I'm doing some renovations in my toilet/bathroom, and am currently pondering the ventilation. Here's the current situation (click pictures to get bigger versions):

My toilet and bathroom are right next to each other (yes, they're tiny. It's a Soviet-era building). The bathroom is on the left, the toilet on the right:


The toilet has a 10cm by 20cm hole in the wall between it and the bathroom:

(source: valts.21.lv)

On the other side of the wall there's a metal pipe placed in the hole:

(source: valts.21.lv)

The pipe spans the width of the bathroom and enters a similar hole on the opposite wall:

(source: valts.21.lv)

That's where the ventilation shaft is. You can also see a smaller hole beneath the pipe - that's the ventilation hole for the bathroom itself.

What I want to do is place fans in both ventilation holes (for the toilet and for the bathroom). The only fans I could find where of this type:

(source: valts.21.lv)

The smallest one had a diameter of 10cm; it's depth was greater than the thickness of the wall between toilet and bathroom (the rear end sticks out a good deal on the other side); and the face-plate (being a square) doesn't fully cover the hole in the wall. I even had to knock the holes in the wall a few mm bigger for the fan to fit in there at all; and the current metal pipe is about 1cm less wide than the hole, which means that the fan cannot possibly fit in it.

That pretty much means I'll need a new pipe. I don't mind, because the old one is pretty unsightly anyway. The new pipe will most likely be circular (I haven't seen rectangular ones like this in the store) and unlike the old one it won't be able to support itself by being placed inside the hole (since the hole is barely wide enough for the fan, there's no room for the pipe). That means that it will need to be somehow attached to the wall and the remainders of all the holes closed.

What is the best way to do this? I can imagine buying the right length of the pipe (which will then need to be very precise in length), and maybe three metal (or plastic?) plates with appropriate holes cut out... but how to stick it all together? And it would also very much then depend on the pipe being the proper length (exactly the width of the bathroom). A few mm longer or shorter, and it's unusable. That worries me.

Is there some other approach or some tricks that can help me?

1 Answer 1


There's a number of things you could do, this is one I would do in your situation. These fans are made to fit inside sheet metal ducts. Go ahead and get 2 lengths of this stuff. You will end up with a joint somewhere in the room, but then the duct can run fully through both walls and there's enough slop in the fit that precise cutting is not needed.

Because of the joint, you will need a hanger support that anchors into the ceiling, which I assume is concrete. You would need a hammer drill to make the hole necessary to insert an anchor stud to which the hanger can be attached.

This is not the most attractive thing in the world, but it can be painted so that it's not too objectionable. Painting exposed ductwork is done intentionally sometimes as a desirable (to some) "look". You could also construct a soffit box to contain the duct, you end up with a sort of stair step on the ceiling. Given the extra work for this, it's easier to like the painted duct look.

To fill in the holes, get some cement backer board and cut to the size of each opening. Also cut holes to fit the duct or fan. For the size involved, the boards can just be glued in place with construction adhesive. You can also glue in some backup cleats behind the boards to increase the glued surface area if you like. The boards should be recessed about 15mm from the wall surface. The remaining space is filled with portland cement plaster in 2 or 3 coats. It is an obvious patch, but it sort of matches the wall and would be hardly noticeable if the walls are to be painted.

  • I understand everything but two points - 1: Why do I need 2 lengths of the duct and a joint? Why can't I get one length (like it is now)? 2: The ducts cannot "run through the wall". The holes are a so perfect fit for the fan itself (in width), that there's no space left between the fan and the wall. And I cannot knock the holes even wider, because there's that metal frame that runs throughout the concrete.
    – Vilx-
    Commented Aug 14, 2013 at 6:03
  • I'm thinking 20ga (0.91mm) wall thickness sheet metal ducts, not very thick. If it overlaps the fan some, then it does not need to go into the fan wall. It still should go through the airshaft wall. The duct is somewhat flexible and will deform slightly to fit the opening. Two pieces because I was unsure you could get a length longer than the room width to fit as a single piece. Now that you are not going through one wall, you may be able to use just one piece with no joint, but I suggest you still provide an intermediate support if the duct is left exposed.
    – bcworkz
    Commented Aug 14, 2013 at 18:55
  • OK, thanks. Btw - another question: upon closer inspection of the hole in the bathroom I realized that it's too shallow to put a standard fan in there. I thought that a PC fan (or similar from a local electronics dealer) could fit there just right, and provide about the same airflow. Could there be problems with humidity?
    – Vilx-
    Commented Aug 15, 2013 at 5:43
  • When you say too shallow, do you mean the fan projects through the wall? The wall thickness is less than the fan depth? Not a problem as the fan will be inside the duct and not visible. Or just that it simply will not fit in the opening's width? As far as effect of humidity on computer fans, they are not designed for humid environments, they might work OK, but they may not last very long. Perhaps consider building out a box to contain a larger fan assembly in the upper corner of the compartment. You can't get much of a fan in that hole, but lots of air flow will go through from a bigger fan.
    – bcworkz
    Commented Aug 15, 2013 at 20:40
  • "Too shallow" == "I cannot put the fan completely in". It hits the rear wall of the duct and sticks partially out of the wall. As for computer fans - I found at a local electronics retailer fans that look similar to PC fans, but have different characteristics. For example, there's a 120mm by 120mm fan that runs off of 220V AC and provides an airflow similar to that of the bathroom fan at around 3000rpm. The cost is about the same too (approximately $20 if converted to US dollars). I think I can widen the hole enough to fit it.
    – Vilx-
    Commented Aug 18, 2013 at 19:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.