How do I get my thermostat to work right?

I've got a thermostat that operates a hot water fed furnace and central air plumbed throught that same furnace. Red and white wires will operate the furnace and fan. Yellow and green will operate the AC compresser and and the same fan in the furnace. The problem is when they are all hooked up at the same time. In AC mode things work fine. In heat mode, the thermostat calls for heat, then after a short delay the fan/blower comes on. At this point, the AC compressor also comes on so that I am heating/cooling in the heat mode. If I unhook the green/fan wire the heat mode will work fine and the compressor won't come on. The green wire is the fan wire which I checked by wiring directly to the red. The strange thing to me is the fan will work in the heat mode without the green wire attached. I assume that means the furnace is running the fan directly. Thanks in advance.How do I get my thermostat to work right?Look Red is your power

Green is the blower

White is heat

Yellow is cooling(A/C)

Unless you have a heat pump the compressor should not come on while in heating mode.

You should have %26quot;auto%26quot; normal position

%26quot;off%26quot; System down

%26quot;fan%26quot; or %26quot;on%26quot; for the blower only

Your T-stat maybe defective so replace and connect as aboveHow do I get my thermostat to work right?Get a new digital set back heat-cool thermostat for $25.00 (more or less depending). It has full instructions, including the 'trick' wire. It will have a heat or cool switch, and a fan on or 'auto' switches.You will save bucks with the set-back feature, and it will pay for itself the first year.Done.How do I get my thermostat to work right?,your water furnace is back feeding( sending) power to the g terminal on tstat, which is connected to y inside your type of tstat. Local ac supplystore should be able to match correct tstat which isolates this g to y connection for you. note this should fix the problem but it is odd for a furnace to do this, and may be caused by a wiring era on installation