MVHR branch system - diameter of rigid metal ducting

[Novice Becky]

What diameter ducting should you use for rigid metal ducting in a branch layout? I'm having a retrofit MVHR system (rigid metal ducting) - see attached floor plan/layout. 

 

I'm using BPC as they are a lot cheaper than the swanky MVHR companies and will pay an installer. However they are saying that ALL the ducts should be 125mm diameter. This is not what I've read anywhere else and it doesn't make sense to me. eg. Heat Space and Light say: "Rigid steel ducts will start at between 200mm – 125mm diameter at the MVHR unit, gradually reducing to 125mm and 100mm diameter ducts to each room."

 

I have questioned BPC:

ME: The layout shows 125mm ducting throughout. Being a branch system, I was expecting larger ducting nearer the MVHR unit. Would it improve airflow if the ducting nearer the MVHR unit were a wider diameter with smaller diameter ducting on the 'branches'? Or is there a reason for it all being the same size?

 

BPC: This is the correct ducting size for your MVHR unit (125mm or equivalent). If you wish to go for a larger size of ducting, it will just reduce the P/A rating. 

 

ME: I'm still not convinced on using the same size ducting throughout on a branch system and I can't find any examples of this.  I thought the main runs of ducting needed to be larger than the branches in order to carry the higher volumes of air at a low velocity (less resistance - making it quieter and more efficient). The branches will be carrying a lower volume of air and where they join up there will be a greater volume of air.  If all the ducting is the same size surely it will be noisier and less efficient as all the air would be 'squeezed' into a smaller space?  This is why when you have radial ducting the entry spigot on the manifold distribution box is always larger than the distribution spigots.

 

I'd appreciate people's thoughts on this. Can anyone explain BPC's thinking?

 

PROJECT DETAILS:

1960s bungalow

Surrey, England

134 M2

Condensation issues (but also want to make my home more energy efficient)

Vent Axia Sentinel Kinetic Advance S (large but chosen because it takes F7 filters (good for my headaches) and it looks to be very efficient for a smaller price tag)  https://www.vent-axia.com/sites/default/files/474957.pdf

Specified rigid metal ducting - I know radial with semi-rigid plastic is quicker, easier and cheaper but I've chosen metal ducting because of headaches linked to plastics and for longevity.

Fan unit and ducting to be located in loft area, insulated ducting.

Another Question: Not clear yet on whether I need to insulate the fan unit mounted on gable wall.

See my intro post today for more details of my retrofit

 

Also any other comments or suggestions welcome.  And where do I need to put rigid silencers?  I've asked for several revisions (see floor plans):

• the ducting to the atmosphere is too close to the eaves. I've asked for the fan unit to be moved to the right (more clearance height in the loft) and for the exhaust and intake valves through the (north-facing) gable wall to be positioned vertically near the RH corner of Bed 4 (I think this will look neater)
• extract ducting to be run diagonally across bed 2 (45 degree bend instead of 90 degree bend)
• extract in hallway (by Bed 3) is 'to balance the system'. I've asked whether I can have 2 extracts in the bathroom (think aftermath of teenage showers) instead of one in a hallway (there is another post on BPC putting extracts in hallways). Is it possible to balance the system just by having more air extracted per ceiling terminal compared to the amount of air being supplied per inlet valve? We have 4 (or 5) extract valves and 7 supply valves.
• Some minor repositioning of ceiling terminals but I am concerned about the one in BED 1 - The ducting run (Lounge to Bed1) needs to be moved down nearer the door because of the restricted roof space.  There's a steel beam across the middle of bed 1 and I was avoiding traversing this but I would like the ceiling terminal away from the door so it needs to go in between the bed and the cupboard. I'm concerned that this would be noisy as it's near a 90 degree bend. I suggested a 45 degree t-piece above the bedroom door with ducting branching off to the study at a 45 degree angle to avoid a ceiling terminal being near a bend. Any thoughts?  BPC didn't like this idea as it's longer
• Add a T-junction for the return air in the Shower room and make a short branch - there are some pipes running diagonally across the shower room and I'm not confident that it will be possible to fit the plenum in line with the straight duct run.

Novice Becky obstructions for MVHR system.pdf

[Gone West]

@Novice Becky I installed a 125mm branched MVHR system in my last house and didn't have any problems. It was a PH so the flow rates were low. I fitted only one attenuator which was on the supply side and doing it again I would fit one on the extract as well.

 

 

Edited October 8, 2023 by Gone West

[Conor]

You can downsize to 80mm for just the run to the final plenum on each branch.  Otherwise best sticking to 125mm for the rest.

 

Also, with 125mm ducting, you don't actually need plenum boxes, just a 90⁰ bend, short length of 125mm duct and put your air valve straight on. Save you a good £20 per room.

[Novice Becky]

Thank you Gone West and Conor. This is very reassuring! So is the decreasing diameter on a branch system just applicable to larger/industrial applications? ie. overkill for 134 M2?

[Novice Becky]

@Gone West No problems with cross talk with just one attenuator? Any cross talk between your lounge and study? How can I predict where I'll need an attenuator - is it where there are short runs of ducting between rooms? I read that you need one between bedrooms. What about noise from a TV?

[Gone West]

I fitted an attenuator to reduce noise from the MVHR unit into the ducting system. I don't recall crosstalk being a problem although I guess it is more of a potential problem with a branched system than a manifold system.