| It is not that simple. Search this forum, and you will find many threads where people are having trouble making programmable thermostats work. Much of the time, it is their own fault, or the thermostat is bad. Many of the problems are with a Honeywell RTH230. Let me give you some background.
Usually there is a 24 volt AC transformer in the furnace with the secondary winding connected to a red wire running to the thermostat and a blue wire, common, to the gas valve, A/C relay, and fan relay. From the thermostat there will be white wire to the gas valve, yellow to the A/C, and green to the fan. The thermostat is wired to switch the power from the red to the white, yellow, and green as needed with the blue completing the circuit. Most thermostats and furnaces have the contacts labeled R, B or C, W, Y, and G for the corresponding wire colors. It may be wired to have the A/C control wires return to the furnace and its controls and then a second wire goes to the A/C unit. Internal wiring may replace the green wire if the thermostat does not give you the option of fan only or continuous fan. Digital or programmable thermostats may need the blue wire connected to them.
That is for gas, forced air. A hot water system will be much the same at the thermostat except the fan on function won't work. When a zone thermostat calls for heat, 2 things must happen, the zone valve must open, and the boiler must come on if not already. If you don't have air, the old thermostats could be wired up with only 2 wires. Check, may be easier to see at the boiler, and don't buy a thermostat that requires the third wire to the common terminal. Other than that, make sure the thermostat comes with directions in English, and you follow them. Also, note where the wires were connected to the old thermostat. Doing those will avoid most of the problems I have seen here. | 
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