I experimented with heat pads on the outside of the orchidarium, but the heat transfer was not good enough, and the amount of heat too small. I still used heat pads stuck to the bottom of the orchidarium, but this was to add humidity, not to raise the temperature in the orchidarium. I also looked into low voltage heating inside the orchidarium, but they just don’t put out enough heat either.
So I did the unthinkable and put a 120VAC personal heater inside the orchidarium, and gave it an umbrella to protect it during watering! I also made sure that it was turned off during the watering cycle, in a slight nod to safety. This provides 682 BTUs at 200-Watts. I know what you are thinking: hair dryers and water don’t go together 🔥 🔥 🔥, and even worse, it is not aesthetically pleasing!
I did consider placing the heater on the top of the orchidarium and making a hole in the polycarbonate top, but the problem is that dry air is then pulled from the outside, which then becomes even dryer through the heater. This hot dry air is not healthy for the orchids, or for your hair either.
I covered the whole thing with a banana leaf from the garden, for which I have a large supply. This improves the aesthetics considerably. But I suppose if you don’t have a banana plant handy, you could put a thick leafed plant in front of the heater to hide it. That was the original idea for the basket – to put soil in it and grow ground cover plants.
There is no temperature control on this heater, just on/off, so I just used a single 5V opto-coupled relay to control the heater. The advantage of using a single relay (as opposed to a block of relays) is that the relay can be attached directly to the power cord, and the control cable length can be adjusted to go to the controller. I wrap the whole relay and power cable splice in electrical tape. Be generous with tape: this is 120 volts!
A GFCI plug should be used with the heater, so that it will turn off automatically if there is a “little” problem, such as a sprayer malfunction filling up the bottom of the orchidarium with water.
Accurate measurement of temperature is needed. I found that it is better to go more high-end on this with an AM2315 encased humidity and temperature sensor, since the cheaper sensors are not accurate enough. It also has to go inside the orchidarium, so being waterproof is useful. This sensor is accurate to +/-0.1 degrees celsius.
In a later blog I will include code for the Arduino temperature control PID loop. The temperature set point is adjustable via a website, but I have set it to 80 degrees fahrenheit. The temperature drops during watering. This gets slightly complicated with a humidity control loop which uses the exhaust fan to take humid air out of the orchidarium, and draws in colder, dry air through the front door vents.
I have heard that having a night time temperature drop helps promote plant blooming, but I have not tried this yet. Anyway, I am getting a temperature drop every time watering occurs.