostat troubleshooting checklist covers fuses and transformers.
The guide also cautions against running the air conditioner when the outdoor temperature is below 50°F (10°C), to avoid compressor damage. For anything the guide does not cover, Honeywell Home’s support site lists current contact options.
9. Frequently asked questions
Does the Honeywell RTH2300 need a C wire?
No. It runs on 2 AAA batteries and has no C terminal. Honeywell’s guide says wires attached to C or C1 on the old thermostat are not connected: wrap the bare end of each in electrical tape and push it back. If you later move to a smart thermostat that needs power, our no C wire guide covers the options.
Can I use the RTH2300 with a heat pump?
Only a heat pump without auxiliary or backup heat. The guide says it cannot be used on heat pumps with auxiliary/backup heat or on multistage systems. For a single-stage heat pump with no strip heat, wire O or B to O/B, set jumper JP2 to match, and place a short jumper wire between Y and W.
Should I remove the jumper between R and RC?
Only if you have a wire for both R and RC. One power wire means one transformer, so the factory jumper stays and joins the two terminals. Two power wires usually mean separate heating and cooling transformers, and Honeywell says to remove the jumper. Our Rh and Rc page explains how to tell.
Where does the B wire go on an RTH2300?
On O/B, with jumper JP2 moved to B, if your heat pump energizes the reversing valve in heat. The guide also says not to use a B wire if you already have an O wire; on some older systems B was the common, which this thermostat does not use. See our B terminal page.
Why does the furnace blow cold air when the heat starts?
Jumper JP1 is probably on HE (electric) with a gas or oil furnace. HE starts the fan with the heat call, which suits electric heat; HG lets the furnace control the fan after it warms up. Honeywell names cold air at the start of a heat cycle as the sign of a wrong JP1 setting.