We use the circular type that attach to the top of the tank. Obviously, they have to be in the greenhouse to work :)

We have had no issues using them.

With a very similar set-up to yours in my greenhouse, I decided to research the issue.

First, to be completely safe and legal, this is probably a question to ask the fire department. But, here is the code on this:
http://www.grainger.com/content/qt-nfpa-58-gas-cylinders

According to this article, http://homeguides.sfgate.com/heating-greenhouse-propane-58413.html , this use is acceptable.

Since my greenhouse is well away from my home (should this turn out to not be a good idea), my 20 lb propane tank, with a small camp tent type heating element attached, is inside my greenhouse with no problems, after a few years of use.

If your code requires, or you want to be extra safe, perhaps you could just add a few extra feet of hose between your tank and your Mr. Heater to keep the tank outside. I built a small wire cage to support my heating element, so adding a few extra feet of hose to the tank would should be a problem.

Thanks so much!

just re-read my last reply sentence-"so adding a few extra feet of hose to the tank would should be a problem." Hopefully you realized my typing error, and that adding hose should NOT be a problem.....

I do hope you have realistic expectations on how much 18,000 btu will help in Indy in january when it's -20 at night, or that your greenhouse is really small. or that you're just wanting it above freezing, or better yet all of the above? I'm in Muncie...

That was my question on reading through this - how long would it take to consume all the propane from a 20 lb. tank in this situation (~ 5 gallons, is that right?)

My greenhouse is 12 x 20 x 8. Right now propane heater is keeping it at 65 degrees on low at night. I am trying to grow basil thru the winter. I am also supplementing with a small electric heater. Changing tanks every 4 to 5 days. This is all an experiment for me. If it gets to be too costly, I will either shut it all down or trudge on and spend way past my budget!

Muncie is only an hour or so north of Indy. I cannot believe you experience -20 at night! Except for last winter...and possible this winter!...,which was way out of the ordinary for us, our winter night temps will occasionally get into the single digits but not negative digits! Yikes!

It's true, the lowest temperature recorded for Muncie is -20 a few years ago and -14 last year. link below...but that is not the norm for here either. if you don't have a larger heater though, you can bet for sure that is going to happen at least a time or two this winter...so do us all a favor in Indiana and get a bigger heater if you're going to really heat this winter....lol...
I'm heating a 1960's 10x20 Janco glass paned greenhouse against the south wall of the 1890's farmhouse, a 30x30 gothic arch 14' tall, and in towards the spring an additional 26x36, and 26x96. At least we switched over to natural gas this year from LP so hopefully we won't run out...come and see us in Muncie if you get a chance!

Here is a link that might be useful: weather lows muncie in

I might just make that drive sometime Sandy! Sounds like you have quite the set-up. I am envious of your big greenhouses and obviously the land to accomodate them!

My original plan was to heat with electricity because propane scares me but my electrician messed up and only pulled one circuit line. Thought I was getting 2. When I was trying to run a couple heaters, with lights, I kept blowing the circuit. So I had to go to propane. I thought about running a line of natural gas because that's how we heat the house but I'm in a neighborhood with homeowner's association who would go beserk if I did it. And I just don't feel like dealing with them! So, I will have to keep an eye on the temps and if I have to go bigger, I guess I will. I have looked at other heaters. And I cover the plants with a thermal row cover at night to "keep some heat."

All an experiment! That keeps me up at night!

Thanks,
Lisa