Debunking “modern central heating problems”
Ok I know I am going to get a lot of Heat for this post but listen and think about this a minute.
I read a lot about how the new centrally heated homes are harder on furniture than the old houses and I think this makes no sense at all and will explain why below. (I have read Bruce Hoadley’s book, very good reference)
The current “wisdom” goes like this: In the old days houses were not sealed tight or centrally heated. With the advent of central heating we have crated a situation that is harder on furniture (read problems with wood/moisture changes and joint failures as the furniture goes through more dramatic changes of relative humidity than the old days.
I say that is a load of non-sense. Before you all jump on me for going against the conventional thinking of our time consider the two situations carefully first.
In the old days: Houses were drafty (no vapour barriers, little or no insulation, fire place for heat, doors and windows were not very well sealed to the frames. single pane glass was all that was available, maybe some lose fitting “storm” windows outside the double hung windows. In summer the house had no air conditioning system and we can expect that all the windows were opened to try to moderate the interior temperature. So it was hot. The high summer humidity would be just as high inside the house as no one had a dehumidifier yet. In winter the low exterior humidity would draw all the moisture out of the house since no one had invented the vapour barrier yet. Heating was generally done during the day and the homes left to cool off at night and then be reheated the next day (hence the night cap -one they wear not drink)
Now in contrast look at modern heating and home air conditioning systems: Now we have vapour barriers to stop (or limit) moisture transfer to the exterior during the heating season. We have air cooling systems (with dehumidifiers) to moderate the extremes of the summer heat and humidity. In many homes in the northern latitudes we have automatic humidifiers attached to the air ducting for adding moisture during the winter months to maintain a fixed level of humidity and comfort in our living spaces. We may set-back the house temperature at night but we certainly maintain a relatively constant temperature by contrast to the “old days”.
So I say the talk that modern central heating is somehow harder on the furniture than the old drafty homes of the 1800’s is pure non-sense. If you can show me some logic I am missing I am glad to listen. Otherwise I think it is time to correct a long held old wives tale.
Robert.
Replies
Robert,
Your thinking is spot on. Temp and humidity are remarkably stable in moodern homes. This is the whole idea of central heat/ac.
Steve
It's not modern HVAC that is the problem, not with AC in the summer and humidifiers in the winter. BUT, the problem with central heating occurred in years between central heating and AC. In the 18th century furniture probably didn't ever dry much beyond the air dried levels perhaps 10% even in the winter, and would only get a little wetter perhaps 14% by the end of winter so wood movement wouldn't be severe. But central heating can bring wood moisture contents down to 6% or less by the end of the winter, but without AC it could have climbed to 14% by the end of the summer. That's a much wider swing, and wood suffered more stress. If
Even in modern living designing to accomodate wood movement is important. Things sometimes go wrong--you have to store furniture for six months, or the moving van breaks down during your late july move. Or the AC needs a part that isn't in stock at the distributor.
agreed
Steve, I think you have a good point here. On another thread a reader was looking for help to repair a true 18th century fold up table that was - wait for it- top split during a trip in a moving van!
I have been building my furniture using a moisture/wood movement guide that I bought at Lee Valley (based on Bruce Hoadley's reference I think) and a quality wood moisture meter. I pay a lot of attention to the frame and panels and carefully work out the movement expected from the season I build to the extremes of the year. After the build I have monitored some of the pieces and find the actual movements to be maybe only HALF of the expected movements. Some of this may be the influence of the finishing materials but I am convinced that the house temperature and humidity swings are really not all that extreme in the modern home.
I hang a few wide cut offs from some table tops on my shop wall. These are between 24 and 48 inches wide, but cut only two inches long and 7/8" thick. I have one each of maple, cherry and walnut. I mark on them the widths measured at various months of the year, putting all the same months together regardless of year. These are unfinished pieces and they too move a lot less than the guide would have me think they should. The shop has a dehumidifier for summer.
Robert.
Robert,
Your contention is incorrect that "the low exterior humidity would draw all the moisture out of the house ".
Winter air holds less moisture ( total weight of water in a given volume of air) than warm summer air, it is true. But expressed as a percentage of the total amount of water it can hold (relative humidity), summer and winter air are both relatively "damp". It's just that cold air is denser, and has less "room" for water vapor. Winter air is only dry when you bring it inside and warm it up. Warmed winter air can hold more moisture than cold air, so its relative humidity is lower than the outside air. Since it can suddenly hold more moisture, it has the effect of pulling that moisture from the objects (like wooden furniture) it surrounds, until they are in equilibrium with the air.
18th century home heating was primarily by fireplaces, which warm objects mainly by radiation, and not by heating the air in the room. (The air that is heated by the open fire rushes up the chimney, pulling more cold air into the house.) So, the relative humidity in an 18th cent home was relatively constant, summer to winter.
You are correct that modern homes and museums, that have heating systems that humidify in winter, and de-humidify in summer, are also relatively constant in humidity levels. The biggest problems with wood movement arise in homes that have forced-air heating in winter with no humidification, and inefficient a/c in summer, allowing for wide swings in moisture levels.
Ray
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