Temperature, Humidity, and Drying

Some generic head-scratching over relative humidity and the heat required to dry wood under various conditions. April 10, 2007

This is a question specifically related to drying and maintenance of proper moisture content in music soundboards, piano in particular, but the concept involved is more general.

In achieving the proper moisture content before various fabrication processes, we use a "hot box," not a kiln, but a temperature controllable environment which heats ambient shop air as necessary to achieve a target moisture content (often 6.5-7) and adjusted for the daily fluctuations of ambient RH. Spruce boards are thin at 5/16". The hot box is not air sealed by a long shot.

In observing the empirical results actually achieved in the box, i.e. the actual moisture content achieved in either a short period of time, or maintained over a long period of time (wood that remains in the box say 1 or 2 months at a time) we notice that according to the equilibrium moisture charts, in really humid ambient conditions during the NE summer, we should need to be running the box much hotter (20-40 deg) than we actually do. Why?

I suspect that there are two things going on in the hot box:
1- Yes, as intended, we are raising the temperature of the ambient air, thus lowering the RH.
2- But that's not the whole story... The warm temperature of the wood must be also affecting either RH of the actual air that contacts it, or some other radiant heat effect (since the wood I think at this point is a secondary radiant heat source).

Any insight on what's really going on here? This question affects not only the above hot box scenario, but would explain why a very low powered heat source used in finished pianos in homes called a Damp Chaser clearly works, even though its seems that it really shouldn't.

Forum Responses
(Sawing and Drying Forum)
From contributor D:
The wood is both a heat sink and a moisture sink. If you are trying to adjust the temperature of the box based on ambient conditions, you have chosen an unnecessarily difficult way to do it. Just put a humidistat in the box, set it at 35% and have it turn the heat on whenever the humidity is above set point.

From the original questioner:
Actually, my question is a conceptual one. I don't have a humidistat in the box, but presently simply check the RH daily and adjust temp once daily as necessary to maintain 35% RH or so. I'm happy with the results and the setup.

For example… During the soggy New England month of July, ambient temp/RH in the shop is often 80 f /70%RH.In order to reduce this ambient air to the target 35% rh, the tables show the temp needs to be raised to approx 108 deg. The hot box is not air sealed so I'm assuming the air in the hot box is the same absolute moisture as the ambient air. However, I never need to raise the temp to that 108f to achieve the 35% RH. The wood stays at 7% mc with the hot box only at 90f or so under extended hot humid periods. Why?

The Moisture EQ tables assume that the target RH is achieved as a simple system where the air is the only thing that affects the wood. The results I'm observing seem to suggest that in reality the system is more complicated. Can the temp of the wood itself also be mitigating the air RH levels by radiant means?

From contributor I:
I think what contributor D means by wood being a moisture sink is that if the humidity in the air goes up, then the wood will absorb that extra water from the air. The amount of water suspended in the air inside your box is actually very small, a fraction of an ounce. If you have 100lb of wood in there, you would need to add approx 1 lb of water to raise the MC by 1%. Although your box isn't airtight, it's not actively ventilated, so the air doesn't change very much. Even if it changed completely every day, it would still take weeks to make any significant change.

Another way of looking at it is... The wood is actually drying the air in the box. The wood can hold a large amount of water compared to the air. If the conditions persisted for months you might notice the humidity creeping up, but unless you are changing the air in the box with vents/fans it will take a long time.

From contributor D:
The change in moisture content is not instant. It takes time and periods of the day when the air ins 80F/70% are actually pretty short. Usually people who are controlling a storage area by humidity install a high limit thermostat in series with the humidistat because going over 95F is usually only required for a short period and not long enough to have the wood absorb much moisture. For example, the dewpoint through the day is about the same as the morning low temperature. At 80F/70%, that is 69F. Naturally the weather can change dramatically, but as a rule, most morning low temps in most places in the USA and Canada are higher and an exception now and then is not enough to worry about.

From Professor Gene Wengert, forum technical advisor:
I do not know where you are getting your data about the amount of heat needed to achieve a dry condition, but something is wrong. Are you using a chart that gives the absolute humidity?

In fact, the air has roughly the same RH, summer or winter, so the amount of heating required to achieve a given EMC will be the same throughout the year. As an approximation, it will take about 25 F heating above the morning's low temperature to achieve 7% EMC, summer or winter. Of course, if you put some wet wood in the box, this will not be accurate for a brief period due to the high moisture coming form the lumber.

From the original questioner:
Thanks. The part of contributor I's response regarding the wood drying the air brings up the second part of my original post.

There has been a discussion between piano technicians regarding why a product called a Damp Chaser, a moisture controlling system installed in pianos in the field (in varying climate conditions), works as well as it does. It seems like it really shouldn't work that well, but it actually performs its task very well. It consists of one or a series of very low powered linear resistance heaters, usually installed on the underside of the piano very near the soundboard. As well, there is a humidification side to the product which consists of a water bath/resistance heater/humidistat.

Given the fact that the piano is in the ambient room air, subject to same air movement and air changes as the rest of the room, you would think that these low powered heaters wouldn't have a prayer overcoming the effects of the air in the entire room.

Obviously, the installation is creating a micro-climate close to the soundboard and close to the pinblock. The usual explanation talks about raising air temp lowering RH. I find that explanation difficult to buy as a simple single dimensional explanation of what's going on because of the air changes in and around the piano.

Rather it seems to me that in addition to the standard air temp/rh explanation, secondarily, the heaters are also gently raising the temp of the soundboard, metal plate (the string harp) and wooden pinblock, as well as parts of the rest of the piano carcass. The raised temp of the wooden members would seem to be affecting the temp of the air close by, and in addition, the wood's ability to suck some moisture out of the air and release it in a very slow and gentle absorption/release cycle would also seem to create the very beneficial cushioning of the ambient cycles. Does this seem reasonable?

From the original questioner:
Gene, I'm trying to understand this concept more thoroughly, so please bear with me while I pursue this just a little longer. Actually, my expectations of how much temp rise was needed to achieve a target rh was generated by mistakenly using an "EMC@stated temp" chart as a "psychometric" chart.

Am I correct in the following reading of the charts?
Given the example 80f/70% RH using a psychometric chart, I get 69f dew point and 103f to lower that RH to 35%. According to your explanation of morning low temp (dew point) plus a rise of 25f roughly to achieve 7% EMC, 69f dew point +25f =94f (roughly). Using a psychometric chart that brings the given air (80f/70% rh) to 94f/ 45% rh, a little high to achieve 7% but in the ballpark, given the cushioning effect of the closed box and wood in the box.

If that's right, I think I have the idea... but the question I posed in my previous post regarding why the Damp Chaser works as well as it does stands. Any opinion there?

From contributor D:
I just looked up Damp Chaser and it is simply a heater run by a humidistat. When it is humid, it heats the air to drop the RH. Don't overthink this.

From Professor Gene Wengert, forum technical advisor:
To the original questioner: your calculations are close to what I get.