Saturday, 3 January 2015


Making a tub-in-tub incubator.

I have built a few of these things in the past and now I have got the design just right!

They work a treat and keep seeds, mushroom cultures, petri dishes and grow-bags at a perfectly regulated temperature!

They cost under £30 to build, if you shop wisely and do the job of much more expensive commercially available products.

You will need...  

One 35 litre plastic storage tub, and one 25 litre one. They should both fit flush into each other. 

(They can be clear or coloured, it dosent matter, I just use clear ones so so I can keep an eye on the water and the electrics)

One 25 watt aquarium heater. I found this from E-bay for £6!

One small aquarium submersible circulation pump (150watt is perfect). Make sure the measurements of your pump allow it to fit in the gap between the two tubs. I found this on E-bay too for £7.00!

One lemonade bottle (for the neck and screw cap)

One tube of No more nails or similar. (And a skeleton gun to apply it)

Twelve 1" size 6 stainless self tapping screws (pan head, phillips or hex drive)

Putting it all together...

Set your thermostat on the heater for 24 degrees. This can be altered later if needed, but it saves hassle if you have already done it. Site the heater in one corner of the large tub and hold down the wire with a bit of sticky tape for now.

Do the same with the circulation pump in the opposite corner, on the same end of the large tub.
Make sure the pump outlet is pointing along the side of the tub and not inwards. Stick the wire to the outside of the tub with sticky tape to hole the pump in position while you assemble the rest of the incubator.

Put the smaller tub into the larger one, They usually have concave and convex ends to aid stacking.
Pre-drill 5mm holes from the inside of the rim of the large tub, through to the outside rim of the smaller one. By slightly lifting and bending the outer rim you can get a screw started in the holes you have drilled. Screw them all in until the gap in between the two tubs is as narrow as you can make it, without straining or cracking the plastic. 
Don't let the screw pierce the inner wall of the small tub!

Time to get your no more nails (or whatever its called; Homebase and B&Q sell it at £1.99 per tube) and start filling in the gap around the edge. There will be the two corners with the pump and heater in that will be much wider gapped than the others. Leave these until last.
Fill the gap in and clean it up so you have a nice neat bead of sealer. This keeps the water from evaporating too much and it keeps dust etc out of the water jacket. The pump won't like muck in it!

Here you can see, I've left the two widest gapped corners, where the electrics are, 'till last.
In the corner with the heater you need to leave enough room to be able to adjust the thermostat. It can be a fiddly job, especially if you have big fingers, so that's why I suggested you set it before installation! Use the No More nails to stick the heater wire in place. When its set you can remove the sticky tape that you put on to hold it temporarily.

In the other end, where the pump is situated,  cut the screw-neck out of the top of your lemonade bottle. This is going to be inserted into the hole and glued in. The capped hole will act as a filler and a drain for the water jacket you have created!

Finish filling all remaining gaps with your sealer and smooth it down to give a nice clean finish.
 A neatly cut sheet of 1" thick polystyrene makes an ideal lid, as the original lids wont fit anymore.
When its all dry you can fill the water jacket up with clean cold water and plug in your pump and heater. Replace the cap on the filler and put the lid on. Its handy to have a thermometer inside the box just to keep a check on how accurate the heater thermostat is.