Critique my MVHR plans - round 2

[Sparrowhawk]

After my last ask, we've negotiated with the neighbours to redirect their boiler flue so we can put the MVHR in the most logical place: the garage. I've also compromised by putting upstairs ducting in the cold loft. This has simplified things:

 

A few notes (or skip to the calculations below)
Upstairs

• I've compromised by putting ducts in the cold loft outside the thermal envelope, as my wife is 100% against any boxing in
• The loft is well ventilated at all eaves (anywhere there is a fat wall on the plan)
  • Positioning supply away from doors makes the pipes run close to vented eaves, so more exposed to cold air in winter. Best compromise is to keep pipes warmer and put near doors?

Between floors

• I've gone for separate distribution boxes on each floor, to minimise number of penetrations into cold loft (airtightness). This is open to negotiation.
• The chimney breast has wood frame round it supporting floor joists, so cannot drill straight through in ceiling void - has to be within the downstairs room

Downstairs

• Got permission to lower part of the ceiling in the dining room & hall to hide ducting
• Need to sort route to downstairs cloakroom - an annoying run to the other side of the house
• Have to get the ducts through an external wall + chimney breast into main house
• No duct to dining room - assuming the flow through from the office & lounge will be enough ventilation

MVHR

• Compromised by putting outside thermal envelope in unheated garage
• Will build insulated "plant room" around it with 100mm PIR to protect from temperature fluctuations
• Fire safety requirements?
  • Can mitigate with 2x plasterboard boxing in the MVHR and ducts?

 

Calculations

There's 2 of us in the house but as it's 4 bed I'm calculating based on 4-5 people.

 

I started with the rule of thumb for 0.3ACH by @Iceverge which gave me great hope - I didn't need massive supply/extract between floors or through solid walls! Then I looked at Part F and the Passivhaus recommendations (which seem v sensible, thanks @JohnMo) and came to a halt. Here's my rooms, displayed based on @jayc89's spreadsheet. The final column is me trying to get supply and extract volumes to match. Editable spreadsheet attached.

 

Questions

1. Downstairs has v little supply, despite people being down there 15 hours/day. Bedrooms having more seems wrong?
2. The utility room and kitchen are effectively a single room (no door between them). Worth reducing utility room extract so the kitchen clears more easily?
3. I am stuck with how to treat duct length for pressure drop figures - do I calculate it from the MVHR to each plenum, or do MVHR to first splitter / splitter to distribution box / distribution box to plenum?
4. Correctly sizing the ducts between ground floor and loft distribution box, and to the Dining room distribution box, is going to be crucial. Does 2x 90mm OD for each of these seem right?
5. How do I best make the loft penetrations airtight? Sealing multiple circular ducts to the ceiling so there are no leaks looks a challenge

 

MVHR calculations for BuildHub.xlsx

Edited March 31 by Sparrowhawk

[JohnMo]

You could do a cascade system downstairs to simplify duct routes etc.

 

Downstairs, supply air to lounge, via 2x coanda nozzles, high on wall next to dining room. A further outlet would be to office (7m2). The 6.8m2 office would have a transfer fan in the wall from first office to second office). This could be a dMEV type fan or something like this which works on demand only.

 

https://www.ventilationland.co.uk/en_GB/p/brink-indoor-mixfan-co2-controlled--up-to-70-mh/17927/

 

Extracts leave as you have them.

 

Upstairs leave as is.

 

Flow rates, you need to balance overall flow and extract - you don't need to balance downstairs/upstairs extract and supply by floor, any excess will go via the stairs.

 

With two people in the house your flow rates overall can be 0.3ACH or slightly lower.

 

Coanda nozzles high on wall looks like this (my lounge made by zehnder) and fit direct onto 90mm duct.

 

 

[Iceverge]

Wowsers!

 

You've put lots of work in here. 

 

My first question is ......and you're going to hate me........is it worth pulling your house apart instead of another system like dMVHR or dMEV? 

 

 

Secondly what is you airtighess like?

 

I'll have a better gander at it later on in the week. 

[nod]

I’m presuming that your airtightness is really good If not it will be a lot of work for little benefit 

[Sparrowhawk]

11 hours ago, Iceverge said:

Wowsers!

 

You've put lots of work in here. 

 

My first question is ......and you're going to hate me........is it worth pulling your house apart instead of another system like dMVHR or dMEV?

 

I've gone over this a lot, and there's 4 reasons I've headed to MVHR

1. The location. We're in an exposed coastal location and when the wind hits, it wind-washes the house by finding a path between any openings. I want to avoid trickle vents on new windows and remove existing extractor fans. AFAICT dMEV/dMVHR cannot prevent this wind washing, because the openings are on different aspects of the building. With MVHR, with the intake and exhaust on the same aspect the pressure should equalise
2. Avoiding adding trickle vents to the new windows
3. Air filtration. We have bad hay fever which meds don't fully combat, and the idea of keeping pollen out of the house for much of the year sounds wonderful.
4. High internal humidity. We've had a dehumidifier running 24/7 this winter which has made a big difference. While many systems help (the standard solution around here is PIV) I figured a flow through the house is going to be important, not just room by room and we need ventilation no matter if it's windy or calm outside.
   • This is complicated as external humidity is often high (both RH and absolute). But, autumn-spring, should be lower than indoors

As an aside on 1) our kitchen extractor fan stops extracting in bad weather if the wind's from the south west.

 

11 hours ago, Iceverge said:

Secondly what is you airtighess like?

 

Better than it was 4 years ago

 

The home made blower door has done great work, and sealing first floor joist penetrations this winter has made one room airtight and a big difference in the others.

 

When it's not windy outside, the rooms I've worked most on get stuffy and need a window opening.

 

This year we're replacing all external doors and double glazing which - after extractor fans - are the leakiest part of the house. I'm lifting the suspended floors this summer and adding airtightness and insulation. EPS cavity fill is coming, and the chimney was removed this winter and will be draft free once we remove the fireplace and seal it up.

 

All of which is to say I don't have a figure, but I have an ongoing programme to make it better.

Edited April 1 by Sparrowhawk

[Sparrowhawk]

10 hours ago, JohnMo said:

You could do a cascade system downstairs to simplify duct routes etc.

Thanks John, that was what I was hoping.

 

The transfer fan is something I can't add though as due to the nature of our work clients need privacy, and I think that fan would break soundproofing. A separate supply to the 6.8m2 office is the best alternative?

 

10 hours ago, JohnMo said:

Flow rates, you need to balance overall flow and extract - you don't need to balance downstairs/upstairs extract and supply by floor, any excess will go via the stairs.

 

Oh good, I thought I had to balance each floor.

 

Great to see what a coanda nozzle looks like, that will blend in nicely with what's planned for that lounge wall.

[JohnMo]

2 hours ago, Sparrowhawk said:

separate supply to the 6.8m2 office is the best alternative?

Yes 

[HerbJ]

13 hours ago, Iceverge said:

Wowsers!

 

You've put lots of work in here. 

+1 to this but  and you seem to have covered most of the key points.

The big question/issue  for me is the detailed routing of the ducts, particularly  downstairs, in locations where you haven't negotiated "permission" to reduce ceiling height. There is still a lot of investigation and detailed work to be done to ensure that you are able to make penetrations in floor joists  without compromising structural integrity BEFORE you start installation.  

 

[Sparrowhawk]

21 minutes ago, HerbJ said:

The big question/issue  for me is the detailed routing of the ducts, particularly  downstairs, in locations where you haven't negotiated "permission" to reduce ceiling height. There is still a lot of investigation and detailed work to be done to ensure that you are able to make penetrations in floor joists  without compromising structural integrity BEFORE you start installation. 

 

That's my bad, downstairs I've got permission to lower ceilings as the ceilings are higher to begin with / the rooms don't have sloping ceilings cutting into their height.

I do need to find out if in the 30cm side of chimney breast I can drill 2x 9cm holes for the ducting (as drawn above) without weakening it. If not then I need a big rethink for how to get from the garage to the dining room.

[Sparrowhawk]

I've updated my spreadsheet, and it looks like 90mm single/double runs will cover all, while double 75mm would break the 2.5m/s rule for the kitchen and en suite extracts.

 

To connect downstairs to upstairs I think something larger than 2x90mm is going to be needed, given those speeds for the full supply and extract volume.

[MikeSharp01]

Let's hope @MVHRDesigner is out there looks like they know their stuff.

[Sparrowhawk]

Am I correct in thinking that when you do a radial ducting system it's the max pressure drop of any of the ducts that matters, not the sum of all the pressure drops?

 

A simple calculation on duct length and flow (assuming straight ducts) gives:

 

The total static pressure is getting close to what a well designed system should have as its max, before adding plenums, bends and more, so I'm assuming I've done something wrong.

[JohnMo]

7 minutes ago, Sparrowhawk said:

max pressure drop of any of the ducts that matters, not the sum of all the pressure drops

Max drop of the worst duct run from MVHR outlet to and including the outlet nozzle / terminal. As this run will also determine your fan running speed the nozzle should be in the fully open position, so its lowest pressure drop.

[Sparrowhawk]

Latest update: I've redone the floor area reducing it for the skeilings, excluding areas under 1m in height and taking 50% of the floor area for areas 1m-2m high. 50% seems to be the done thing, but makes that part have an effective ceiling height of 3m instead of the 2.5m elsewhere 🤷🏻‍♂️

 

That gives a floor area of 142m² (previously 162m²)

 

ADF for a 4 bed (Table 1.3) = 133.2m³/h

ADF on floor area (1.24a): 154m³/h

ADF boost extract rates (Table 1.2): 156m³/h

And 0.33ACH of the volume is 120.6m³/h

 

So designing for a minimum ventilation rate of 154m³/h:

 

 

This gives distribution of (downstairs = lounge/P office, upstairs = bedrooms)
  

 

For 2 of us in the house I expect to need to turn this down (based on forum members' experiences), but I think we should still have 40m³/h into our bedroom which will need it rebalancing. We will see.

 

Nevertheless, I thought I'd best calculate a boost mode. I arbitrarily chose 1.3x (the Passivhaus figure is 1.42x). This results in:

 

with distribution as

 

Should I keep boost mode under 2.5m/s? If I should, Boost mode means doubling more duct runs.

 

This leads to unit size. Blauberg talk about a 40/70 rule, where "The trickle rate is no more than 40% of the MVHR units full speed. The boost rate is no more that 70% of the MVHR units full speed.". Is this a common rule of thumb? 159m³/h is 40% of 397.5m³/h. 207m³/h is 70% of 295m³/h. I'd been thinking to use a Zehnder Q350 (rated by PHI for 270m³/h) or Brink Flair 325 (rated by PHI for 251m³/h). Given my floor area's only 142m² getting a result that says "increase unit size" sends me looking for a mistake.

 

Looking at the initial quotes I got based on 155m2 floor area, BPC specced an "Up to 425m³/h" unit (will be less once F7/G4 filters are added), GBS a Zehnder Q350, HS&L a Zehnder Q350, and CVC a Brink Flair 400 (rated by PHI for 313m³/h). So a bit of a spread.

Edited May 23 by Sparrowhawk
Added details from initial quotes