Heat recovery unit for 80mm fan size.

Heat Recovery Unit - For warm clean air in, and cold stale air out.

Commercial units to recover heat from air being exhausted from a room or building exist and are several hundred pounds/dollars to buy. I decided to see if I could make such a unit by designing one and 3D printing it.

The basis of it is an air-air heat exchanger. An 80mm fan blows stale air from the room through the device to the outside, this would reduce humidity if used in a bathroom for example. However the heat from your room is then lost, so this device recovers the heat by passing the air through around a hundred plastic drinking straws. This creates a large surface area. The second 80mm fan sucks cold clean air from outside over these straws recovering the heat from them. The result - warm(er) clean air is brought into the room.

Efficiency of around 70% can be expected. So if the inside air is 20 degrees C and the outside air 0 degrees C, then the air coming in will be at around 14 degrees. Which is well worth the trouble. Condensation might form inside the plastic straws, this where the moist inside air is cooled by the outside air. The fan should push it towards the vent to the outside, so I have included a condensation-collection tray which drains to a 6mm OD fitting for a drain tube to the outside.

Printing this, the important part is the straw endplates. Print your first layer very slowly and as precisely as possible. I am using 5mm drinking straws and making the holes 5.25mm seems to fit them very well. These will take some considerable time to print.

After that print the body parts and lids. The straw endplates fit into the heat exchanger body, you can then insert the straws, cutting them off to length once inserted with a sharp knife. The outside air body can then be attached to the side of this unit, use a little silicone sealant in places to ensure the units are air tight to one another. Bond on the lids (use the silicone if you wish). Attach the fans so that fan 1 blows air from in the room to outside through the straws, and fan 2 sucks air into the room from outside over the straws.

You probably ought to run the unit continuously especially if in a bathroom so select fans that push plenty of air (but a very high air flow will not necessarily improve the working of the unit the heat exchanger can only exchange so much heat over any given unit of time), but are also quiet and reliable. I have chosen 80mm as a plethora of PC fans exist to choose from.

The unit only requires 12v power supply to run the fans so only around 0.4A is required so only around 5W power draw in total.

8th April 2020 - I designed and printed this about 3 years ago. I finally installed it and it works great.

30-12-2020 - I just added the actual stls. Oops. Also, I have added a few more parts, which can make a side by side duct to mount the unit through the wall in place of one of those rectangular standard vents you sometimes get especially in rooms with boilers/furnaces or open fires. One is the complete duct, the other is the ductwork in two parts for easier printing.

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