Mechanical heat recovery ventilation damp problem?

Reply to the comment left by Ararat at 06/01/2024 - 20:14
Does anyone complain that the incoming air is cold?

Reply to the comment left by Slooky at 06/01/2024 - 21:09Drawing fresh air from a loftspace has mixed opinions even without internal gutters. Lofts get very hot in the summer and drawing that hot air into the flat in the height of summer is not always welcome.
If possible, draw your fresh air from a North wall or roof aspect, discharge the stale anywhere as long as its nowhere near the fresh air inlet. If you suffer from a prevailing wind, try and locate both inlet and exhaust in a sheltered location, wind pressure on either inlet or exhaust will play havoc with your flow rates.
Obviously this means working at height, you may need permissions or scaffolding and/or roof access. If you go through a wall (I did) your going to core drill a 150mm diameter hole with a very chance of falling Masonry. If you go through the roof using a roof vent you going to need to get on the roof, remove some tiles, fit adaptor, replace tiles.
Speaking to a civil engineer friend in the pub (where all problems are solved) about internal gutters, his first comment was about dead pigeons which tend to end up in them. Legionnaires may be only one of a number of problems, I think PIV is a no no.
I've enabled the contact form on my account, if you wish to use it I can send you detailed images of my own setup. But on reflection, if you go MVHR, I think ModularGuys clip duct solution would be a lot cheaper than my radial duct system. The radial system is nice but expensive.

WE have installed both of Nuaire's PIV's ( Flatmaster and loft version) in apartments with some success, however we have had some issues with noise and cold draughts. If they are set at a low speed, the loft PIV is hardly noticeable, but the Flatmaster was installed in a cupboard in a bedroom which was my mistake, otherwise works a treat and as this is also a locally listed building in a conservation area, we had to maintain wooden sliding sash windows and the PIV helps to reduce draughts at the window.

We have installed aluminum framed, heavy glazed sliding secondary glazing with laminated glass which we have found to be brilliant. Really good sound attenuation even with a much smaller spacing to main window than recommended and significantly reduces the potential for condensation on the windows. Would definitely recommend these if the conservation officer agrees.

As for the HRV system, if you can install it without too much hassle then I would say that is the best option for controlling internal air quality and humidity. It is econaomical to run and delivers tempered air to the internal space. As state by others, you would normally extract from the 'wet' areas and supply to the bedrooms and living room, but if it is difficult to get the ideal installation the proposal to supply the bedrooms and extract from the corridor would be a sensible compromise. Delivering tempered fresh air to the bedroom will lower the humidity and reduce the potential for condensation to occur and displace the air out of the bedrooms into the hall where it can pick up additional heat before passing through the heat exchanger before being finally exhausted to atmosphere.

Keep noise down so tenants are not tempted to switch it off, by making sure that the unit is not too small and can run effectively at a low speed. Make sure all ducts are within the insulated envelope of the building to avoid heat losses and that the supply and extract ducts from the HRV top outside and a well insulated type to avoid condensation on the outside of the pipes are these will be at outside air temperature.

Hope this helps.

i would suggest you have a maintenance issue. The rear wall is slightly damp from a source and is taking longer to dry out. As it's not an active leak it might be harder to find. Some top culprits for top floor, minor roof defect, flashing, chimney, parapet wall, gutters, gutter hopper, pointing, flat render and render edges, window, window frame and cill.
You're better off finding the source first and using the dehumidifier after fixing to dry the room quicker otherwise you can mask the issue and it will worsen over time.

Reply to the comment left by Andrew Rudge at 06/01/2024 - 21:57Just catching back up with this thread. Andrew and ModularGuy have provided a lot of very sound advice for MVHR installs and I can only support the virtues of MVHR to solve the problems you have, and add the following:
As others have said you extract from wet (bathrooms & kitchens) and supply to dry (receptions and bedrooms). The reason BPC might have suggested with your flat to extract from the common hallway instead (the intermediary between wet and dry) is that it sounds like you have useful fans in the wet rooms already which will be extracting some of the moist air at source, but inevitably the airflow as Andrew mentioned for moisture laden air in your flat (with the arrangement of South and North facing rooms) will be towards the bedrooms so what isn't captured at source will make it's way via the hallway into your bedrooms.
Add to the that the moisture created in the bedrooms themselves (which will stay there) breathing overnight, laundry on radiators, hair drying etc etc and your walls are wet.
The reason to lag and make all the duct runs between the rooms and the MVHR unit as short as possible and as well lined as possible is to maximised the efficiency of the heat recovery effort (so the entire system in effect) - lose the least, send back the most. The loft runs to the MVHR to and from the outside air intake/ exit points are of less significance.
For this reason I would be cautious about locating your MVHR in a cupboard on the floor down from the delivery ductwork lattice. It's likely to increase the run lengths to/fro, may be more likely to be heard and can (likely) be more easily accessed by tenants (switched off because they think they're paying for it despite your reassurances, or just tampered with). Locating it for ideal efficacy (centrally in the loft) would be my priority - hopefully in a spot where you or your husband can get at it for maintenance.
Lastly, altho air will be incoming into the bedrooms there are a couple of things to consider that may leads to tenant complaints: firstly if it's noisy and keeps them up: think MVHR unit location again, well secured and lagged ductwork (vibrations from ductwork knocking on rafters etc), and secondly if the 'warmer' air flows directly over their bed (esp the pillow area or down the wall behind it). This air will inevitably be cooler than ambient (esp in winter) and no-one likes to sleep under a draught (esp one bringing cooler than room temp air) so locate the ceiling admittance values strategically in bedrooms.

Reply to the comment left by SupaSarge at 09/01/2024 - 13:03
Thanks for your response. There certainly is a lot to think about with the position and runs etc. ideally we would like to site the actual unit in the loft but it is not a conventional loft. The roof level is very low and most of it is crawl space!!! Not really something I can manage. We thought the cupboard was ideal as the unit said it is for fitting in a cupboard. It is on the end of the hallway so is not close to the main bedroom. The runs to the second bedroom which is always used as an office is next door to the cupboard. I'm not sure where to vent new supply in the main bedroom. I can avoid the bed if we vent next to the offending north wall or even if we vent on the opposite wall. Would it be better to vent directly next to the north offending wall or would venting on the opposite wall which is 4.4 metres away still allow circulation on the north bad wall?
I've now had a requite from BPS and wow big increase in price since they last quoted 3 years ago. There's a lot to still get our heads around but we are definitely going for the MVHR.

Reply to the comment left by Slooky at 09/01/2024 - 13:59You want to position the fresh air flow close to the worse (most moist) area so the new air circulates expediently across the damp wall and carries the moisture away (makes it airbound) and off to the hallway for collection.
Given the impediments in the loft, have you looked into decentralised MVHR (or d-MVHR)?
These are wall based fans that essentially treat each room (wet areas) using a ceramic core within the wall fan which (in theory) acts as a heat exchanger, heating up by expelled air and warming up outside air before sending it internal (from the heat that's retained in the ceramic core).
They are supposed to be "80% efficient at heat exchanging" but I've never seen compelling evidence so naturally sceptical.
For wet rooms they work with two fans in one unit, one exiting air the other incoming - working together same time. For dry rooms (aka "habitable / living") they usually work in pairs located in different rooms to create airflow (air changes) with one on and one off (alternating or otherwise every minute or so) to maintaining a balanced airflow.
Again, given your room arrangements (north and south) it sounds like your bedrooms are essentially 'wet' rooms so I'm wondering whether a simpler solution (cheaper) might be to install a wet room version in each bedroom and have them come on during the day (via timer) to ventilate the rooms when ppl aren't in them. You may hit a challenge with LBC however drilling new wall cores - "altering the appearance" etc. They will be noisier I expect ,hence operate via timer maybe.
If there's a wall penetration in the kitchen and bathroom already, perhaps upgrading the fans with d-MVHR could be an alternative - see whether that brings the moisture problem under control. You could also use them in conjuction with a dehumidifyer housed but vented in the cupboard and piped into a nearby sink or shower drain so as not to require emptying. That combination might do the trick.
Just thinking aloud really Slooky, note: I have no experience of them in practice, efficacy etc ........ but perhaps one of the great minds on this trail might and be able to colour this idea in or just usher it swiftly towards the bin instead 😆

Reply to the comment left by SupaSarge at 09/01/2024 - 14:32
Thanks. Yes we have discounted any kind of fans in the bedroom. The noise would annoy me so no reason why I should inflict it on a tenant. My husband is quite happy to do the duct runs and the unit in the loft but I am the one who would rather have the unit in the cupboard as they say it can be sites there. I think the next thing is to clarify everything with BPS given all the information which I've received on this site.

No problem, good luck with it all Slooky and do let us know what solution you settle on and how it ends up if you’re able 🤞🏻👍🏻

And … if anyone has any first hand experience on d-MVHR I’d be really interested to learn more 😊

Reply to the comment left by Slooky at 09/01/2024 - 13:59
Our designer put bedroom supply vents as far away from the door as possible but no closer than 300mm to a wall. So if a door is at one end of the room the supply vent is in the opposing corner. A bit awkward on mine because of the pitch of the roof, but we managed and it worked.

I also looked at these Standalone single room units but they were to pricy for me, not to mention all that core drilling!

It sounds like the unit in the cupboard is the solution for you .

I, sadly, have to disagree with SupaSarge regards not needing to Insulate the main supply and exhaust to the unit. On the supply side, fresh cold air on the inside of the pipe will create a cold surface when the pipe is inside the building, a potential for condensation to form. Similarly, but less so, the exhaust containing cooled extracted air will also create a cold surface. Suitable lagging will come in the kit.
All my loft runs are under 300mm of insulation.

Something else to mention. Extracted damp air will condense out as it passes through the heat exchanger, the MVHR unit will have a drain to discharge the condensate, there is a maximum run length for this drain, possibly 6M in 40mm waste pipe. I managed to plumb mine into the soil stack. Something else to consider.