this post was submitted on 21 Jun 2023
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We do have HVAC, in a heat pump. It's running all day. A complete system like HRV is a bit too much for now. The dry setting is annoying as it is cooling. Not handy mid winter.
You mean a split system? A heat pump usually refers to a split system which doesn't have the V in HVAC as it just recirculates air. HRV is the ventilation from the attic thing, please don't get that.
HVAC generally refers to ducted aircon with mechanical ventilation. The "heat pump" sits in the attic. Fresh air is taken in from the outside through the soffit. Ducts run from the "heat pump" into each room.
If a ducted system is too expensive then you should install plain mechanical positive pressure ventilation from the outside (not HRV/DVS) Depending on your house it is possible to DIY this using stuff from Bunnings. You could even link the fan to your Home Assistant and get it to activate when the humidity reaches a certain threshold. This should fix your problem. If it causes the house to get cold by doing this then you could install a mechanical heat recovery device.
Thanks mate, that is very useful. Yes, we indeed only have a heat pump.
I looked into HRV/DVS alternatives some time ago, and Cleanaire was recommended on e.g. Geekzone; which gets the air from outside & heats it. Quote was $4-5k which is a bit steep as we're not sure how long we'll be living here.
A DIY option for a PIV isn't too bad. I could leave it running overnight and use the heat pump to keep the house above 16c.
I also bought a small dehumidifier as a stop gap.
I've left the heat pump running on dry / heat cycles overnight (1 hour dry to 17c, 1 hour heat to 19c). Didn't do anything, RH at the end was exactly the same as the night before.
Oh wow mechanical heat recovery systems have jumped in price. They were 2-3k a few years ago. The way mechanical heat recovery works is kind of like penguin legs. The inflow and exhaust airflows pass by each other. So the warm exhaust air passively heats the cooler inflow air. So they are quite basic things that are very cheap to run. I guess another kiwi tax. Done some googling for pricing and it does appear 5-6k is the going rate now. E.g. another option: https://www.smooth-air.co.nz/home/hex390
What about something like this: https://www.smartvent.co.nz/why-smartvent/ that should also improve the humidity. Doesn't say price though. They seem to have a variety of options including positive pressure which should be much cheaper than a heat recovery system. https://www.smartvent.co.nz/smartvent-positive-advance/
Here's some more information about heat recovery systems and why DVS/HRV from the attic are shit. https://www.smarterhomes.org.nz/smart-guides/heating-cooling-and-insulation/ventilation-and-heat-recovery-systems/
Thanks for the details!
Smooth air has kits for just ventilation for 1.5k for 2 rooms. Smartvent is more expensive, like 4k, they have some pricing.
I don't need a lot of bells and whistles. I can automate it myself. Just an outdoor outlet and an indoor outlet, ventilator and two pipes for just the living room.
I read your article, they say HRV/DVS is bad for heating, but not if you use it just for ventilation?
One other easier option is to just have a simple diy DVS by using the air from the attic. I'm going to check the moisture levels there first.
Sorry to bother you again, but wouldn't something like this work? Instead of connecting to the heated room as inlet, I'd connect it to the outside. Of course has to be water proofish.
https://www.mitre10.co.nz/shop/weiss-heat-transfer-kit-1-room/p/175013
That should work if you bring it to the soffit instead of a different room. However youre paying extra for the temperature sensor. Don't get the air from the attic, it's not healthy air, I don't care how good the filters are, it is something unheard of elsewhere in the developed world. I would rather have nothing than air come in from the attic. Please rid yourself of the idea of venting from the attic.
I have these links saved in my bookmarks when I first researched this 10 years ago and retrofitted a ducted HVAC system in my previous house.
https://www.energyvanguard.com/blog/Case-Closed-Get-Those-Air-Conditioning-Ducts-out-of-the-Attic
https://www.nrel.gov/docs/fy10osti/48163.pdf
In many countries in Europe houses actually have to have their ventilation/insulation performance tested and pass the test (e.g. blower door for air tightness). We have gotten better at insulating but our builders still don't generally understand the importance of mechanical ventilation. Cue Mouldy homes chapter 2.
Balanced heat recovery mechanical ventilation > positive pressure ventilation > negative pressure ventilation > nothing > air from the attic.
Just my opinion. I don't work in the industry. Just a geek.
Thanks. I had a look at those articles. I'm not an expert, but they seem to talk about the temperature / lack of insulation in the attic, not about the air quality. They recommend putting the pipes inside the thermal enclosure.
I think you're still right though, better to get air from the outside.
I did some reading, and I found this article where they talk about lack of attic ventilation being a bad thing. Overseas it seems to be a standard now. Think next step is to measure the attic as well to see if I need to ventilate that as well.
Another read about attic ventilation here
Oh that's a slightly different thing I think. It is referring to ventilating the attic with the outside. Not ventilating the attic with the living space which is what the DVS/HRV thing is. Ventilating the attic with the outside is generally a good thing for various reasons. I might have messed up the terminology. But I don't think attic ventilation between the attic and outside will solve your problem.
Yeah, definitely don't want air from the attic.
I read somewhere else that just a positive pressure system is not sufficient, you need a balanced one, especially in well insulated houses. Ie fresh air in, stale are out. If I install something I want it to be right.
That would mean a couple of thousand dollars.
Food for thought. Going to try with opening windows more often first and a small dehumidifier, especially when drying clothes.
Positive pressure is definitely not as good. But if you plan ahead you can always modify into a balanced system later. In my last house when I installed positive pressure I also added an additional vent in the soffit in case I wanted to turn it into a balanced system in the future. But the positive pressure solved the condensation problem so didn't end up upgrading to balanced. You could also do balanced without mechanical heat recovery.
Maybe as an experiment try blow a fan pointed out an open window and see if it makes any difference to the humidity?
Thanks again for all the tips, much appreciated! I learn more and more about it.
Yes, will try the fan.
What type of system did you install? Just a simple fan or a more sophisticated one? If you want to be ready for balanced I guess your fan needs to be ready as well?
I installed ducted air con and instead of having a separate system for ventilation, for cost effectiveness I routed it from the outside through the air con system. So the aircon pulled the air from both the return vent in the house (90%) and from the outside (10%). You only need a small amount of mechanical airflow from the outside to reduce moisture levels. And had another vent hole with cover on the soffit for future but not ducted into the air con.