Then it just means that you're overventilating! We emit much more steam (through cooking and showering for instance) than we consume oxygen/emit CO2 so controlling the humidity is the main purpose of ventilation, air renewal comes for free as a byproduct of that.
My system doesn’t over ventilate - we sometimes have too high CO2 in the bed rooms, but humidity is way too low in winters, sometimes below 30%. That means getting sick more often and having irritated airways at times.
For reference, it is currently 84% relative humidity and -20°C outside where I live. This is about the same absolute humidity as 5% relative humidity at 22°C (i.e., inside).
An all you need to bring that up to above 50% is 2cl/cubic meter of water!
A sponge drying up in your kitchen sink is enough to raise your kitchen's air humidity by 10%. Shut down your ventilation and you'll see, you won't suffocate but instead you'll get mold starting to pop-up. Moisture is the reason why houses have ventilation system in the first place.
As I said elsewhere in this thread, though, there's a problem with “dumb” ventilation systems though: they can't really adapt to big variations in outdoor conditions, and as such they tend to suck way too much air out of your house than needed during the cold days.
> A sponge drying up in your kitchen sink is enough to raise your kitchen's air humidity by 10%.
That's simply not correct.
I go through liters of water a day with my two humidifiers just to try to raise humidity by around 20 percentage points. In a small urban apartment that isn't much bigger than some people's whole suburban kitchens.
A damp sponge isn't going to do a thing, and I can't imagine where you would ever have gotten the idea that it would.
Moisture is not the primary reason for ventilation, except above showers -- it's to prevent CO2 buildup along with other toxic gases like CO and VOC's.
> I go through liters of water a day with my two humidifiers just to try to raise humidity by around 20 percentage points. In a small urban apartment that isn't much bigger than some people's whole suburban kitchens.
No surprise, that's because your water gets vented away…
My brother had a broken ventilation for a whole northern England winter in a flat he rented (and the landlord was too busy fixing this shit up), he had massive humidity issues with fungi spores making him sick before he understood what the problem was, and he'd tell you how much discipline it takes in manually venting your house by opening the windows to keep things from molding!
> A damp sponge isn't going to do a thing, and I can't imagine where you would ever have gotten the idea that it would.
Hey you know what, just do the math by yourself, it's just one pV = nRT away ! But of course, this is assuming you're not removing all that water directly as it evaporates.
> Moisture is not the primary reason for ventilation, except above showers -- it's to prevent CO2 buildup along with other toxic gases like CO and VOC's.
Maybe have a look at your local building code and see how the ventilation requirements are made. I've refurbished a house by myself and I did just that, it turns out the regulations are built on water extraction, as CO2 won't realistically kill or harm you, CO only matters in kitchens if/where you have gas stove (and in my country, this is subject to additional ventilation requirements in the kitchen itself independent of the house's ventilation), and VOC are only a recent concern. That's also why there have been hygrometer to pilot ventilation for a while.
Even with our cross counterflow enthalpy exchanger it can get somewhat dry in the bedroom in winter. The device in the OP would probably require an additional humidifier.
Isn't it the other way around? 50% humidity means that air contains 50% as much moisture as it, at a given temperature. Raising the temperature means that the air can now hold much more moisture.
Bringing in cold air at 50% humidity, then warming it up to room temp makes the humidity fall, leading to dryer air indoors than comfortable.
Yep. Humidity will tank to 25% here in winter. I have two humidifiers fighting the HRV continuously when it gets cold. As I understand it an ERV controls moisture as well, but such a module for my system costs over $4000.
But household activities like cooking, showering, drying laundry, or even just washing the dishes etc. generate tons of moisture, and this moisture is the reason why your home has ventilation in the first place: to get it out and avoid mold!
There's a problem with “dumb” ventilation systems though: they can't really adapt to big variations in outdoor conditions, and as such they tend to such way too much air out of your house than needed during the cold days (and it also tend to be designed to suck cold air into dry room first, and get out from wet rooms, when you want it the other way round when it's very cold).
> In the hot and humid summer you're definitely trying to reduce indoor humidity.
No, you can't do that with ventilation when it's hotter outside than inside actually, that's not how thermodynamics works! But we don't care about that, because in the summer you don't have cold walls or window where water vapor can condense and let mold grow.
> But in the winter when it's bone-dry? A hot shower barely makes a difference.
The reason why it doesn't make a difference is because all the moisture is vented away by your ventilation system! And that's because that's what it's designed to do! Stop it and see how it goes! For the record a single wet sponge drying up in your kitchen is enough to raise humidity by 10%! You barely need 2cL of water per cubic meter to have 50% humidity at 20°C.
As I said the problem is that in winter, ventilation system often ventilate way too much.
Also, they are often designed so the cold and dry air enters in the bedrooms/living room and the warm/moist air is extracted in the kitchen and the bathroom, and because of that the rest of the house doesn't get any of the excess moisture of these places. This is done because the designers wanted to make sure that the humidity level never raise too much in the room, because again humidity will ruin your house and health pretty quick (having air that's too dry isn't very good for your lungs, but having fungi spores in the air is much worse!)
I think you are agreeing with the parent comment with a tone of disagreement.
They say they let cold air in during the winter because they want to lower the humidity.
Then you say that, if you let cold air in (and then let it heat back up again) then you end up with lower humidity.
The only thing you might disagree on is exactly what humidity you would like inside your house. But that's subjective. (As it happens, I agree with them: I often find it too humid inside during the winter, because I've restricted airflow to keep the heat in.)
My house currently is sitting at 35% humidity while being very poorly ventilated (~900ppm CO2). In the summer, it’s around 50% with the same level of ventilation. This generally has been the case everywhere I’ve lived; in the summer, you’re cooling air, which (all else equal) increases the relative humidity of that air. In the winter, you’re heating air, which decreases the relative humidity of that air.
It's “less than ideal” level of ventilation, but it's also very far from “very poorly ventilated” (it's not even above the target level set by workplace regulation in my country, which is 600ppm above the baseline).
ERV is apparently the term used for HRVs that also exchange humidity and keep it in/out as required, though I'm not sure that makes much logical sense since E is for Energy and H is for Heat, no mention of humidity.
Possibly just different terms used in different countries where the humidity is a bigger problem (very hot and/or very cold outside air).
This current OpenERV product appears to use dessicants for this purpose but might be an optional add-on?
A perfect ERV will recover the heat (occurs naturally by a temperature difference) and the latent heat (associated with condensation/evaporation).
i.e. water vapour in the outbound flow should be transferred the inbound flow.
This is to avoid energy loss that would occur when moisture condenses (i.e. the latent heat) by using an adsorbant material to capture moisture before it escapes, or a membrane that allows moisture in the outbound flow to pass to the inbound flow.
Does very much depend on your location. In the UK a dehumidifier is still often a good idea in the winter because although our humidity drops, its nowhere near enough. Inside can still be high 50's, in older properties it'll never really go below 60%.
Really a lot of our older homes shoud be retrofitted with a MVHR unit to help things as ventelation is awful in most houses. I'm actually quite surprised a lot of landlords in the UK don't do it as theres always a fight between them and their tenants who don't necesserily want to leave a window open all day in the winter to stop mold.
Positive Input Ventilation (PIV) units seem to be a new thing for landlords to battle this problem.
I think it basically trickles in cold (and therefore drier) air into a central space to reduce humidity, like an (slower, quieter) extractor fan in reverse.
Seems a bit of a conflict of interest still, especially if the tenant is paying for heating.