Question: what are man made sources of heat?
Answer: Gas Fired Water Heater. Here at Radiantec we often recommend the use of domestic water heaters instead of expensive boilers. ... 
Oil Fired Boilers or Water Heaters. Oil can be less expensive than gas in some locations at this time. ... 
Wood or Coal Heaters. ... 
Solar Collectors. ... 
Radiant Retrofit. ... 
Electric Heat. ... 
Geothermal.