If an electric couldn't keep up with an 8x10, even a little electric heater, I have to wonder how well insulated your greenhouse is. Esp. given that you live in Dallas. Your January is like our November.

It's easy to get a lot of power out of propane heaters, but do realize that they're a lot trickier to deal with than electric heaters. You can probably get one for $100 on ebay that has a thermostat and can heat that greenhouse just fine. If you want to buy it locally, check Menards, but expect to pay notably more.

You'll also need at least one propane tank (two is better so that you're not in a rush to fill it when it runs out). 20lb are standard, but you can get bigger portable ones (30, 40 lbs). U-haul-type places can fill the bigger ones. If you're in an isolated area, you might just get a big tank (100lbs or more) and have a truck come out to fill it for you. A 20lb tank will cost 40-60$.

When you get it, you first need to figure out how to connect it; different heaters will take different kinds of connectors, and some are pretty similar (for example, flare vs. normal cylindrical). You'll also need some teflon threading compound (not tape -- the stuff is a liquid). You may pay $20-$30 for hoses (see the costs adding up? You save money in the long run, but the initial costs are higher). Tighten with a wrench, not pliers (make sure you have the right size wrenches), and don't over-tighten. There are three ways to test for leaks: do all of them.

1) Listen. If you hear a hiss, you did it wrong.
2) Smell. If you smell gas, you did it wrong.
3) Put soapy water over the joint and examine closely (in good lighting). If you see tiny bubbles, you did it wrong.

Once that's done, it's time for installation. This obviously depends on your heater. Many thermostats aren't for specific temperatures -- they're "Low to High" knobs. You have to guess and see how hot it keeps the greenhouse. For me, my minimum temp (10C) is about halfway between "Off" and "Low". *Keep Flammable Things Away From The Heater*. Should seem obvious, but you'd be surprised. This means below, above, and side to side, not just in front of it. Behind is usually safe, because that's how the heater is typically mounted. Also, protect the heater from water from your watering system -- but again, not with anything flammable!

I stress this because, unthinkingly, I once propped up my water shield (made of al. foil) with a piece of foam board that I had around, underlain with foil. I figured that above the heater was relatively safe. Once day, I happened to look at it, and noticed that its underside was partially blackened. Bad idea. :)

Finally, get a carbon monoxide detector. Mine has been on the fritz lately -- the moisture is causing problems, I think (it first went off on a day that I was refilling the propane tank -- i.e., the only time when the heater *wasn't* burning. ;) That morning, I had noticed that water had collected on it, so I think it's shorted. I should try and find a model designed for greenhouses...). Modern heaters have low oxygen sensors (rather sensitive ones, too -- mine goes off if the O2 level goes below 19.5%), so in theory they shouldn't ever allow CO production, but when it's your health that you're dealing with, it's best to play it safe.

Modern heaters will have all sorts of other sensors, too. Mine has a temp sensor, an O2 sensor, and a pilot light sensor. All the reason to only buy a modern heater, never an old one.

As for the perreneal "Ventless" vs. "Vented" debate: I work with ventless, and have never had any problems. So do many other people here. Some people worry about CO or ethylene, but I've never had any problems. Give them a fresh supply of O2, and they work quite nicely. Most importantly -- more important than their lower cost and their higher efficiency -- ventless heaters add critical CO2 to the greenhouse. Greenhouses are often short of CO2 in the winter.

Take care, do things right, and you'll have reliable winter heating, ample CO2, and lower heating costs. :)

I Have Been Known to "preach" the value of a vent free propane heater.It's all I Use Here in zone 7b.Like Karen said,once you have it set up,you just adjust the thermostat,and forget it for the whole season.It works without any electricity,so when our ice storms hit,it keeps on working perfectly.I've been through several ice storms,with power out for many nights,and never lost a plant.Thats the main reason I've got one.You probably could get by with a really small,maybe a 12k btu heater.

ProCom makes a variety of ventless propane heaters. Harbour Freight carries them (some may even be on sale right now).

Here is a link that might be useful: ProCom ventless propane heaters

Glenn,
So as not to repeat a discussion of unvented heaters, I would highly recommend that you visit the thread on the garden web entitled "does anyone supplement CO2 and how".

To repeat the above, read the small print in this thread and also read between the lines. "Give them a fresh supply of O2, and they work quite nicely." Give your ventless/vent free/unvented heater a vent and you will be OK.

We have been talking vent free heaters for years now. I was the one that started it 6 or 7 years ago. Just goto Lowes or Depot or your building supply store or some place that has a heater. Get one! Hook it up, check it and make sure you have the right connections, and your ready to go.

You can get propane or natural gas and you can get many different sizes based on your size greenhouse and min low night temps.

It todays world, gas is about the only way to heat a greenhouse and feel safe that your plants are ok.

Trigger is telling you the truth.

So far I've found just 3 small vent-free propane heaters with thermostats (thinking Radiant is the way to go vs. Blue Flame which heats the air?). Haven't found a good/cheap source for any of these yet (around $200!!!):
Glo Warm GWRP16T
Empire SR-10T
HearthRite HR-10TL

Appears these last two have the same OEM.

Does anyone have recommendations for specific 10-15KBTU heaters (vent-free) with thermostats???

All of them heat the air..., depends on the size of your greenhouse and the lowest outside temp your area drops too, plus wind speeds.

I went to Harbor Freight's website and found a 28,000 BTU Propane BLUE FLAME and a 25,000 BTU Propane INFRARED RADIANT. Both are under $200. Which one is better. I too have a similar sized GH that I need to heat. I also use an electric heater but would be interested in switching to Propane. Thanks.

Here is a link that might be useful: Harbor Freight Tools - GH Heaters

I have a radiant, although to be honest, I was initially looking for a non-radiant heater out of concerns that the IR might be hard on my plants. However, I got a good price on a radiant heater, and it's worked great. I think either should work fine for you.

I didn't want such physically big heaters as the HF ProComs, and besides I don't THINK we need a high-BTU-output unit cuz it only gets below freezing a few days out of the year here (So. Central TX). Our GH is the 10x12 HFGH...

Problem is, the smaller the heater, the less likely you can get a thermostat with it, and I would think a thermostat is a must-to-have to avoid cooking one's plants. So the units I cited end-up being somewhat pricey because the demand for them is not that great.

I believe that some of the little 9000 BTU "Buddy" heaters have thermostats.

Ebay is a good way to get heaters for cheaper than buying straight from a store, if cost is an issue. Remember that the heater isn't the only cost -- tanks aren't cheap, and even the hose and adaptor costs can add up.

Not sure if you saw my post - homedepot.com is having a sale on their "charmglow" unvented gas heaters. Do a search at homedepot.com