Hey all. I haven't posted in AGES but since I've had some experience with blue-flame ventless heaters AND I recently got my greenhouse version 2.0 up and running, I thought I would put in my two cents. I've got a 30k btu propane ventless heater in my 8x16 gh, and it does a good job of keeping things nice and toasty. No problems with yellow leaves. The two biggest problems I've had (now solved) are 1) heater turns off due to internal sensor's detection of low oxygen and 2) 10-20 degree temp fluctuations between on/off cycles of the heater. Problem #1 is the biggie; no heat, no plants, right? Back when I first built my gh several years ago(version 1.0), it was 8x24, plastic on all sides, well sealed. The heater kept things warm enough but the gh was so big that the heater was on almost all the time. The oxygen was rapidly depleted (I could still breathe fine but the heater thought otherwise) and the heater turned off in the cold dead of night, plants died, etc etc. Now, I read about having a vent (read: gaping hole that let what little heat did stay in the gh out) *near* the heater, but experimentation (leaving door open a crack) showed me that that wasn't a reliable alternative. What I did was to seal off most of the air vents along the bottom of the heater except the large one on the back. Then I piped a dryer vent hose from the back of the heater to a one-way air exhaust (used for dryer installs, but in reverse) connector mounted in a hole in the gh wall. Thus, cold outside air only comes into the gh when needed by the heater, which of course is warmed by the flame, and the only air I'm consequently venting is the coldest air near the floor (going out under the door) or via the inflation fan for the double layer plastic. Since my solution's implementation, I've had zero problems with the heater cut-off. Problem #2: Temperature consistency. The ventless heater's designers seem to assume that it will be used in a relatively well-insulated space. This is to say that it's assumed that even though the temp will go down over time, it will do so in a slow and controlled manner. Therefore, the temperature-sensing bulb has time to come or go to whatever room temperature there is, and turn the gas on based on the thermostat setpoint. The occupants of this hypothetical room would probably notice things getting a little cool just as the heater turns itself back on. Greenhouses tend to be more lossy than an insulated room, therefore the temps drop more quickly than the heater's thermostat is designed to respond to. My greenhouse v2 is muuuch better at keeping in heat than the first one, but temps that *average* 65 degrees still really vary between 50 and 75 because of the effect described above. My solution is to have an electric heater as well, set so that it kicks on at a slightly lower temperature than the gas heater. The electric heater is just a Walmart special and puts out a fraction of the heat of the gas heater, but it serves to smooth out the bigger temp. dips... giving the gas heater's thermostat time to cool down and turn on the heat. Heat comes on, room warms up past the electric heater's setpoint and it turns off. The electric bill doesn't suffer too much and I have to make fewer trips to refill the twin propane tanks. It's good all around. :) If anyone's at all interested, I could post some pics of my setup. Thanks for letting me babble. It's great to be back into the swing of [green] things!

I spoke with a guy at Southern Burner today and due to variables such as the amount of sun in any given day, night time lows, greenhouse insulation and ... you get the idea. He did tell me the C series would be sufficient and their run time on a 20lb. tank (the grill size) is about thirty hours. That would equate to 150 hours on a the 100 lb. size. I figure around an average burn of around 8-10 hrs/day so thats about 2 tanks a month. Again the amount of sun and the duration the unit burns to maintain 50 degrees in my case will impact the total usage. I'm guessing a 2- 100lb. setup would be ok, I only plan to heat from March til May, but who knows if things decide to grow that could change LOL! By the way their number is 1-800-375-5001 and the guy I spoke with was Glenn, very informative and seemed anxious to help me out.

Barb -13 degrees....Isn't that why they invented florida? good luck with your heating situation. IMHO 80 degrees is a tad too warm. Most large growers shoot for 60 at night and 65-70 days. Also with that unvented heater make sure you have some fresh air coming into the house even if it is cold. You're enviroment is going to get pretty stale with the heater running all night and no new oxygen coming in. Kind of a catch 22.First you try to plug all the leaks then they say let some fresh air in. I'm not sure of the amount you need at 30,000btu but a piece of 3" pvc or abs pipe sticking through the endwall and ending somewhere around the heater should help.Don't worry about that learning curve, you never stop getting new ideas and info on this forum Steve. Always remember keep the green side up.lol

A friend and I tried to correct HDD for other temperatures. If you assume that the curve is a sine wave over the year, going from the coldest in feb(july), to the hottest july(feb) and with a mean defined by the HDD then you can draw a graph and work out the area under a given temperature (or integrate asin). Unfortunately, this doesn't include the variance of the temperatures (especially if you have things like lakes to put lumps in the sine wave). To see this, if the year were only two days long then 100F, 35F would have a HDD(65) of 30; similarly 50F, 50F would have a HDD(65) of 30. But if you want HDD(50) the first is 15, the second 0! In practice the weather is better behaved, but not enough. In conclusion, if you want HDD to say 60F, that is doable fairly easily, but for a greenhouse you are more interested in HDD to say 45, and the error is too much to be very practical. Having said that, if you have monthly climate data, you can use the mean minimum as a reasonable starting point for working this out. Better is to use Karen's gmodel to get quite accurate estimates of performance. Perhaps if you are interested in that, and are intimidated by the code, tell me (or perhaps Karen?) your postcode and some basic details and we can try it out. (This is useful to us to tune the model over a wider sample) Re: CO2, all the commercial growers I know seem to not care in the slightest about ethene from natural gas heaters (which are more common here). There is at lot more ethene in natural gas than LPG (often called propane, but actually a mixture of lots of similar hydrocarbons) Carbon monoxide is a danger, as is high levels of CO2 in well sealed greenhouses. Both are monitored. I have no idea the effects of things like NOx (smog), although a 10 second search indicates that at least N2O and NO are plant active. Practically, unless you are trying to get 100% recycling of everything (perhaps for your space station or biosphere 3 project) the natural leakage rate of greenhouses seems enough to make this not a big deal. Has anyone tried using fermentation for heat and CO2? An aquatic plant grower I know brews a lot of beer for this reason, but he says that you need to know a lot about both to make it economically attractive. Re 2.5 kW heater, I forgot that the US has only 110V and the lower power ratings that result.