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#16
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Greenhouse oil heat math question
I second this one from Pat. I'm pretty sure I use far less than heat than
you do, although I do still wince everytime we have to fill up the LP tank .... but I suspect I spend way more for summer cooling, even without cool pads. Cutting corners on either expense hasn't worked out well for us -- keeping the plants at their best, we've found, is more cost-effective. And for us, at least, it's more than bud-blast on Phals. E.g., an extra 3 degrees F of heat greatly reduced our leaf-loss on the vandaceous plants. That said, 65F is warmer than we try to stay. We haven't found that 58F hurts anything, including budded Phals* -- except ME, when I have to be out at dawn. I'm not set up for anything under 75F! Fortunately, the plants are tougher than I am. Kenni The other factor that affects us probably more than you is the likelihood of really BIG diurnal shifts, esp. if we don't get the place opened back up early after a cold night. Despite an overnight low in the low 40s [outdoors], the high can get into the 80s the next day. THAT will blast some buds. If you think your greenhouses grow better with two heaters; increased sales from better plants or faster growing plants will most likely more than pay for the additional oil. Humidity will be more uniform in a greenhouse that has uniform temps. How many blasted buds does it take to cost more than the oil savings? |
#17
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Greenhouse oil heat math question
Al,
If temperature distribution is your primary concern, why not just run more fans They're probably less expensive to operate than the heaters. The key to efficiency is to match the heating capacity to the greenhouse demand. The ideal is a heater that puts out the exact amount of energy that the greenhouse loses, such that it is on constantly. Obviously it's impossible to have such a device, as outside temperature changes and sunlight affect the balance. As there is no way to actually meet that ideal, one tries to size the heater to be able to keep up with the demand under your worst case scenario - in my case with an "even worse than that" built in safety factor - knowing that it will operate less efficiently under "better-than-worst-case" conditions. If one of your heaters has the capacity to heat your greenhouse, it is more efficient to heat exclusively with one, and have the second as a backup with the thermostat for it set lower than the primary. If each your heaters has half of the necessary capacity, using them both with be immeasurably less efficient than running a single unit with twice the capacity, the difference being the amount of oil that has to be burned in order to heat the extra mass of metal. -- Ray Barkalow - First Rays Orchids - www.firstrays.com Plants, Supplies, Artwork, Books and Lots of Free Info! "Al" wrote in message ... So you all are saying that I am burring more oil by what I am doing: by running two identical heaters at 63.5 instead of one heater at 65 to maintain an average of 65 in the space I will spend more money? "heating double the mass of steel and running each for half the time" uses more oil? The extra BTUs required to "maintain the part of the greenhouse 2.5 degrees above normal would not be made up with the savings of only heating the other side to 2.5 degrees below normal" so I will use more oil? Lacking some calculus equation I could not understand anyway, I'll take your word for it. :-) I do kind of see what you are saying. So the smartest thing I can do (since I was dumb enough to buy these inefficient heaters anyway) is to set one heater at 65 and one at 63 so the second unit would only come on if the outside temps/winds stressed the first unit's ability to maintain the temperature or failed to start at all. My goal is to use the least amount of oil while knocking the fewest buds off the phals. I do like the nice even heat distribution I get with both heaters. I kind of think some of the bud loss I got last year was from marauding cold dry drafts in the circulating pattern. I will have to look into a way to distribute the heat from one unit more evenly. Those clear plastic tubes that hook in front of some heater brands and run down the length of the greenhouse dispensing heated air though little holes will not work because they can not be attached to the front of my heaters. The manufactures told me so last year. If these things would just hurry up and rust away, I would invest in a better method of heating; probably a boiler with fin-tubes of circulating water running under the benches. "Ray" wrote in message ... It is less-efficient to run two. When a heater comes on, you pay to heat the heater in addition to heating the air. If you plotted a curve of the percentage of energy spent heating the hardware versus heating the air, it gets more efficient with longer individual run times. The longer the burn time per start, the more efficient it is. That's why it doesn't pay to buy a 100K BTU heater when your need is for a 50K unit - it may heat the air really fast, but it has to do so more often. The ideal would be a heater that generated heat continuously at the exact rate the greenhouse lost it. With two heaters, you are heating double the mass of steel and running each for half the time for each burn cycle. -- Ray Barkalow - First Rays Orchids - www.firstrays.com Plants, Supplies, Artwork, Books and Lots of Free Info! "Al" wrote in message ... ps. logically I come up with the same btu's must be needed to reach the same average temperature, but is this the truth? "Al" wrote in message ... Hi, I have a 30 by 100 foot greenhouse. I have two heaters, forced air heaters, that burn oil. One heater alone is enough to heat the greenhouse (on all but the coldest nights). They hang on opposite side of the long greenhouse and the un-used one is a back up incase the primary one dies. Two thermostats hang in the middle of the space, 50 feet from the heaters about midway way from the floor to the roof. One thermostat controls one heater each and they are independent of each other. There are four temperature monitoring stations in the greenhouse that are independent of the on/off thermostats that are situated to read the two coldest spots and the two warmest spots. The four spots are gathered from plant level around the bench levels, (not extremely close to heaters or tucked down in cold corners) I can run the south wall heater with its thermostat set at 65 (in the middle of the greenhouse) and the north end of the greenhouse will register around 63 and the heated side will register around 68 but the air is circulated by LOTS of fans and the center air mass where the thermostat is, is 65 degrees. The four temperature sensors around the greenhouse average 65. The same pattern emerges no matter which heater I use; the far side is cool and the heater side is hotter but the middle is 65. If I set *both* heaters to come on at about 64, both sides of the greenhouse heat to around 68 and the middle of the air mass turns on and off both heaters at 64. And they don't seem to cycle on and off as often, and it seems there is probably more radiant heat available from two heaters but that may be an illusion because the outside night temperature varies a lot from night to night and this has to effect how the heaters cycle on and off. ....anyway the average temperature based on the 4 monitoring stations now comes to just under 66.... I have to set the thermostats just about 63.5 to get an average air temperature of 65, so it matches the average air temperature of the single heater number. Here's my question, so by running two identical heaters at 63.5 instead of one heater at 65 am I using more oil, less oil, or the same amount of oil to heat the same space to the same average temperature? there has to be an equation that will answer this question. Anybody know how to figure it? |
#18
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Greenhouse oil heat math question
Reading this thread makes me glad I went through the extra trouble and
expense of putting in hot water perimeter heating. With only one small fan to keep the air moving the GH stays at 58 to 61 F except when the sun shines then I have to vent on days above 25 F. |
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