MVHR intake and exhaust vent position

[Stones]

It is my intention to order my MVHR unit and ductwork by the end of the week.  The ventilation plan (kindly provided by BPC before any order has been placed - outstanding service from the team there, extract of the relevant area attached) shows the air intake and exhaust vents positioned on the same length of wall in the soffit / overhang. 

MVHR vent position.pdf

Unfortunately, there are a couple of issues in respect of the position as currently marked.  The first is that I have no soffit / overhang to speak of (the trusses currently being fitted do overhang the ICF walls as they are by 195mm, but the walls still have a 100mm layer of EWI, batten and timber cladding to be fitted on).  The second is that the vents are currently shown on the prevailing windward side of the house.

As I see it I have two options in respect of the soffit / overhang issue.  I could bring the intake and exhaust air ducts out of the insulated loft/service space I am creating, down the outer face of the ICF block wall before the EWI and timber cladding goes on, terminate with a 90 degree bend and finish with a grille or hood once the EWI has been fitted around the ducts and the cladding is on, even if it meant creating a decorative cladding box around the duct to give me sufficient depth to return the 90 degree bend out .

The second option is to put both ducts through the roof.  Ultimately, whatever I do means penetrating the insulated layer.

The issue of both vents being on the windward side seems easy to solve, by moving them to the leeward side, except this is where our entrance door is, and I'm not sure how sensible it is to terminate the ducts, either side of our door.  There is also the potential issue of how close the vents will be to each other (duct centres of 1.8m)

Any thoughts?

[Jeremy Harris]

I think I'd be inclined to put them up through the roof.  Almost certainly the easiest option and probably means fewer bends, too.  It doesn't matter then if they are on the windward or leeward side, as they'll be at 90 deg to the wind anyway.

 

[ProDave]

Related to this, the easiest for me would be to put them both through the east facing wall. It would be harder to do, but part of me says to put them through the south wall as the air there is more likely to be warmer.  Likewise the worst place to put them would be through the north facing wall as that's much more likely to suck in very cold air.

Or am I talking nonsense?

[Jeremy Harris]

The air temperature tends to be the same all around the house, as it's usually always moving to some degree, even on a "windless" day.  The sun doesn't really directly warm air very much, it warms surfaces that then warm air, but the flow rates through the MVHR terminals is high enough for there to be very little heating effect, I think.  Our intake is on the North wall, under the eaves, and last week it was drawing in air at 29 deg C.

[Stones]

 

12 minutes ago, ProDave said:

Related to this, the easiest for me would be to put them both through the east facing wall. It would be harder to do, but part of me says to put them through the south wall as the air there is more likely to be warmer.  Likewise the worst place to put them would be through the north facing wall as that's much more likely to suck in very cold air.

Or am I talking nonsense?

Part of my thinking in terms of windward or leeward side.  I know someone locally who put one vent through his roof and the other through his wall, which caused huge imbalances when the wind gusted.  I'm therefore keen to keep both on the same roof or wall, provided I have sufficient separation between the two.

https://www.roofingsuperstore.co.uk/product/ubbink-ub16-universal-ubiflex-tile-vent-anthracite.html

https://www.roofingsuperstore.co.uk/product/ubbink-ub47-universal-150mm-insulated-vent-terminal-tile-slate.html

seem to be the main contenders.  Not sure which would be best.

[ProDave]

It's not uncommon to get a still, cold, sunny spell here in winter.  Open the back door and the air is warm and the ground is frost free.  Open the front door on the north side and it's like opening the fridge, and there's permafrost building up where the sun never shines.

There is one road near here that I avoid in winter. It's in the shadow of a hill so never gets any sun for nearly 3 months of the year, never gets salted, and that can build up a really thick layer of ice that stays for weeks.

There is a huge differene here between north and south sides.  Granted when it's blowing a hooley from the west it will make no difference.

[Jeremy Harris]

Bear in mind that there will always be a significant "breeze" blowing in and out of the MVHR terminals, roughly half the volume of your whole house every hour, through a 150 to 160mm diameter hole.  That tends to over-ride the near-ground level static air, in my experience. 

[Stones]

Any thoughts on which of the two types of roof vents I linked to would be best?

[Stones]

Just bumping this one to see if anyone has views about which type of roof vent would be best;

https://www.roofingsuperstore.co.uk/product/ubbink-ub16-universal-ubiflex-tile-vent-anthracite.html

https://www.roofingsuperstore.co.uk/product/ubbink-ub47-universal-150mm-insulated-vent-terminal-tile-slate.html