I looked at green houses this Friday

What size do you think is the best? I am tettering between a 14 x 22 or 8 x 22. I went to Turner Greenhouses in Goldsboro.
Also what works best for you the poly sides or the fiberglass?

Lavina

In 7a, I would want twinwall if using the GH during the cooler months. I'm not sure I have ever seen twin wall fiberglass glazing...twin wall would mean going with polycarb.

Size does matter...go with the bigger one if you can afford to buy and run it. There is a thread on calculating greenhouse costs, I think it is referenced in the sticky on top.

OK, it isn't there. It is here: http://davesgarden.com/community/forums/t/644362/

I agree, go with the big one if you can. I don't think I've ever heard anyone complain about having a GH that's too big!

SB I just read the thread that you linked to, and came up with an observation about the formula you posted. Your numbers assume the heater is running 24/7. Mine rarely do. If the sun is out, even on a cold day, thermal heating will warm the GH, and the heaters will shut down. I would say they only run (with some exceptions in late winter, of course) ~12 hours a day. I'll give ya the fact that the formula gives a 'worst case scenario'). =)

I'll definately agree to go with the bigger of the two sized GH's, if they can afford it. I'd love to have one about four sizes larger than what I have (but I'll take what I got!).

Not eggsactly, Eggs. You are correct in that the formulas assume heaters running 24/7. They must, because the best we can get is an average temp. That average is over 24 hours, thus the calculations must assume the heaters run 24 hours. They won't, obviously, but OTOH, there will be times that they will run more than average, when the temps are below average. It evens out.

If we had temps broken down by 12 or 6 hour periods, the calculation would be more precise. If we had average night temp and average day temp, we could do the calculation twice and add the totals.

This is my first year with the GH, so don't have any real hard data, but this should be close.

My KWH usage was up 900 kwh this period over last. 11/09-12/08.

My heater draws 5,700 watts/hour.
900,000 watts divided by 5,700 watts = 157.9 or 5.26 hours per day.

We pay .037 for everything over 1,000 kwh in the winter so my cost is approx. $33.00 per month. Maybe I will crank it up another 5 degrees.... lol. Anyway it is less than I thought it would be.

This is to heat a 12x14 GH in Zone 7b. The GH foundation is built up on 6x6's raising the ceiling height by 2'.

The north side is insulated with foil backed bubble wrap and the GH covering is twinwall poly.

I'm not the math wizard here, so I will trust SB and the others to verify this. :)

Nautical

Lavina, sorry for the threadjacking!

Nautical, you left out the one last thing we need, that is the temp you are holding in the GH. Just for fun, I ran the calc with the following assumptions (and I swear, I didn't rig the numbers): the end wall is the north wall, and with the bubble wrap you end up with improved R value of 2.5; 4mm twinwall with an R value of 1.42; to keep it simple, I assumed the 6" base has the same R value as the rest of the wall; min temp of 60F inside; average temp of 50F outside in November and 41.5 in December for Conway, AR (got that from the web).

Average hourly heat requirement 4059 BTU -> average hourly heat requirement of 1.1893 KWH
1.1893KWH * 24H/day * 22d in November = 628 KWH for month of November.

Same calculation for December gives avg hourly heat requirement of 7510 BTU -> avg hourly heat requirement of 1.7408 KWH. 1.7408 * 24H/day * 8d in December = 334 KWH in first 8 days of December.

334 + 628 = 962 KWH for that monthly utility bill. That's pretty close!

Merry Christmas.

Lavina, I too am sorry for the thread jacking!

SB I can only assume that you have been naughty and not nice, as doing this complicated math on Christmas Eve can only mean one thing......Santa didn’t bring you anything!

WOW! You never cease to amaze me.....that goes for probably most of us. Yes that is very, very close, considering that you did not have the REAL data to work with.

So, that forced me to think about the real data and what it would take to get it. Well we won't talk about how long it took, but here it is.

First the easy part, the north wall is the 14' wall. No insulation on the ends or south side.

Now for the temps:

Average outside temp, from 00:00 on 11/9/2008 - 23:30 11/30/2008, was 51.89403409.

Average outside temp, from 00:00 on 12/01/2008 - 23:30 12/08/2008, was 45.6609375.

Average GH temp, from 00:00 on 11/9/2008 - 23:30 11/30/2008, the was 68.55681818.

Average GH temp, from 00:00 on 12/01/2008 - 23:30 12/08/2008, the was 61.609375.

Anyway, I am most amazed at how close the numbers are.

Merry Christmas to you and your family, and to everyone else here.

Nautical

Hey no problem with thread hijacking as some of my best information has come from those high jackings.I'm still a dumb bunny and new to this stuff so I need all the help I can get. I don't know what I need so could miss things, there fore if you all tell me stuff I am better off.

Lavina