I am confused about the difference between warm and hot. When the temperature is very hot people say " Πο (sp.?) Πο ζεστή". Here it means warm.?
Ζεστός is both warm and hot and, I agree, it gets confusing which one is the best to use. Burning hot is καυτός so you can also say that hot=καυτός (this also works metaphorically for a good looking person). When the weather is hot people say "Πω-πω ζέστη!"
Ζεστό can mean warm or hot, depending on the context. The same thing happens in French with the word chaud. Some languages just don't differentiate between the two. Similar to how English does not distinguish between the colours γαλάζιος and μπλε, we just say blue for both.
Is καλοριφέρ a specific kind of heater, that is a radiator. I think there is also σόμπα to heat a room?
Yes, καλοριφέρ specifically means radiator. In order for the difference to be clear, take a look at these two.
Google image search results for καλοριφέρ https://www.google.gr/search?q=%CE%BA%CE%B1%CE%BB%CE%BF%CF%81%CE%B9%CF%86%CE%AD%CF%81&source=lnms&tbm=isch&sa=X&ved=0ahUKEwiSpOjg58LfAhWGb1AKHemmBrUQ_AUIDigB&biw=1532&bih=670
Google image search results for σόμπα https://www.google.gr/search?biw=1532&bih=670&tbm=isch&sa=1&ei=Qj0mXIXkOpLcwQKbwJP4Ag&q=%CF%83%27%CE%BF%CE%BC%CF%80%CE%B1&oq=%CF%83%27%CE%BF%CE%BC%CF%80%CE%B1&gs_l=img.3..0i30j0i8i30l3j0i10i24.38749.40139..40349...0.0..0.517.1428.4-2j1......1....1..gws-wiz-img.......0j35i39j0i67.Oih6cg3GHiE