Thanks for the thoughts guys, and thanks for the link Graham.
It seems one of these is spot on anyway - condensing dehumidifiers don't take out much water at low temperatures.
Checking again today my unit is still only shifting about 1L in 24hours, and with air changes in what's a fairly tightly sealed shop it seems to have stabilised at just over 70%RH. Which is down maybe 5%, but certainly not the 45% RH needed to dry wood in the shop to what's needed.
In retrospect that's 100% in keeping with the psychrometrics. The condensing temperature is determined by the refrigerant used, but either way it can't drop below freezing point or water removed from the air will simply accumulate as ice on the coil. But if you start with already cool air (a) you can't drop the temperature all that much more (and the more you drop the temperature the more moisture drops out), and not only that but (b) down at the low temperature end of the psychrometric chart the %RH lines are approaching horizontal - so that the rate at which a given temp drop moves your air towards saturation/condensing is much reduced vs. as at higher temperatures.
Put another way - domestic dehumidifier units are no doubt optimised towards performing best at 70deg F or so. i.e. in indoor conditions. Which no doubt explains why industrial/construction units are in comparison so large.
So some tentative conclusions:
1. If you want to maintain dry conditions in your shop in Winter that will help dry or maintain timber down around 7% (and your winter humidity levels are 70% or thereabouts as is the case for anybody in the British Isles experiencing Atlantic weather) it's probably best to think in terms of maintaining indoors type temperatures.
2. More to the point - if you don't you can expect your timber to equilibriate at around 16 - 17% EMC. (that's what the psychrometrics and EMC data predict, and what both Graham and myself are seeing)
3. The exception is the very dry frosty Winter conditions experienced in some parts of the US, and at times in continental Europe. In that case the air is moving in from much colder areas, and having already been dropped to very low temperatures and reheated a bit as it moves South (and in the shop) will deliver very low %RH in the shop.
4. Worse still - in Summer if you experience relatively high humidities in the 60 - 70% range or higher this will no matter what the temperature is likewise leave you with 16 - 17% EMC or higher.
5. Living in a desert/hot and dry area is an exception to this, again as in parts of the US.
6. If you can't heat the shop in Winter, or are using a domestic dehumidifier that does not have the capacity to dry the air in the entire volume then plan B is perhaps to build the previously mentioned small heated kiln.
It's possible though that with higher temperature conditions in Summer that with a domestic dehumidifier running at its rated full (e.g. 18L in my case) capacity (the usual dodgy capacity claims apply here too it seems) that it might be rather more effective in dehumidifying the entire shop.
Failing that a larger industrial/construction type unit is required, or maybe more to the point a properly designed air conditioning system. Either is likely to be very expensive to install and run - and may well dtruggle to work reliably in the dusty atmosphere.
This still begs lots of questions though.
1. It seems that when we buy timber from a merchant that's been stored in ambient conditions for any decent period of time that no matter what's said about kiln drying it's probably at 16 or 17% moisture content (EMC).
2. If it's lower (e.g. at the 8-10%) claimed for kiln dried wood, that's because it's just out of the kiln, and in that you can be sure that (a) it's going to take up quite a lot more water, and (b) may well warp while ding so.
3. If you don't happen to live somewhere which by some quirk of fate has cold dry Winters and very dry Summers (inland Canada, the desert or somewhere) and your piece is destined for a centrally heated or air conditioned house then you'd better work like mad in the hope of getting finished while it's still dry enough (a) not to mess up your work, or (b) to be OK indoors.
4. Allowing kiln dried timber to rest in the shop to equilibriate is often advised. It no doubt has the effect of making the moisture content more uniform (presuming it wasn't in the first place), and will at least prevent in changing while you work - but (exception as above) don't expect it get any drier, or less likely to create problems in a centrally heated house.
The inescapable conclusion in all of this has to be that most of us are playing Russian roulette with timber condition. The one basic is that wood takes up and loses moisture according to the %RH of the air its immersed in.
This means as Graham says that unless you have some alternative strategy that your shop conditions should be such as to deliver an equilibrium moisture content the same as that in the place for which the piece is intended. Probably best actually in fact if both the temperature and %RH are the same.
This still begs the original question. Few of us are set up to control conditions this well, and anyway given the fact that in use they may be exposed to variable conditions we have to design to accommodate movement. It's pretty clear though that we can't hope to accommodate e.g. 5% movement in many pieces, so some control of conditions is required.
I wonder what the high end guys are doing? Do they run air conditioning?
ian
It seems one of these is spot on anyway - condensing dehumidifiers don't take out much water at low temperatures.
Checking again today my unit is still only shifting about 1L in 24hours, and with air changes in what's a fairly tightly sealed shop it seems to have stabilised at just over 70%RH. Which is down maybe 5%, but certainly not the 45% RH needed to dry wood in the shop to what's needed.
In retrospect that's 100% in keeping with the psychrometrics. The condensing temperature is determined by the refrigerant used, but either way it can't drop below freezing point or water removed from the air will simply accumulate as ice on the coil. But if you start with already cool air (a) you can't drop the temperature all that much more (and the more you drop the temperature the more moisture drops out), and not only that but (b) down at the low temperature end of the psychrometric chart the %RH lines are approaching horizontal - so that the rate at which a given temp drop moves your air towards saturation/condensing is much reduced vs. as at higher temperatures.
Put another way - domestic dehumidifier units are no doubt optimised towards performing best at 70deg F or so. i.e. in indoor conditions. Which no doubt explains why industrial/construction units are in comparison so large.
So some tentative conclusions:
1. If you want to maintain dry conditions in your shop in Winter that will help dry or maintain timber down around 7% (and your winter humidity levels are 70% or thereabouts as is the case for anybody in the British Isles experiencing Atlantic weather) it's probably best to think in terms of maintaining indoors type temperatures.
2. More to the point - if you don't you can expect your timber to equilibriate at around 16 - 17% EMC. (that's what the psychrometrics and EMC data predict, and what both Graham and myself are seeing)
3. The exception is the very dry frosty Winter conditions experienced in some parts of the US, and at times in continental Europe. In that case the air is moving in from much colder areas, and having already been dropped to very low temperatures and reheated a bit as it moves South (and in the shop) will deliver very low %RH in the shop.
4. Worse still - in Summer if you experience relatively high humidities in the 60 - 70% range or higher this will no matter what the temperature is likewise leave you with 16 - 17% EMC or higher.
5. Living in a desert/hot and dry area is an exception to this, again as in parts of the US.
6. If you can't heat the shop in Winter, or are using a domestic dehumidifier that does not have the capacity to dry the air in the entire volume then plan B is perhaps to build the previously mentioned small heated kiln.
It's possible though that with higher temperature conditions in Summer that with a domestic dehumidifier running at its rated full (e.g. 18L in my case) capacity (the usual dodgy capacity claims apply here too it seems) that it might be rather more effective in dehumidifying the entire shop.
Failing that a larger industrial/construction type unit is required, or maybe more to the point a properly designed air conditioning system. Either is likely to be very expensive to install and run - and may well dtruggle to work reliably in the dusty atmosphere.
This still begs lots of questions though.
1. It seems that when we buy timber from a merchant that's been stored in ambient conditions for any decent period of time that no matter what's said about kiln drying it's probably at 16 or 17% moisture content (EMC).
2. If it's lower (e.g. at the 8-10%) claimed for kiln dried wood, that's because it's just out of the kiln, and in that you can be sure that (a) it's going to take up quite a lot more water, and (b) may well warp while ding so.
3. If you don't happen to live somewhere which by some quirk of fate has cold dry Winters and very dry Summers (inland Canada, the desert or somewhere) and your piece is destined for a centrally heated or air conditioned house then you'd better work like mad in the hope of getting finished while it's still dry enough (a) not to mess up your work, or (b) to be OK indoors.
4. Allowing kiln dried timber to rest in the shop to equilibriate is often advised. It no doubt has the effect of making the moisture content more uniform (presuming it wasn't in the first place), and will at least prevent in changing while you work - but (exception as above) don't expect it get any drier, or less likely to create problems in a centrally heated house.
The inescapable conclusion in all of this has to be that most of us are playing Russian roulette with timber condition. The one basic is that wood takes up and loses moisture according to the %RH of the air its immersed in.
This means as Graham says that unless you have some alternative strategy that your shop conditions should be such as to deliver an equilibrium moisture content the same as that in the place for which the piece is intended. Probably best actually in fact if both the temperature and %RH are the same.
This still begs the original question. Few of us are set up to control conditions this well, and anyway given the fact that in use they may be exposed to variable conditions we have to design to accommodate movement. It's pretty clear though that we can't hope to accommodate e.g. 5% movement in many pieces, so some control of conditions is required.
I wonder what the high end guys are doing? Do they run air conditioning?
ian
