Raising the air temperature in a terrarium?

[Numbersix]

Hi all,

 

I am trying an experiment out with a terrarium I have made for CP's and I'm looking for some advice on heating it effectively, specifically raising the ambient air temp.

 

I have a number of small terrariums indoors with highland and lowland neps in that work well with natural daylight and ambient indoor temperature. However I tried growing some other CP's like the petiolaris complex sundews this way and they grew very etiolated and did not enjoy this! Not having the room indoors to setup a more serious terrarium for them I decided to build one in the garage utilising a 160L fish tank I had spare. I set it up as a palludarium specifically with a pool with utrics in and a banked soil arrangement for the sundews etc.

 

My problem is the garage isn't heated and although the plants are alive they are obviously not enjoying the temperature which I am struggling to raise over 15 degrees C. The tank has a large heat mat set above the base reservoir which warms the soil and an aquarium water heater heating the water to 28 degrees. A fogger and fans help to distribute the warm moist air when running but really I need to raise the air temp by at least another 10 degrees for it to work. At any other time of year the ambient temp may help this but for now I need to find another way.

 

Lighting is LED's so no heat is emitted so I wondered if it was an idea to add a vivarium ceramic heat lamp to raise air temp or alternatively a basking bulb which also puts out the correct light for plants. Has anybody tried this? I need to know if these are likely to add the temp increase I need or whether I would in fact be wasting my money?

 

Just to confirm there are no reptiles/amphibians in the tank, it is purely to grow CP's.

 

Rich

[pmatil]

I think the bulbs are not suited for humid environments. Otherwise they might do the trick it the power is adequate. I have made my own terrarium heaters from car seat heating pads, heatsinks and fans. Currently I heat the terrarium to 28 C during the day and the heating capacity is about 90 watts max. + the lights that are inside the terrarium, 2 x 18 W T8 CFL tubes. It's a rather large terrarium, measuring at 1 m (H) x 0.8 m (W) x 0.5 m (D). I keep it in my living room which does not see temps below 19 C during the winter. The heating units are hanging from the roof of the terrarium blowing air toward the plants. Depending on your setup you could use similar heating pads on the terrarium walls maybe.

 

Here's a picture of the heater unit:

 

[Numbersix]

That's an interesting idea with the car heat mat's, I hadn't thought of anything like that! I did wonder if the ceramic heaters and bulbs would be suitable as most reptile enclosures are dry and mine is very humid with the fogger etc. I will look into how to build something similar to yours .

[nicmanism]

Hi Rich,

I'm thinking of doing a similar thing. The main problem is going to be heat loss through the sides of the terrarium. Glass is not a very good thermal insulator. I've done a few approximate "back of the envelop calculations" (assuming conduction is the only heat transfer mechanism and perfect mixing within the terrarium) to estimate the heat losses (hoping that I remember my undergrad heat transfer theory - if there are any heat transfer experts out there then they should feel free to check my calculation).

 

q=k.A.dT/s

where:

q=rate of heat transfer (W)

k=thermal conductivity (W/m.K)

A=surface area (m^2)

dT=temperature difference (C or K)

s=thickness of material (m)

 

Let's assume:

a) the dimensions of your terrarium are 40cm x 40cm x 100cm so it will have a surface area of 1.92m^2

b) the thickness of the glass is 4mm = 0.004m

c) you want to maintain the inside of your terrarium at 25C

 

Then if the temperature in the garage drops to 0C (i.e. dT=-25K)

q=(0.8W/m.K) x 1.92m^2 x (-25K) / 0.004m = -9.6 kW

 

So if the temperature in the garage drops to 0C you will need to supply heat at a rate of 9.6kW if you want to keep the inside of the terrarium at 25C.

Wow - That is a lot. You would need three kettle heating elements to supply enough heat. That would fry your plants unless you have some ridiculous amount of air circulation. I'd hate to think what the heating bills are for an outdoor lowland greenhouse in the UK.

 

You could dramatically reduce the amount of heat you will need by insulating the system. For instance polystyrene has a thermal conductivity of 0.033W/m.K

 

If you were to cover the entire outer surface of the terrarium with a 1cm thick layer of polystyrene then the equation for thermal resistors in series becomes:

 

q = dT.A /  ( sg/Kglass + sp/Kp )

where:

sg = the thickness of the glass = 0.004m

Kglass= the thermal conductivity of glass = 0.8W/m.K

sp= the thickness of the polystyrene = 0.01m

Kp= the thermal conductivity of polystyrene = 0.033W/m.K

 

q = (-25) x 1.92 / (0.004/0.8 + 0.01/0.033) = -155W

 

So with 1cm of polystyrene insulation you would need to supply 155W to keep you plants at 25C in a 0C garage.

If you were to use 2cm of polystyrene insulation then only 78W would be required.

 

So my recommendations would be:

a) Check my calculations. I was taught heat transfer 20 years ago.

b) Re-work the numbers for your set-up i.e. plug in the correct numbers for the thickness of glass, dimensions of tank and minimum temperature the inside of your garage is likely to experience, etc

c) Build a wooden cabinet big enough to contain the entire fish tank. You could have hinged doors on the top (for access and watering) and front (for viewing)

d) Line the cabinet with a 2cm thick layer (or what ever the re-worked calculations suggest) of polystyrene

e) Place the entire tank within the cabinet. An air gap between the cabinet and tank will further improve insulation (but this is a much more elaborate calculation so I can't tell you by how much).

f) Immerse a 100W (or what ever value the re-worked calculations suggest) aquarium heater in the water within the tank. Heating the air isn't the best way to go since the thermal mass of air is so low. But if air circulation within the tank is good then the air will be warmed up very quickly by the warm water.

g) (Very important) have some kind of temperature monitor with a low temperature alarm that will warn you if the temperature drops below a desired set-point. I've seen quite a few sob stories on these forums from people who have either fried or frozen plants due to a lack of a back-up temperature monitoring system (I fried my favorite nepenthes last year).

 

Good luck.

Edited January 19, 2016 by nicmanism

[manders]

Nic, some of your calcs look a bit dodgy, e.g. 9.6kw would probably heat up a good sized greenhouse of at least 200ft2 let alone a small fish tank, but i did the same thing with a fish tank the other year, no need for a wooden cabinet, just put 2 inch thick polystyrene around the sides of the fish tank, that will keep the heat in.

 

I find the best way is to use a soil heating cable, heat transfer between air and glass is a lot lower than transfer between water and glass, so the more water you have in contact with the glass sides, the more heat duty you will need.  100W should be ample if you insulate it.

[nicmanism]

Ah that is even better than someone checking the calculations - Manders has first hand practical experience.

 

Yes the 9.6kW figure does seem high but I've checked the calculation and the figures are correct but the equation does assume perfect heat transfer inside the system from the heat source, through the air and onto the inside of the glass. And in reality convection will not provide these ideal conditions. So the 9.6kW figure does look overly conservative.

 

But I guess both Manders and I agree that you definitely will benefit from insulation (probably 1-2 inches) and will need around 100W of heat input.

 

Before you risk your plants you could easily set up a test system: Place a heat source in a baking dish (or some other heat proof vessel) within a polystyrene box. A high/low thermometer inside the box you will give you a reasonable idea of whether your heat source is powerful enough and whether your insulation is adequate.

 

Good luck and let us know how you get on.

[manders]

Ah that is even better than someone checking the calculations - Manders has first hand practical experience.

 

No need to check, you ignored the biggest factor by far which Is the insulating effect of air... (on both sides of the glass)

Edited January 20, 2016 by manders

[pmatil]

Oh, and I do have an insulating mat (car soundproofing mat) over the sides of the heated terrarium and an old blanket on top of the terrarium. They make a huge difference on keeping the heat inside. Ventilation is kept at a minimum (basically only during the night when the heating is off. This reduces condensation too).

 

The reason I'm using 12 V heating mats is that I'm a bit paranoid about mains powered heating cables in the soil. My heating system has several safety circuits. First the controller (ebay) has over-temperature alarm which disconnects the heating via a safety circuit (relay) if the temperature goes over 38C or so. Second the heating units each have 2 over-temperature thermostats in series so if any of them trip the whole safety circuit trips and cuts off the power. And last each of the units have a thermal fuse which needs to be replaced if it blows. The safety circuit needs to be "armed" by hand if tripped.

[Numbersix]

Many thanks for the additional input guys. Having read what you all recommend I agree that 2" of polystyrene insulation should allow the temperature to be much less influenced by ambient air temps so I will have a crack at that.

 

What seems difficult to achieve with the converted tank is that , given the problems I have faced, the tank lid has to be retained to fit all the components in. However the LED rig I have doesn't allow an even spread of light at that height above the plants. I had planned to suspend it above the tank but that would mean a glass top to keep heat and moisture in and then I have the problem of no insulation and poor airflow. For now I think I will see how I get on with the tank as it is with added insulation.

 

This is all an experiment anyway to determine the viability of growing in this situation. I have some petiolaris complex sundews alongside tougher utrics and pings and despite the low temps they haven't kicked the bucket yet. They are not putting a lot of growth out understandably but what leaves have emerged are better than they were in my windowsill terrariums with the lowland neps.

 

I will let you know how it progresses.

 

Rich

[pmatil]

Then you should put the lights inside the terrarium. If they can handle the humidity of course. My CFLs are Arcadia Ultra Seal (IP67) so they do fine and my LED lights are too (I made them myself). The lights are hanging from the glass walls using ziptie mounting plates and superstrong double-sided tape. The lid consits of 2 glass panels with corners cut to allow cables and tubes through. This way everything is inside the terrarium and I'm free to insulate the lid without problems.

[Numbersix]

Well once again thank you all for you advice. I will keep all lights built into the lid for now. I have added 2" polystyrene insulation under the tank, around the sides and on top. I am pleased to say that by adding 2" of insulation the heat mat alone is keeping the tank maintained at 25 degrees C despite the lower ambient temps in the garage. I am sure as ambient temps rise throughout the year I can reduce the insulation in order to prevent things getting too warm.

 

Just in addition to this I wondered what sort of photoperiod you guys run. At present my lights are on from 8am-11pm. I assume you could in theory have them on constantly. I also run the fans off the same timer. Is it worth running the fans this much or should I just have them cut in and out throughout the day?

[pmatil]

I've always used 12h photoperiod but I think most people use 14h or more. You could run the fans constantly if your heater still keeps the temp and humidity up. For me I have a separate timer for the fan. It runs during the night every 2-3 hours, 1-2 hours at a time. This way the humidity stays high but will not drop too low. Heater is off during the night so temp drop doesn't matter - in fact it's even better as it causes the temp to drop faster.