Minimum Length of Ducting with In Line extractor Fan

Nearlyold

Bought a Manrose in line extractor fan MF100T. Instructions say it must fitted to a minimum duct length of at least 500mm both sides of the fan but gives no explanation why that is. I'm guessing it's related to preventing something being poked into the fan. There isn't 500mm of duct from the hole in the bathroom ceiling to the fan, just a 90 degree coupler and two short lengths of duct

Vent Axia sell an identical fan but their instructions say 

"The unit is designed as an in-line duct fan to be positioned between lengths of ducting. Short duct runs terminating close to the fan (i.e. within 1.5m) must incorporate suitable guards unless the fan is mounted higher than 2.1m above the floor."     No Mention of a minimum 500mm duct length

The fan in the loft is more than 2.1m above the floor & there is ceiling diffuser to act as a guard

Is there any reason why it wouldn't be okay like this?

Mr.Generous

I cannot think of any reason why a minimum duct length is required for a ceiling fan. Anyone stupid enough to poke something in - unlucky. And I doubt much would happen anyway. Very low power and plastic blades.

Long duct runs can suffer from condensation, I'd keep em short and resist the urge to pust anything into the fan blades.

ThisIsWeird

I cannot think of any reason for this instruction. Yes, it has unprotected fan blades, but you'll have a grill on the duct's inlet in any case, if only for aesthetic reasons?

It can't have anything to do with air turbulence either, as their own typical installation diagram shows it with flexible ducting bending at 90o just beyond the inlet grill.

You could always ask technical@manrose.com

But I'd personally ignore the rule if it helped the installation.

womble12345

I think it is due to noise, I fitted the same and its so powerful I hear a rushing noise as the air is sucked in, maybe if it was mounted further away it wouldnt be as nosiy?

Kiran

I think,  its due to the fact that inline extractors are more powerful in terms of extraction rates than a standard through the wall extractor that it gives a volume of air without restriction (the vent cover or grille) to prevent the airflow to the extractor being restricted and the motor burning out. As long as you have plenty of free air flow it shouldn't make  difference. I have 2 extraction point in my bathroom (1 over the shower and  over the toilet) so double the free air, one of the points is basically 300mm up through the ceiling and loft insulation and then turns into the inline connection. It works perfectly

stuart45

It's more efficient with a longer pipe due to the tunnel effect of creating a vacuum behind it. Same principle as a chimney.

ThisIsWeird

Good afternoon, 

I hope you are well,

This is due to it increasing the extraction rate with ducting as it can essentially vacuum through a tunnel.

When there’s no ducting on the end they will not be as high of an extraction rate. 

Kind regards

Zennon MacDonald

Manrose Technical Support​​​​

Email: technical@manrose.com Tel: 01753 691399 Web:https://www.manrose.com

I can't claim to understand this, but it's what the experts say.