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mdf

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  #2086799 10-Sep-2018 11:55
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We installed a positive pressure DVS-branded system at our old house (this was well over 10 years ago). This was a 1900s-build, 1970s semi "modernised", wooden cottage. Timber piles into clay. No insulation and generally rubbish joinery (i.e. typical first home). We had serious dampness issues when we first moved in to the extent where mould was growing on leather jackets in one of the wardrobes. 

 

After installation, we had 6 weeks of no obvious change and me thinking we had bought a lemon. One morning (i.e. literally overnight) there was no condensation on the windows any more and the house felt noticeably drier. House also became easier to warm. 

 

As I understand it, this works something along the lines of the following:

 

The DVS we had was about as simple as it got - fan would suck ("warm, dry") attic air through a filter sock and push it into the rooms. Very basically, if you mix 1000 L of 50% humidity air with 1000L of 0% humidity air you end up with 2000L of 25% humidity air. Mix another 1000L of 0% humidity air and you get 12.5% humidity etc. Our rooms were far too leaky to hold the overpressure of the additional air, and so inevitably it escapes under and around doors, window cracks and through floorboards. Obviously it doesn't push 1000L all in at once, it is a very gradual process. You're not pushing out the moisture per se, rather mixing in additional dryer air constantly thereby reducing the moisture content of what's left (homeopathic home ventilation?).

 

Eventually enough drier air was pushed through to take care of the additional moisture we had retained by curtains, wall linings and likely even the floor boards. The DVS didn't push moisture into these, rather the drier air shifted the equilibrium enough for the additional retained moisture to evaporate off.

 

Newer houses (post 2004 apparently) are much more airtight. So far as I have been able to figure out, the problem with these for positive pressure systems is not that the moisture is pushed into furnishings or anything, rather that with no cracks etc. for the overpressured air to escape, the pressure pushed by the fan (which is by design very low) will be equalised by the room. You probably wouldn't get backflow but you're not getting any drier air pushed in. So it shouldn't make anything worse, but it might not be very effective.

 

Disclaimer: I am not a home ventilation technician. This is what I have gleaned from reading and thinking about this. Why do I read and think about this sort of stuff? I suspect there may be something wrong with me.


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  #2086800 10-Sep-2018 11:57
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networkn:

 

Hi.

 

Thanks for your information. I am not disputing your account or experience, simply trying to understand the situation fully. I wasn't aware you had windows open until today, that explains where the moisture goes. It's the missing "piece" per se. 

 

How many windows do you have open per room, do you know?

 

 

No worries.  I really agonised over this same thing a few years ago before installing our system.  I would have installed it at the time we built, but my heatpump guy said our house wasn't suitable for an install.  On paper it's a tricky one (raking mono-pitch with no eaves and no roof-space), but it has been done (who needs cupboard space anyway).

 

I've got one window in each bedroom open - they range from 500mm deep x 1100mm wide to 1000mm deep x 900mm wide.  

 

Other measures that were installed when we built the home are a rangehood in the kitchen, extraction fans in both bathrooms (bathroom windows both cracked open as well) and an externally ducted dryer.  Unfortunately, because we don't have a garage, we have no other space to dry clothes, so they spend a lot of time drying in our Living room during the winter.  (This is a big no no, but we have to do it).  The only other "problem" are the house's occupants.  There are 4 of us and we all expel a lot of moisture laden air into the house.


 
 
 
 




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  #2086801 10-Sep-2018 11:58
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Very interesting reading @mdf thanks.

 

 




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  #2086802 10-Sep-2018 11:59
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We found a great thing we intend to install in our bathrooms, which already have extractors, but it's a $115 "silent" extractor, that moves about twice the air that our current extractors probably do. I hate the noisy ones we have now, great find at the home show. 

 

 


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  #2086803 10-Sep-2018 12:01
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networkn:

 

We found a great thing we intend to install in our bathrooms, which already have extractors, but it's a $115 "silent" extractor, that moves about twice the air that our current extractors probably do. I hate the noisy ones we have now, great find at the home show. 

 

 

 

 

That sounds great!  What's it called?  I can't stand our noisy extractors either!


mdf

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  #2086804 10-Sep-2018 12:02
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And to chime in on a previous comment, we never cracked the windows in winter and still noticed the positive effects. 

 

At our current place (bigger but not renovated and with some moisture issues in a couple of rooms - i.e. a typical second home), we have ducted central heating that includes a fresh air bypass. Completely different system but sort of works along the lines of a balanced pressure system. We run this for a few hours during the day if we are out and that is enough to make a positive difference. i.e. you don't necessarily need the systems to be running 24/7 for them to work effectively.

 

I am pretty sure someone else has chimed in with running their system on a timer effectively.

 

i.e. If you did have to crack windows to make the system work effectively, you could do this for a few hours a day (say while out) and still notice the difference. House might be cold when you get home but you could heat it very efficiently. Obviously doesn't suit every lifestyle though.


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  #2086825 10-Sep-2018 12:08
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mdf:

 

And to chime in on a previous comment, we never cracked the windows in winter and still noticed the positive effects. 

 

At our current place (bigger but not renovated and with some moisture issues in a couple of rooms - i.e. a typical second home), we have ducted central heating that includes a fresh air bypass. Completely different system but sort of works along the lines of a balanced pressure system. We run this for a few hours during the day if we are out and that is enough to make a positive difference. i.e. you don't necessarily need the systems to be running 24/7 for them to work effectively.

 

I am pretty sure someone else has chimed in with running their system on a timer effectively.

 

i.e. If you did have to crack windows to make the system work effectively, you could do this for a few hours a day (say while out) and still notice the difference. House might be cold when you get home but you could heat it very efficiently. Obviously doesn't suit every lifestyle though.

 

 

Older houses are very "leaky" anyway, so don't require windows to be cracked open.

 

To be clear on my comments regarding the windows being opened.  Our system works with them shut as well.


 
 
 
 


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  #2087148 10-Sep-2018 20:07
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networkn:

 

We found a great thing we intend to install in our bathrooms, which already have extractors, but it's a $115 "silent" extractor, that moves about twice the air that our current extractors probably do. I hate the noisy ones we have now, great find at the home show. 

 

 

What is the make / model of it? I have a Mixflo MF150s, it does 166 liters per second / 597 cubic meters per hour on high, low I guess a bit less than half that. It's very effective, but I wouldn't call it near silent. The fan itself isn't all that loud, but the air rushing around makes some noise.

 

I'd never use an integrated fan / light, most are too puny.


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  #2087185 10-Sep-2018 22:06
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We got this one when we re-did our bathroom.  It has separate switches for each fan so can be operated independently.  The in line fan is pretty quiet, the one in the intake not so much.  Easily keeps our 3x2.5 bathroom clear. Good WAF too as no visible vent.

 

https://www.weiss.co.nz/products/weiss_FV120SS

 

 

 

 





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