Refining my design - some basics / principle-type questions

[Great_scot_selfbuild]

Ok, so I'm revisiting some of my MVHR design and I've found myself circling the following questions, without really finding the answers from any of the manufacturers sites (happy to be pointed in the direction of any reference material):

 

Whilst I mention ender here, I'm not fixed on them as a brand - happy to take advice/ lived experience on performance / noise of other brands.

 

1. What is the largest number of 90mm ducts you can feed from a single manifold (single MVHR unit - looking at Zehnder Q600 sort of size)?

2. I'm interested in a quiet system; when suppliers were quoting for 75mm they were insisting on twin ducts per vent purely to meet the BR spec. I have now managed to confirm I can fit 90mm (through holes in beams - I had previously worried 90s wouldn't fit, but they do). If I fit twin 90 per vent, will this just make it even quieter by reducing the flow rate per duct even further but still achieving the BR requirement at the single vent?

3. My investigation into attenuators previously was because I was looking into a branch system and needed them to avoid cross-talk; would an attenuator on a 90mm duct as part of a radial system reduce the noise?
4. I have a limited number of holes through the glulam beam, and so it would be preferable to achieve quiet running using attenuator on a single duct rather than running twin ducts.

5. If I had twin ducts feeding into a single vent, is there a single attenuator that would receive two ducts and then go into a single vent?

6. Has there been any study (formal, or just based on experience) into the noise difference between attenuator and multiple ducts.

 

For upstairs, almost all our vents will be wall vents. We have 90mm studs and the duct is 90mm (having offered up, it'll fit). Where we put the ducts in the stud wall, I plan to line the wall with 9mm OSB either side before plasterboard. The studs will have rock wool sound insulation.

 

7. How much space is needed for the 90degree wall vents?

8. Although the 90mm zehnder duct (off-cut) I have offered up looks like I could get it to work, I would also consider the flat cross-section ('Flat 51' as zehnder call theirs), but note that the actual cross-section is less than 90mm; if I use these, do I need to double up (not according to their spec sheet), but I'm concerned about space and the practicality of bending them into the space available.

 

Thoughts?...

 

[JohnMo]

Lots of questions 

 

The basics and that's where you start from.

 

First it's about pressure drop management, a higher pressure drop the fan in the MVHR have to work harder to give a set air flow.

 

Second when you use a plenum and individual pipes to rooms, the strings in parallel, so you will have one single run which sets the pressure drop - it's generally the longest pipe run and the highest flow rate. This is the route you need to concentrate on and get low. Then make sure all other routes do not exceed that pressure drop.

 

Noise comes from two sources

A. Fan noise - as mentioned above

B. Duct noise from air velocity. This is generally managed by looking after pressure drops.

 

So get your data sheets, look at pressure drops for ducts, bends and terminals. Get data sheet for fan speed/pressure charts for the MVHR.

 

Before you do that you need to understand each terminal flow rates.

 

You can massively reduce duct length by using coanda terminals. These will throw air from one side of the room to the other following the ceiling before coming down.