Mechanical heat recovery ventilation damp problem?

I'll defer to modular guy as he's clearly experienced. The only thing I'll point out is the 73mm duct is arguably easier to find space for, it's only corregated on the outside, it's actually smooth bore and anti bac lined making it stupidly expensive compared to rigid clip together.
As modular guy said, and I said, the system will need setting up. A half decent calibrated aninometer and hoods will cost you a couple of hundred quid, there is no other way. This is covered by part F and building control will want the data from the aninometer, at least mine did.
Good luck.

Reply to the comment left by Andrew Rudge at 03/01/2024 - 18:51
Thanks for your comments. Regarding the fitting it is a top floor flat with access to the loft so no need for ducting to run down walls or through floor spaces or taking up floors or chopping through joists. Simply duct runs in the loft, ceiling vents in the room and a vent through the roof for fresh air not attic air. The unit I am looking at can be fitted in a cupboard which is floor to ceiling and the initial duct runs will go through the ceiling into the loft. Rest assured I haven't just picked from a catalogue it's suited to the size of the flat and I would further investigate with the company before buying. I have already spoken to BPC and they are the ones who said to push fresh air into the bedrooms rather than extract the bad air from the bedroom which confused me. They were made aware of the problem and had a floor plan of the flat. They were also made aware that I already have a cyclone 7 humidistat fan in the bathroom and an ordinary kitchen extractor in the kitchen (which I realise are only basic). I will of coúrse go back and speak with them but I wanted to find people who use these systems and ask how they solved the problem when the problem rooms are not actually the kitchen or the bathroom. Apparently the vents also have automatic closures for fire integrity if needed.

Reply to the comment left by Andrew Rudge at 04/01/2024 - 00:07
Andrew,
I agree with you. I wasn't trying to imply 73mm isn't a good product, and I know the stuff your referring to, with flexi external and smooth internal - it's nice stuff, but expensive.
In small flats, 73mm is probably ok. In larger installs, to get the airflow, the MVHR unit will have to run harder, which can create noise problems.
I think we both agree, that finding somewhere for the ducting is the key challenge in retrofit

Reply to the comment left by Slooky at 04/01/2024 - 09:16
Your job sounds fairly straightforward given your floorplan.

As for the condensation. Think about it, the condensation is generated in a warm environment, the kitchen, the bathroom. That moisture will hold in the air because its warm, that air will naturally travel around the flat and condense out on the first cold surface it comes to, glass is usually the coldest, old North facing solid 9" walls won't be far behind.

The MVHR will stop all of that condensation but will probably take several weeks for the building to dry out, all of that excess moisture is everywhere, carpets, walls, plasterwork but in time you'll see the relative humidity come down and the place will feel warmer.

And I fully agree with Modular Guy, the job is all about the ductwork but yours should be fairly straightforward.

Keep your main feed in and exhaust to the unit as short and straight as possible and insulate them, but all this detail will come from the design.

Reply to the comment left by Andrew Rudge at 04/01/2024 - 10:56
Thank you. It is worth hearing your comment because that is a thought that has occurred to us. The tenants are instructed to always close the bathroom door after they have showered (and during!) And instructed to always use the kitchen extractor precisely because we thought about the wet air travelling to the coldest walls. But seeing someone else say it has changed my thinking. I will definitely have another chat with BPC who said supply the bedrooms and emphasise where the damp is etc

Reply to the comment left by Slooky at 04/01/2024 - 12:48
Another point worth mentioning is how much moisture we sweat and exhale during the night, up to 1L each. Bedroom windows streaming in the mornings? That's just us breathing.

And of course, the elephant in the room. Drying laundry, they all deny it but they all do it.

MVHR will deal with all of that.

So good to see lots of helpful landlords! Your problem is identical to one of my top floor flats with a similar layout. My problem was solved with a piv unit in the loft and a single vent through to the landing ceiling. I cannot sing its praises highly enough! Every time I visit the flat it smells like fresh air and has completely eradicated the problem!
And not expensive to install - the electrician used the light fitting for the electric.

Reply to the comment left by Luna at 05/01/2024 - 17:50
Yes we looked at that system first but I had heard that tenants complained because the fresh air coming in was cold? I also wonder what happens in the summer. I've been told that they have an auto switch off when the heat in the loft rises over a certain temperature.

Reply to the comment left by Slooky at 02/01/2024 - 19:11Secondary glazing (especially the lightweight acrylic & magnet versions) are highly effective - as their mounting creates the optimum air cavity (void depth) and acrylic conducts less heat than glass - great for noise reduction also - but they of course don’t solve the moisture problem they just dramatically reduce (if not eliminate) condensation on streaming windows. It manifests elsewhere such as the top of curtains or wallpaper.
Listed buildings will soon be permitted to retrofit double glazing into sash windows (rule changes afoot - including solar panels!) but whilst that seems like a great idea it actually isn’t and we always steer customers away. Very narrow air cavities (poor performers), high DGU failure rates due to large heavy DGUs set in slender (45mm sash frames) that are moving parts (now much heavier despite counterbalancing) altogether asking for trouble.
We also never recommend acrylic magnet SG solutions for tenanted properties as panel care (when removed in summer) becomes an issue since easily scratched or damage if not looked after.

Reply to the comment left by SupaSarge at 06/01/2024 - 10:22
It has been said before that secondary glazing while it might stop most condensation on the glass, as a consequence the wall will suffer more. So I think the secondary glazing comes after sorting out the ventilation. I have not heard that listed buildings will be allowed DG. Is this something that is dependant on the views of a particular council or mandatory for all councils. I am in support of changes being allowed to allow a decent standard of living and being told to hang a heavy curtain is so archaic they should be ashamed of themselves. I feel it won't suit all windows to retrofit DG, as you say the space will be so thin. I would however support complete replacement of the windows with DG units with appropriate dimensions to allow the DG to do a good job. We have over the last few years been slowly refurbing 3 windows which are 3.6 metre high each with 2 sashes. It has involved completely removing architraves and shutters replacing rotten wooden lintels (which in itself has involved removing bricks and render etc). Huge job. Sash weights have been replaced to get the correct counter balance. Moving the sashes is now easier. However I can not imagine what it would be like if the panes were replaced with DG. Very heavy. We have looked at the magnetic panels with the intention that we would place and remove them depending on the season however we were told the size of the windows is too big to ensure they were safe. We have also looked at secondary glazing in some flats but it seems that the shutters would them be obsolete. The price of anything is also so crazy we are just refurbishing our lowest price rental flat but decided to re-use the old secondary glazing because the replacement quotes came to approx 4k for 2 windows. The only upside is that our council has said anything listed is EPC exempt