heres a suggestion ,
At my Dad's , his garage is also attached to his home , it is insulated so we tapped into the ductwork for oil furnace that heats the house and ran 2-6" duct's to the garage from the basement so they blow out near the floor , and added shut-offs on the ends , now he just goes out before he wants to work an opens up the shut-offs and goes has breakfast and the garage will warm up to comfortable enough to work and is plenty warm enough for him an he is in his 60's.
He doesnt like it too warm when he tinkerin.
I dont know what kind of heat you have for your home but maybe bleeding a bit of heat from the house when needed may be an option ?
