Question: Like many others, temperatures between the two levels of my house are uneven. This is particularly noticeable as spring arrives and the upper floor (above ground) enjoys solar heating (so the heater never runs) and the lower (in ground) floor cools off. Frequently, I manually turn on the furnace fan at the thermostat to circulate air between floors and this basically achieves the goal. But then I forget to turn off the fan override. So, I'm wondering if I can install a second thermostat on the lower floor to automatically control the heater blower only when the temperature falls below a given threshold on the lower floor. 

Givens:
1) I have a recent model high efficiency natural gas heater
2) my current Thermostat is Honeywell TH4110D1007 Programmable Thermostat
2) access to the heater on the lower level is trivial

Assumptions:

1) I can add a second thermostat in parallel

2) I would only make some connections from the new thermostat

3) 24v Power is supplied with the Red wire

4) Heat is controlled with the White wire

5) Fan is controlled with the Green wire

6) I'd be using a simple programmable Honeywell thermostat

7) I'd need to manually set similar time periods and temperatures as are set on the existing "master" thermostat.

Note: In my current plan, I am NOT trying to turn the heating on - just the fan. 

So, my specific questions:

A. Has anyone ever done this?

B. Would I connect the Fan wire or would I actually cross connect the thermostat's Heat connection (W) to the fan control wire. i.e., to get automatic fan control, would I use the G or W wires to signal/control? Or, am I totally mixed up?

C. Am I risking my entire system for something risky?

D. Is there another better way to achieve the goal?

Answer: 
In theory this should be easy -- have the second thermostat connect the R and G wires when it calls for cooling, and the fan will run.

Here's one reason it might NOT work, and it's subtle/non-intuitive so I thought it was worth mentioning: Some thermostats short the Y (cooling) and G (fan) wires together whenever they're in cooling mode. They do this so that they can use one relay to turn on both fan and A/C during a call for cooling. (Relays are relatively expensive components.) This shorting happens whenever the thermostat is in "cool" mode, whether it's actually calling for cooling or not. (Although if you set such a thermostat's Fan switch to "ON" instead of "AUTO", the short is removed so as to run the fan only.)

Where there's only one thermostat in the system, this shorting behavior does no harm. But if a second thermostat is added and it calls for "fan", the power will go through the short in the first thermostat and turn on the A/C as well. I have this exact arrangement in the house I just bought, and I cannot run the fan independently from either thermostat -- it will always run the A/C as well. If I unwire either thermostat (or set it to "heat" mode), I can then run the fan without the A/C.

So you might try shorting R (or Rc) and G with your existing setup to make sure that the A/C does not come on as well.

Source: personal observation confirmed by isolated testing on Robertshaw 9600 programmable thermostat
I added a second thermostat to run the fan in heating months. It did start the air conditioner as mentioned in several articles, so I just disconnected the air conditioner at the breaker box and it is working fine. I also jumped the white and green so I could manually turn on the fan, if needed.