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Electrician's Arms
Trainee Access
Got a few questions if anyone's got the time!

1) for starters I need to find a decent quality, silent running (so am guessing metal cased/bearings etc) axial fan, 4", which is small enough to fit above some kitchen cupboards (haven't measured it but at a guess I'd say it's about 150mm-ish clearance), to extract from ducting branched from two rooms (wc and utility). Any suggestions?

2) unit above is going to need to have a means of boost or humidity control but across both rooms independently (obviously, once it's in boost it's doing for both rooms, I mean the trigger method)

3) repeat of 1 & 2 only with some heat recovery and based above two bathrooms so have slightly more space to play with but not loads as it's more a crawl space than a loft! This could use more robust 6" metal flexi pipe.

This is to rescue a complete b's up installation recently installed into a new build by Msrs Bodgit and Scarper who didn't have the first clue what they were doing and it's now a bit late to go pulling ceilings down to replace faulty whole-house ducting runs!! Oh, what fun we have.

Cheers, R.


Regular EF Member
Sussex by the Sea
I deal more with commercial HVAC systems than domestic so cannot help with the fans. Do not really get involved too much with ductwork or fans either but..... I am sure you will have thought of this but.....

Axial fans are not particularly good with long duct runs as they cannot cope with high static duct pressures so depending on duct run and layouts you may not get anything like the manufacturers rated airflow actually extracted. Centrifugal fans are better in this respect.

Unless the duct run has been carefully designed, or dampers included to allow balancing, or unless each leg of the ductwork is more or less identical the suction on each leg will differ. One room will get more "suck" than the other.

A humidistat in each room wired in parallell will give simple control.


Admin and gender confused
Staff member
South Africa
Axial and quiet are two words that generally don't go together. As already stated depending on the system static pressure an axial may be off curve unless it's higher rpm in which case you're back to the noise issue. I would look at either a radial type or even a forward curved centrif fan. Check out System Air's offerings, they do a nice line of compact boxed centrif's. Maybe
this one if overhead space is an issue. Really you need to make some volume and static pres calculations before committing to a fan. I would phone their closest branch and explain your problem. Please be warned they're not the cheapest fans on the market but you get what you pay for. They also have a range of heat recovery units as well.
Good luck (it sounds like you're going to need it) :)

**Edit** You can reduce noise at the expense of air volume by using a speed control. This may require oversizing the fan to accommodate.
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