This is my first year trying Poinsettias.(300 of them)
Is it safe to use the Blue Flame Vent Free Propane heaters with Poinsettia in the greenhouse ?

As long as the heaters are firing properly there shouldn't be an issue. Poinsettias are sensitive to ethylene which can be emitted by the heaters in excess if they aren't firing properly.

I have been using the type of heater you describe for over 30 years now in two of my smaller greenhouses and they have caused no problems. I do keep a strict maintenence program on the heaters and check/repair them every fall to insure that they burn clean with the proper flame color for each type of heater. Allowing a small amount of fresh air into greenhouses daily ensures proper burning also. Down here on the Texas Gulf Coast that is rarley a problem in the Winter as we heat at night and have to use the exhaust fans many days as our Winters are very mild and greenhouse temperatures can get in the 95F+ range even in January and February.
...Michael Pawelek

Michael & Ecke, thank you for your info. I live in WI so we do see the cold. I talked with local heating companies and they are strong against using this type of heater as they say it will kill the plants. If anyone else lives in this area, have they used this type of heater?

You have to be careful with vent free heaters. The gases produced from burning alkanes which include CO, CO2,and H2O may be reacted further if reburned to produce C2H4 (ethylene). If you do not properly vent these gases you will basically have a ethylene generater. This has been a severe problem with easter lily crops in the past as lilies are extremely sensitive to C2H4 build up. As far as poinsettias go I don't know if the amounts generated would have an impact. Susan Han @ Umass did some work on this about 10 years ago. You might want to call her.

Thank you for the info grower nick. I'm going to stay away from them.