CO2 via a NG tankless water heater

The local shop suggested that a natural gas CO2 generator was gonna make my rooms so hot I would never cool it. They had plenty of them for sale and I appreciate their candor. What was suggested was using a tankless water heater as a CO2 generator as the water passing through acts as a heat exchanger. I gust bought a 120v solenoid so I assume I could use it to trigger the water heater to come on. I have a CO2 controller that would turn on the solenoid, then the water flow turns on the water heater. They say these are 85% efficient which should mean only 15% as much heat entering the room as a traditional CO2 generator.

Anyone with experience with this? I am assuming I would have to waste the hot water because I would hate to CO2 starve my girls cause I am a "dirty hippy" and don't use enough hot water.

On the bright side these units on ebay seem to be cheaper than a generator.

Pleas let me know your experience.

i found this somewhere on here i plan on useing this when i get my new room
but ihave to find a way to make it apply to a small grow room
maybe a smaller tap

maybe if you get 2 fans and regg. them into the dute 1 leading into the grow room on a 12/12 timer and the other leading out side (where it should be going) on an opposite 12/12 timer, both placed at the ends of the "T" splitter. when the one for the grow room cut on drawing co2. the one leading out side will cut off. the timers will run on the same sch. as the lights in the growroom. then when the lights in the grow area cuts off so will the co2