Greenhouses have been used for 2,000 years, because they work.It has never been known because it is not true.
Stoves, ovens, space heaters, etc. turn energy into heat. Heat is thermo energy, and uses energy units.energy cannot be converted to heat because they aren't the same quantity kind and have different units of measure.
Are you thinking of temperature? Temperature is heat over mass, and would use a slightly different unit. That being said, you can create temperature with energy also.
In the real world, insulation exists. Humans have known about it for maybe a hundred thousand years.Heat is not anything that can somehow be contained/captured/trapped/ensnared/held/retained/enclosed/entombed.