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how do you heat your greenhouse

Posted by thinkdirt z5ohio (My Page) on
Thu, Jan 25, 07 at 17:01

Hi everyone,

I live in Ohio and I recently purchased a new greenhouse 10 X 14 walls made out of waffle plastic panels, it is quite different from what I am used to. I currently have 2 heaters in it and it doesn't seem to keep the temperatures quite as consistent as I would like. And of course DH is starting to complain about the electric bill going up.

I was considering looking into a commercial type heater, but I am not sure what to look for. Please give me your opinions on varies types that you use.

JoAnn


Follow-Up Postings:

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RE: how do you heat your greenhouse

thinkdirt,
If you can provide more information on your conditions- minimum outside temperature, desired inside temperature, and if your "waffle plastic panels" are 4 mm twinwall polycarbonate (such as with the harbor freight gh), then your heating requirement can be calculated. If your panels are indeed 4 mm twinwall polycarbonate, and just guessing at a 10 deg. outside temp and a 50 deg. inside temp., then you need about 13000 btu/hr heater. The type of heater depends on your preference. Most 110 volt electric heaters produce about 5120 btu/hr, so you would need 3 , under the conditions estimated. Another option would be propane/natural gas heaters, but the required btu/hour would be the same.


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RE: how do you heat your greenhouse

I have larger grhouses ( 25x65, and 25x100) but I use the vented forced hot air propane heaters. I am not sure if you can use a forced hot air in a smaller grhouse, but I love them because I can use the hot air to germinate the seeds and cuttings. When the thermostat (set up halfway down the grhouse) is at 60-it is usually about 70 or more under the seeds. They are also very easy to hang on 4x4 beams about 12" off the ground. The BTUS of the heaters are 100k and 200k


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RE: how do you heat your greenhouse

thinkdirt,

If I chime in now I might be pre-empting folks like Orchiddude, but you might also want to consider a ventless propane heater. There's an interesting thread or two started recently by MollyD about them. On the vented side, there are some smaller propane/natural gas forced air heaters that pump out 20k and 30k btus. They sound to be appropriate for your gh.

I had also looked into building a waste veggie oil burner for heating the gh but 1) I'm not that good with an arc welder and 2) they're so variable in how much heat they put out that I can't imagine using them for primary heat.

Electric is going to be the most expensive option in the long run but probably the lowest maintenance. Granted, the biggest maintenance I have is making sure my propane tank is full, but that's still more than *click* *heat*.

You could try wood, even... if you like running out and stoking the fire when it's 20 below outside. I imagine it could be an effective secondary heat option, mitigating the need for a propane or electric heater to fire up for at least a few hours into the night.

Hope this helps!


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RE: how do you heat your greenhouse

This is my first year with a greenhouse in zone4. I started out with a electric heater when I did not need much. I changed to a propane one but it did not have a thermostat on it so it got to hot and all the heat went to the ceiling. I have a 12X18 Now i have a smaller propane with a thermostat. I also have a floor stand fan running right in front of it to draw the heat midway instead of it all going to the ceiling. I also put in a sheet of plastic where the roof starts to keep more heat down where the plants r. I have a small electric heater in case something should happen with propane....like running out....but it has not had to run yet. I keep my greenhouse at about 62-75. It varies. I have orchids in it and i have more than one in bloom at the moment and a lot more with buds. All seems happy. I did put a solar pool cover over the top but the wind has blown it half way back so it is not doing much and all it did when it did cover was block a lot of light.
This is my first winter. I am learning a lot and hopefully next winter will be more prepared and will be able to keep it warm cheaper. oh yea. The downside. It is costing me around $190 a month for my propane but I love my orchids and think it is worth it. What helps is I live with my son and DIL and so I dont really have any bills to worry bout so can afford to do this.
I also have a tank of water which is very cold(bad idea there)with a fountain to help with humidity and also a humidifier which I fill twice daily but it is keeping my humidity high enough for the orchids to survive and be happy.
I dont know if this is helping any but a propane heater also has a shutoff switch in case of carbon monoxide. Do not get a propane heater without a thermostat or the shut off switch.


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RE: how do you heat your greenhouse

My experience may be of limited use, since I have just a 6x8 greenhouse, but I'll share for what it's worth, especially after last night's 9-degrees (unusual here) and high winds.

I use a 1500-watt convection heater. The greenhouse walls are 4mm twinwall polycarb with 1" foil-faced foam insulation board on the north wall and north third of the east and west walls. All panels are edged with foam weatherstripping tape and every gap in the frame (that I could find) is filled with caulk. There's insulation (plus a hardware cloth critter barrier) around the perimeter where foundation meets ground (these are hidden by the gravel). The whole thing is wrapped in a clear solar pool cover, except the door, of course, but there are strips of that same material hangning just inside the door (like you'd see in a commercial cold-storage room) ... not pretty and a little awkward if your hands are full, but it's surprising how much it helps. There are also several gallons of water tucked inside for thermal mass.

We'd hit a low of 9.5 by 5:00 a.m. and the greenhouse was 42.3. It's to be a little warmer tonight (17 degrees), and not so windy, but I draped a string of outdoor Christmas lights around the most tender plants ... so I can sleep tonight, lol.

I hope you're getting some helpful information. We've found the pleasure and fresh produce from the greenhouse well worth the investment of resources; I hope your husband soon feels the same way. (Mine isn't a gardener, but he's hooked anyway, sometimes retreating out there just to sit and read.)

Diane


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RE: how do you heat your greenhouse

Oh how nice you all or to try to help me, what great info.

Custaroble, you are right my Hubby bought the one from Harbor freight, so I quess i will probably need three heaters and let the eletric bill soar LOL, only have to add one more Lol.

I think I really need to study this some more what great advise from everyone. If I can keep this greenhouse at about 50, I will get my seeds started soon.

JoAnn


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RE: how do you heat your greenhouse

Are you sure it's 10 x 14? Harbor Freight's biggest model is 10 x 12. If 10 x 12 is indeed the size you have, here are some numbers to help you size the heater correctly:

The dimensions of your wall and ceiling space gives a square footage value of 554.

The lowest temperature rating for USDA Zone 5a is -20F. If you need to keep the inside temperature at 50F, that's a temperature differential of 70 degrees on the worst nights. I'm pretty sure Harbor Freight uses 6mm twinwall, so that would require 25,207 BTUs to heat at the desired differential. A 1500-watt heater only puts out 5,120 BTUs, so running three heaters leaves you short about 10,000 BTUs. 15,000 BTUs would only get you about +44 degrees, or about 22F when it's -20F outside.

Here's another option. You're not going to get any appreciable light through your north wall, or the NW corner. If you insulate those sides well, you reduce the square footage of the panels experiencing heat-loss essentially to about 426 square feet. Running the same numbers as above, it now would require 19,383 BTUs on the worst nights. Your three electric heaters would now give you a temperature differential of +55 degrees, so you could expect an inside temperature of 35F when it's -20F outside.

If you think -20 is a bit low, then you can adjust the minimum temps upwards. Also, if you're only using the greenhouse as a cold frame, it's possible that you would be starting seeds after the worst of your winter is over.

Another issue. I'm not an electrician, but I think that three 1500-watt heaters would dangerously overload a single circuit. Something to check out.

Hope this helps.

Bruce C.


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RE: how do you heat your greenhouse

Whoops. I have a HF 10X12. I guess that was wishful thinking of me mentioning a 12X18. One thing I will do next year is put the bubble wrap in. The kind made for greenhouses. U use it year around if u want. I know it is not cheap but I think it would be worth it.
We also caulked everything but I believe that HFs greenhouses r not really made for the North country. U have to do what u can to make it more cold air proof. I also have plastic where my doors r. Inside and out. It is a little hard at first to get in and out but once u get the idea and use clothespins to keep plastic closed it works great. The end of my greenhouse faces the end of the garage. My son used glass windows to enclose it so that no cold wind could get in either.
By next winter I will have some changes made and hopefully it will help. The plastic that u put for a ceiling also helps a little bit and every thing u can do to help keep it warm without costing a fortune is worth the extra money. Plastic is basically cheap. I used painters drop plastic.
Hope u get more ideas so hopefully i can use them also.


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RE: how do you heat your greenhouse

BC, What great technical info, i am going to print that post out for future reference. Thank you.

Puffymom, thanks so much for your info and I think you are right about HFGH in the north LOL. It sure is an adjustment to what I had before but eventually I think we will get it.

I am thinking about trying Drop ceiling strofoam panels to insulate the inside walls. Just a thought. The plastic drop cloths on the roof have blown off twice already. I have tried clothespins also and a little wind pops all the pins off LOL.

Will keep trying until I get it right.

JoAnn


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RE: how do you heat your greenhouse

I don't know if drop ceiling panels would be a good choice, as most are made of a very pourous material, so would tend to absorb water and might also be a breeding ground for mold and mildew. There are several types of foam panels available that would be better for this application and they're not expensive. I selected the 1" foil-faced foam panels since the foil facing reflects light back into the greenhouse during the daytime and (hopefully) reduces radiant heat loss at night and on cloudy days.


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RE: how do you heat your greenhouse

Another option is a natural gas fired infrared radiant tube heater. They come as small as 40K BTU input which would give you about the right heat capacity.

http://www.rg-inc.com/pdf/Tech-Sheet/Vantage-II-tech-Spec-Rev F.pdf


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RE: how do you heat your greenhouse

There is a great post somewhere on GW (can anyone put a link to it?) about why infrared heaters are not as desireable as blue flame. Seems the infrared heats up the plants and anything in the greenhouse before it can heat the air so your plants can be hurt in the process. The blueflame heats the air first. Others can explain the science of it better than I can.
I now have the blueflame heater and my plants seem happy.

As an aside I've been using an electric heater as my backup and this week discovered that it was my weakest link so to speak when I found myself relying on it and then the power failed. I've been wondering about the effects of kerosene on plants and the use of it as a backup heater.

MollyD


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RE: how do you heat your greenhouse

Well, After this extreme cold spell in Ohio and WoW is it cold.

I insulated the walls with 1" thick Styrofoam and what a difference it made, my heaters are now doing the job and I am about to start some of my seeds.

Thanks everyone for the info.

By the way the temp today is 17 degrees and the GH is toasty about 75, I guess now I have to start opening the vents to cool it down during the day when the Sun is shining
LOL.

JoAnn


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RE: how do you heat your greenhouse

In the sage words of chris from iowa: If you're heating at night and cooling during the day, you need to rethink your design...


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RE: how do you heat your greenhouse

In my PA microclimate I can only use kero to afforably heat use GH as a cold frame. (we hit the 5a zone this past week)
I wanted to mention the company Farmtek.com, they have bubble panels for insulation as well as allows the light in.
I am really considering these for earlier use of passive solar with the supplemental heat.


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RE: how do you heat your greenhouse

I know this is an old thread but if anyone is checking, I am just wondering if anyone has tried using compost in a small gh to supplement or provide passive heat? I'm thinking composting straw, etc near the 3' knee wall. Can't stand there anyway, wondering about putting a work bench over top of 3x3x(anything over)3' long boxes. I'm thinking I will incorporate lift open doors in the knee wall on the outside of the house. Heating is more of an issue here than cooling and I figure I can remove the composting material as needed. Old style hot frames (very similar to cold frames) were heated this way except that crops were grown right on top of composting materials. I'm not trying for year round plants but would like to use it about 9 months of the year.
Thanks,
June
PS: I've also thought that I might make one section that opens on the top from the inside. It should be a good place to start seeds.


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RE: how do you heat your greenhouse

I wouldn't be willing to give up any GH space for compost. I don't think it would keep my GH warm enough on cold zone 6 nights.


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RE: how do you heat your greenhouse

junembe,
""I am just wondering if anyone has tried using compost in a small gh to supplement or provide passive heat?""

Somewhere on here is a thread where someone built a hoophouse on a knee wall of straw bales, ran several hundred feet of 1 inch (I think ) of water pipe inside it and then dumped a few feet of straw on the ground inside it.

As the straw composted it heated the greenhouse AND they got hot water too!

If I could find it I would post it, honest!

I did a few searches, and found so many heating catfights it made me smile....

In all these years nobody has perfected it.

:)


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RE: how do you heat your greenhouse

"In all these years nobody has perfected it."

Yes they did Chris. The Romans figured it out over 3,000 years ago and their design is still the best heating method of all: Radiant. Warm the floor, and the rest follows.


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RE: how do you heat your greenhouse

If your greenhouse has a decent height you may want to consider blowing the heat from the top down to the floor. You can either attach a normal greenhouse fan high on one side and angle down towards the other side, or get one of the types that hang from the ceiling and blow directly down. Making a more even temperature in the greenhouse may save some heating costs.

Here is a link that might be useful: StratoJet


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RE: how do you heat your greenhouse

The comment about the Romans caught my eye, and I'd like to know if anyone here has experience with radiant hot water heating in a greenhouse floor. We put a system in a log cabin that my husband just built and have been very happy with it there, so I'm considering using radiant floor heat in a greenhouse -- a project that's currently in the planning stage. The size of the GH will probably be about 26' x 40'. (It gets bigger the longer I plan.)


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RE: how do you heat your greenhouse

karen: There is a typo in my post. The Roman's built most of their vast array of radiantly heated baths during imperial times, so it should read 2,000 years, not 3,000.

But the fact remains that the proof of the effeciency of heated floors has been around for a very long time. However, as effecient (and clean) as it is, it's also pricey, although in the long run, cheaper to operate and as fuel costs go through the roof, any additional up-front costs to affect less use of any fuel, the shorter the pay- back time becomes.

As long as the building is properly insulated, there is no reason why heated floors would not be as effective and efficient in a GH as it is in any other structure and if the Romans could have circulated hot water or oil under their floors, instead of hot air, they would have. They understood the concept of warm air rising and had the engineering knowhow to build radiant heated buildings, as well as plenty of labor to keep the fires stoked. What they lacked, and we have aplenty- were circulating water/oil pumps.


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RE: how do you heat your greenhouse

I've seen a huge greenhouse used to grow and maintain tropical plants using crushed stones for a floor with radiant heating. They used forced hot water running through plastic flexible tubes, and it seemed to work great. They kept a high temperature and had no problem. Although, the greenhouse used insulated glass, so it kept the heat in very well.


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RE: how do you heat your greenhouse

You can see some pics of instalation a radiant floor in my BC Greenhouse 12 x 20.

Here is a link that might be useful: radiant heating


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RE: how do you heat your greenhouse

Thanks for all the comments.

Hemerocallia, I found your greenhouse installation photos earlier today when I was searching on BC Greenhouses. It looks very much like what we put in the cabin. Please forgive me if I ask a question that's explained on your website, but it's been a long time since I studied French. What temps do you get with the system, and do you have a supplemental heater for the coldest periods? I talked with BC Greenhouses yesterday (and now anxiously waiting for a quote) -- they said that in floor radiant heat works well, but the floor would be too hot for comfort if I tried using it without a supplemental heater in our coldest weather. I see that you poured a slab, but I'm leaning toward gravel/crushed stone, as noted by Greenhouseman. I'm planning on double paned glass and will spring for low E if I can afford it.


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RE: how do you heat your greenhouse

Hi Karen,
I live in zone 4. The winter temperature is very cold. We have many days under -20 f.

The night I keep the Greenhouseat at 50 F.and the day at 70F. I maintain the floor temperature at 105 F. (it's very comfortable) We need a heater for the extreme temperature below -20 F. Also the heater remove the excess humidity


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RE: how do you heat your greenhouse

Thanks, that's exactly the kind of information I've been looking for.
Karen


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RE: how do you heat your greenhouse

Haven't heard much mention of passive solar, but what about rock walls for solar passive heat? We are still designing, but are planning to do rock walls on the house wall and north wall (solid insulated). Split fielstone that is half rocks is rather expensive, but I've just gotten a new idea from seeing a fire station's exterior walls. they're pea gravel pushed into mortar. so my idea was to do this and then have clusters of fieldstones here and there to save money. the other solar heat idea is to have a fieldstone bin of rocks (made with cattle panels or some other strong wire mesh) under the work bench. the rocks are free since I live in farm country. the third idea which is probably exclusive to my design (but you never know) is to use unused heat loss from my woodstove. Our woodstove in the house is directly on the inside wall of where the greenhouse will be. A box called a "Magic Heat" can be intalled into the flu pipe to vent excess unused heat. I will be piping this box out through the house wall into the greenhouse. the only disadvantage is that if you left the house for too many hours and your fire went out, then obviousy so would the heat. A portable electic heater is planned for backup. these are just some ideas for free heat, I hope they will work! Kat in Iowa


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RE: how do you heat your greenhouse

Karen, Here is a rough translation of Hemerocallia's website without the photos..John

After a few years of reflection and research we decided to build a greenhouse. You will find in this page the results of our research and experiments by hoping that can help you if ever you were tried by this adventure.


We chose a greenhouse (20 ' X 12 ' x8') in kit of company B.C. Greenhouse which has worked in the field for more than 50 years and whose comments on the various forums which I consulted were extremely eulogistic. We could note that their reputation was not overrated. I give them a note of 99 out of 100.

We will add our comments before long concerning the costs: Preparation of the ground, electric wiring, plumbing, pump, lighting, unit of heating, ventilation, table of culture. Obviously as it will be our first year will be necessary for you to have patience in spring at the cost of the heating.

The assembly of a greenhouse in kit requires very good manual provisions. People who as me have difficulty in go up a table from Ikea must be well surrounded. Moreover I would like to benefit from it to thank the large Andre for his invaluable assistance.


Here some photographs of the adventure .......

Beginning of work with May. Hiding of wire and the water pipe with 48 " (120 cm.) as well as the compaction of the ground with 30 tons of rock 0,3/4. (that takes a good pair of arm)


Reception of wood 6 X 6 (15 cm. X 15 cm). for the foundation. Wood is treated and I added a brown dyeing to him. As of the reception of wood you must install it as soon as possible because it will warp and wire with retordre will give you (with the André help) If you can allow it to you I advise you the purchase of cedar of the west which contains a natural condom.


The assembly of the structure is rather simple when you have a Andre:)


The greenhouse with the polycarbonate covering and installations of the windows of ventilation.

The greenhouse seen of with dimensions


Interior

I am supplemented the construction of the tables of culture as well as the installation of the shelves for sowings.


Installation of the heating erasing (warm water), but before we posed 3 '' (R15) of rigid insulator "blue" to prevent freezing from penetrating inside the greenhouse. To note the metal squaring on insulator for attaching the pipes well


652 "ty-raps" later and the ankylosés fingers the installation is finally is finished.


Junction towards the pump and the water-heater which we will install in September


Cardiac concrete ............. to abstain from. We shoveled and raked up 3 concrete rods I did not even have time to take a photograph. The floor with an average thickness of 4.5 ''

To attenuate the demarcations I will put a dyeing gray beads before the installation of sealing


Heating with warm water with its multiple valves and wiring.


Seen of close to the pump and heating water


On the left back up heater of 3000 Watts for the extreme temperatures (- 20 c)

On the right extractor fan when the temperature reaches 35 C.


Lighting system of 600 Watts to the MH


Electronic ballast for lighting.

This ballast releases little heat and uses 35% less electricity than the mechanical ballasts


Water tank which I use as thermal mass to maintain more constant a temperature


Count of culture which I covered with plastic to use it the made-to-order of a basin.

That will enable me to sprinkle the seedlings by bottom.


Small island power station with some plants


Switch for wet condition.


Electrical outlet for wet place


One of my many ventilator


The adventure continues ..........................................


Hémérocallement your!!!!!!!!!!!!!!!!


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RE: how do you heat your greenhouse

Build planting beds on continuous insulated elevated tables and install in the soil beds 1/2' PEX tubing connected to an instantaneous tankless water heater.It also helps if you can excavate into the foundation soils several feet for added insulation,especially if you are building on a southernly sloped hillside.This will allow for natural drainage.You can preheat this with a passive solar heater,too.Use a 1/6 hp 115v Taco pump with a Goldline electronic thermostat and 10k sensor located in the planting bed.This way you are only heating the plant roots and not the air space.This will encourage the plant growth better than heating the air.It is much more efficient as well.

Here is a link that might be useful: INDEKSOLAR POOL HEATING


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RE: how do you heat your greenhouse

Hi Thinkdirt. This may be a couple of years late but could be usefull this coming winter. I have had lots of greenhouses mostly the 100' type and I found one of the cheapest easiest ways to heat your favorite plants and save btu's is as simple as moving all your fav plants to the front of the house and drop a piece of poly from the ceiling to the floor just past your plants. I only heat the front 15' of my 125' house through the coldest part of the winter then gradully move the poly back as I fill the house. It's simple but it works. It also gives you a place behind the poly in the cold end for the potted plants you usually cover up outside for the winter. Good luck. Steve


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RE: how do you heat your greenhouse

I've used a ventfree propane heater for many years in my 14 x 20 greenhouse-with great results!!Here in georgia,I only use about 140 gallons of propane a year.I maintain about 50F at night.The ventfree is a very inexpensive one-lowes and home depot sell them.As a added bonus-it doesn't use any electricity-which comes in real handy around here when we have those ice storms that knock the power out for a week!!!Mark


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RE: how do you heat your greenhouse

Where can I find the calculation for BTU?

10x16
tempered glass
insulated concrete floor
Home-attached (north side of greenhouse is my house)
Zone 4-5 (min outside temp -10F)
Desired inside temp 55-60F


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RE: how do you heat your greenhouse

See other thread. Surface area is 600 freestanding, less one 16 x 5 side wall (against the house), so let's assume 520 sq ft.

-10 min temp
55 inside temp
single layer tempered glass R value of 0.88

=40104 BTU.

SB


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RE: how do you heat your greenhouse

I've been playing with a few of the calculators I've found and I appear to need somewhete between 24,000 and 30,000 BTUS....dependin on how you count the wall of my house that the greenhouse is attached to. Electric simply won;t cut it from what I see. I've just started to look at Propane heaters (Direct-vent, wall mounted, thermostat, no electrical required). Seems like once you cross the 24,5000BTU area, they all require electricity. Any advice on a good on-line source for propane heaters with these sorts of stats?


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RE: how do you heat your greenhouse

If you need 7-8kw of heating for a 160 sqft greenhouse.. improving the insulation should be at the top of the list ;)


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RE: how do you heat your greenhouse

My GH supplier is estimating around 30,000BTU. Not sure what you mean by 7-8 kw. My north wall is my heated/insulated house and I'd really prefer NOT to have bubble wrap hanging if I can avoid it. Insulate it how?


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RE: how do you heat your greenhouse

junembe, i have a small 6x6' gh and i was thinking along the same lines as you to use compost as a heat source. i think this fall i am going to dig up my dirt floor about a foot deep, put a layer of compost in and then backfill on top of it so i can just set my plants in there this winter. (it will be good to get some good exercise too!) it should keep the floor warm for a few months anyways and all the extra bubble wrap and tin foil i put in should help some too. I am trying to get by without using a supplemental heat source this winter because my mother would freak out if i drove up her electric bill. : ) As soon as i move out on my own though, my gh is coming with me and if this method works i wont need to ever use a heater. I like your idea about the bins though, it might save me a lot of digging. : )

cheers, Riley.


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RE: how do you heat your greenhouse

Im building a wood lean to greenhouse with double poly walls. I came up with a plan to heat it using half wine barrels with aquarium heaters. I also plan to insulate my floor using semicomposted horse manure and staw. I liked sunworksco idea about pex pipe through the soil so I will connect a pump and make a soil bed for my peppers and tomatoes. What do you guys think about this form of indirect heating. I will also use the wine barrels for aquaculture.


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RE: how do you heat your greenhouse

farminggreen:

Presuming your use of the term "aquaculture" means growing fish for the table, unless you live in an area that never drops below freezing, I would have grave reservations about using aquarium heaters to raise water temps high enough to radiate sufficient heat to serve as space heaters, without heating the water to a degree that would kill the fish.

Also, aquarium heaters are not really made to raise water temps much over 10 - 12 degrees above ambient air and why every Discus breeder I know has to use multiple heaters in their larger tanks, or risk having their 90 deg. loving fish keel over in winter, even with room temps in the 70s.

I suppose it could be done, but you could end up spending far more on the heaters and electricity to run them than if you simply installed a space heater in your GH.

So it's really a case of your expected air temps and the volume of warm but not hot enough to kill the fish water necessary to radiate enough heat to warm your GH.

However, also keep in mind that regardless if baseboard, old fashoned cast iron stand ups, or underfloor- radiant heat circulates hot water or oil in a closed system. Fish need as much open surface as possible, which also accelerates both evaporation and heat loss. The cooler the ambient air in relation to the tank, the faster it will evaporate and require topping off, and the more wattage to maintain the optimum temp. for the health of the fish.


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RE: how do you heat your greenhouse

Save yourself a heap on heating costs and use trout in winter, they`ll be very happy with 10C-15C water temps :)


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RE: how do you heat your greenhouse

I got a Handi-House 12x24 ft. hobby g/h with corrugated polycarbonate (not a twin-wall, but single-corrugated) walls and roof and wooden floors. Walls are 6'8" and the middle is 100". Wall-to-wall is almost 12x24 ft. a few inches short. I am in Z8A/B where temps are couple nights a year into high or mid teens and the rest are 20s F and up. My g/h gets a lot of shade from the tall oaks nearby in autumn/winter. Greenhouse Megastore suggested that I need a 5kW - 14000 BTU electric heater (MODINE), yet another place suggested 22000 BTU. I am confused. Could someone PLEASE suggest what would be my BTU requirement?

Also, my g/h manufactured by Handi-House who foolishly manufacture it with 20 Amp - 110 Volt receptacles and wiring. Now I must do a very expensive and hard upgrade to 30, 40 or 60 Amp wire and 240 Volt receptacles, because the 5kW Modine heater is 21,3 Amp, plus a couple lights and a fan in that greenhouse. I wish I knew all that before I purchased this greenhouse and before the electrician connected it to a 110V-20Amp service.


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RE: how do you heat your greenhouse

You'll need a sub-panel in the greenhouse to wire that heater into, but the rest of the wiring is fine. Do you just have one circuit coming into it now, and no panel? The other option for the big heater is to wire it directly into your house's panel. But if you already have several 20 amp circuits wired, it would be easier to just use smaller 1500 watt electric heaters, one on each circuit. They are much cheaper, too.

The link below should answer your btu question. You'll need the surface area and r-value of each wall, expected outside low temps, and required inside low temps. Be prepared to be shocked at how much energy is required to heat a plastic house.

Here is a link that might be useful: Heat Loss Calculator


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