Down the rabbit hole, what external inlet and outlets grills and duct size.

[Tims]

I am looking to finish the install of my MVHR over the next couple of weeks (need it done before we have family staying for Xmas and I go back to work  in January after a couple of years of playing builder) and am sourcing the last bits. I've moved where I am locating the unit from an upstairs roof space cupboard (West (a little south) facing aspect  for roof vents,) to a utility room and will put the exhaust and intake through a North (westish) facing gable wall on the ground floor, as the roof where the inlets and outlets were going gets very hot in the evening in summer when it gets the sun. I'm going to keep the manifolds where they are in this upstairs walk in cupboard as the new room ducts are in and its a central location, and supply the manifolds from the ventaxia  sentinel kinetic advance MVHR unit via 160mm diameter Spiral duct boxed in along the bootroom/utility room ceiling giving me a 3m feed to the manifolds with 1 * 90^o bend and 2 *45^o bends to get me over a steel and up to the manifolds. All of the pressure calculations for this look good and the air velocity stays nicely below 2.5m³/s.

 

I've now got my self lost down a rabbit hole on the external vent side of things. Doing the system pressure calcs on a system pressure calculator spread sheet I found on the web last year from (Mas ltd) I came to the conclusion that I could get away the relatively small duct runs to the outside world in 125mm duct (smaller holes in the wall no worries about lintels etc) as the grill/louver dominates the loss and I guessed that I can live with a 3.6m³/s velocity for a small distance in a utility room. Then I noticed that MAS had fixed the grill loss at 10pa for all duct sizes in their spread sheet and that in reality the loss from the grills is huge at maximum designed ventilation rate and would alone exceed the passive house design documents 80pa upper system pressure drop ( We're not a passive house and don't have hopes of ever being but they seem to have lots of useful information and pointers in the right dirrection).

 

if anyone's read this far 

 

• What external grills are people using? What's the loss and noise like with them?
• What size ducts to the outside world are people using?
• On brick walls at what point in duct size are people starting to knock bricks out and fit concrete lintels over the duct?
• Is anyone using one of the combined inlet/outlet ducts like Paul Heat Recovery are selling and are they any good or should I stick to a slightly longer exhaust duct rout to get vertical and horizontal separation (exhaust above inlet).

 

Thanks in advance for any feedback/wisdom anyone's got..

[Tims]

So Lindab have a nice calculator on their website it looks like 2 * 150mm YGC  wall grilles are a good compromise and keep the velocity at 2.5m/s to boot.

[Tims]

Also SE mate has confirmed not to worry about the hole sizes at 160mm in the positions I'm putting them so jobs a good un.