It would depend I suppose on how warm you want the greenhouse kept and with a thermostat I dont see why you would need to go smaller. If its really cold out the heater wont have to work as hard to maintain your preference in temperature

Determining the size of heater needed depends on the size of greenhouse (square feet of surface area), covering material (polyethylene, polycarbonate, double layer,twin wall, etc) lowest desired inside temp. and lowest outside temp you want to use. With those numbers, it is relatively simple to calculate btu requirement.

heebiejeebie,

Well, what can I say?

""I've done the research and I've made up my mind already.""

Your research should have told you what size heater you need, depending on your required minimum inside temp, your expected lowest temp in your area and the insulation value of your greenhouse.

Your research determined you purchase a 25k btu unvented propane heater.

Your research also told you the clearances around the heater you chose.

In a 6x8 where are you going to put it?

dear chris,

thank you for being incredibly unhelpful to this post.

Good points, tho.

You are right, a thermostat should theoretically keep your temp relative even, but with an oversized heater you have the problem of hysteresis. Hysteresis can be metallic, electric, or magnetic. It is when you apply a force or heat or current or magnetic field to an object, then remove the force and the object goes back to its original state...but not quite. Nathan can give us more information on the physics perhaps, but basically what will happen is that an oversized heater will cycle on and off too rapidly or too frequently, causing premature failure of either the metals or electrical components. It is one of the reasons you don't put your thermostat too close to your heater.

heebiejeebie:

If you haven't yet used the heater and think you may have bought one too large and can return it, do check out the easy to use calculator on the ACF website before you make your final decision.

It's designed for people who are NOT physics majors, but proved to be highly accurate.

I used it to calculate my heating needs in a 12 x 16 Cross Country GH, and when the H/AC pro who installed our new home system advised us about which heater to buy, he came up with the same BTU recommendation.

Unvented gas heaters, regardless if natural or propane gas labeled safe for use in a home or garage should also be safe in a GH and are relatively cheap.

Their only real drawback is the need to assure some fresh air in the heated space, so unless you have sealed your GH like a tomb, you should be fine.

chris

take a chill pill!!!!!!!!!!!!!!

"incredibly unhelpful"

I gave you the information you needed. On the OTHER thread I did the numbers for you.

The straight answer is that heater is about FIVE times larger than you need, and because of the space you have to leave in front of it empty, you will lose half your greenhouse.

stressbaby is correct, an over-sized heater will 'hunt' as the gain is too high. The reason this is bad is because every change of state (on to off say) damages the heater due to differential expansion (some parts shrink more as they cool, putting stress on joins). Electric heaters don't have this problem. If you are going to use a heater in your greenhouse (as I've said elsewhere, I don't believe they are a good idea, especially for hobbiests) you really should work out your maximum heating power requirement. An oversized fuel based heater will be more inefficient and potentially cause more problems due to incomplete combustion (such as ethylene production).

I don't really mind if the heater wears down quickly. I can afford to buy a new one if that happens.

Ventfree seems to be the only option for me. Electric is too expensive in our area, I'm not willing to pay $600+ for a vented heater (twice the cost of the greenhouse!). Furthermore, I won't mind TOO much if the plants die. I doubt the heater will kill ME, and I'll install a carbonmonoxide detecter. If the thing explodes, well...

Then we'll talk.

Home Depot has already shipped the thing. I'm going to look at it. I think I'll like it. If not, I'll sell it on ebay, and probably make a profit.

Well, whatever happens, be sure you post your experience in our survey/poll/study! Good luck. SB

PS - in terms of taking up too much space, the thing is about 2 feet by 2 feet and a few inches deep.
Yes, I know you have to keep the area in front of it open. I was planning on using that area for a walkway, anyways. In the months I use it the only things I'll be growing are seedlings. I don't overwinter very many plants. Later I will take out the heater and grow more summer vegetables in there. And get a fan.

Do fans create as much drama as heaters do?

HeebieJeebie, you're just trying to get us all in trouble, aren't you? LOL

Yesssss Heebie...oh gosh dont mention fans or water, that is worse. LOL

This best thing to do here is take the boys advice, they are telling you the truth. Take the baby back and trade it for something smaller. You dont need this big. Just do it and you will be happier later. They are just trying to help ya, in a crazy sort of way.

The most useful advice I can give regarding fans is to pick the biggest fan you can get, because fan efficiency is sort of proportional to the length of the blades (a 12" fan blade is 12% efficient, ish). Obviously this diverges for blade lengths of 100 inches :)

Regarding water, I only use water collected from a special mountain stream on a full moon near easter using a pure silver jug polished with yaks milk by a tibetan monk. I suggest you do the same.

orchiddude,

I have changed my opinion and attitude. I now fully support heebiejeebie's choice of a heater.

It is his choice, I did what I could.

As to fans, I have a slightly used 30in 1/3hp fan I am willing to sell for five bucks, (you pay shipping) but only if someone installs it in a HFGH 6x8.

(really glad no small animals, plants or humans were injured in the making of this thread...)

heebiejeebie,

Pity you bought a blue flame heater though, a radiant in there would be a lot more fun.

RE: Fan blades. Based on Ohio State University research, "fans should not be purchased or evaluated strictly by the fan blade diameter. Air delivery of 39 different commercially available fans showed that air delivery varied from 6400 to 10000 CFM."

Always expect some research source from individuals posting information.

http://ohioline.osu.edu/aex-fact/0112.html

cuestaroble,

I thought the way you work out the size of the fan(s) rewuired was to do all the complicated math and know exactly what was required. Then you buy any one that fits in the hole you made when it is on sale.

Aw drat, nothing on TV... bored....

Heebie, how much are you paying per KWh for electric and how much per gallon will you have to pay for propane?

Chris,
My only point was that random thoughts concerning fan efficiency and size should be based on some applied research. However, after considering your more practical ideas, I have decided to espouse your point of view. If I may paraphrase, " cut a hole and put in a fan to fit". For the hobby grower among us, perhaps that is a good tactic.

cuestaroble,

I did it the scientific way, I calculated, and calculated, I waited until the cheap fans were on sale, got them, installed them.

When they turn on is it right that the greenhouse door slams shut, then your ears pop?

Seems to work for me now, as soon as the door moves I stick my fingers in my ears and yawn....

P.S we are doing water tomorrow orchiddude,

chris, that's why you should have an airlock into your greenhouse. That way it can double as a hyperbaric chamber for use after diving in your thermal storage pond. Nothing on TV here either (apart from the cat, who likes the viewpoint).

LOL...

Ok, so lets recap what the h3II is going on.

heebiejeebie bought the WRONG size heater. DO you understand that heebiejeebie?

heebiejeebie wants to burn the place down! Do you understand that heebiejeebie.

Chris wants to blame me if you burn your greenhouse up, do you understand that heebiejeebie.

I am not taking no part in any of this cause yall dont understand the rules. You got that everyone!

So bring the damn thing back to the store and get something for your size greenhouse. In a 6x8 Gh, and a 25btu heater, your going to blow the freaking yard up!

Get a heater at walmart for $28 and use that.

IF yall cant play by the rules, please dont listen to me.

From now on, do not LISTEN TO ORCHIDDUDE

heebiejeebie I bought a 10K BTU heater for my tall 10x12 greenhouse, which is about 2x the size heater I need for the volume, but it's the smallest I could find with an automatic thermostat.

I'd guess your heater is 5 or 6 times bigger than you want. If it is a true Vent-free unit (I didn't look at the link) then before all the oxygen is used-up in your GH the thing will turn-off by itself via the Oxygen Depletion Sensor. So my expectation is that unless you run it at its lowest setting, and even possibly then, it is going to short-cycle on either-or-both of the t-stat and the ODS.

If it doesn't work there, maybe you have a three-stall garage you could use it in! ;)

Geting serious here....

heebiejeebie,

I asked you to tell us how much you pay per KWh for electricity and how much you would have to pay for propane.

OK, I won't run the "numbers" for you, (you don't want me to) but I am sure others here will.

When you factor in the cost of installation of whatever heater is best, the running costs, the maintanence costs, the availability of the correct sized heater for your needs, etc.. I have a gut feeling the electric option may be not as bad as you think.

"what if the power goes out" you ask.

Well depending where you live, if you have enough thermal storage in there you can ride through a couple of days without power. (we are talking plant survival, not what the plant wants) Any longer than that then you won't really care about the greenhouse, your comfort and survival will be first on your list of priorities.

Post your cost per KWh and cost per gallon of propane and the others will help.

orchiddude,

You said it!

""IF yall cant play by the rules, please dont listen to me.""

We need to establish the rules for unvented. I know them, you know them, others know them.

You make them work, others make them work, I know I can install one (well I need three, but that is another story) and I know I can operate them and be safe for both me and my plants.

However can we legally specify what a rookie needs? I mean there are so many variables.

You want to risk it?

chris

I think you just stepped over into the Twilight Zone.

However can we legally specify what a rookie needs? I mean there are so many variables.
You want to risk it?

#1. Anybody who reads anything on forums, must take the information they get and do what they will with it. If a dump ass takes my advise and then blows himself/herself up, its not my fault. I give advise based on my experience. I think my experience counts a little because I have been honestly doing what I preach. So, I think we need to step back into the real world and get on with finding out what heeibeeijeei has done, but I suspect that we will never hear from them again. LOL

I cant figure out why people run and hide when we start these little talks. I mean this all about good clean fun, right! I mean, nobodys mad at anyone right? Just good ole clean talk about good ole gas heaters.

You know, I was just thinking, I need to copy all this stuff cause next year when we tackle this subject again, I can just copy and paste. LOL

This is really a great forum. I love the greenhouse forum. Great bunch of folks, even if Nathan has to prove everyday that he has a PhD....LOL ( I am joking with you Nathan )

Right, Orchiddude. People, don't run and hide! I'm giving you the chance to help me build the gun to shoot MYSELF in the foot on the other thread! Nobody's coming to the party! To tempt you, I'll tell you that I have a new analogy over there.

I'm in zone 4, have a glass 8 by 12 foot greenhouse, not sealed well.

I have a 30,000 vent free propane heater,that I only use
last week in March through middle of April for seedlings.

During the day, if the sun isn't out, it usually stays on pilot.

It does come on a lot in the night. It really doesn't blow heat out far from the heater, it goes almost straight up so I don't have to worry that much about it being too close to the plants.

I have to put it about 1/3 of the way towards one end, otherwise the rising heat makes my auto vent open up. I usually use about one 20lb. tank every 2 days in March, then it begins to last longer when it gets into April.

The longest it lasts is 4 days, and then I usually turn it off during the daytime, and relight at night.

It keeps my gh at just about 55 in the night, not much more. The pilot stays on at around 50 or higher.

I try and rotate 3 tanks so I don't run out. I don't bother with large tanks since I only heat it for 3 weeks or so. I put my orchids in it for the spring-fall.

I'm posting this so you can have an idea of what you might be looking at, although your house is half the size of mine, maybe this will be helpful.

M

Chris,

30 inch fan. Omgz. I'll have the most bling-bling g-house in the ghetto omg! Yes, I live in a bad part of town. No, I am not growing illegal things. Now that I think of it: propane+the ghetto+meddling neighbors=boom

The heater.... was an impulse buy. Ok. I admitted it.

Like a good girl, I ran to my daddy (who posts here as justray) and he convinced me in a much more loving and convincing way, that yes, I would blow up my neighborhood.

Can't return it (internet purchase), so I'm going to try to sell it on ebay, maybe for a profit.

Chris, you impaitently asked me twice how much electric costs. I don't know. I have a LIFE outside of greenhouses. I don't feel like finding the bill right now. Maybe I'll look later to provide amusement so you guys can run the numbers.

I'm about to throw up my hands and start the seedlings under lights, inside, like I did last year, where it is WARM. And let the G-house sit idle.

Heebie,

Despite suffering some collateral damage, you got good advice...now relax and do your own thing.

Some of us want to make this GH thing out to be rocket science when it isn't.

Did you see my message about the T-bolts?

SB

Yeah I just saw it. Thanks so much! I'll be needing about 10 that would hold on 1in insulation, I think I've found some bolts at home depot that would work.