Heat Recovery Ventilation with small diameter electrical conduit as ducting

[Bwip]

Hi all, I'm wondering if it's possible / practical to use flexible 25mm conduit as part of a Heat Recovery Ventilation system.

My challenge is that I have a stone cottage in the far north of Scotland with a flat roof lean-to extension which I've just finished insulating and draft proofing. I fitted new hi-spec double glazing but there's still condensation forming on windows. I have an externally venting extractor fan in the bathroom, but not one in the kitchen (it wasn't practical to vent). The relative humidity in the house sits at about 70% - it's high, but not unusual this far north.

I could solve the condensation problem with trickle vents but the strong winds and freezing temperatures make that a less than ideal solution - even a crack of an opening lets a gale of wind in. Running a dehumidifier stops things getting too bad but it's not a permanent solution.

So, I was thinking of installing a small Heat Recovery Ventilation system. There's a loft that provides ventilation access to three of the four rooms (including the kitchen which could be vented via the HRV), but the bedroom is in a flat-roof extension with no practical way to route ducting.

However.... when renovating the extension I ran 5 lengths of 25mm flexible conduit from the loft to 5 plasterboard boxes in the bedroom walls (3 in the bottom corners and 2 high in walls) as I had planned to run cables for TV, speakers etc at some point in the future but wasn't sure of how the furniture would be laid out.

So, the advice I'm hoping for is, do you folks think it's possible / practical to use these 25mm conduit runs as ventilation ducts? I've done some 'fag packet' calculations taking into account the length of the conduit runs and I think I could get a combined flow rate of about 280 litres per minute which I believe is within the realms of possibility to incorporate into a small Heat Recovery Ventilation system.

Thanks in advance!

 

[stem]

The smaller diameter pipe over that length would need a high pressure to move that quantity of air due to the resistance losses in the pipe (made even worse if there are any bends. Also, bends should be gentle with a large radius, and not the sharp 90 degree bends used in electrical conduit). Therefore the pressure required is probably going to be over 10 times more than can be generated by a HRV's centrifugal fan.

Even if it were possible to generate the required pressure, unless the pipes were sealed, the joints are likely to leak and / or blow apart, and It would also be noisy. 280 litres of air a minute I reckon would have a velocity somewhere in the region of 7m/sec along a 25mm pipe.

Choose a HRV system suitable for the duty and then follow the manufacturers instructions as regards the diameter of the duct. AFAIK 100mm diameter will be the smallest size you could use.

 

[Bwip]

Thanks @stem. I think my figures were a tad optimistic! The conduit is the flexible uPVC corrugated stuff, so the internal surface won't help with the flow, there's at least 2 x small radius 90° bends in each run, and a couple of the runs are about 10m long. The runs should be airtight though - I believe I used unjointed lengths for each.

After a bit more research (and blowing through the conduit from the loft!), I think the best I could achieve in the room with the electrical conduit is a bit of a trickle - which is better than nothing I suppose, and might just be enough to move the air enough to keep down on the condensation in that room.

As part of the HRV system I'd be venting out from the kitchen, which is the main source of internal moisture, and in to the living areas, so I'm thinking that the conduit to the bedroom would be a bit of a bonus!

 

[stem]

I doubt with a centrifugal fan over that distance you will even get a trickle. For balancing, generally with 100mm duct it is only capable of supplying one or at most two vents. Larger ducts are usually used for supplying multiple inlets/outlets. Often 150mm connected to a manifold or plenum that is then split down to individual 100mm connections.

You would also be putting a major strain on the fan/s because of the resistance they are working against and are designed for. I don't know what the effect of that would be, but it won't be for their good.

The idea of HRV is that it utilises a gentle flow of air using a duct, with a low resistance so that the airflow and return are balanced, the air goes through the heat exchanger for heat recovery to take place, instead of taking an easier route and thus skipping the heat exchanger.

With what you propose the air will take the easiest path of low resistance via doors, windows, cat flaps, letterbox, chimney etc.