Am I on the right path designing MVHR vents for apartment?

[Giko]

Hey all,

 

I recently bought an 45m2 apartment with unfinished interior, it came with 191m3/h MVHR unit with only external ducts (125mm) installed, internal ones are up to me. So after reading a lot about it (this forum was a bliss!), I made a design with calculations: I chose to have 2 double inlets in living room and bedroom, as well 2 double outlets in kitchen and bathroom, and 1 singular outlet in wardrobe, all this with 90mm flex (semi rigid?) 90 mm vents. Now if my calculations are correct, it may not be enough to run unit at 100% load without any noise, as air velocity through vents would peak at ±3m/s (as I am aware it should be no more than 2.5m/s). Now on the other hand, I am unlikely to use unit on the max very often (perhaps only when I have guests over). So I am thinking to add 1 additional outlet in hall (next to exterior door), and 1 more in living room (close to bedroom door), would that make sense? Or perhaps I am just overthinking? Never had seen MVHR working live, and wouldn't want to regret it later, If I didn't add additional vents when I had opportunity.

 

Attaching calculations and design, I would also appreciate tips on how to improve it. Thank you.

 

Also a note, that I am not from UK, thus regulations in my country are a bit different - actually, there are no strict regulations here for residential places, only recommendations.

[JohnMo]

Your ach seems very high, you need less than half that, when you have finished.

 

I would extract from the kitchen and bathroom only, and supply an equal amount to bedroom and wardrobe. The living room will be continually washed with fresh travelling from the bedroom and wardrobe as it travels to the extracts points

 

So do 47 and 29 m3/h in the kitchen and bathroom as extract, and supply the same to bedroom and wardrobe.

 

Short runs will only need one 90mm duct.  So bathroom and wardrobe.

 

Once you get signed of by building control, you could have those rates. Get a CO2 meter to monitor.

 

[Mike]

9 hours ago, Giko said:

with only external ducts (125mm) installed

It looks like they're installed through opposite externals walls? If so I'd be looking to change that, to have them both on the same elevation. Otherwise you effectively have a wind-tunnel, where the wind can create a substantial pressure differential between vents, making it difficult / impossible for the MVHR unit to properly control the airflow.

 

 

[Giko]

22 hours ago, JohnMo said:

Your ach seems very high, you need less than half that, when you have finished.

 

I would extract from the kitchen and bathroom only, and supply an equal amount to bedroom and wardrobe. The living room will be continually washed with fresh travelling from the bedroom and wardrobe as it travels to the extracts points

 

So do 47 and 29 m3/h in the kitchen and bathroom as extract, and supply the same to bedroom and wardrobe.

 

Short runs will only need one 90mm duct.  So bathroom and wardrobe.

 

Once you get signed of by building control, you could have those rates. Get a CO2 meter to monitor.

 

Thank you for advice, but if I understand correctly, then 1 duct would only be enough for normal Unit operation, but let's say if I have guests over, and now there are 6 people inside, so according to guidelines I will need to increase it to 180m3/h, and that may get loud with single, or even two ducts. Basically, I want to get it setup, so whole MVHR system works quietly even on max load. My second concern is, if I put supply in wardrobe, then some unpleasant smells may travel to living area (e.g. it's normal to keep seasonal shoes in there, that may be stinky).

 

It seems it's very popular to rely on airflow through whole house (e.g. bedroom in, bathroom out), but I wonder how closed doors may affect airflow pressure? E.g. you sleep in bedroom with closed doors generating a lot of CO2, at the same time there's ~40m3/h air supply above your head, so can't it happen that new air will mix up with CO2, just because it can't leave the room fast enough due increased pressure? Perhaps it's more efficient to have supply and exhaust in the same room (at opposite ends)?

 

13 hours ago, Mike said:

It looks like they're installed through opposite externals walls? If so I'd be looking to change that, to have them both on the same elevation. Otherwise you effectively have a wind-tunnel, where the wind can create a substantial pressure differential between vents, making it difficult / impossible for the MVHR unit to properly control the airflow.

 

 

The wall, where exhaust duct is leaving, is not an external one, there are other apartments/shared hall behind it. Though, I have no idea where duct goes after that (I guess to the roof).

Edited November 4, 2023 by Giko

[JohnMo]

2 minutes ago, Giko said:

closed doors may affect airflow pressure

All door have a small undercut at the bottom, about 10mm between floor coverings and the door. Closed doors make no difference. There is zero pressure increase.

 

3 minutes ago, Giko said:

supply in wardrobe, then some unpleasant smells may travel to living area (e.g. it's normal to keep seasonal shoes in there

The constant air flow, leaves bacteria in a place it doesn't survive. So smells don't tend to be an issue. 

 

6 minutes ago, Giko said:

Perhaps it's more efficient to have supply and exhaust in the same room (at opposite ends)?

I would say no, the whole idea is the flow of fresh air, dilutes the humidity and CO2, continuously, the further it travels the better. CO2 and humidity disperses very quickly it doesn't just sit there.

 

10 minutes ago, Giko said:

now there are 6 people inside, so according to guidelines I will need to increase it to 180m3

You would never ventilate at 180m3/h. And the guidelines don't say that.

 

You only have a max required airflow rate that is equal to the bathroom and kitchen extract rates. Boost would be 20% more than normal flow, no more is required.  You don't size the ventilation, on the off chance you get a gang of mates coming round.

 

If you have six people over and it gets stuffy, you open the window!

 

I have a small MVHR doing two bedrooms and ensuites, I used two of these as a plenums

https://www.paulheatrecovery.co.uk/product/7178/

 

So it runs - MVHR to plenum with a silenced duct, to 90mm duct, the duct to outlet plenum/nozzle. And similar for the extract. My longest duct run is about 9m and it's fine as a single 90mm.