Don't have inlet to the bathrom - will feel like a cold draught, when wet.

Posted By: enerk1) Can I use a large unit (suitable for 120m2 house) use partially or would that cause some issues?

As well as area, its usual to deal with MVHR in terms of internal volume, since they output mĀ³/hr and adequacy is rated in terms of ACH. But in general turning and MVHR down doesn't cause issues, in fact it's usually beneficial. Ours is nominally 300 mĀ³/hr but we don't normally run it above 125 mĀ³/hr and never above 165 mĀ³/hr.

2) Does providing exhaust through the roof make sense? Im worried about positioning ducting upwards.

It can be done. The no-no is having the intake and exhaust on different walls, or one on a wall and the other on the roof. They need to experience similar external pressures to each other regardless of wind direction.

3) What unit to choose?

Horses for courses I suppose. I live in a PH so we have a PH-certified Brink unit. The benefits are independent 3rd-party certification of performance. Good airtightness; quiet; well-built. etc. But PH cost more than some others.

4) Ideally I would want a unit I can control over wifi but do I actually need it?

Well mine doesn't have it, so no you don't need it. But it might be convenient sometimes.

5) Either the unit would be supplied with good filters or have an option to upgrade it depending on needs.

Look for washable filters. The slots in mine are thick enough that I can put either a G3 or a G4 or both together (which is what I do on the intake) or an F7 filter.

6) Noise - needs to be quiet!

As mentioned; PH tests for noise level.

Look at https://www.ncm-pcdb.org.uk/sap/pcdbsearch.jsp?pid=34 or https://database.passivehouse.com/en/components/list/ventilation_small for specs. Talk to suppliers about system design, and if possible get a professional system design.

About your plans in particular: 1) do try to plan to add some insulation! It is really important for comfort. Why no cavity insulation? 2) You won't need a terminal on the landing: airflows from the inlet terminal through the room it is in, then around the door (remember to leave sufficient gaps), through the landing and through another door into the wet rooms. 3) Consider semi-rigid ducting with a smooth built-in lining. It's smaller and easier to fit than rigid ducting. We have Ubbink.

Posted By: enerkPosition of my house is such that I can't put the exhaust and intake on a wall hence thinking about roof. If I run it through sofits (which would probably accommodate max 100-125mm shall I then double it?
If i go with roof how to make sure it doesn't cause mould? I know that with bathroom extractors it does cause issue often but would it be different here because it's always on?

Our MVHR unit uses 160 mm ducts. We're 145 mĀ²-ish so a bit bigger than you and we have some fairly high ceilings (approx 440 mĀ³ IIRC) but those are PHPP values so perhaps measured differently. But I'd think 100-125 mm was too small, yes. Low velocity is important for noise.

Dunno about your mould question. Generally the idea is not to have condensation in the external ducts, I think. And the continuous air movement helps prevent mould. But I have no idea what the issues are with vertical ducts.

Oh, forgot to mention. If you're planning to mount the MVHR in the loft do remember you need to change/wash the filters regularly (every 3 months where we are) so plan for access. And remember it and ductwork will need insulating if in cold/hot areas.

How much water can be produced over week? In southwest I don't think freezing should be a big issue. long term I can probably plumb it indoor. Short term I could use to collect it. The biggest issue at the moment is planning exhaust and intake ducting to outside.

The condensate drain can go to any convenient indoor waste pipe. Be sure to use a dry trap, not a U-bend wet one.

Yes, wet traps dry out unless used regularly (i.e. water flows through them). In a house with MVHR operating this is more of a problem than condensation itself, if our experience is anything to go by. We make sure we tip some water down all showers and basins and the bath once a month or so. An MVHR unit won't be producing any significant condensation much of the time so would dry out.

I'm not sure what you're proposing in your last sentence? You mean omitting the trap and connecting it directly to a sink? I believe the trap is specificed by the MVHR manufacturer in the installation instructions.

Posted By: Peter_in_HungaryConnecting the MVHR drain pipe to a sink outlet above the trap or to a washing machine up-stand drain

I suspect in most case it would be difficult to do that whilst keeping the condensate drain flowing downhill.

Downhill! And having mould in the exhaust vent wonā€™t be a hazard and may not occur as warm air is blowing out all the time

Is there not a bit of confusion here between the air ducts and condensate drain when it comes to talking of downhill?
If not iā€™ve broken the down hill rule on the ducts between the unit and intake and exctract ducts through the roof, along with several others ( inlet and out let are on different faces, some of the distribution ducts within the house are compromised in that I couldnā€™t run twin ducts to the furthest inlet, the duct runs are pretty convoluted) but these were all pretty much unavoidable if i was to stick with the buildings layout and construction.

As a result iā€™ve no doubt the system doesnā€™t operate as efficently as it could have, however it controls humidity levels very well, the comfort and ā€œsenseā€ of fresh air in the house has been remarked upon by several people.
The system is effectively inaudible at normal levels, barely noticeable at boost and canā€™t be considered a nuisance on boost ( but can be heard).

The house still needs finishing and i expect things to improve once a couple of obvious ( and large) air pathways are dealt with ( not that the house will ever be close to being considered air tight by GBF standards).

I decided on MVHR primarily to deal with any potential moisture/ humidity problems resulting from attempting to insulate an old listed building that has been much altered and a used in its lifetime, my view being that it was sensible to remove moisture rather than have to deal with any problems it may cause later.

The only thing that iā€™d have done differently and which may get done once everything else is finished , is to have fitted direct extraction to the outside from the cooker hood rather than relying on the filter/recirculation and mvhr when cooking.

Djh - 80%+ of cooking is no problem especially if I use purge function and most of that will be done without using the hood, itā€™s the remainder that can be problematic , mainly smell from stir fries and fish, but iā€™m still tinkering with the inlet/ outlet settings and am hopeful ,when the last couple of jobs thatā€™ll greatly reduce the air coming into the house via the structure are dealt with, airflows will alter enough to improve the mvhr extraction efficiency in the kitchen.

Posted By: djhI think that the intake and exhaust ducts should basically go downhill if they run through a wall

The terminal should have a rain screen, so rain entry shouldn't normally be a big issue, though an outward slope is still a good idea.

However disposing of any condensation in in exhaust duct is a concern. Most condensation is likely to occur within the MVHR unit itself, but some could occur in the duct. The recommendation is therefore that the exhaust duct should either fall towards the unit (where any condensation will be drained via the condensation pipe), or fall towards the outside, or one then the other.