MVHR fierce supply rates with restricted flow - what am I doing wrong?!

[DownSouth]

We are setting up our MVHR. No issues using an anometer and the extract rates are bang on. However for the supply we are having to restrict the flow through the pipe to such a point that it’s all coming through a small gap, creating noise and it’s throwing the balance off.

zehnder say to commission at the medium speed (their default is 235) but we just can’t get the supply right. Should we be adjusting it to the total air flow needed for this actual build which is ~160.
 

we’re giving up for today  but grateful for any tips! 

[DownSouth]

https://www.international.zehnder-systems.com/en/spotlight/detail/6-common-mistakes-when-installing-and-commissioning-ventilation Our problem is that all the supply valves are effectively being throttled to get down to the per room flow rate.

[JohnMo]

The system are you running semi flexible ducts via a plenum box or rigid branch system?

[Nickfromwales]

2 hours ago, DownSouth said:

We are setting up our MVHR. No issues using an anometer and the extract rates are bang on. However for the supply we are having to restrict the flow through the pipe to such a point that it’s all coming through a small gap, creating noise and it’s throwing the balance off.

zehnder say to commission at the medium speed (their default is 235) but we just can’t get the supply right. Should we be adjusting it to the total air flow needed for this actual build which is ~160.
 

we’re giving up for today  but grateful for any tips! 

I assume this is a series system vs radial? 
 

Do you have the same number of supply outlets as extract?

 

The outlets closet to the unit need to be strangled back, to allow airflow to eventually arrive at the very furthest ones, and this makes them audible / noisy; another con / caveat when deciding between series and radial.

 

Radial for me every time, too many fleas with the series dog. . No need for cross-talk elimination (more £££) either then. 

 

If the system design has balanced the number of in vs out then are you doing this with doors open. Have you done the undercuts on the bottoms of all internal (vented) spaces?

[DownSouth]

It’s a radial semi-rigid duct design which is continuous from the unit to the room valve, testing with all doors / windows shut and cut outs under internal doors to allow flow. Same number in /out. 

2 hours ago, Nickfromwales said:

The outlets closet to the unit need to be strangled back, to allow airflow to eventually arrive at the very furthest ones, and this makes them audible / noisy; another con / caveat when deciding between series and radial.

@Nickfromwales would this comment apply to radial set up? 

[JohnMo]

On a radial system set you longest run first, that is your index run. Not sure how your MVHR unit adjusts fan speed, but you really want your index run with a fully open terminal and flowing the correct amount by adjusting fan speed. Then set the others ideally work from the next longest and finish with the shortest, adjusting terminals. Then check them all again and do any fine tuning needed.

[Nickfromwales]

2 hours ago, DownSouth said:

It’s a radial semi-rigid duct design which is continuous from the unit to the room valve, testing with all doors / windows shut and cut outs under internal doors to allow flow. Same number in /out. 

@Nickfromwales would this comment apply to radial set up? 

Oddly you'd think these would be on par, but the radial seems to offer much less of an issue here from my direct experience.

 

I am overly meticulous with my MVHR specifications, as I need to stand by the promises I make to my clients as to how well (and silently) it will perform. One of the biggest problems with MVHR is bean-counting, and not allowing a good bit of headroom resulting in operational fan speeds of well under 50% of the unit capacity.

 

Try lowering the fan speed and work from the bottom > up, and aim for less than 0.5 ACH at trickle. How many m2 is the dwelling?

[DownSouth]

Thanks. It’s 216m2 over 2 floors. 

[Nickfromwales]

15 hours ago, DownSouth said:

Thanks. It’s 216m2 over 2 floors. 

Turn the fan speed down a lot lower, and try again.

[DownSouth]

@JohnMo @Nickfromwales thanks to you both. Adjusted the preset fan speeds for the low, medium (preset ‘2’) and high settings. Then retested the supply (extract was ok) starting with longest run, worked back from there checking balance after each one. 
Kept it on setting 2 for a bit, now on the lower setting which is 30% less, working perfectly 😊