I would like to add a downstairs thermostat on my furnace. Will it hurt my electronic/programmable thermostat upstairs to simply wire another thermostat to the furnace? I understand the voltage would be sent back to it but it wouldn't go anywhere that it doesn't already when the upstairs thermostat kicks on. I was told it for sure wouldn't hurt the old dial thermostats but want to check and see if I am going to kill my programmable one. Any info on this would be appreciated.

Such an arrangement would apply voltage to the W terminal of the thermostat when it wasn't calling for it. If the thermostat actually has small relays in it, it shouldn't hurt anything. If instead, it is solid state, I can't say. Might be a good idea to carefully read the instructions and maybe check with the manufacturer.

The thermostat makes a loud click when it kicks on and is at least 10 years old so I would assume it has relays. I think I will give it a shot. Thanks for the info.

I would like to add a downstairs thermostat on my furnace. Will it hurt my electronic/programmable thermostat upstairs to simply wire another thermostat to the furnace? I understand the voltage would be sent back to it but it wouldn't go anywhere that it doesn't already when the upstairs thermostat kicks on. I was told it for sure wouldn't hurt the old dial thermostats but want to check and see if I am going to kill my programmable one. Any info on this would be appreciated.Many Programmable stats have a remote sensor opption.They are a 2 wire S1 & S2 simple hook-up.You may need to disable the main sensor for set up to work properly.

I don't see where this would help you. You will just end up overheating the area with the least heat demand. Pretty much just like the problem you are trying to resolve.

If possible you need a zoning system installed, aprilaire makes nice setup

if possible you need a zoning system installed, aprilaire makes nice setup
Sounds like a better plan, but still not easy to retrofit and make work good.