fb wrote:
Having used a single Southern Burner to heat an 8x12 greenhouse for
many years, we find it can't quite keep up with the heating demand in
our new 12x16 greenhouse. We'd like to add another heater that does
not require a chimney pipe. Rinnai makes a couple of direct vented
models that are probably more efficient than a Southern Burner, but
with the added complexity of fans and considerable electronics. We
wonder if the humid atmosphere would damage such a sophisticated
heater. Please share your experience using a Rinnai or similar direct
vented heater in a greenhouse. Fred in Lakewood, Colorado

While I was browsing the net for greenhouse info I stumbled onto this site.

http://ygh.home.att.net/heater.htm

They're using a Comfort Glow model #CGP10TL Propane Heater which has the
advantage of needing no electricity to work.

Not sure if they still make them but I'm sure they have something
comparable in natural gas.

Question is, do you really need the fans, buttons, bells and whistles?
Or would it be better to set up a static system centrally located which
will heat by convection.

I know my first choice.. In fact, I'd picked up one of those oil filled
electric radiator heaters to heat a badly insulated addition to our
house (which is my office I'm in all the time) and it heats the whole
room without issue. Makes quite a bit of wind, just by convection.

Anyway.. Just my 2.5 cents.

Here's the website.
http://www.comfortglow.com/

Just scroll down to the vent free units. I'd opt for blue flame as you
don't want the heat directing out to the plants, but up to heat the room.