Flower Pot One Way Air Flow Valve

I recently fitted a 100mm extractor fan in a bathroom, having drilled a 120mm hole through the inside wall i found i had come through into the eves of the roof.  No problem I'll vent it through the sofit board and fix a standard grill to the outside. until the owner pointed out that the house was on top of a hill and the wind blew directly into the back of the property. 
Having searched the internet for a solution to the problem with no success, i thought what i needed was a dog flap for a 100mm pipe.
the flap would have to open one way only within the circumference of the pipe. so a slot or funnel was required.
A flower pot would be ideal, and as it turned out the pot tray would make the flap, the sloping sides would centre it back into position and cutting the pot at an angle would let gravity weigh the flap down.

Materials
1.  A 200mm length of plastic 100mm ducting pipe or a sweetie tin.
2.  A 100mm plastic plant pot.
3.  A 90mm pot tray.
4.  Apoxy resin glue.
5.  Stiff wire or a long pin nail.
6.  A tube of super glue.
7. waterproof cloth tape.

Tools
1.  A multitool, grinder or hacksaw.
2.  A pliers, long nose would be better or a plumbers grips
3.  A permanent marker pen.
4.  Wet and dry sand paper or emery cloth.
5.  and a scissors.

 

To mark out the angle on the pot, first position the tray so that the base of the tray will cover the hole after the pot is cut.
Next match the angle of the side of the tray with the side of the pot. 

The cloth tape reel can then be used to help mark all the way around while holding it to your original marks.

The pot can then be cut with the multi tool, grinder or hacksaw. and sanded to clean off swarf.

The pot tray can then be checked for clearance.

To remove the bottom of the tin, I took it in the house and used a tin opener.
If using plastic pipe you don't need to do this, one it don't have a bottom to cut and two tin openers work on tins. 

After cutting the tin or pipe check that it fits over the fan outlet. Cloth tape can be used for a tighter fit.

Fitting the pot into the tin or pipe trimming or packing may be required.I had to cut the rim off using a scissors to get it to fit but you could use the cloth tape wrapped around a few times to make the pot wider for larger pipes or stiff foam cut into a ring to bridge the gap for existing pipes in situ. Next project maybe.

The pot tray can then be cut to gain clearance in the pipe to allow the flap to open. about 5mm a rim  would be sufficient.
this will self centre the flap when closed. i made mine 4mm in the top and 6mm in the bottom to add weight.

I cut the lid originally to hold the flexi hose in place but as you will see later i stuck the pot in the wrong end and had to come up with another idea. Wear safety goggles sparks go everywhere. 

. Mark a line inside the pipe (tin can) for the position of the flower pot

I placed four drops of super glue on the line inside the pipe/tin can and stuck the rim i cut off the pot to the line to form a stop leaving a gap for the hinge to pass and left to dry.

I bent a galvanized pin nail to form a loop or square as mine came out, leaving a tail to glue to the pot, this will form the hinge.

Placing the flap into position i drilled a hole through the top of the flap and the pot 2mm from edge of tray/flap, Then placing the wire hinge through the hole and tacking in place with the super glue on the inside of the pot, when dry i applied two part apoxy glue to fix permanently in place. 

Next i applied apoxy glue to the stop inside the pipe, then pushed the pot into the pipe until it was tight and taped in placeand left to dry.
later i Checked that the flap moved by blowing and that there was a sufficient void for ventilation.  i applied more glue to seal the pot to the pipe. this could be silicone sealant in existing pipes but making sure it doesn't get on the flap mechanism. i could not see any problem with the fan pushing against the flap when was closed because of existing wall flap covers. but the funnel shape would decrease flow efficiency by half. Perhaps someone has come up with a better idea.  Let me know, .

I gave the tin can a coat of enamel Spray paint and formed the tin lid to fit inside the flexi pipe. i used waterproof cloth tape to fix the valve to the fan and to secure the flexi pipe and after testing fitted it into the wall. Sorted no more wind but not much suck either. needs a bigger pipe and a larger pot with two hinges i think. This took me an hour to make from bits in the shed, now I've got to find a new tin for my loose nails,  have fun.