I installed a pellet stove in my family room and now I want to power a relay from the pellet stove's room blower fan, so I can start the HVAC fan in the house with a short from the relay.
I put my meter on G and C while the fan was on and there was 24 volts AC. There was nothing between G and C when I turned the fan off but there was 24 volts between R and G when the fan was off.
Originally I thought all I needed was a short between G and C to start the fan but that didn't work. So is it possible to send a short to my thermostat to turn on the fan?
The small brown wire goes to the heat pump and the larger brown wire goes to the fan and aux heat inside the furnace.

 

I think you need to connect R to G for the HVAC fan.

 

So with the fan "off" there's 24 volts between G and R and nothing between G and C.
With the fan on there's 24 volts between G and C and no volts between G and R.
It's like C and R are switching when the fan button is pressed on the thermostat's console.

 

I can explain the readings but have you tried connecting R to G?

 

@sghioto 's diagram in post #2 is correct.

What would the voltage read across a closed switch?

 

Common functions of thermostat wiring...

https://cielowigle.com/blog/thermostat-wiring/

as mentioned already #2 is right

 

Ok so the fan motor went on when I removed the blue wire coming from the heat pump from the common input.
So that works all I need is an open circuit from the common and blue wire to turn on the fan.
Thanks for the help Ian

 

Everyone pat Ian on the back for helping Ian figure out what was going on. It's not like everyone that produces a heating appliance builds it exactly the same way as the next company or something.

There's standards, and then there's the non standard...

 

Caution!!! Heat pumps may be a bit different from non- heat pump systems. But the proposed solution looks OK to me.
But just to be safe: , my suggestion is to measure the 24 volt transformer voltage when you have the fan running, to be sure you are not overloading something. If it does not drop way down then all is OK.