My 1700 sq ft costs about $400 per year to heat to 40F. That's with ave outside of about 30F. Since cost is proportional to the average difference in inside vs outside, it would cost roughly $800 and $1200 to heat to 50 and 60F respectively.

It's your goal of 60F that gets expensive.

Our temperatures were bitterly cold
Cost for natural gas heater for the period December 18th to January 18th - approximately $100. - heated to 50F (min heat setting) but got warmer on sunny days.

Cost without knowing size doesn't help much. All other things equal cost will be at least somewhat proportional to size. Although I suspect that a greenhouse 10% the size of mine would cost more than 10% as much to heat. The reason being the smaller greenhouse will have a larger surface area from which to lose heat.

This post was edited by fruitnut on Wed, Jan 29, 14 at 21:29

My greenhouse is a palram single layer polycarb - 6x8 - approximately 324 cubic feet.
I've read a lot of posts here referring to the gh getting colder than it is outside - mine (presently unheated) was .8F colder than the outside this a.m.- I suspect it was just because it wasn't keeping up with the outside temperature rising.And just that.

Forgot to mention that very few sites recommend heaters by the square foot and that is why I used that particular reference.

Last year I heated the twinwall part (400 sq.ft.) for about $100, mostly firewood. This year I added 200 sq.ft. of leaky used window glass glazing and the heating may be over $400.

So to answer some of your questions rht123...

If you already have a large propane tank, it would be cheapest to run a line from that to the GH and use that fuel. Of course you will need to refill more frequently, but generally big tank propane runs less than the tiny BBQ tanks pricing. You may want to consider getting a second big tank, so that you can fill it during the summer when prices are cheaper.

In theory, if you have thermal mass, that will give off its heat at night until the GH gets down to the temp you've set your heater thermostat to. Then the heater will come on and keep everything at that temp. So you won't be heating the thermal mass with the heater. But in cold, cloudy stretches, that thermal mass won't be helping much either. And if you ever let the GH and thermal mass get below that thermostat temp, then your heater will be bringing that thermal mass back up to temp too.

It is best to use some of the more sophisticated heating estimators. Ones that ask you for not only the floor area, but the glazed area and type of glazing (or R-value of the glazing). If you know those numbers, and the temp difference between inside and outside, you should be able to get a pretty good estimate of the needed BTU's. Then if you know your heater efficiency and the price you pay for fuel and the fuel's BTUs, you can get an accurate estimate.

I love this one. It's what I use for budgeting.

Here is a link that might be useful: My favorite greenhouse heat calculator

Sandy:

I like that heat loss calculator. According to it the heating cost is proportional to the surface area of the greenhouse, not square footage.

And of course heating cost is directly proportional to the average difference in inside vs outside temperature.

I'd agree, it looks correct. I put in the numbers for my new GH, and it came up with approximately the same number of BTUs that I had calculated manually.

Well, that sure does help, especially the calculator. I couldn't really provide a surface area because all that was still up in the air but this thread really gave me some direction. Thanks for all your help.

As long as you are not running your heat through your heat sink/thermal mass, you should be fine. Thermal mass keeps things a steady temperature so while it may not "heat" your greenhouse" on cloudy days, it does help maintain temperatures. If you were to run your heat through the heat sink you would be directly heating your thermal mass and cheating the air of heat. As already stated, heating thermal mass is not efficient. However, heating the air of a greenhouse that has lots of thermal mass is a lot more efficient than just heating more open air and air that is adjacent to more outdoor surface area.

I have a berm on my north wall to limit the above grade surface area of the greenhouse. I also have an indoor berm of sorts in the form of terraced raised beds. The thermal mass and excellent thermal envelope has kept the temperatures steady in New England winter weather with no heat other than the sun. I just discovered a door left open a crack by a construction worker (!) and the greenhouse temperature is 50 degrees and the outdoor temp is in the teens! Luckily, I have no plants in there yet. -Still painting and our Subterranean Heat and Cooling System is not running yet. But I am pleased with the performance with just Passive Solar design working for me. So far, it has cost me nothing to heat but I did invest in building a greenhouse with R28 walls, R42 ceiling, triple pane windows, double pane skylights, below grade design w/ berm to take advantage of the earth's steady temperature and a SHCS.

A good resource for building is www.GreenBuildingAdvisor.com. They don't have anything specific to greenhouses, however, they have all kinds of information about thermal mass, air sealing, insulation, high performance windows, Passive Solar, etc. They have great forums to ask questions too.

Impressive results Curlygirl! I'm also a big fan of the no heat/cold climate frame of mind. Sounds like you have really done your homework. Congratulations!

Thanks so much, Karin_mt! I am still holding my breath a bit since I was hoping to grow some temperature-sensitive fruit trees and I won't know how the heat sink of the SHCS performs for another year+ but I am pleased so far.

I have long held the same view. Having a greenhouse that can maintain temperatures without a lot of technology and fuel is the way to go. I could not imagine the pain of losing all my fruit trees do to an extended power outage in the winter or if fossil fuel prices get so high that I can no longer afford to heat my greenhouse and then have to let fruit trees die. Of course, I could use the greenhouse for season extension only but I'd still have to deal with temperature volatility.

Can't wait to get my fruit trees! Having a lush greenhouse environment in the northeast this time of year would do wonders for my state of mind! :)

Do you have a greenhouse or plan to build one?

Agreed on all counts CurlyGirl!

For me, the thing that appeals most about the GH is the uber-efficiency of growing our own food right here at home. Everything we do is geared toward maximizing food without a major energy outlay. To me, personally, heating a GH all winter defeats the purpose of being efficient so I've never been interested in that pathway. I can see why folks do it, so I'm not passing judgement, but it's not for us.

Yes, we do have our GH already, a Riga in Zone 4 Montana. We get by with a nicely insulated foundation, stacks of 5 gallon water buckets for thermal mass, and a predominance of sunshine in the winter. I also make a hoophouse within the greenhouse using metal hoops and many layers of fleecy frost blankets. This has allowed us to keep lettuce alive even in this cold winter. (Although I brought most of the lettuce inside during our recent dip to -27, but amazingly, even the lettuce left in the GH survived just fine.)

The next thing I'd like to do efficiency-wise is vent excess GH heat into the house in the winter. So often the roof vents are open and perfectly good 85-degree air is venting to the atmosphere. I would love to have that warmth coming into the house instead. So I'd like to figure out a way to do that before next winter.

What kind of fruit trees are you getting? How exciting!

Venting into your house sounds like a great idea! I have heard there can be problems with it but it sounds like your situation will work very well. I like the idea of harvesting the wasted heat and then sealing the house off from it when it gets cold. -That is really efficient!

We are going to see what we can get away with growing in our greenhouse. At the very least we can grow citrus but I'd like to grow mangos, avocados, miracle fruit, carambola, ice cream bean, cacao, cinnamon, black pepper, etc. We'll also do seed starting for our outdoor vegetable garden. We built a root cellar along with the greenhouse so that we can store some vegetables and fruits for long periods of time -some all the way into the spring. Do you have a root cellar? It is a great way to store some appropriate crops for long periods of time with no energy.

You are lucky to have so much sun out in Montana. In Massachusetts we have a lot of cloud cover and we have a lot of trees that limit the low winter light we do get.

Good luck with your getting your excess heat venting into your house for next winter!

Sincerely,
Curly Girl

Thanks Curly Girl - I am looking forward to following your progress as you settle in to your new setup.

We've tried a couple of different configurations of jury-rigged root cellars and we've settled on putting coolers in the unheated part of the garage. Seems to work just fine. Once it warms up in March we don't have much left so we move it into the fridge at that point.

Hi I need help. I want to rent a greenhouse, but can't figure out the cost. The floor area is 200,000sqft. Suppose min temperature is 2c. How much will it cost annual? $500,000? And how many heaters needed? 30? Then cost for heater will be ?75,000(suppose $2,500 for each)?

The greenhouse use propane for heating.

My greenhouse heating costs vary by 5 fold between extreme years. Although I use firewood to heat with portable propane backup the weather determines extent of BTUs needed to keep crops going. One year I might get by burning less than a chord of wood and on another year I'll burn 4 chords and still need 6-8 propane tanks filled to get through the Spring months (April - May). I still haven't figured out why on some nights a 160,000 BTU stove will easily keep my greenhouse 40*F above outside temps , yet on another night I can barely manage to keep the same greenhouse 10*F above outside temps.

I question if all that attention to detail really matters as long as you keep vegetable plants alive. With my early tomatoes I might strive to keep temps at night above 60*F but if temps drop to 40*F for an hour every few nights I get the same outcome. I get far better results relying on cheaply generated heat and circulation fans than trusting in extra covers or heat sinks. At least I know that there are no cold spots and I won't run out if heat when it is needed most.

Heating needs are not just determined by the inside/outside temperature differential but by windchill as well. Any warmer than air temperature object will cool faster (thing of blowing on a spoon of hot soup) with an increase of air hitting it. The higher the insulation rating (windows are insulators, just not very good at it) the slower the heat loss. Also in windy conditions, more cold air leaks into the heated envelope than on calm days.