2 extractions ducts in a bathroom one done by an existing dedicated extractor other done by new balanced ventalition system, is it a good idea?

Forums › Home Workshop DIY › 2 extractions ducts in a bathroom one done by an existing dedicated extractor other done by new balanced ventalition system, is it a good idea?

Cherios

50 posts

Geek

#318785 20-Feb-2025 17:12

Need some advice, hopefully there's some HVAC experts hanging around.

Early 2000s brick and tile single level, single glaze windows, 160sqm floor area.

I am going to install an a balanced ventilation system ( looking at lossnay vl350), but I have a dedicated extractor in my bathroom already.

I have a few options:

A - keep the existing extractor as is and install another return duct for the lossnay, resulting in 2 extractors in the bathroom. Installer says if going this route I should ideally leave the existing dedicated extractor off, but gives me the opportunity to turn it on when it's not sufficient in getting rid of steam in long showers etc. / lossnay is not running.

B - take out the existing bathroom extractor and repurpose the duct for the lossnay. More expensive, probably with this option need to change the switch and remove/isolate existing wiring.

C - No return duct to the lossnay from bathroom keep as is. But higher electricty consumption as two separate systems running.

Leaning towards A, but if I duct it into the lossnay and its not sufficient, I will then have to run the dedicated extractor, there's a chance the systems will starve each other.

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427 posts

Ultimate Geek

#3345280 21-Feb-2025 13:27

I've done A.

I have a lossnay system exhaust grille in each bathroom (2x) and another in the central hallway. The bathrooms are doing about 10 L/s each continuously.

I've left the exhaust fans in place and these are used only used during showers as a boost. There are backdraft dampers in the exhaust fan ductwork. I reckon he exhaust fans probably run for 30 minutes a day so the power is negligible.

It's not best practice in an airtight passive house but my house is an 80's house so neither here nor there to me.

Edit: I've got a couple of teenagers and they can spend a half hour in the shower with no fan running and turn the bathroom into a wet steamy mess! So the extra fan helps to clear this quicker when required.

tweake

2391 posts

Uber Geek

#3345354 21-Feb-2025 17:24

first thing is the house is probably so air leaky a balanced ventilation system won't gain you much for the extra cost. i don't know what part of the country your in, but i would be inclined to spend money on improving insulation than using an hrv or erv to save energy. especially as the house will be well below standard (assuming it stock)

one big problem with A is lossnay sucking air back down the bathroom fan. you would need to fit a good damper to stop that, tho some new fans have that built in.

it really comes down to what your trying to achieve and what you want to spend.

tweake

2391 posts

Uber Geek

#3345355 21-Feb-2025 17:32

Cherios:

Need some advice, hopefully there's some HVAC experts hanging around.

Early 2000s brick and tile single level, single glaze windows, 160sqm floor area. ( looking at lossnay vl350)

that model is nearly double the size you need. the vl220 is plenty big enough. you only need 53l/s (assuming 2.4m walls) and i would go slightly less at 35l/s (the legal minimum) due to the air leaky nature of the house. edit: i would also keep the air flow small if your in auckland/northland otherwise you will just bring in more humidity and make the house more humid/damp.

Kickinbac

427 posts

Ultimate Geek

#3345508 22-Feb-2025 00:31

0.5 AC/hr is the current accepted air change rate for mechanical ventilation. So for 160m2 and 2.4m ceiling that is 53 l/s.
I disagree with Tweake, the VL350 appears to be a good selection as will be running on about medium speed (it has 4 speeds). Selecting an MVHR to run on medium speed is better as you don’t want to have to run it on high speed to get the design airflow. More risk of it being noisy or not achieving the airflow. Plus and more importantly is noise and efficiency, the fan is quieter and the heat exchanger and fan are more energy efficient at lower speeds.
The duct design is very important to get right. Check out Solutionair.co.nz, they sell the Blaufast duct system which is a great duct system for this purpose.

The vertical Lossnay are also easier to service. Most people don’t want to go into the roof to access the filters.

Kickinbac

427 posts

Ultimate Geek

#3345509 22-Feb-2025 00:37

tweake:

Cherios:

Need some advice, hopefully there's some HVAC experts hanging around.

Early 2000s brick and tile single level, single glaze windows, 160sqm floor area. ( looking at lossnay vl350)

that model is nearly double the size you need. the vl220 is plenty big enough. you only need 53l/s (assuming 2.4m walls) and i would go slightly less at 35l/s (the legal minimum) due to the air leaky nature of the house. edit: i would also keep the air flow small if your in auckland/northland otherwise you will just bring in more humidity and make the house more humid/damp.

There is no legal minimum ventilation rate for a house providing there are opening windows. You don’t even have to have a bathroom exhaust if there is an opening window.

tweake

2391 posts

Uber Geek

#3345525 22-Feb-2025 10:48

Kickinbac:

0.5 AC/hr is the current accepted air change rate for mechanical ventilation. So for 160m2 and 2.4m ceiling that is 53 l/s.
I disagree with Tweake, the VL350 appears to be a good selection as will be running on about medium speed (it has 4 speeds). Selecting an MVHR to run on medium speed is better as you don’t want to have to run it on high speed to get the design airflow. More risk of it being noisy or not achieving the airflow. Plus and more importantly is noise and efficiency, the fan is quieter and the heat exchanger and fan are more energy efficient at lower speeds.
The duct design is very important to get right. Check out Solutionair.co.nz, they sell the Blaufast duct system which is a great duct system for this purpose.

The vertical Lossnay are also easier to service. Most people don’t want to go into the roof to access the filters.

its nearly double the airflow. even the smaller models you would still be slowing down to get the required rate. they should not be noisy etc unless they are a poor product. down side of slowing the fan speed down is also reducing the static pressure, then you become even more reliant on duct work.

not sure with that exact model, the other models i looked at, the "4 speeds" you can change each "speed" to whatever you like. the only time you really want an oversized unit is if you want a really big boost mode (eg burnt toast mode).

tweake

2391 posts

Uber Geek

#3345526 22-Feb-2025 10:57

Kickinbac:

There is no legal minimum ventilation rate for a house providing there are opening windows. You don’t even have to have a bathroom exhaust if there is an opening window.

🙄🙄

the legal requirement is based around what is a good ventilation rate. so doing less, even tho in this case you can legally do less, is a silly idea as its poor quality ventilation. (keep in mind if you run mechanical ventilation you don't open windows).

the 0.5ach is often used because its more ventilation without much in the way of cost penalty. however as its a heat recovery only, there can be a humidity penalty.

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Kickinbac

427 posts

Ultimate Geek

#3345529 22-Feb-2025 11:30

tweake:

Kickinbac:

There is no legal minimum ventilation rate for a house providing there are opening windows. You don’t even have to have a bathroom exhaust if there is an opening window.

🙄🙄

the legal requirement is based around what is a good ventilation rate. so doing less, even tho in this case you can legally do less, is a silly idea as its poor quality ventilation. (keep in mind if you run mechanical ventilation you don't open windows).

the 0.5ach is often used because its more ventilation without much in the way of cost penalty. however as its a heat recovery only, there can be a humidity penalty.

🙄🙄🙄🙄

So what standard are you referring to for the legal requirement? NZS4303?
Air change rates for ventilation, particularly balanced heat recovery ventilation, are based on scientific research, and experience to give a number that is good practice.

tweake

2391 posts

Uber Geek

#3345536 22-Feb-2025 12:33

your saying that because they have windows they don't have to do the legal ventilation amount, implying they can have WORSE ventilation.

this is the building industry mindset that "legal minimums are the way you do it". which of course is not true. we are allowed to go above the legal requirements.

Kickinbac:
Air change rates for ventilation, particularly balanced heat recovery ventilation, are based on scientific research, and experience to give a number that is good practice.

that is what sets the legal minimums. however it does not set "good practice" as that is always debatable and changes as theory's and research comes and goes. so the 0.5 ach is not set in concrete, you can run 0.4ach or 1ach if you want. that is a good thing especially when dealing with situations the balanced ventilation was never really intended to be used in.

a good example of this, is the use of recirculating range hoods in air tight homes. that was "good practice" because it was presumed air tightness was more important than a ventilating range hood. however thats found to be not true so its no longer the "good practice". well except nz where we still do it because its cheap and houses are there to make money from, not be healthy.

Cherios

50 posts

Geek

#3345545 22-Feb-2025 13:31

tweake:

Kickinbac:

0.5 AC/hr is the current accepted air change rate for mechanical ventilation. So for 160m2 and 2.4m ceiling that is 53 l/s.
I disagree with Tweake, the VL350 appears to be a good selection as will be running on about medium speed (it has 4 speeds). Selecting an MVHR to run on medium speed is better as you don’t want to have to run it on high speed to get the design airflow. More risk of it being noisy or not achieving the airflow. Plus and more importantly is noise and efficiency, the fan is quieter and the heat exchanger and fan are more energy efficient at lower speeds.
The duct design is very important to get right. Check out Solutionair.co.nz, they sell the Blaufast duct system which is a great duct system for this purpose.

The vertical Lossnay are also easier to service. Most people don’t want to go into the roof to access the filters.

its nearly double the airflow. even the smaller models you would still be slowing down to get the required rate. they should not be noisy etc unless they are a poor product. down side of slowing the fan speed down is also reducing the static pressure, then you become even more reliant on duct work.

not sure with that exact model, the other models i looked at, the "4 speeds" you can change each "speed" to whatever you like. the only time you really want an oversized unit is if you want a really big boost mode (eg burnt toast mode).

I ran my numbers based on branz best practice, 0.35 to 0.5 ach, most generally most base it on 0.5ach, so for me 37 - 53 L/s. so according to that vl250 will meet it just find but will require you run at almost at higher settings which is not ideal, more noise, worse efficiency etc.

So next step up with be VL350 at running at a lower/medium to get the same 37 to 53 L/s. Low/medium speed is recommended based on Mitsubishi's datasheet. There online calculator/ brochure even recommends this this model.

Also if I am reading it correctly the lossnay has 4 speeds according to this datasheet.

tweake

2391 posts

Uber Geek

#3345556 22-Feb-2025 14:36

having a look through the manual to see how mitsi intend it to be used. i can see why they recommend such high air flows, because they use a boost and a purge mode. that really comes into play when using co2 sensor. their "normal" is fan at minimum speed. so you run low speed for normal, then high speed for boost and max for purge which is really shoving a heap of airflow. however i doubt your going to use boost or purge unless you use it to replace the bath fans.

it has 4 preset air speeds, each preset can be changed. odds are your going to run it off balance, ie more incoming air than outgoing due to being an air leaky house. you don't want such a big fan that you you run the inlet at minimum and can't run the exhaust any lower. you need some wriggle room for adjustment.

Cherios

50 posts

Geek

#3345561 22-Feb-2025 15:17

tweake:

first thing is the house is probably so air leaky a balanced ventilation system won't gain you much for the extra cost. i don't know what part of the country your in, but i would be inclined to spend money on improving insulation than using an hrv or erv to save energy. especially as the house will be well below standard (assuming it stock)

one big problem with A is lossnay sucking air back down the bathroom fan. you would need to fit a good damper to stop that, tho some new fans have that built in.

it really comes down to what your trying to achieve and what you want to spend.

I live in Auckland, around Alfriston. Main reason with going for the lossnay is I have an existing ~15 year old pp Hrv setup. House is on a concrete slab, has wall insulation and the previous owner previously redone the roof insulation. I also have 2 heat pumps, 1 in the lounge and master.

Liked the idea of built in fresh air bypass, possible savings and non roof space air. To replace it with another good pp system+ fresh air bypass would be more expensive, just the smart vent pp by itself is pretty close to the lossnay. So the lossnay balanced system was more attractive.

You raised a good point of it sucking air back down the bathroom fan. Now that I think about it, If going Option A I would probably need to add/have a damper for the bathroom extractors which I think they all have, makes no sense to me to re do it just to add a damper on top just for the extra lossnay return, might aswell take the extractors out at this point and go option B. So I think the best course of action would be option C, save a few bucks for now and when the extractors die, repurpose the duct to the lossnay.

tweake

2391 posts

Uber Geek

#3345570 22-Feb-2025 15:38

Cherios:

I live in Auckland, around Alfriston. Main reason with going for the lossnay is I have an existing ~15 year old pp Hrv setup. House is on a concrete slab, has wall insulation and the previous owner previously redone the roof insulation. I also have 2 heat pumps, 1 in the lounge and master.

Liked the idea of built in fresh air bypass, possible savings and non roof space air. To replace it with another good pp system+ fresh air bypass would be more expensive, just the smart vent pp by itself is pretty close to the lossnay. So the lossnay balanced system was more attractive.

You raised a good point of it sucking air back down the bathroom fan. Now that I think about it, If going Option A I would probably need to add/have a damper for the bathroom extractors which I think they all have, makes no sense to me to re do it just to add a damper on top just for the extra lossnay return, might aswell take the extractors out at this point and go option B. So I think the best course of action would be option C, save a few bucks for now and when the extractors die, repurpose the duct to the lossnay.

that shows just how overpriced the pp systems are, tho there is cheaper ones. also the balanced systems do not come with good filter setups. you need a prefilter for them. the balanced systems filters are just to keep the core clean, not filter the air. they are way to small for decent air filtering. get a decent filter box and decent filter. check out recent thread on that. sales people like to leave the need for extra filter out to make it look cheap.

the problem with doing C is your have the cost of installing the exhaust which won't be used when you change it to the bath room (plus need to be sealed off). i would go straight to B. if your bath fan has its own switch, then that can be used as a boost button for the lossnay. just not sure how they wire it on those.

i see Panasonic now have their one here. might be worth checking out. unfortunately i have no details on what models they have here. there is a few other brands around to.

being in auckland, i would keep your ventilation rates down. start with 0.35ach. thats because the ventilation system will bring in a ton of moisture as well.

Handle9

11386 posts

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#3345758 22-Feb-2025 21:31

tweake:

Kickinbac:

There is no legal minimum ventilation rate for a house providing there are opening windows. You don’t even have to have a bathroom exhaust if there is an opening window.

🙄🙄

the legal requirement is based around what is a good ventilation rate. so doing less, even tho in this case you can legally do less, is a silly idea as its poor quality ventilation. (keep in mind if you run mechanical ventilation you don't open windows).

the 0.5ach is often used because its more ventilation without much in the way of cost penalty. however as its a heat recovery only, there can be a humidity penalty.

Please quote the standard or law that is applicable for your supposed "legal requirement."

Cherios

50 posts

Geek

#3345813 23-Feb-2025 00:45

I think he's referring to nzs-4303

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