Mhvr units- t system or semi flexible ducts

What follows is just my opinion, I don't have either, I have just read bit here and there along the way so it may be all B.S

Leaving aside the obvious differences in ease of fitting and such like, the key design consideration, IMO, is to keep the velocity to a minimum as it's velocity that creates the noise.
I am talking about normal design flows at 3 am in bedrooms.

So depending on room sizes, you may find that the semi-flexible ducting has a more limited through-put at a specific velocity than its rigid counterpart.
This may require you to have two flexible drops to a room as opposed to a single rigid, or an increased velocity.

The rigid will have less resistance and will be relatively easier to clean should the need arise.

Then you have the issue of silencers, an item often dropped in Irish designs due to cost: " Sur you will never hear it missus"

My limited understanding is that silencers work better on larger, rigid ducts than on the smaller flexies.

On the layout issues, its is patently easier to thread the flexies than the rigid and this often reflects the MHVR design as being somewhat of an afterthought.
This is also manifestly obvious when I go into attic spaces and see the ducting fitted with 4 by 90 degrees angles because someone forgot about that big timber joist that we can't bore through.

In some cases, these, Lock Ness monster lookalikes are never insulated, giving rise to condensation in the ducting.

Final point, keep it all inside the conditioned air space.

If I were doing it, I would go rigid as far as possible
Hopefully the experts will join in at some point.

The following might help:


http://www.greenbuildingstore.co.uk/...t-maintenance/
Full report attached
Blog summary
"...There are a lot of really interesting points raised in the study, which we hope to cover in future blogs, but to start off we wanted to look at its findings regarding operation of MVHR systems, where two major issues were raised:

The first was that, due to a lack of communication with the occupants there was a misconception that the fans in the MVHR systems were expensive to run. This lead to half the occupants in the study disabling the system; the most common reason was out of concern for the operating cost of the MVHR.
The second issue was related to delays with filter replacement. A study of Passive houses in Wimbish development looked at the effect that a poor maintenance programme had on the MVHR system. The systems used the PAUL Focus unit which has constant volume flow fans, meaning that when dirty filters partly block the duct (increasing the resistance to the air flow), the fans push harder to ensure a constant air flow. However this means that the fans consume more energy and also get noisier. Also, at some point, if the filters are very dirty, the fans reach their limit and less air is supplied, reducing air quality and putting the system out of balance, which then also reduces thermal efficiency. Replacing filters at correct intervals is therefore important for the energy use, noise and air quality.
In the study data from the Wimbish Passivhaus project’s MVHR systems, the fan power was almost doubling before filters were being changed, with the fans reaching their limit and supplying less air...."

Could be useful for some of the guys over the wall who want to do MHVR with a nilfisk

From page 85
"Common MVHR Shortcomings

Specification
Wrong type of fan installed
Poor manufacturing of components
Lack of summer by-pass function
Poor control interface/ occupant inadequate control

Installation
Inadequate adjustment of control settings Failure to insulate ductwork
Failure to securely affix fan.
Deviations from design
Failure to connect ductwork to outside terminal Supply/extract ducts installed wrong way around
Supply/extract vents too close (short circuiting)
Outside supply/extract vents too close together- recirculation of exhausted air
Pollutant sources within 2m of supply grill
Poor sound installation/ silencers not installed properly
Over-use of flexible ducting (bends in ductwork) Contamination of ductwork during construction
Leaky joints
Air supply and/or extract vents not locked in place/ marked; wrong vents used
Insufficient gradient on condensate drains
Lack of traps (condensate tubes)

Commissioning
Insufficient and/or inaccurate commissioning

Maintenance
Inadequate access for cleaning Insufficient changing of filters
Lack of dedicated trade body/ accredited training for servicing/installation

Occupant Knowledge/Use
Inadequate occupant knowledge of ventilation system
Occupant(s) turning system off altogether or at certain times of the year
Tightening/blocking of supply/extract vents Inadequate use of ‘boost’ mode
MVHR system operated in lowest setting

Performance
Problems with noise, particularly in bedrooms Thermal comfort, perceived draughts, overheating"

The installation issues list is scary