Basics of humidity control - Info/FAQ

Gombessa Wow, what a lot of great information! I definitely learned quite a bit from reading this.

One very basic thing I didn't understand — why is it the relative humidity that's important as opposed to the absolute humidity?

@rogerch where do you live? In the US, AC commonly refers to cooling. There may be people who use it to refer to heating but they are using it mistakenly, because the complete term HVAC is heating, ventilation, and air conditioning.

Since becoming a piano owner, I have lived in Japan and in the U.S. Midwest (including the northern Midwest, central Midwest, and southern Midwest) and in the U.S. Southeast. In Japan, humidity control was challenging but made do-able by the very efficient and capable mini-splits that are so common in most houses.

In US, in the last house we owned, we replaced the HVAC. And in the last rental house we lived in, we had a hell of a time controlling the RH. So I’ve spent a lot of time reading about HVACs, monitoring my own systems, and also talking to HVAC installers.

So my understanding is specific to the regions I’ve lived in of course, but with that caveat in mind, in my understanding, it’s both common knowledge as well as my direct experience that AC, as in cooling, lowers the RH.

Unless there’s something wrong with the system or the house. Which is not a minor issue, and not that uncommon. (Our last rental house fits in this category and is one of the reasons I was in a hurry to move and one of the reasons I’m so happy that the house we just bought has such a good HVAC).

So with regard to the system, it’s also common knowledge among HVAC installers that an oversized system can often result in the AC portion not working as intended, such that when the AC is on, the indoor RH actually increases.

So AC (cooling) resulting in an increase in humidity (an increase to problematic levels, that is) usually means either that the system is not well-suited to the house, or else that there’s some moisture problem in the house. In the first house we owned, we lived in the southeast where it was hot and humid in the summer and cold and damp in the winter. And the house was about 60 years old. So after I upgraded from an acoustic upright to a grand, I replaced the HVAC, had the crawl space encapsulated and installed a whole house dehumidifier. I had hygrometers all over the house and monitored it closely and there was a big improvement.

In our last rental house (which we just moved out of two months ago), there were problems with the house and the system. The system was oversized, and I’m pretty sure that the crawl space had an unmitigated water problem so it was always damp. Because it wasn’t my house, I couldn’t do anything about either of those problems, but if I were the home owner, I would have done what I did in our last house — replaced the HVAC and had the crawl space encapsulated. I would bet that this would solve the problem, but if it didn’t, I would have then had a whole house dehumidifier installed.

In our new house, which is only about 30 years old, the previous owner had the crawl space encapsulated, and the basement is treated (i.e. in the finished parts of the basement, there are HVAC ducts and vents and returns). Because it’s a newer house then previous house i owned and the previous rentals i lived in, the windows are better and the insulation is better, and the HVAC is well-suited to the house. So on the main level, humidity control has not been a problem, and when the AC is running, the RH drops in all rooms where I have hygrometers.

Ok, sorry, I’ve gone on much too long here! It’s just that i have spent a lot of time and effort — and money —working on humidity control …

So, back to you @rogerch what area of the world are you in? I’m guessing if it’s the U.S., maybe you’re in the southwest? Also I notice that you wrote that your RH goes up to maybe 40 or 50, I don’t think that’s a problematic increase and so I wouldn’t say it means your hvac is oversized or something. In the last rental we lived in, the one with the problems, with the AC on, the RH would jump up to 75% or 80% — so I started using a free-standing dehumidifier. It was awful! (And we don’t even have to talk about how unpleasant it is to have a free-standing dehumidifier going 24 hours a day in your living room… nothing like the noise of a jet engine to make your house cozy…)

navindra why is it the relative humidity that's important as opposed to the absolute humidity?

Ooh good question… I do think absolute humidity is important, as in, it’s not completely irrelevant.

But my understanding is that moisture moves around (for lack of a better term), and it’s RH that determines how it moves… if the RH is high, it’s easier for moisture to “move into” the piano … i.e., be absorbed by the wood and cause the wood in the piano to swell (and we all know why that’s bad).

If the RH is low, it’s easier for moisture to “move out of” the piano, i.e., for the wood to dry out, which causes the wood to shrink and in worse cases, crack.

When the RH is around 40-50%, that’s when moisture moves to or from the wood the least.

It’s different for other materials (say, fabric for example). So I think this is why it’s RH that matters, not just absolute humidity. Because as the temps go up and down, and the RH moves accordingly, it changes the ability of the moisture (that’s already there, the absolute humidity) to move into and out of materials in the room.

Here’s another interesting (and I believe relevant) tidbit… in the paperwork for my Venta humidifier (note, not a de-humidifier), it explains that when you first turn on the unit and start putting more moisture into the air, the first thing that happens is that soft materials in the room soak up that moisture — curtains, plush furniture, carpeting. During this time, hygrometers may show little or no increase in RH. But once these items have soaked up enough moisture, then the additional moisture coming from the humidifier starts to stay in the air more and that’s when you’ll start to see the RH go up on your hygrometer.

So there’s probably a better, more technically accurate way to explain why it’s RH that matters, but hopefully this helps a little bit…

Oh, one more thing to add (sorry for posting so much, there’s nothing I love more than a leisurely Saturday morning breakfast and posting on Piano Tell…

So, back to the question of why an AC causes a drop in RH.

Normally — say, outside in the open air — if the temps drop, the RH increases and if the temps rise, the RH decreases. This is because cooler air can ‘hold’ less moisture, and hotter air can hold more moisture. That’s why many climates are more humid in the summer and less in the winter. (But not deserts, I don’t know anything about deserts and someone else can chime in on that matter).

So you would think that AC, i.e., cooling the air, should cause the RH to rise.

But the important point is that with an HVAC, the cooling is artificial — first, warm air is blown across a cooling coil (with refrigerant) and then the air is moved around the house by artificial means — the air handler, which is like a massive fan.

Any kind of movement of air can contribute to a drying effect, hence the AC causing a (usually beneficial) drop in humidity even though the temp dropping would otherwise cause a rise in humidity.

In a desert climate, this is different because it’s so much drier outside and I don’t know how the parameters are different. (It’s also why I am super curious about whether @rogerch is in a desert or desert-like climate)

With regard to heating, it’s already less humid, and on top of the air being moved around by the air handler (the fan) the air is also being heated. So even though an increase in temps outside lead to an increase in overall RH, indoors with the heat on, it leads to an overall decrease in RH because the air is being in both artificially warmed, and artificially moved around — both of which result in a drop in RH.

So basically, HVAC systems, whether in heating or cooling mode, usually result in a drop in humidity. It’s just that in the summer, this is usually beneficial for pianos because the starting point is a higher RH. But in the winter, the starting point is already a lower RH, so usually the drop in RH caused by heating is bad for pianos.

So, in conclusion (you all thought I’d never finish!) absolute humidity is important and relevant, but it’s relative humidity that has the more direct impact in the piano.

I live in Montreal, Canada, where the indoors are dry in the winter and wet in the summers. In addition to humidifiers (during the winters) and dehumidifiers (during the summer), another thing that helps is to keep the indoor temperature on the cold side (less heating) in the winter and on the hot side (less AC) in the summer! I think this is the easy/stupid version of what @ShiroKuro said earlier, but it works for me. My grand piano has amazing tuning stability.

Rubens I live in Montreal, Canada

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Oh nice! Montreal is a beautiful city. I lived there for a decade. It's amazing. Pierre Marchand is not too far away and Esmonde White as well... would love a report on Oliver's pianos with the Fandrich action one of these days.
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However, after a decade in Montreal, I could easily survive "winters" elsewhere in a t-shirt and shorts... I don't miss those -40 deg winters at all and it's not like the summers don't get hot either. I would have thought it would be hard to do optimal piano climate control under those Montreal conditions, but it sounds like you're doing just fine based on the other thread!