Hi here's what I want to do
I have 24 vac furnace thermostate programmable I want to put in parallel a preset timer say 15minutes with a rocker type switch (eg levitron 6652-1w)
This so if I want to get the chill out when I first get home I can still have my prog therm
Untouched

Lets say the heat and blower are separate wires at the thermostat, we'll say green for blower and white for heat. When you turn thermostat to heat, Then the red turns on blower and heat contactor.
So for the timer if not 2 pole, you may want to use a 2 pole relay, so when set for 15 minutes, the red make contact with blower on one pole, and the red makes contact with heat contactor on other pole, When time expires, thermostat will take over.

I just looked through the specs:
6652-1W > Timer Switches > Residential Grade > Lighting Controls (Box Mounted) > All Leviton Products from Leviton Electrical and Electronic Products
It switches 120 volts out, you will need a DPDT
Relay with a 120 volt coil.
Power from the timer goes to relay coil, contacts to the 3 wires from the thermostat.

Intermatic makes mechanical timers which do not use any voltage to operate. Check out their website for dbl. pole mfg. numbers.

Don't know if its too late. That switch is expensive, and being a switch, it may be hit accidentally by someone not knowing what the switch is. I would not have the switch easily accessible, A cheap way would be a little timer from Altronix, under $20, and will allow a 2 conductor low voltage wiring from switch to Air Handler, Timer will be in air handler, and get power from there.

Use a programmable thermostat. Any home improvement center has them. Mine is a Honeywell and works well.