kitchen extract doesn't need to be above the hob. it's extracting moist air not cooking smells. In the barn we have the kitchen extract near the sink, drainer, & dishwasher, where the steam and evaporating water is. Above the hob we have a recirculating cooker hood with grease then carbon filters. Some on here have cooker hood extract to outside, I don't know how that works in a Passive House with MVHR but I'm interested.

BPC were actually generous with the materials estimate, I have an entire roll of semi-rigid ducting unused, but I also worked out carefully a lot of the other stuff e.g. insulated ducting to outside exhaust and supply air.

We also have a supply terminal at the far end, diagonally opposite corner

Posted By: Dominic CooneySome on here have cooker hood extract to outside, I don't know how that works in a Passive House with MVHR but I'm interested.

It doesn't. It would blow the airtightness. We do as you with a recirculating extractor. The MVHR extract definitely should NOT be above the hob.

I've got half a roll of duct left too.

PH requires an extract or supply terminal in every room. In our plant room I just left the duct exposed and taped over the end to restrict the flow as desired. A terminal seemed a bit of a waste :)

All our ducting runs are in the first floor joist space. Our joists are metal web, which makes running the ducts easier. Terminals downstairs are all in the ceilings. Extract terminals upstairs are high on the stud walls, and supply terminals are grilles in the floor. It works well. We used semi-rigid ducting too.

If I had wooden joists I'd make that clear to the system designer, and I'd probably be expecting to have a single big duct run (well one supply and one extract) feeding smaller ducts to individual terminals. The big ducts would probably have to be insulated ones in the roof space.

Our clothes dry well in the bathroom (with an extract terminal) on wet days like today. We have a Sheila's Maid in there.

We have extract in all wet/damp areas i.e utility kitchen bathrooms and supply to every other room. The kitchen extractor is over the hob flush to the ceiling and recirculates back to the room about 2 m away. There is a MVHR extract about 2 m away from that to pick up the "clean" moist air. We have a press button switch next to the hob which puts the MVHR into boost mode. All "wet" rooms have a booster switch. The extracts including the cooker needs to be at ceiling level the flusher the better as the warm air rises and "parks" at the ceiling the extraction is much better like this. Although we are not passive house we adopted the principles of PH. I used rigid Domas ducting 150 round and 125 round feeding 204x60 rectangular ducting. One circuit for downstairs in the ground floor ceiling the 1st floor in the loft. I originally bought plastic outlet valves but they were rubbish so got better quality Lindab metal ones. In the kitchen you will need a fire extract valve.

Posted By: Doubting_ThomasEveryhabitableroom - it is acceptable in Passivhaus to use what's called a 'cascade ventilation' strategy

Hmm, sorry, I was thinking about the need for ventilation in *every* room - "All rooms within the thermal building envelope must be ventilated either directly or indirectly (transferred air) with a sufficient volume flow rate. This also applies for rooms which are infrequently occupied by persons" and forgot about cascades. But cascades through a room would be fairly unusual - it's interesting that the only diagram in the passipedia article is one pointing out where it is NOT suitable! Most living rooms and the like tend to be on at least one external wall, often two, and have at least one corner that isn't in an air circulation path unless there's an air inlet. We do cascade through the hallway, which is a lot more common situation.

Posted By: revorIn the kitchen you will need a fire extract valve

Hmm, we don't. More important to me is that we have a filtered extract terminal in the kitchen - it gets dirty far more quickly than any other room.

Posted By: Victorianecohe reckons I shouldn't be using MVHR as the house (cavity wall) is going to be so leaky, you'd never recoup the costs....

The Passivhaus Trust did a research paper on this which concluded MVHR is still a good idea for the moisture control and air quality benefits, even if it doesn't pay back on pure energy savings.

https://www.passivhaustrust.org.uk/UserFiles/File/research%20papers/MVHR/2020.04.27-The%20Case%20for%20MVHR-v7.pdf

You start to save on the energy from below 5m³/m².h @50Pa which is still pretty leaky.

Might be worth you getting an airtest done to establish where you are. Ours was about ÂŁ200 and I think you can probably find cheaper.

Posted By: VictorianecoAn ASHP specialist came out today and he reckons I shouldn't be using MVHR as the house (cavity wall) is going to be so leaky, you'd never recoup the costs....

But you're planning to do a big retrofit and you said you believed in airtightness, so that won't be the case, yes? MVHR will be required!

I think when I looked into FreshR type system it was quite expensive (but probably less hassle to install? - no ducting) maybe better for retrofit where you don't want to disurb any of the existing building fabric?
Because we were doing an entire barn coversion, we bit the bullet and put in a whole-house system.
There was some compromise on the exact position of supply and extract terminals in some of the rooms (due to constraints of the existing building fabric) but we did manage to get a supply terminal to the outside wall and furthest corner in each of the far ends of the building (lounge, kitchen and bedrooms) with all the extract rooms close to the centre of the building, and a centrally located MVHR unit in the utility room. In and Out to outside air went through the side wall via insulated ducts in the first floor void, only a couple of metres.

My biggest issue is the joists are 8" x 2" so I couldn't drill 75mm holes in these and the joists run both front to back in some rooms and left to right in others.

I have received a plan from BPC which looks great, now it's just trying to achieve it with minimal disruption.

But you're right, it's better to fit it now and have the option and I can hold fire on purchasing the unit itself perhaps..

Posted By: VictorianecoMy biggest issue is the joists are 8" x 2" so I couldn't drill 75mm holes in these

You can drill a 6.75 mm hole in a 8" joist without restriction of the location (unlike notches which are limited to 1/6 of the joist depth but not in the middle 1/3)

If it really makes a difference I would have thought a 7.5mm hole mid point of the depth at the end of the joist would work (even if it means screwing and glueing a batten either side of the joist under the hole, talk to your local friendly SE). Having drilled the hole it would then be another matter of threading the pipe through, probably only in short sections so the trouble of installing the pipe for anything other than a couple of joists probably isn't worth the effort.

The regs I use quote -
"Holes bored in joists shall not be within 2 inches (51 mm) of the top or bottom of the joist, and the diameter of any such hole shall not exceed one-third the depth of the joist. Notches in the top or bottom of joists shall not exceed one-sixth the depth and shall not be located in the middle third of the span."

The UK may have different regs

The EU countries all adopted Eurocode 5, including the UK, about 15 years ago. All metric now... Maximum hole dia allowed now is 0.25x joist height, so 50mm in a 200mm joist. But not allowed in zones around the ends or midsections of the joists, which is ideally where you'd want to put the services..

Victorianeco, have you looked at decentralised mhrv? No ducts, so good for retrofit.

Here's a particularly fancy one:
https://www.paulheatrecovery.co.uk/comparison-of-recommended-mvhr-units/cascade-mvhr/

Any idea on pricing?

a central MVHR with radial fixing will have SUPPLY to every bathroom and living room, EXTRACT only at bathrooms, kitchens, any rooms with taps etc like utility, nothing for hallways, and a door undercut in all rooms, the MVHR will be pressure balanced so that fresh air from the MVHR goes into bedrooms / living rooms, then the air flows out using door cuts, and that air goes into extract rooms via door cuts, then out of the extract rooms via ducting to MVHR. Cooker hood should be recirculating only and bathroom / kitchen fans disconnected. So basically a single duct will go from or to every habitable room. Ducting can go in false ceilings, parallel to beams, be boxed in (eg making an alcove feature around it), run via attic (if insulated enough) and up or down disused chimneys. You would only need both supply and extract in one room if you are having a different design or if you are having a fully soundproofed room or airtight room for some reason.

We have been warned that for retrofit many free designs are very poor. They should all have supply ducting marked on. Otherwise where would the heated fresh air go after going through the MVHR.

Posted By: djh

Posted By: stonecolda central MVHR with radial fixing will have SUPPLY to every bathroom and living room

This is simply wrong.

I read it as a typo. You shouldn't underestimate modern autocorrect!