How long can you hold your breath? If long enough to do everything you need then it might be OK. Otherwise do it right and vent the exhaust gases outdoors. Someone else might enter and not know the danger. Mine is vented outdoors.

Yes you can use the unvented type. I have been for the past 20 years and they work great. Things to know. All gas heaters burn O2 and give off water and CO2. Your plants take in CO2. No problem there. All gas heaters today have an O2 sensor on them, so when the O2 level is below heater specs the gas will shut off. No problem there.

If you have a TIGHT greenhouse then you will need an opening to allow O2 to come in. THere is a formula out there that will tell you the size of square inches needed per 1 BTU. So if you have a 30,000 Btu heater, then you will have so many square inches that you will need to allow air to come into the greenhouse. Now if your greenhouse is not TIGHT, then you might have enough O2 coming in and you would not make another opening. So just hook up your heater to the gas line, and fire it up, if it burns all night then you have plenty of O2 and if it cuts off, you will need to make sure you have the proper size opening. You can do a google search on the formula per Btu and do the math.

There is no problem with using gas heaters in the greenhouse, I grow orchids and palm trees and I have NEVER had a problem with the heaters or the plants. Plus you can save a ton of money.

One secret...the more you have in the greenhouse the cheaper it is to heat. THe more dead space you have the more costly it is to heat. So pack it out and fire up your heater.... you will be glad you did.

Orchids in the greenhouse with gas heater....they bloom like crazy and last for months. Gas is the only way to go.
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Yes make sure you have air access or your heater will go out in the middle winter if your greenhouse is tight. My smallest greenhouse has this type of heater, and luckily it's a lean to up against the house, so we open a basement window and put a box fan blowing from the basement to the greenhouse to take care of the heater shutting off. Maybe you can do something similar.

The bigger worry for you would be carbon monoxide, not dioxide. Burning natural gas is prone to produce CO when there is insufficient oxygen. And burning anything works better with a constant supply of cold outside air, anyway. You might try installing some dryer hose to the outside and connecting that to the air intake on the heater. It's also worthwhile to spend a few bucks on a CO monitor.

NO Cole Robbie...your biggest concern is NOT carbon monoxide! I dont know where you get your information but that is NOT true. Gas heaters today do not work like the ones made 50 years ago or even 75 years ago. We had a big discussion here on Gardenweb back in the early 2000's about all this. You are simply are not telling the whole story which leaves people in fear and not understanding the process.

You also say. "Burning natural gas is prone to produce CO when there is insufficient oxygen."

Thats wrong! It doesnt make carbon dioxide without oxygen. It must have oxygen to make anything. Its not prone to, it does create carbon dioxide and water in the presents of oxygen, this is not a bad thing. It is suppose to do that. But it does not create carbon monoxide and NO you do not need to get a CO2 monitor, there is nothing wrong with CO2. Your plants use CO2 and the water goes to humidity. Its not even that much to begin with so you will never see it anyway.

So, lets take a lesson on how carbon monoxide is made since we have a someone that does not understand it.

When natural gas burns "completely," carbon dioxide and water is created as the byproduct. This is safe if vented directly to the atmosphere, however, if a gas appliance is not venting properly the carbon dioxide may displace the oxygen in an enclosed space creating a hazard of asphyxiation. Also, if the flame is impinged, or prevented from completing combustion, soot, carbon monoxide and aldehydes are formed.

Carbon monoxide is a chemical produced from the incomplete burning of natural gas or other products containing carbon.

So with all that said lets look at the heater and find out how it works.

Todays heaters have a CUT OFF oxygen sensor and once O2 levels fall below a preset level, the sensor will cut the gas flow off which will cut the flame out and stop all gas from burning. This stops the heater from heating and nothing happens. This will NOT happen 99% of the time because YOU will have a good air flow. If you dont have a good air flow, then the heater will not work.

Your heater does NOT continue to burn if there is NO oxygen available. Therefore you are NOT going to get carbon monoxide. You do not need any sensors unless you just want them. If you hook your gas line up properly and your heater works, then there is NO problem with the heating. If you find that you have a lack of Oxygen coming in, then you can make an opening and allow some air to come in. If you do not have enough oxygen, then your HEATER will NOT run!!!!!! Its really simple!!! Dont let others fool you. Look it up on the web if you dont believe me.

To be honest, I am amazed at the wealth of information out there on heating with gas but because of the old type heaters from the 50's and 60's that did not have oxygen senors on them and caused problems, people still think that same technology applies today. It simply doesnt.

After reading my post let me clarify it a little more.

You also say. "Burning natural gas is prone to produce CO when there is insufficient oxygen."

Yes this is true,CO=carbon monoxide, CO2=carbon dioxide, but No it wont happen because the oxygen sensor will shut the heat off.

It doesnt make carbon dioxide without oxygen. It must have oxygen to make anything. Its not prone to, it does create carbon dioxide and water in the presents of oxygen, this is not a bad thing. It is suppose to do that. But it does not create carbon monoxide and NO you do not need to get a CO2 monitor, there is nothing wrong with CO2. Your plants use CO2 and the water goes to humidity. Its not even that much to begin with so you will never see it anyway.

A simple trick to try is to take a match and light it, then place it on a concrete slab and put a glass over it, once the oxygen is burned up, then flame goes out, there is no carbon monoxide present. If you want carbon monoxide you need to do a forced burn, a burn where there is no oxygen in the presents of carbon atoms. This is simply not going to happen in a greenhouse setting.

http://www.epa.gov/iaq/co.html
Sources of Carbon Monoxide:
Unvented kerosene and gas space heaters

"Todays heaters have a CUT OFF oxygen sensor"

Yes, they do. But I bet that little sensor is made in China and costs about fourteen cents. It's not wise to risk your life on one cheaply made part. At least buy a $12 CO alarm so that you can have two different cheaply made Chinese electronics standing between yourself and death-by-gardening.

In addition to the carbon monoxide potential problems mentioned earlier, sulfur dioxide and ethylene can occur with the use of ventless heaters in greenhouses. These gases are toxic to plants at relatively low levels.

Here is a link that might be useful: ventless heaters in greenhouses

cuestaroble--- ventless does not mean you wont have O2 in the greenhouse to burn. Ventless is a marketing word that is used to say you dont need a pipe sticking out your roof. You still have air circulating around. There is no harm in using these heaters in the greenhouse provided you have the correct air exchange. If I can do it for 25 years then anyone can. There are no toxic gases. These are heaters that you use in your house, with a window cracked.

There is no worry about anything toxic to your plants. Our seedlings are all started in that greenhouse for the last five years and they do just fine. As long as there is enough air in there for the heater to burn there is enough for you.

Thank you Sandy!

Sorry but there are enough potential problems here to cause concern. Just what are the advantages of ventless over vented? Very little that I can see other than being cheaper. Professionally designed and built greenhouses have vented heaters. And just because ventless has worked in many cases doesn't mean it's safe for the plants or anyone inside.

All im saying is they asked for input, I have this kind of a heater and we have used it for several years and it works for us. We had to blow air in there with a fan so that it didn't run out of air and go out when it was cold outside...
. If you have used an unvented heater and had actual problems post it here so that the original poster can have actual user input because that is what the original poster wanted to know. My experience has been overall good with the farmtek procomm unvented gas heater.

Lots of good info here. I've been thinking about getting one of the vent-less propane heater. What night temp. Do you try to maintain? Also what is your average night outside temps range? Also how much propane do you use per month at these temps? I have used a oil filled elec. heater the last 2 years. It does okay. But it cost $50 to $100 per month to use it here. Oct. Not needed. Nov. $40 to $50. Dec. $60 to $75. Jan. $75. to $100 Feb. $60 to $75 Mar. $40 to $50. My green house is small 8'x8' I grow salad, cucumbers, and tomatoes just for fun and to give me something to do till spring garden time. LOL

mrdoitall:

Trying to grow warm season crops, cucumbers and tomato, in winter is expensive no matter how you heat. My heating costs are about the same as yours for a 1700 sq ft greenhouse. But all I grow is cool season crops and fruit. I'm heating to mid 30s when outside averages upper 20s. If I heated to 50s-low 60s for warm season crops the cost would be about 5-10 times as much.

At your cost it's still a pretty cheap hobby in my view.

Rob Orchiddude
Could you post a photo of you greenhouse fresh air vent?
I am tryin to deal with a overtight Greenhouse and not enough fresh air.

The major difference between vented and ventless heaters is their efficiency, IMO. Ventless heaters are 100% efficient; all the heat goes into the GH. Vented heaters efficiency ranges around 80%; you loose some potential heat up the chimney/vent.

For GH use, the ventless heaters would seem ideal, in that the extra CO2 they give off is a plus; in fact you can buy CO2 generators for GHs which are basically a ventless gas heater.

However there is always the potential of any gas burning heater producing CO. It happens more when O2 levels decline but can also happen if the burner has clogged air ports or extra gas is being injected for some reason. So it is prudent to keep a CO monitor around, just in case. BTW vented heaters can also produce CO, which can find its way into the main heated space, but this is less likely to happen than with ventless ones.