Another thought is reducing the catalyst size even further to a size that only maintaing 1500ppm. For instance:

If 3000 BTU maintains 34,000ppm @ 1 air exchange per hr then it would only take 132 BTU to maintain 1500ppm CO2.

At 21,591 BTU/lb in propane a 15lb propane bottle last 200 days maintaining a grow room of moderate size at 1500ppm!

Should this thread be moved to the advanced growing section?