Mechanical heat recovery ventilation damp problem?

I have MVHR in an older property and it works very well. By design you extract from 'wet' rooms and supply dry rooms. A wet room is a kitchen, bathroom or WC. Air flow rates from each supply, extract will be very different. Supply rates to a bedroom will be half that of a living space, extract from a bathroom twice that of a kitchen. The whole system is designed to suit your space and will need to be set up and calibrated.

Slooky, MVHRs do not extract air from one room and pump it to another. They extract air from some rooms, typically bathrooms and kitchens and vent this warm, damp air to the outside, passing over a heat exchanger. That heat is used to warm air drawn in from the outside to feed other rooms, typically bedrooms and living areas.
A previous commenter is right - retrofitting this is expensive and hugely disruptive, if you’re planning on a whole house system.
You can now get single room MVHR which are easier to install but will leave you with a visible unit in your bedroom and something on the wall outside.
You can also get purge vents, brought out to allow new builds to meet a new section in the building regulations around overheating, which are designed to bring in lots of fresh air, but there’s no pre-heating, like with an MVHR.
Or, you could simply fit a bathroom / kitchen Xpelair type unit to simply such the damp bedroom air out.
If NUAire (who I consider as the Rolls Royce of MVHR) are reluctant to recommend PIV because of internal gutters, are you sure these are not part of the problem - they can be a nightmare

Reply to the comment left by ModularGuy at 03/01/2024 - 16:47
Thanks for your response. I am aware that the bad air is pulled out and fresh air pushed in as per my post. I also favour them because it has been said that the heat exchanger is cheaper to run than the alternative system which actually has a heater.
It's a nice flat and the thought of putting vents and units in the bedroom will spoil it which is why I am considering the mvhr unit rather than PIV. Tenants would have no reason to switch it off. We also intend to run it of the landlord electric supply so the tenant doesn't pay the running costs. You are the second person to say it is very expensive and difficult to retrofit but no one has said why?

Reply to the comment left by ModularGuy at 03/01/2024 - 16:47
By the way the company who said a piv couldn't be installed in the loft quoted legionellas disease. However we are quite happy to vent/duct directly through the roof and have a unit sit in the loft. I have also recently seen one which can sit in a cupboard (we have a floor to ceiling cupboard in the hallway) which ducts into the loft so I assume it can take from the bedrooms and push fresh warmed air back into the hallway next to the bedrooms. I really would like to hear from someone who has the same problem. I have looked at the individual units and they are small and tidy but use a heater which is expensive to run

MRXBOX95AB-WM1 WALL MOUNTED MVHR UNIT
Has anyone any experience of this unit?

Reply to the comment left by Slooky at 03/01/2024 - 16:55
It's expensive because it's time consuming to fit. Upstairs isn't so bad because you can run ductwork through the loftspace. These ducts are typically 73mm diameter, are flexible, sort of, and come In 50m rolls, it's like wrestling a python. These ducts run in pairs for extract and supply to living spaces, singles for bedrooms.
The problems and cost involve getting into the kitchen for instance, you can't just bang big holes into your joistwork to feed duct through. Your going to be pulling up lots of floorboards and boxing in ducts as they drop down walls.
My system is quite compact but still has 24 duct runs and 200m of it.
My project involved a complete back to brick refit and it was still hard work.
People like EPC can design your system but nobody knows the fabric of the building and where best to run duct.
Have a look on buildhub, quite a few have done diy jobs that will give you some foresight.

Reply to the comment left by Slooky at 03/01/2024 - 17:10
You can't just pick a unit out of a catalogue, it has to be sized to the building. You have to calculate the size of the building, calculate how much air you need to shift, then find a unit that will shift that volume of air running at 30% load.
If you look on buildhub they will show you how to do the calculations, or you could use somebody like EPC ventilation who will give you the calculations and a selection of suitable units to choose from.

Reply to the comment left by Andrew Rudge at 03/01/2024 - 17:26
Just to add, MVHR is covered under building regulations, part F I believe. If you google the relevant documents they will tell you the minimum amount of air you need to shift for each room. It's no good you curing the mould issues if the inmates then die of hypoxia!

Sorry, it was BPC Ventilation, not EPC.

Slooky, I build social housing for a living and we fit MVHR units in every new home, so I’m a big fan and have fitted hundreds.
The MVHR unit can vary in size from something the size of a carry-on suitcase to something more like a sideboard!
I’m our smaller houses we fit a unit which is actually combined with the cooker hood, so sits in the kitchen, and on larger properties we usually tuck the unit in the roof space, but we’ve also put them in cupboards. You need to be able to get to the unit to change filters, at least annually and we recommend every six months.
The expense comes in running the ducting. Our homes are designed with voids above the ceiling to run the ducting. In a retrofit, if you’re a single floor flat, with the loft above, then that’s relatively straightforward, but if you’re more than one storey or your not immediately below the loft you’ve got to find somewhere for the ducting and air admittance valves (the things that sit in the ceiling to deliver or extract the air - usually one per room).
I saw someone suggest the ducting is c73mm. I’ve never managed to do an installation with ducting that small. As standard we use 125mm round or the oval / rectangular equivalent and on bigger properties we use 150mm. The other poster also suggested the flexi ducting. We never use this. We use the clip together rigid. It avoids the python wrestling the other poster noted and the corrugated internal surface of most flexi pipe massively reduces the performance of the system. You don’t want to create turbulence in the pipework. It creates noise and problems in balancing the system.
So, most of the expanse comes from the ducting install.
The other issue is that rooms like bathrooms need the facility to boost from a background trickle rate to a higher boost rate, to remove humidity. This can be achieved using a sensor in the main unit (most have this facility as an optional extra) or by connecting it to the light switch, much like a shower fan. It is possible to set the trickle rate to run permanently above the boost rate, which I’ve done for simplicity a number of times, but that can be noisy. In bathrooms / toilets without windows, there must be an opportunity for a manual boost - building regs say “in rooms where the primary pollutant is odour, there must be a manual boost”! 😂 This means an extra switch, although we often do that using wireless switches to avoid complex wiring costs.
NUAire or other suppliers / installers will usually size system and ducting for you, and the Part F building regs documents, particularly the latest edition, have some really simple guides and the docs are free on line.
Finally you MUST commission the system. If found NUAire approved installers who don’t understand how to do this. It’s not hard to do, but it does need a bit of kit known as an anemometer to measure the air flow at each admittance valve. NUAire instal docs describe how to do it, although they do miss a couple of important bits.
On your other point, PIV would also have some of the ducting challenge / expense although you only duct to certain rooms. I guess the legionella risk would come from the fact that most PIV units draw air from the loft space rather than directly from outside.
Does the landlord, or anyone else do a legionella check? In commercial properties, like our factory, we’re obliged to check annually.