I was wondering how to hook up a three wire baseboard heating system to a thermostat the colors are white, green, and red. When I touch the white and red together the heater turns on and when I touch the green and white the heater turns off. I just recently bought a new programmable thermostat and all there is, is letters and I don't know where to hook up the wires

There are special thermostats for baseboard heaters. Don't just grab the first stat you see at the store. Read the packaging carefully as to what heating system it is designed for. That same package will give you detailed wiring diagrams. Your white wire is a neutral, no power. The red and green wires, are they coming from the heater or coming out of the stat?

The three wires are all supposed to connect to the thermostat. I bought a universal thermostat that should work. I put the white wire in the R connector and the red wire in the W connector and the heater turns on its just when I try to turn it off the thermostat says heat off but the heater is still running. I know that if a connection is made with the white and green then the heater turns off. Its just I have a problem with the thermostat because I don't know where to plug the wires in. on my old thermostat there was no labels so I can't go by that. It was the old mercury thermostat

Also I have to add, the three wires then go to a controller type device by the boiler(heater) when heat is activated (red and white) on the thermostat then a dial turns on the controller type thingy and the heater turns on. When heat is turned off on the thermostat (green and white wires touch) then the controller dial turns back and the heater turns off. This was accomplished in the older thermostat that when the temp would go down past a certain temp the mercury would go to one side which would connect the white and red wires and then on the other side the white and green would be connected when the temp got high enough to make the mercury tilt to the other side. I don't know if any of this helps but I'm trying because the instructions on the thermostat don't tell me anything

There are several different voltage ratings of thermostats. The old thermostat should have a voltage and amprage rating on it. Conventional thermostats are designed to switch small 24 volt currents. The wires that run to them are usually 18 gage. Most baseboard heaters are 120 or 240 volt and require thermostats that can switch the full load at the voltage. The wires that run to them are much larger, 10-12 gauge. In a 240 volt heater, the white wire is not a neutral, but the other hot leg. The green is always a ground in 120/240 volt wiring. In a conventional thermostat, it controls the fan. It sounds like you have a 120 volt heater. The green should be a ground. If you touched a hot white from a 240 volt heater to a ground, it would short and pop the breaker. What are the terminals of your thermostat labeled, R, W, Y, and G? If so, it is meant for 24 volts and likely you have burned it up. It you have 4 terminals plus a green screw, it is a 240 volt thermostat and will work for 120 volts. Two of the terminals should be labeled L1 and L2. Connect the red wire to L1. Connect the white wire to the terminal on the same side as L1 of the other set of terminals. Connect the green wire to the green screw in the frame of the thermostat. Post back if that doesn't work.

I forgot to say this is a water baseboard heater. I don't know if this changes anything

Hot water systems never use a 240 volt thermostat. So do you have R, W, Y, and G? Is this a zone system? Some zone valves use a double throw relay, red is common, white opens the valve, and green closes it. On a conventional thermostat, R is connected to W when it calls for heat. Normally I don't think anything is connect to R when it is satisfied. Read the directions and see if it has a setting to switch the red to another terminal when the heat should be off. Otherwise, you will have to add a relay. For more details, see the sticky at the top of the heating forum.

Never is a bad word. Sometime, somewhere there may be a hot water system using 240 volt controls. Likely not yours.