Need greenhouse heater advise

[Gtlevine]

I completed my 16' x 20' greenhouse and am looking for a heater. Home Depot sells propane units that require a 100LB propane tank that are forced air type for about 400 bucks, quite pricy. The label says a 100LB tank will last 30 hours of continuous use if the heater is on the low setting. With the cold we have had, I'm afraid the heater may need to run all night, that means 25 gallons of propane will last only three or four nights, for $75, were talking close to four hundred a month.

For those of you with experience heating a greenhouse, what type of heaters work best, the costs per month, etc... If you have a brand in particular, let me know your thoughts as well. My greenhouse is 320 SQFT and 9 feet tall so keep this in mind when recommending a model.

Gary

[epicure3]

(Gtlevine @ Jan. 15 2007,19:04)

QUOTE

I completed my 16' x 20' greenhouse and am looking for a heater. Home Depot sells propane units that require a 100LB propane tank that are forced air type for about 400 bucks, quite pricy. The label says a 100LB tank will last 30 hours of continuous use if the heater is on the low setting. With the cold we have had, I'm afraid the heater may need to run all night, that means 25 gallons of propane will last only three or four nights, for $75, were talking close to four hundred a month.

For those of you with experience heating a greenhouse, what type of heaters work best, the costs per month, etc... If you have a brand in particular, let me know your thoughts as well. My greenhouse is 320 SQFT and 9 feet tall so keep this in mind when recommending a model.

Gary

Hey Gary:

You're observations on the forced air heaters is dead on the money. I bought 3 and sold the kids. I opted for kerosene instead of propane. I got mine at The HD, so the prices are correct. The large one  I got holds about 10 gallons of kerosene. On high, that thing flamed out in about 7 hours (probably depends on how cold it is outside). However, I don't regret it. I look at it as more of an investment. Time will se if it paid off.

[tropicalb]

Gary....

exactly how warm do you want to keep it in there?

[Gtlevine]

(tropicalb @ Jan. 15 2007,19:21)

QUOTE

Gary....

exactly how warm do you want to keep it in there?

My low outside is usually 40F to 42F, I will base my requirements on those temps. I would like to keep it at around 50F maybe 55F, so I need a heating unit to raise the temps about 10F to 15F. I don't mind spending a hundred bucks a month on fuel or electricity, but I don't want to spend four or five hundred so hopefully something is out there economical.

Gary

[JD in the OC]

Propane has the advantage of producing carbon dioxide which benefits the plants right?  I can't see electricity being cheaper than gas either...

Low in Riverside on Sat night was 30.9 deg F.  I'm at 1400 feet elevation.

Lake Elsinore got down to 21.  

JD

[tropicalb]

Gary....

the set-up that i rigged up just for this last cold spell (and man is THAT an understatement!) seems to work just fine...

My greenhouse is single poly with wood framed ends...20' long X 11' wide (not so much smaller than yours).

Even with it getting down to 29F just outside the greenhouse, inside the greenhouse it never got below 42F during the last few days...not bad.

The heater is an oil-filled electric one...got it when it was on sale at Home Depot for $29.00 (it's now back up to $60 regular price I think):

I leave it on at the highest setting (and it sucks 1500 watts..still waiting to see what it's going to actually cost me to run it), and the addition of a small oscillating fan on the top of three stacked cinder blocks just in front of the heater is what really made the difference in keeping the whole inside of the place warm.

I'll post some pics tomorrow of how i actually have it set up now....it's not pretty but it works.

[BS Man about Palms]

Gary- and Burt too!

This past cold wave gave my "Halogen light" theory a full test!  I only have the 6,6" x8' greenhouse, but all I put in was a 250 watt light "on the floor" pointed at about 45deg angle and a tiny fan behind it. The lowest I hit in the green house was 37F. that was at LEAST 7F warmer than outside, with the tiniest light I have!  (I only had a temp sensor under shade cloth, with a sheet over it and a 500W light "outside" the greenhouse, it hit 32F.) (For some sick reason between laziness and curiousity, I DID NOT take in my Kentiopsis Magnifica or Piersoniorum, they were under the cloth, and at this point are showing no damage)

I stand by my statement that biggerlights will work better and can be quickly placed anywhere, won't run out of fuel either. Will need electricity though!.

[tropicalb]

I hear ya Bill...halogen might be the way...you are living proof!

I think I may end up giving it a try a little later this winter...I mean, even 750 watts of halogen lighting is gonna be half the price of running the 1500 watts it takes to keep this radiator hot...but I will say it has done a damn fine job of keeping it warm in there. I could run it at the 800 watt setting, but it's just been so damn cold I don't want to run the risk of it getting below 40F in there.

I'll let you know how i really feel about it when i get the SDG&E bill.

[Neofolis]

My 4kW natural gas heater keeps my 4m x 2m (13' x 7') polytunnel at 30°C/86°F without being turned up very high.  It can do this when it is -5°C/24°F outside, assuming it is calm, which it tends to be at that temperature.  It is was on full power all the time it would cost £31.00 per month to run and, because it is plumbed into the mains gas supply, I don't have to worry about running out of fuel or trips to get bottles refilled.  Gas is much cheaper than electricity here, otherwise electricity is far more convenient in terms of installation and portability.  Any non permanent fuel source is going to a pain at some time, especially if you have to go away at any time.

My heater cost about £160.00 and is flueless, so other than a bit of plumbing work for the gas supply and providing sufficient oxygen to the burner, which I do via a piece of plastic ducting, it is very simple.  The only bi-product is water vapour, which might be beneficial in terms of humidity, but in reality it is such a small amount it makes no difference.  The one down side is a steep temperature gradient, which can be overcome with an air circulation fan, which should be beneficial to any greenhouse anyway.

[tropicalb]

Like i said...not pretty but it sure works...Here is the setup in all it's temporary glory Gary:

[Gtlevine]

Thanks alot, when you get a grip on the costs, let me know.

Gary

[BobbyinNY]

Electric, in my opinion, is the way to go.. It's SAFE, and with the right thermostat, can be relatively inexpensive... I use a 220-volt, 30,000 btu Dayton Heater in my 15x15x12 biosphere and it is fantastic.... Our average low has been around 32f and it's been keeping it at 70f inside.... Tonight will be a test when our low drops to 19f with a 28mph wind.

[Walt]

Gary:

It's hard to tell how much heater (BTU output) is required unless one knows the following: 1. The inside design temperature you want (say 50F).  2. The average low temperature anticipated outside  the greenhouse (say 20F). 3. And probably most importantly, what is the heat loss per square feet of wall and roof in BTU/hour during the two above parameters. Also, there will be heat loss through the ground/floor, too.

Somewhere on the web I once saw rough figures of wall/ceiling heat loss for various constructions of greenhouses, but I can't recall where at this moment. Possibly, you can do a Google search to find it.

Once you find the heat loss through the walls, based on indoor-outdoor temperatures you will encounter, figuring the gross output of a heater(s) will be easy.

The above being said, I will say this:

I have a small greenhouse (12'x12'x8'-6" high). The structure is treated 2x4" (except the ridge board) and is covered with those clear rigid corrugated plastic panels you buy at Loews, Home Depot, etc.

I heat my greenhouse with one infrared propane heater. The type that mounts directly to a 20# propane tank. The heater has three settings: 7500 BTU/10000/BTU/12500BTU.

At the high setting I can keep my GH around 40-45 degrees when the outside temperature is in the high 20s to low 30s.

Because your GH is roughly 2.22 percent larger than mine, I would recommend using several smaller heaters rather than one large heater, so there will be less hot spots and better heat distribution.

Plus, if one heater were to fail, you have the others to give you at least some protection.

But getting down to price, let's assume you pay $15 to refill a 20# propane bottle (.75 cents/pound of propane). If you pay less (I do), then all the better.

One pound of propane has a gross heat value of 21,669 BTU. Hence, a 20# bottle will contain 443,380 BTU.

My infrared heater (on high setting at 12,500 BTU/hr.) uses .57 pounds of propane/hour, and will roughly last 35 hours at the high setting.

So if you were to use three of these heaters at high setting, you could expect to use 1.71 lbs./hr. or in other words, $1.29 per hour.

You can do the rest of the math to find the estimated costs based on your anticipated use.

Electric heaters are clean and hassle free, but they don't put out much BTU/hour. For instance, a 1500 watt (120vac) space heater produces about  5,115 BTU/hour. A little heater like this would be okay for say a GH of maybe 8'x8' in square footage, but not for mine, I tried it. Two heaters might have worked but they would have to be on separate circuits and my GH is too far away from electric. When I tried a space heater I powered it off a gasoline generator (what a monumental waste, but it was all I had at the time).

Kerosene heaters have firing efficiency losses, but all and all are good.

I happen to use all three of the above methods, depending on the situation at hand.

Lastly, my greenhouse isn't really a greenhouse per se. I only use it to protect palm seedlings during the one or two worst nights each winter (so far there have been none, thankfully). I only use my propane heater once the temperature inside the GH gets down close to 35 degrees.

If you want to maintain 50-55 degrees all night long (on nights where the outside air will drop below 50-55 degrees) than I can see where fuel might get costly. On the other hand, if you are growing stuff that won't be damaged at 35 degrees, then I would expect your winter fuel bill (propane) to be very reasonable.

Hope this helps.

Walt

[Gtlevine]

(BobbyinNY @ Jan. 16 2007,15:41)

QUOTE

Electric, in my opinion, is the way to go.. It's SAFE, and with the right thermostat, can be relatively inexpensive... I use a 220-volt, 30,000 btu Dayton Heater in my 15x15x12 biosphere and it is fantastic.... Our average low has been around 32f and it's been keeping it at 70f inside.... Tonight will be a test when our low drops to 19f with a 28mph wind.

This sounds like much less trouble than propane, not thrilled about dragging tanks to be refilled. Do you use a fan with the heater? Does the heater run all night or is it on a thermostat. What material is covering your greenhouse? Have you determined the cost as of yet to run the heater?

Thanks,

Gary

[Matt in SD]

Gary,

I don't know the best way to heat but I have a few general thoughts/coments.  

First, if you are growing plants in your greenhouse that will eventually be planted outside, I think keeping the temps above 50F is not necessary and may even be a bad idea.  Phil and Jerry grow some pretty tropical stuff in their greenhouses without heating.  Also, given our normal winter temps, keeping above 50F will be very expensive, while keeping above 35F or 40F will mean the heater will barely ever run.  If you are growing Cyrtostachys, you'll probably have to keep it even warmer than 50F.

I have found that in my unheated greenhouse, very small seedlings (less than 4 leaves) have trouble through the first winter and take a while to really get established because of it.  So for germinating seeds and growing seedlings through the first winter, I would just set up an enclosed box inside your greenhouse that you can heat and maintain at a warmer temp (I'd say 60F minimum).  I currently have a fish tank with bottom heat that I'm growing seedlings in that stays at ~80F 24 hours a day and the seedlings are growing like they do in Hawaii.  I think Jason (Kamipalms)  set up a pretty cool seedling box for this purpose.  Once the seedlings are past about the 4 leaf stage they seem to go through the winter in my unheated greenhouse without any setback.

Matt

[BobbyinNY]

This sounds like much less trouble than propane, not thrilled about dragging tanks to be refilled. Do you use a fan with the heater? Does the heater run all night or is it on a thermostat. What material is covering your greenhouse? Have you determined the cost as of yet to run the heater?

Thanks,

Gary

Gary,

this is ALOT less trouble than propane, and alot safer....  The only thing you'll have to do is get an electrician to wire you up a 220-volt circuit.  As Walt said,  110-volt heaters are a maximum of 1500-watts (5000btu) and you'd have to get 3 or 4 of them and put them on separate 15amp circuits... 220 is the way to go. Cheaper to run, and heats my greenhouse up like you're in the middle of the tropics....  I figure all together, my heaters are about $200-250 extra/month. but that's for all (3) greenhouses I have set up and the other 2 of them each have (2) 110-volt heaters - but I don't maintain a very high temp in the other ones because I'm just trying to keep the palms alive - those are at 40f min

[Gtlevine]

Bobby, which model number do you have? I like the idea of one heater.

Matt, I think you are correct, better to have plants somewhat hardened of by some winter cooling. I plan to compromise by setting the heater thermostat to 45F, that way I can maintain plant growth somewhat, harden the palms off for eventual planting outside, and save on energy costs.

Thanks for the suggestions,

Gary

[Logolight]

Here's how I protect my palms from the cold.  I put a 100W bulb at the bottom of my small green house and it keeps the temperature about 7 degrees warmer than the outside temp.