I was wondering what the difference is between the verbs греть, обогревать and нагревать. If I'm not mistaken all three verbs are imperfective. When I search google images for обогревать, it seems to be used for heating up a room, while нагревать seems to be used when cooking stuff. But what about other situations?
You use "греть" when you conduct heat to a thing, but not neceserilly making it warmer, for example, "warm clothes warm you up in winter" - "теплая одежда греет в зимние месяцы".
You use "обогревать" when you want to convey the same meaning, but applied to a space, a room, or a system, and not a singular thing, which is the case with "греть". For example, the furnace heats the room in winter - "отопление обогревает комнату зимой", or "the fireplace heats the cabin" - "камин обогревает домик". Once again, without neceserilly making it warmer, just maintanining the temperature.
Now, you use "нагревать" only if you are talking about actually making the thing or a space/a room/a system warmer, increasing its temperature, not just conducting heat and maintaining warm temperature. For example, "the fire started slowly heating up the air in the cabin" - "огонь начал медленно нагревать (разогревать) воздух в домике", ("air" is needed here) or, "the warm clothes we put on started warming up his body" - "теплая одежда, в которую мы его одели, начали нагревать (разогревать) его тело".
The word "разогревать" is better to be used in these examples, because "нагревать" is more formal and is usually used in technical texts, while "разогревать" carries an additional meaning of bringing the heat back where it's needed (like going from low body temperature to normal after putting warm clothes on), or to it's desirable amount. I would not use "нагревать" there. But in a sentence like "the burner heats up the water in the bulb to the temperature of boiling" - "горелка нагревает воду в колбе до температуры кипения" it is completely fine