MHRV system - ducting questions

leo738

Hello all,

I'm about to start retrofitting a MHRV system to our house(3 yrs old). I realise it's a big undertaken & the efficiency is dependent on the airtighness of the house. Having said that however we occasional have significant issues with humidity in the house so I'm hoping this may help solve the problem.

I've a couple of question about the ducting:

From what I understand rigid ducting is preferable over flexible as it allows more efficient movement of air. However is it recommended to use some flexible ducting to reduce noise transmission (privacy) from one room to another?
Is it best to have a radial arrangement from a central hub or a spur arrangement from a length of ducting? Is there a recommended max or min length of the spur?
I understand that the ceiling valves should not be located too close to a wall to prevent down draughts. Is there a recommended min distance?
What time of fireproofing should be used in the ducting (I understand the system should be shutdown from the smoke alarm system)

If anyone can think of other pointers they'd be much appreciated!

Regards

Leo

SemperFidelis

I did this recently on a 1970s built bungalow. Made a huge difference to our humidity/mould issues.

I used a small bit of flexible duct to connect to the MHRV unit. I used Rigid to the outside and to distribution boxes (1 for extract, 1 for supply) and semi-rigid from the distribution boxes to each room. The semi-rigid ducting is smooth on the inside and can bend a bit so its easier to install than rigid, it also has a smaller diameter so for the kitchen I have two runs of semi-rigid to get the required airflow without the noise. Extract runs to kitchen and bathroom and supply to all other rooms and hall.

If you use the distribution boxes you don't get noise transmission from one room to another otherwise you can use a silencer on each outlet.

There's an app on the play store called Vent Tools, it will help with calculating duct sizes and airflow/noise levels.

I think for the ceiling valves its something like 300mm from a wall but I placed them in the centre.

Calahonda52

Nice piece of work there:SF!


Just a few questions to help the OP, and others.

1. Have you an extract fan over cooker in kitchen?
2. Have you any open fireplaces?
3. What modifications if, any, did you make to the internal doors to facilitate internal air flows?
4. Have you any silencers, even between the unit and the distribution boxes?
5. where did you locate the unit?
   

ps
nice app from a good stable!
K value is new to me: what did you make of it?

leo738

Anyone tried oval ducting?
Or flexible ducting with a smooth interior?
How about the ducting with fixed valves & the rate set by restrictor in the distribution boxes?


It would seem from my initial reading the bigger the ducting the better as the fans don't have to work so hard. Is this correct?

Leo

SemperFidelis

1. I have no Extractor over cooker, a kitchen refurbishment is my next big project kicking off soon. I'll probably go with a recirculating hood. Also toying with the idea of extract to atmosphere through attic using some of the leftover 150mm ducting and inline fan and a motorised damper to close the duct when not in use.

2. No open fireplaces, stove in living room so I didn't close the wall vent in that room completely.

3. There are vents above my internal doors so no changes to doors themselves but I did stop the vents from opening to the attic through the stud walls with some insulation and airtight tape. I believe a 10mm gap is required normally.

4. One big silencer from unit to inboud distribution box.

5. Unit is in the attic, all rigid ducts wrapped in insulation. Semi-rigid ducts and distribution boxes are under 200mm rockwool insulation. I have some PIR around the unit too.




Example of temps from January;

Supply from atmsphere to unit 0.6
Supply into house - 15.6
Extrach from house - 18.3

SemperFidelis

The semi-rigid ducting I used is the flexible one with smooth anti-bacterial surface on the inside. Comes in 50m coil with a 63mm internal diameter. Easy enough to run but watch the bends aren't too tight.

I don't think the fans work any harder with smaller ducting but the air will move faster and generate more noise for the same rate of airflow.

You need to workout the airflow required in each room and then you can calculate what size unit and ducting you require. I did up a spreadsheet with all the calculations and worked out the airflow for each room. For Kitchen I needed two valves and one of those has a double feed of the semi rigid duct in order to get the correct flow rate.

Calahonda52

leo738 wrote: »

Anyone tried oval ducting?
Or flexible ducting with a smooth interior?
How about the ducting with fixed valves & the rate set by restrictor in the distribution boxes?


It would seem from my initial reading the bigger the ducting the better as the fans don't have to work so hard. Is this correct?

Leo

Each room needs to be balanced so needs to be done at room level.

There will be less resistance in a bigger duct but the main problem, as noted earlier is speed increase as duct gets smaller.

Volume is X section area by speed
So if duct is 100mm diameter: X section area is 0.00785m2
So if duct is 060mm diameter: X section area is 0.00282m2
which is a factor of 2.78
so if speed for the first duct is 1m/s, then it is 2.78 m/s for smaller duct for same volume of air