Can anyone help with my MVHR saga?

[Deniance]

Hi everyone I need some mvhr help please…

 

ok so about 10 years ago I looked into MVHR and got all excited and I bought a Brookvent air cycle 1.1 , the green one, I did a bit of research and armed myself with the thermal ducting from Nuaire and all the various bits and bobs, I got as far as installing all the ducts in the loft , then I read that mvhr was pointless as the house is 150 years old and will never be airtight and all that jazz, so my project kinda got mothballed and everything went in the loft and just left!

 

well, fast forward 10 years and I had a small flat roof bathroom extension built, and I needed an extractor fan, so I started thinking about the mvhr unit I have in the loft and whether it is worth trying for MVHR again?

 

We had new windows fitted and our condensation levels are getting out of hand, my partner also bought a co2 monitor with an alarm that beeps, and in the middle of the night it starts beeping, not every night but some nights it alarms and on the sticker it says it’s at medium danger for c02 levels.

 

So temporary connected the mvhr unit with some flexi, switched it on and connected it to the old ducts in the loft and got some air blowing to the bedroom, it’s been 2 weeks now and the c02 alarm hasn’t gone off at all and condensation has been reduced by about 50% so it may be worth installing it properly just to get some ventilation running in the house.

 

So everything I read 10 years ago I’ve forgotten 

 

so where do I start redesigning the mvhr?

 

do I need to start with house total volume to see if the unit is even big enough? 
 

it’s only a 3 bed terraced house

im extracting from the bathroom and utility /kitchen which is about 1metre each away from the unit, so just two extracts, the vent to outside is straight out the wall so it’s only 300 mm from the unit, the fresh intake is only about 1.5metres away, and it’s supplying 4 rooms with fresh air, (3 bedrooms and a living room)

 

the ducting that’s already installed is 125mm holes on the unit that will go to 125mm round adapter that changes it to 220x90mm Nuaire ductmaster all the way through in a branch system, and I think at the end of each branch in the loft it goes back into a 125mm round adapter for connection to the rooms

 

the duct runs are probably 4m to bedroom1, 9m to bedroom 2, 13m to bedroom 3 and then if I drop from the loft to the living room ceiling it’s probably 18m? (The 18m run takes 4 90 degree bends to get there)

 

sorry for the long boring post I guess I just need to know the first step, which I guess is measuring all my rooms and corridors and getting a house volume and then comparing that to the unit specs? Anything else I need to measure or look for?

 

if you’ve got this far thanks for your time!

[JohnMo]

The easy stuff

1. Work out house volume in m3

2. Divide to 3, that will give you a min flow rate in m3/HR.

3. Work out the flow rates to each area. Use attached as guidance.

[JohnMo]

You need to calculate pressure drop - use duct suppliers table for your duct and bends to calculate the max pressure drop and ensure that is ok for the fans, the bigger the pressure drop the harder the fans have to work and that means more noise.

 

Are you going through a manifold section? If not you may need attenuators between rooms to stop cross talk.

[Deniance]

8 minutes ago, JohnMo said:

Thank you very much, I’ll measure all rooms and stairs and hallways after work!

[JohnMo]

You don't need to be that exact, measure your house internal footprint multiple by height will be a near enough ballpark. 

 

Didn't mention your extract and supply rates should match each other.

[Deniance]

4 minutes ago, JohnMo said:

No manifold , if I remember right it was so long ago I don’t think manifolds were a thing! 
 

the unit has 125mm connections , there’s 400mm of insulated flexi that joins into round adapter that changes it to 220x90, this runs throughout with no change in size, there are a few bends though, tried to keep it to a minimum but impossible to keep it straight with no bends, so I’ve ended up with a 90 straight off unit to go left , 90 up to the loft, 90 to go across the loft , 90 to come down from the loft to living room, then an outlet on that which I guess is another 90 so to speak , that’s the longest run, the. The 3 bedrooms are tee offs the main run in same size duct with no bends  and end at a square to round adapter which I guess is a 90 so to speak, 

so I’ll make a list of all the bends and lengths of duct , go to Nuaire site and right down the pressure loss for each one then add it all up? What do I compare that with?

[Deniance]

6 minutes ago, JohnMo said:

You don't need to be that exact, measure your house internal footprint multiple by height will be a near enough ballpark. 

 

Didn't mention your extract and supply rates should match each other.

There is 1 metre of attenuator in each duct before the terminals of each room, it’s the Nuaire attenuator that slides inside 1 metre of duct, heavy black rubber thing, looks like it takes up most of the area inside the duct, but I guess that’s what it’s designed to do?

[Deniance]

3 hours ago, JohnMo said:

You need to calculate pressure drop - use duct suppliers table for your duct and bends to calculate the max pressure drop and ensure that is ok for the fans, the bigger the pressure drop the harder the fans have to work and that means more noise.

 

Are you going through a manifold section? If not you may need attenuators between rooms to stop cross talk.

How do I calculate pressure drop?, I’ve found all the values in the Nuaire catalogue for each bend and length of ducting, do I add the entire branch system together or do you calculate each run and use the longest run

 

ive drawn the entire branch system out and added the values of each item  onto the drawing, and if I simply total them all up together I get 159 pa for the supply branch, 122 pa for the longest run, may need to draw it properly to get exact figures 

[JohnMo]

You basically use the most onerous run length. Not all of them, as they are parallel.

[ProDave]

I am a more practical sort of chap.  I didn't do any pressure drop calculations, I just connected it all, then spend a few hours with the borrowed anemometer measuring flow rates at each room terminal, and adjusting the terminals and adjusting fan speeds until I got flow rates that met the BR required rates.

 

In practice after that, I reduced the fan speeds as it was slightly audible.  Several years on no ill effects from running at slightly lower than BR required rates.  I gather passive house required rates are lower.

[Deniance]

18 minutes ago, JohnMo said:

You basically use the most onerous run length. Not all of them, as they are parallel.

Righteo,I’ll crunch the numbers after work , thanks 

[Deniance]

17 minutes ago, ProDave said:

I am a more practical sort of chap.  I didn't do any pressure drop calculations, I just connected it all, then spend a few hours with the borrowed anemometer measuring flow rates at each room terminal, and adjusting the terminals and adjusting fan speeds until I got flow rates that met the BR required rates.

 

In practice after that, I reduced the fan speeds as it was slightly audible.  Several years on no ill effects from running at slightly lower than BR required rates.  I gather passive house required rates are lower.

That’s what I did as a temporary measure and I managed to get air to blow out of the pipe!

[Gone West]

6 hours ago, Deniance said:

so where do I start redesigning the mvhr?

It's pretty straightforward, seems like you've got it mostly sorted. I used 125mm branched ducting in my house changing to 204mm x 60mm for the rooms with vaulted ceilings. After it was all installed I set it up roughly to test it and then bought a Testo 405i anemometer to set it up more accurately.

 

 

 

 

 

[Deniance]

2 hours ago, Gone West said:

It's pretty straightforward, seems like you've got it mostly sorted. I used 125mm branched ducting in my house changing to 204mm x 60mm for the rooms with vaulted ceilings. After it was all installed I set it up roughly to test it and then bought a Testo 405i anemometer to set it up more accurately.

 

 

 

 

 

That’s neat, thanks for that

[Deniance]

Ok so I’ve done a bit of measuring, not sure if I’ve done it right but here goes…

 

Total volume of house = 213 m3

divide this by 3 to give

min flow rate of 71 m3/hr

 

from the table JohnMo supplied earlier for rules in wales, a 3 bed 4 person home requires a supply air of 76 m3/hr or 21 l/s

 

im extracting from 2 rooms , a bathroom and a kitchen, which require  47 m3/hr 13 l/s and 29 m3/hr 8 l/s

 

So my supply is 76 m3/hr and my extract is also 76 m3/hr (21 l/s)

 

I’ve estimated my pressure drop on my longest branch of ducting is 61 pa.

 

I’ve used the figures from the Nuaire catalogue for each piece of ducting and bends etc, they list pressure drops from 5 l/s to 80 l/s for every item so my system will be running at 21 l/s so I chose all the values they gave for 25 l/s, and I’ve ended up at 61 pa, the supply grille on this long branch  was the biggest pressure dropper at 30pa (assuming 10mm opening)

 

I then found the airflow performance curve on the Brookvent air cycle 1.2 Manual, if I follow 61 pa across the curve graph it says the unit will supply 60 l/s or 216 m3/hr or 127 cfm. (This would be speed 5 out of 5 I’m assuming?)

 

if you then look at the acoustic chart, for the unit to output 60 l/s it would have to be on speed 5 out of 5,

If I then turn the unit down to run at 21 l/s it should be at speed 2 out of 5 and have a decibel of 10 to 19 db at a 3 metre distance

 

i hope all that makes sense!

 

Do these figures sound ok?

 

 

Edited October 5, 2023 by Deniance
Grammar

[Deniance]

2 hours ago, Gone West said:

It's pretty straightforward, seems like you've got it mostly sorted. I used 125mm branched ducting in my house changing to 204mm x 60mm for the rooms with vaulted ceilings. After it was all installed I set it up roughly to test it and then bought a Testo 405i anemometer to set it up more accurately.

 

 

 

 

 

I’ve just googled the testo you mentioned , how does that work? I though you had to use the ones with a little fan on top? It says it reads in meters per second? How do set it or convert it to read litres per second at each outlet?

[JohnMo]

So your pressure drop is well within the limits of the MVHR, so all fine there.

 

Your furthest away grille will be as far open as it goes, you then tune the output of the MVHR to get the flow rate you want. Then close in the other rooms outlets to get them where you need, then repeat a few times until all are ok.

3 hours ago, Gone West said:

Testo 405i anemometer

Goes into a duct, I used a Testo 425 (it will read in flow rate also), used a 150 to 125 adapter and some 125mm duct and secured the probe in the center of the duct with a cable gland.

Edited October 5, 2023 by JohnMo

[Deniance]

15 minutes ago, JohnMo said:

So your pressure drop is well within the limits of the MVHR, so all fine there.

 

Your furthest away grille will be as far open as it goes, you then tune the output of the MVHR to get the flow rate you want. Then close in the other rooms outlets to get them where you need, then repeat a few times until all are ok.

Goes into a duct, I used a Testo 425 (it will read in flow rate also), used a 150 to 125 adapter and some 125mm duct and secured the probe in the center of the duct with a cable gland.

Right I’ll get on with the install asap see if I can finally finish it off , thanks fir all the help , I’ll reply to this thread no doubt when I’m trying to set it up!

[joth]

11 hours ago, Deniance said:

I read that mvhr was pointless as the house is 150 years old and will never be airtight and all that jazz, so my project kinda got mothballed and everything went in the loft and just left!

So, bit late now but fwiw I think this statement is one of it'll never pay for itself as it can't recover enough energy. 

But if you've already bought it, it's a sunk cost and justifying paying for itself is moot. It only needs to pay for the energy it uses to operate, and arguably if it's managing co2 and humidity to safe/comfortable levels it's already earning it's keep and any heat recovery is bonus. 

Great to see it revived, it's unfortunate you got put off the project last decade!

 

One thought: if the gear or ducts have been lying around they maybe rather dusty inside, worth trying to clean any you can before installing.

[Deniance]

3 hours ago, joth said:

So, bit late now but fwiw I think this statement is one of it'll never pay for itself as it can't recover enough energy. 

But if you've already bought it, it's a sunk cost and justifying paying for itself is moot. It only needs to pay for the energy it uses to operate, and arguably if it's managing co2 and humidity to safe/comfortable levels it's already earning it's keep and any heat recovery is bonus. 

Great to see it revived, it's unfortunate you got put off the project last decade!

 

One thought: if the gear or ducts have been lying around they maybe rather dusty inside, worth trying to clean any you can before installing.

Originally it was going to be loft mount but then changing filters would do my head in and the in and out would be close to chimneys and smoke, and the condensate was an issue as I knew the upstairs bathroom was getting moved downstairs, so yeah I didn’t have motivation to complete it, but new extension out the back made us gain a utility room between new extension and the house , it made sense for the in and out through the wall, makes sense for condensate, made sense for filter changes, just made the ductwork about a million times harder getting it up to the loft but hey Ho I’m sure I’ll get there in the end! All ducts were taped up when left and the unit had protective caps on it

[Gone West]

12 hours ago, Deniance said:

I’ve just googled the testo you mentioned , how does that work?

I made a cone to put over the vent and the Testo sends the readings to my phone and I print them out.

 

 

 

 

[Gone West]

12 hours ago, Deniance said:

It says it reads in meters per second? How do set it or convert it to read litres per second at each outlet?

IIRC it printed out both, but if not, to convert, it 1m3/h = 0.277778l/s. I did this five years ago so I'm a bit rusty.

[Deniance]

2 hours ago, Gone West said:

IIRC it printed out both, but if not, to convert, it 1m3/h = 0.277778l/s. I did this five years ago so I'm a bit rusty.

That’s great thanks, everytime I’ve read about setup it’s always people using the ones with little fans on the top and cutting certain size cones etc, are these ones more sensitive to lower air flows?

 

also all this setup on the unit and all the figures used to supply and extract rooms, are they just for the unit running at normal speed and boost is just an extra function? If that makes sense?

[Gone West]

2 hours ago, Deniance said:

all the figures used to supply and extract rooms, are they just for the unit running at normal speed and boost is just an extra function?

Yes, normal speed for the unit you have.

[Blooda]

5 hours ago, Gone West said:

I made a cone to put over the vent and the Testo sends the readings to my phone and I print them out.

 

 

 

 

Snap.. we used the same set-up

I saw a youtube video on this that I cannot find now, but it is best to use an average, by setting to average and going across the whole cross section.

 

Also we cut a small hole in the outside inlet and outlet, and tested so we could ensure the whole system was balanced.