Balcony winter storage – heating and air circulation upgrade

Last winter I put trees into the cupboard and took the task too relaxed way and the result was not near what was needed.

The warming in the lowest shelf did reach the middle and upper shelves but the difference between the temperatures was too much, even so much that the upper shelf was below 0 while the lowest shelf with the trees was +7 degrees C. I decided to try to fix the situation.

Several things were improved:

TemperaturE trigger

In the first installation my old warmer had the thermostat inside its body. As such it was not possible to set the temperature of the end point but rather warm up the lowest shelf and then hope it will be enough for the rest of them. That wasn’t the case.

The newer warmer has the electronic thermostat (versus bimetal earlier) which is in the end of rather long cable so it is possible to position it wherever needed. I placed it in the second shelf, not the highest one and not the lowest one (3 shelves for the trees).

Air flow

Old installation relied on the natural air flow. Warm goes up, cold goes down. Yes, it did go up and down but not in dreams enough for it to function well.

I installed 6 CPU fans to solve the case (suggestion by Jani). The fan sucks the air through the big whole on the sides of the body and blows it through a narrower slit on the bottom.

3 of them (1 shelf each) I installed on the left, to suck the air from the shelf space and blows it downwards to the next shelf. That is for the old/colder air.

The other 3 I set into the lowest shelf with the warmer on its right so they can suck the warm air and blow it upwards to reach all shelves.


On both sides I added some “styrox sponge” type of insulation. Nothing extreme but better than nothing. I will increase this more.

The front doors remains transparent without insulation for now. I want to keep it this way so some light still reaches the trees.

End result

Here is visible how it looks like and where the fans are:


Red arrows show where the fans are and their pointing direction is the direction of air flow


Lowest shelf, view from outside


Lowest shelf, view from inside


Left fans, view from inside


Two upper shelves

My son soldered all the fans cabling. The fans are installed kind of temporary style as I experimented a lot and this was the end position with the best result. Perhaps, just perhaps, I will set them better next year….oooo, famous last words 😀

Temperature monitoring

To be able to follow how the shelf behaved during the cold nights I ordered one ethernet manager and some temperature/humidity wireless sensors from The things work from the box and are small enough to be easily hidden for whatever purpose it is intended. All sensors contain also motion detector so their usage is much wider than bonsai storage temperature monitoring.

One of them I put into the middle shelf where the warmer temperature sensor is.

Below is example of several days of temperature monitoring:


The peak in the blue graph was when the sun hit the shelf. It was a rare occasion that during November we have so much sun and it happened once/twice. The space is so small that the temperature rises substantially in half an hour.

To solve this problem I installed a temporary “shade” 2m in front of the cupboard so that direct sun cannot reach the shelf:


That worked well 🙂 No more sudden peaks.

I tested the warming to 20 degrees C when outside was around 1-2 degrees and it went pretty fine. I can rely on this installation to provide enough of warmth even when the temperature drops to -15 to -20 degrees C. Of course, cost of heating would rise in that case.

The up-to-date real-time temperature in the balcony winter storage can be seen below:


So far the temperature remains at the set 5 degrees C +-1 degree which is very good.

While the outside temperature is 5 degrees lower the upper shelf is colder 0.5 degree and the lower shelf is warmer 0.5 degree which is very good.

When the outside temperature is around 15 degrees lower than inside of the shelves the upper/lower shelves are +3 and -3 degrees compared to the middle one at 5 degree C.

If it shows that the difference grows more I will add more insulation which should make it more equal. I positioned a non-cloud-connected thermometers on upper and lower shelf so I can follow their temperature extremes.


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  1. Jan Tammisto

    This is very interesting, and judging from the charts works very well. Temperature is constantly +5 degrees? This arrangement could be the game changer for growing bonsai in balcony in our climate.

    • tato

      Thanks for comment Jani!
      Yes, the middle shelf is having temperature between 4 and 6 degrees, basically being around +5 degrees Celsius all the time. I tested it for 20 degrees difference between inside and outside and it managed to maintain +5C without problems. Considering I need 30 degrees to be sure it will work for any winter I need to improve the insulation a little bit but otherwise it works very very well. Also, the soil doesn’t remain wet all the time. I have to water it lets say every 2-3 weeks which I think is good so the humidity is not too high. The balcony is just 1-2 degrees warmer than outside but it does protect from winds. Can’t estimate how much value that brings but if the balcony is opened constant wind would cool it down faster.

  2. This is the ****! Sorry, had to be said. I will be interested to see how this performs come winter. Could be the missing solution for tender temperature trees needing 0 to +10 Celsius winter temperatures for dormancy.

    I would be interested to try this if it could be made to work with scant wind protection. Probably need to build the cupboard from ground up but at least the tricky heating/ventilation system is solved. Could spice up the ventilation to deal with temperature spikes with this: Scavenged double glazed ventilation window with insulated seams could also work. Depends if I can find one – and at what price.

    • tato

      > This is the ****! Sorry, had to be said.

      Thanks for the comment!

      Yes, better, double glazed window would be better. Though heating might be able to cope with it still with double glazing is better efficiency and also the temperature difference between closest and furthest point from the heater is lower.
      The solution described is the 2. iteration so lets see how it will evolve.
      Today I am buying a double layer poly-carbonate board ( put to the front and also will add styrofoam boards on the sides too so the insulation would be better.
      The temperature sensor is in there all the time and the graph will show how it behaves. The manually checked upper/lower shelf temperature I will update there after some time when I see how it behaves with the new insulation.

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