Is the waste heat in the air?
If it is a warm and even humid room otherwise, there is a product, name escapes me. This device is an AC unit/dehumidifier which sits atop the DHW heater and uses the hot gas waste heat from the refrigerant process to pre-heat the DHW incoming cold water. (Rather uses the incoming CW as a water-cooled condensing medium).
If I come across the name I will post it but others I am sure know the one I am talking about.
There also were a line of Free-heaters from Spiro (the spirovent people) and Meuller refrigeration specialties, to take the same hot gas waste heat from commercial refrigeration and use it also to pre-heat incoming CW to DHW systems. We specify the Meuller line on commercial kitchens but I see no reason it cannot be adapted to other AC processes.
Depending on how hot the room is, a more passive coil situation could be used, but the refrigerant boost is more positive.
How hot IS this room? It may be as hot as my grandfather used to say: "Hot as a June Bride in a Feather-Bed". Now that, my friend, is hot. :)
"If you do not know the answer, say, "I do not know the answer", and you will be right!"
-Ernie White, my Dad
