Propane heater for greenhouse

We finished putting our greenhouse together last month and moved all of our plants into it shortly there after. It's 13'LX10'WX8'H and made of Polyethylene Fabric. We're in Central Tx, zone 8a, winter can be rough with temps in the 20s or maybe colder at times. I found a heater at Lowes and 18000BTU that says the maximum heating area is 400sq ft. I know that's about 1/3d of my greenhouse so I was thinking possibly getting two of them. We have all the plants mostly in about 3/4ths of the greenhouse. I've posted a few photos to show what I mean. Does it sound as if this heater would work for us? I also have about 5 layers of weed barrier plus 2" of pea gravel on either side of the patio block walkway.

Chris, I hate to tell you this, but your greenhouse is 130 square feet (13L x 10W). If the heater covers 400 square feet, it should be plenty big enough.

I'm, looking for a heater for my greenhouse. What's the number or link for the one at Lowes?
Jo-Ann

now that's a good math correction! big enough is wonderful

Thanks for correcting me Jo-Ann, I thought square footage was LXWXH. The link is http://www.lowes.com/pd_131258-51644-F274815_0__?productId=1132081&Ntt=propane+heaters&pl=1¤tURL=%3FNtt%3Dpropane%2Bheaters&facetInfo=

I have a feeling that the square footage is unimportant. You really should know how many cubic feet the heater can handle. If your GH is 10x13x8'H then the cf. would come to 1040 cf.

Ken

That's what I figured out earlier for instance when I was buying the bags of pea gravel. Each bag was .5cf and I wanted it 2" deep. So, I had to change the dimensions to inches then go from there.

Ken is right about figuring the heating requirements. Here's a link to a greenhouse heater calculator to determine your minimum btu requirements. Chris, you're in the same climate as me. Blazing hot to freezing cold. http://www.greenhousecatalog.com/greenhouse-heater-calculator

BayouBeadery, I was going to recommend the same link. When I did the calculations on mine here in Pa. I came up with 14K BTUs. So I bought a 20k. The calculations are based on the cubic feet and the type of glazing. My heater was adequate last winter in spite of the extreme polar winds we got. I made many improvements this past season as can be seen here:
http://davesgarden.com/community/forums/t/1291535/ and I estimate my fuel use is 1/3 to 1/2 of last year.

Ric, I must apologize for not posting a reply until now, I sure wish I could get a notification when a reply is posted, maybe I need to change a setting somewhere so I do. Anyway, all my plants made it through the winter with my little heater. Most of the time I was able to keep it on low with with days when I couldn't shut it off I could get about 7 days of running 24hrs on two 15lb tanks. But as soon as the sun would come out even if it was in the 30s outside it would heat up into the 70s quickly on the inside. I'm slowly moving some of my larger plants such as my Angel Trumpets outside now since the weather is warming up and have my tomatoes plants that I grow in tubs inside for now. My Columbines are already blooming. Anyway, now I'll have plenty of time to make the improvements that I need to so that it's better insulated for next winter.