Can I use one of those hobby greenhouses that department stores sell for wintering my citrus and other tropicals. I tried a web site heat claculator and it said for a 8*6 greenhouse it would need to have 15000 BTU so that if the lowest temp outside was 0 F and max. temp. inside would be 80 F Can some clear this up to me. I can't beleave that it would take that much heat for such a small area, even though I am in Ontrario.

Valenciaguy:

I only have experience with a small mini greenhouse, but if you're using this calculator:

http://www.littlegreenhouse.com/heat-calc.shtml

...and it's accurate, you can cut the required BTUs in half or two thirds (down to 6,000 to 8,000 BTUs) by going with the highest R-value covering and going for a lower interior temperature, like 60F or 65F. Why would you want to get the temperature at 80F anyway? Citrus will produce new growth anywhere above 13C (55.4F).

I have it at 80 as a max. just incase but I would really only need about 70 because I have some other tropicals that need higher temps than citrus. I just re tried it on the site and I put a min. temp outside as -13F (we have had that before but it was with wind chill without wind chill it would be 0 F) and max. inside temp as 70F and it would be 10000BTU(lowest 0F)12000BTU (lowest -13F) BTU would I be ok by going with a heater from a department store that have 1500watt power usage(Which has about 6000BTU). Should I use lowest temp. with wind chill or without wind chill?

Thanks a lot if you can understand my ramble.

Valenciaguy:

First of all, I can't attest to the accuracy of the calculations on that particular website. I would assume you would use the ambient temperature (i.e. the temperature of the still air, without the wind chill factor taken into account), though the ACF website doesn't give any indication one way or another.

Second, I'm missing two variables necessary to do the calculations you have done to come up with your BTU calculation. I don't know the height of your greenhouse, which is necessary to calculate the cubic capacity upon which the calculation is based, and I don't know what kind of covering you have. Assuming you have the 4mm polyethylene, for example, to get a 70 F temperature reading in your set up (6,000 BTUs) would mean that your greenhouse is only about one foot high! If your greenhouse is six feet high with 4 ft. walls, then you would need something like the best insulation listed (16mm twinwall polycarbonate) as a covering to be able to get 70 F when it's 0 F outside.

How tall is your greenhouse and what is the covering made of?

well I was trying to figure out what kind of greenhouse I would be buy for my needs and how much it would cost to run. On ACF website I chose the Juliana 450, I used the the spects. they provided on the website for that model with its 4mm twin polycarbonate and the BTU came to 9471. I just can find stores that supply heaters with that BTU range, they usually come to 5200 BTU and I dont need the 19000 BTU that ACF supplies.

Valenciaguy, most of the heating calculators go by the surface area of the GH and not the cubic capacity. The ACF heating calculator has some misleading wording, IMHO...the third box is labelled as "maximum" inside temp. Really what you want to put in this box is the floor temp, the lowest tolerable temp in the GH.

These calculations will give you your heater requirements in BTU/hr. The standard 1500W inexpensive heaters we can buy at Walmart produce 5120BTU. Remember that one of these heaters will draw 12.5 amps and therefore you will only get one heater on a single 110/120V 20amp circuit. Two heaters will likely require two different circuits. It looks to me as if two of these heaters might suffice.

Calculating energy costs is a different matter, a different calculation...but it can be easily done. All of these heating calculators use the same formula: Q=(A*deltaT)/R where R is the R value of the glazing, A is the surface area of the structure, and delta T is the difference between outside and inside temps. To figure monthly heating costs, you just use the same formula, but instead of using the lowest outside temp, you plug in the average outside temp. This will give hourly BTU, which you would multiply by 24 (hours) and then again by 30 (days in the month) to get the BTU/month. Then you can convert BTU to watts and if you know your kWh energy costs, you can figure out the monthly electric bill for the GH.

If I had a small greenhouse that had the dimensions of 8 X 6, I would purchase some of the 8 X 4 polyisosynurate insulation boards with the silver coating on both sides and cover the greenhouse sides and top every evening. For years I had a solar greenhouse that I covered the southern glazing every night. It only took three or four minutes a night. The removable insulation boards will drop the heating of the greenhouse to almost nothing. Also in Canada, I would cover the north walls with this type of insulation permanently. North walls do not gain light, they lose light. Your hardest task is not going to be heating the greenhouse, it is going to be cooling the greenhouse during the summer. Small greenhouse over heat very easily. - Millet

Millet's suggestions are great, and not just for those in Canada! Insulation of the north wall will look like this:

One inch rigid foamboard will add roughly R-6. This will substantially reduce your heating costs/requirements. There are ways to calculate heating costs with the insulation, it's just a sum of the separate calculations based on each of the different types of surface area of the GH...Q=((A1*deltaT)/R1+(A2*deltaT)/R2...)

I had an 8 x 16 ft greenhouse that was only glassed on the south side where I had 4 sliding glass doors leaning against the frame --inclined about 20 degrees. I used the same material Millet is talking about for the back and I uses the corregated clear fiberglass for the roof and sides. Extra heating was not needed in our climate, cooling was easy-- just prop the sliding glass doors open with a block of wood at the top. I also used misting in the summer when I was growing plants from seed for winter flower beds.

The aluminum coated foam material is cheap and easy to work with.

Skeet

Well for me cooling it in the summer doesn't really matter because I bring most of my tropicals out in the summer anyways and if it did get to hot I would keep the door open etc. I will try it because I am running out of space in the house, and i will try it with one heater but before the plants are in it.

I this is in an thead of mine but i just read up agian about some studd and was wondering would 4mm twin wall polycarbonate glazing for a greenhouse be enough for me so that i could keep the air temp to around 70-75F or should i go with a higher value for here in zone 6.

The southern wall of my greenhouse is glazed with twin wall polycarbonate. The east and west 3-foot kick walls are covered with twin wall polycarbonate glazing that is laminated with 1/2 inch silver coated foam board. Twin wall polycarbonate only has a R value of 1.5, therefore even if you use the twin wall glazing the greenhouse will still require additional insulation. As far a winter heating, I would strongly recommend installing a heater that has the capacity to always maintain an inside greenhouse temperature at least 60-F (16C) higher than the outside temperature. This provides enough heat variance to protect your tropical plants on extremely cold nights. One last thought, keeping the greenhouse door open on hot sunny days will not be enough to cool a small greenhouse. If anything a roof vent and side vents will be as efficient as possible, if you do not intend on installing cooling pads. -Millet

Thanks so I guess this means just buying a hobby greenhouse is not enough, where do you buy the stuff that you use millet for your greenhouse? Also when i use the calculators online the higher R value I use the more BTUs it says I need to have shouldn't it be the opposite. Also wouldn't 10mm polycarb be good?

Fortunately, the Denver, Colorado metro area has a strong commercial greenhouse economy, with many commercial growers. The areas largest greenhouse is over 1,000,000 sq. ft. in size, and several other greenhouses in the 600,000 sq. ft. range, plus many greenhouse growers in the 50,000 to 100,000 sq. ft. range. Therefore, there are two large suppliers, American Clay Works ( www.americanclayworks.com/ ), and Denver Wholesale Florist/Greenhouse supply that sell almost every item required by the industry. My greenhouse is a Nexus greenhouse http://www.nexuscorp.com/ . However, don't get discouraged. Before I was able to purchase a commercially produced greenhouse, I build my first two greenhouses myself, out of wood and some corrugated glazing. They did not have much of any type of environmental regulating equipment, to maintain a good growing environment. I grew by the "seat of my pants." When one has a will, one will find a way. Just get started, and something will develop. - Millet

Thanks a lot Millet and I definatly going to try and start and see what happen but for first couple of cold days I am going to see what the temps. inside would be without my plants in it just as a precaution. Also can i get the polyisosynurate insulation boards from American clay works?