External Ducts fall away from MVHR - mitigating mould risk

[sheeners]

Hi 

 

our MVHR unit will be in a tight space where external ducting for exhaust and fresh air intake need to dip down below a rafter before reaching the roof vent. I've read that the fall of the ducting should be towards to unit so that any condensation can run through the unit drain. That's not going to be possible.

 

It's a warm loft and I'm planning on insulating the pipes. Possibly also have an enthalpy exchanger.

 

Question: what is the condensation risk inside the external ducts and what mitigation options do I have? I have read about inline drainage in the ducting at the lowest point. However, I cannot find any such material to buy. 

 

Thank you for sharing your thoughts.

 

Edited November 25, 2025 by sheeners

[markc]

Generally you don’t want any “pooling” points in ductwork, but if air is always moving then any condensation that does form should be quickly moved as it evaporates again.

[JohnMo]

If you read the manufacturers instructions, they will mist likely tell you to add a condensate drain at any low points. If not I pretty sure building regs will.

 

A 5 second search

https://www.google.com/search?q=150mm+condensation+trap&oq=150mm+condensate&gs_lcrp=EgZjaHJvbWUqCggBEAAYChgWGB4yBggAEEUYOTIKCAEQABgKGBYYHjINCAIQABiGAxiABBiKBTINCAMQABiGAxiABBiKBTINCAQQABiGAxiABBiKBTIKCAUQABiiBBiJBTIKCAYQABiABBiiBDIKCAcQABiABBiiBDIKCAgQABiABBiiBDIHCAkQABjvBTIHCAoQIRiPAjIHCAsQIRiPAtIBCTExNjk4ajBqN6gCD7ACAfEF5ygzgMLm-90&client=ms-android-oneplus-terr2-rso3&sourceid=chrome-mobile&ie=UTF-8

 

[Mike]

10 hours ago, sheeners said:

I've read that the fall of the ducting should be towards to unit so that any condensation can run through the unit drain.

It should either fall towards the unit, or towards to the wall vent - but you can also have some falling one way, the rest the other. So maybe you create a high point instead of a dip?

 

10 hours ago, sheeners said:

external ducting for exhaust and fresh air intake

It's only the exhaust that's susceptible to condensation, so don't worry about the intake duct.