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Apologies if this is a silly question but I can't find anything regarding the use of differing duct sizes on the site.

 

The majority of rooms in our house require a single run of concealed 75mm semi rigid duct to satisfy building reg extract rates whilst keeping velocity low enough to minimise noise - at least in theory. Our kitchen/lounge/diner is larger and has a vaulted ceiling with a volume of 214m3 which is considerably more than anywhere else in the build. My calculations show the room needing 150mm diameter (or similar) ducts. This isn't an issue as we plan to use exposed galvanised in this space however I'm a little unsure if it is OK to mix duct sizes on a system or will it cause problems?

 

Flow velocity at the outlets can be balanced in theory to within 1/2ms of other but the smaller rooms have an ACH of 2-3 whereas the largest room is 0.4. Is having such a varying range for this also an issue?

 

Many thanks

 

 

 

Edited by bissoejosh
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The standard way to do this with a 75/68mm radial duct system is to double up the ducts.  The terminals will usually accept either one or two ducts, so for any high flow rate extract rooms you just run two ducts in parallel from the plenum.  If you need more than that then you will need to add extra terminals to the room anyway, as the maximum flow rate through a ceiling terminal becomes the limiting factor, not the duct size.  In general, standard size ceiling terminals can only handle the flow from up to about 100mm diameter duct, or two parallel 75/68mm ducts. 

 

The flow velocity has to be kept below 2.5m/s everywhere if you want to avoid flow noise.  Anything over 2.5m/s, even locally around a terminal, will start to make a noticeable noise.  This probably isn't an issue for high rate extraction (for example to meet building regs for a kitchen), as that's only a boost extract rate, so will only be on for relatively short periods when needed.

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It would certainly be easier if using concealed runs to just double up for the large room. Unfortunately as we are having exposed ducting in this space it isn't possible so I need to find a way to join a 75/68mm distribution box to a 150mm duct without somehow upsetting the flow rates or causing noise. Not really sure that is possible unless I could take 2 outlets from the distribution box and branch them into the 150mm.

 

More research required I think. 

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In that case I think I'd be inclined to switch from a radial configuration to a conventional rigid duct configuration for the whole house.  I strongly suspect that trying to balance a mixed configuration will be a nightmare, especially with conventional larger bore ducting on the extract side and small bore radial ducting on the fresh air supply side.  Whilst a conventional duct configuration needs a bit more up-front design work to get the flow rates in the right ball park, I think it may well be your best option.

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It may be possible to mix systems.  The issue being that the kitchen lounge diner requires both extract and supply.  Extract by the kitchen and supply elsewhere.

It is possible to convert from semi-rigid radial duct-work to large bore by using a Ubbink DB208/DB206 distribution box in reverse depending on the size of duct-work:

 

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Simply connect the galvanized duct to the distribution box the run as many semi rigid lines as as required for the correct flow rate, flow limiters to balance teh system as inserted in teh main distribution box or in these adapters if required.

 

Then run semi rigid elsewhere, doubling up as required.

 

Not as it was originally designed but would work.

 

 

 

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Interesting. Another idea I've just had would be to tee into the supply/extracts just before they go into the distribution boxes as they are likely to be a much larger bore at this stage - possibly even the same with careful planning? That would be a simple enough join and keeps the distribution box for the smaller bore semi rigid duct. 

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I can't help thinking that mixing systems is likely to result in a system that's really difficult to balance, bearing in mind the need to ensure the need to ensure that the flow rates remain balanced with changes in ventilation rate (from background through to full boost). 

 

One thing that needs care is making sure that there is no tendency for a branched-off larger duct to create an imbalance as the flow velocity increases, due to possible dynamic changes in flow induced by a local velocity change past a junction.  If the index branch changes due to a change in velocity past one of the junctions in the ducting, then it may well be near-impossible to balance the system properly.

 

 

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