Low Tech Greenhouse Automation : Cooling, Warming, Watering

The purpose of this instructable is to explain how I automated some basic actions (cooling, heating, watering) of my greenhouse using low technology.

I built a greenhouse to save some of my plants from frost at winter time and to open gardening possibilities (but mainly because I thought it would be cool in my garden and fun to built). I live in south of France not far from the sea, winters are mild and summer may be hot (especially in a greenhouse). In this instructable I explain how to:

1. protect from over heating : bring ventilation automatically when it gets too hot in the greenhouse
2. protect from frost : automate heat when it freezes outside
3. self water plants with a network of ollas (pitcher irrigation)

All this...without electricity...without any battery, nor arduino...

SPOILER ALERT

To save you some reading, if you are just curious and do not want to read the details of those automation setup, here is some general information :

1. For automatic ventilation I used greenhouse window openers that are commercially available, they are based on a "wax_motor" using the expansion of wax when it goes from solid to liquid. The are pretty powerful (can lift up to 7 kg) and do not need any power.
2. The automatic heating system is a bit more innovative, inspired by the wax motor I created an ice motor, which is triggering a heating system. The heating system is inspired from those reusable hand warmer, pouring water on calcium chloride heats. Calcium chloride is cheap and easy to find, it is sold in bags for home dehumidifier.
3. Self watering is done through interconnected clay pots buried into the ground. One of them is auto fed from a rainwater tank, proper level is adjusted with a floating valve...extremely simple automation.

END OF SPOILER ALERT

September 2022 update : please note that I improved, or let say thought of a different way to install the irrigation system, check this instructable for the self watering part : https://www.instructables.com/DIY-Low-Cost-Floating-Valve-for-Low-Tech-Irrigatio/

Obviously a greenhouse is needed for this automation...I decided to build one.

Automatic vent opening devices are easily found (at least on internet) at a reasonable price (around 20$). They open at 20 - 25 °C (68 - 77 °F) and allow some little adjustment in this range.

Be careful, some are adapted to vertical windows, some to roof opening (see comments on pictures for explanation).

They are based on a "wax motor" using the expansion of wax when it goes from solid to liquid. The are pretty powerful (can lift up to 7 kg) and do not need any power. I used two to open part of the greenhouse roof and intend to use two more to open lower flap vent to create some air flow.
The installation is pretty straight forward:

• one bracket of the opener is attached to the opening part of the roof (a plate is inserted between the poly-carbonate plate and its U shape edge).
• The other is attached to the frame of the green house (see pictures for details).

The automatic heating system is a bit more innovative and homemade.

It is made of two components:

1. a thermal actuator placed outside the greenhouse that will trigger when it is freezing outside.
2. a heating system triggered by the outside actuator. It will be placed closed to the plant I want to protect from frost, providing little heat to stay above freezing temperature.
   

This is assuming we have some insulation in the greenhouse (ie with bubble wrap), in my case I also placed a rainwater tank inside the greenhouse which brings thermal inertia (but is also very convenient for watering), therefore, it will freeze outside before it freezes inside.

I initially thought I could use a ready made thermostatic valve used in radiators. Some have frost detection but they open at 6-8 °C (42-46°F) which is too high for my use, I do not want heat to start when it is 8°C outside, this is far to high, so I decided to make my own.

The thermal actuator is water based. Water expands of about 10% in volume when it goes from liquid to ice. The actuator takes advantage of this volume increase to push some command cable.

The thermal actuator is made with a tube of copper with one cap at one end. On the other end we have a piston and a spring to facilitate its movement back. The piston should not be too tight: I used a leftover of PTFE which contract less than copper when temperature decreases, if too tight, the copper tube would tighten the piston and prevent proper movement. I also used marine grade grease (it does not mix to water) to lubricate the inside of the tube and the piston to make sure ice would not stick to the copper surface and block any movement.

Grease and smaller piston are crucial for the success of the actuator...my first tests without this were very disappointing as the actuator had no displacement when frozen.

I think a piston in oogoo could have been good option, easy to make an probably efficient...to be tested.

The heating system itself:
There are various options to heat : burn something, heat through a resistor (but requires electricity) etc. I thought it would be interesting to use heat from a chemical reaction (like hand warmers that can be reused).

Mixing calcium chloride with water is an exothermic reaction, it should provide a significant amount of heat. Calcium chloride is easy to find, it is sold in bags for home dehumidifier. It has to be manipulated with caution, please check safety notice, basically do not eat, dot not put it in your eyes, but safe enough to be sold for home de-humidification. Another advantage is that theoretically we should be able to dry up the product during summer time, taking advantage of the sun and reuse it the next winter.

Cut a "small" tube (small enough to freeze quickly) Mine is about 18cm, this will allow to have about 15 cm of water and will provide a displacement of about 1cm.

Close the tube at one end (I soldered a cap, epoxy could be used)

On the other end we have a piston, a 3mm screw, a brass tube (4mm outer diameter, 3mm inside diameter) a spring, a cap with drilled in its center at 4,5 mm. It is assembled per the picture.

Whenever possible, I brazed brass for my assemblies. There are a some very good Instructables to learn how to braze with silver. I should have learned brazing earlier, it is easy and provide very good assemblies :

Brass and Copper soldering Instructable
Basic silver soldering with durable joints Instructable

Measure the distance between the cap and the bottom of the piston (4,5 cm for me) and fill in water in the tube up to the top minus this distance. We want the water just below the piston.

Now we need the piston displacement to go inside the greenhouse. This can be achieved with a flexible tube and a cable (like bicycle breaks). I used some PTFE tube and piano wire sliding inside. I had Brass tube (4mm ID) it is a perfect match to my PTFE tube (see pictures). A little piece of aluminum is drilled with a 5mm hole and a 3mm hole. The 3mm hole is used to insert the brass stick which is attached to the copper tube, the 5mm is used to insert the PTFE end tube and secure it to the actuator (see comments on pictures) .

I use a 2L bottle. On the top part I sealed a silicone tube (easily found, they are sold to air circuit of aquariums), it will be used to let air go inside the bottle. on the bottom part a silicone tube is also sealed, water will go through it when air is allowed in the bottle.

As a rule of thumb, you can plan on using the same weight of calcium chloride as water.

After this ratio, adding water would not provide significant heat. This is what I understood from some reading, and did not check the source. If some reader have some chemistry skills, their comments and correction would be very welcomed.

Based on that, as I plan on using a calcium chloride that can hold 1kg, I will fill the tank with one liter of water. Once the bottle is empty it means I can replace the calcium chloride and refill the bottle.

The end of the cable is used to push a on/off valve that let the water drip in the calcium chloride tank. The valve is home made, I could not find anything smooth enough on the market. The principle is simple, it is a brass rod drilled at 8mm on one side and 4 mm on the other side. A 3mm screw go through a 4mm brass tube, the head of the screw is covered with oogoo using the 8mm hole as a mold (I coated it with oil to easily remove this plug once the oogoo is cured).

Once the plug has been molded, we can drill the brass rod from side to side for water inlet and outlet (I inserted and soldered 4 mm brass tube to allow easy water tube attachment. I used a 5mm diameter spring 0.4mm thick. It is smooth enough to be operated by my thermal actuator and strong enough to pull the plug back into position and stop water leak.

Before installing I made a few test in the freezer to verify it was triggering properly the water command but also was returning to a closed position when warming up.

Calcium chloride loves water and humidity, it has to be isolated from ambient air or it will quickly absorb humidity and will not heat anything... I used a glass box with a lid, the top is drilled to let the water tank exit tube to go in.

IMPORTANT NOTICE, make sure you have this over pressure release mechanism :

I added a small "check valve" which would allow a release of over pressure when temperature will increase inside the tank, but will also allow air to escape while water flows in. It is very important to have this check valve, without it, over pressure in reservoir could break it, or at best pop up the lid and the heating system would soon be useless because it has absorbed all humidity around.

The check valve is made of oogoo, a nut for some weight and a small piece of pipe.

It is only laying on the corresponding hole of the lid (see pictures).

September 2022 update : please note that I improved, or let say thought of a different way to install the irrigation system, check this instructable : https://www.instructables.com/DIY-Low-Cost-Floating-Valve-for-Low-Tech-Irrigatio/



While looking for some watering systems, I discovered the ollas technique, also called pitcher irrigation. The principle is that clay pitchers are buried into the ground, they contain water which will diffuse to the soil through the clay. They have to be refilled once in a while.

The purpose of my setup is to automate the refill with a simple floating valve. Floating valve can be easily found, they are used in toilet water tanks or automatic water filler mechanism of swimming pools, tanks, aquariums.

They can be connected into a network, I installed a floating valve in the first one, as a result the level of water is identical in all the network and auto filled from the rainwater tank.

The main issue I faced was to have a good seal between the connecting pipe and the pots.
My first attempt was with a polypropylene pipe (commonly sold for micro watering systems) and connectors and oogoo to have a seal with a pot hole. Unfortunately silicone does not stick to those polypropylene pipe and my setup failed the leak test.

I tired the same but used brass connectors and a stronger sealant but failed again the leak test (I waited 24h for the sealant to cure, I should have allowed more time, this may have been the reason of the failed leak test.
I ended up using leftover from plumbing project and sealed with epoxy...and successfully passed the leak test.

The drawback of this is that it would be hard to setup a larger network of pots or the extend the current one.

There is room for improvement here....

One pot of the network receives water from the rainwater tank, water level is maintained with a floating valve.

A gardening hose links the floating valve to the rainwater tank. The floating valve is attached to a flower pot saucer that I used as a lid.
The lid needs to be heavy enough or the whole setup will float and drain the water tank...

Level is adjustable through a small screw on the floating valve itself.

See pictures for details.

Prepare the soil, bury the pot, verify a good level between the pots, link to the water tank.

All pots are covered with lids :

• it limits water evaporation
• protect them and keep the inside clean
• avoid turning the greenhouse in a massive mosquito farm

I realized my greenhouse was not very green...

I rushed to the plant market this morning to get a greener photo. I may have to read some gardening instructables too ;) so far my best gardening skill is lawn mowing with my hovermower

Those windows opener and the watering system rock!