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Do shop heaters raise relative humidity?....
http://www.luthiersforum.com/forum/viewtopic.php?f=10101&t=19408		 Page 1 of 1
Author:  Chris aka Sniggly [ Thu Oct 30, 2008 7:11 am ]
Post subject:  Do shop heaters raise relative humidity?....
......I have to get a shop heater and was wondering about this. Any input?

Thanks,
Chris
Author:  Mitch Cain [ Thu Oct 30, 2008 7:17 am ]
Post subject:  Re: Do shop heaters raise relative humidity?....
Propane heaters increase the moisture content in a room quickly. If you go from a cold shop to a warm one very quickly with propane you will have a lot of condensation on the tools contributing to rust issues if you don't stay on top of it.
Author:  Pat Foster [ Thu Oct 30, 2008 8:25 am ]
Post subject:  Re: Do shop heaters raise relative humidity?....
I do see a rise in RH with my Heater Buddy, but haven't seen condensation. I'm in a pretty dry climate, so it might depend on where you are.

Pat
Author:  Mike Mahar [ Thu Oct 30, 2008 9:05 am ]
Post subject:  Re: Do shop heaters raise relative humidity?....
Normally raising the air temperature will cause the relative humidity to drop. I'll sometime turn on my shop heater if the dehumidifier isn't keeping up. My shop heater is part of my home hot water heating system so it doesn't add anything to the air except heat.

Do propane heaters emit water vapor? An electric heater won't emit anything and running one should cause the relative humidity to drop.
Author:  Hesh [ Thu Oct 30, 2008 9:27 am ]
Post subject:  Re: Do shop heaters raise relative humidity?....
My new shop, in it's initial trials, is maintaining 72F and 43 - 48% RH 24/7...... :D Me - I'm not proud...... :D

Right - warmer air will hold more humidity and unfortunately what we have to do, at least with current technology, to dehumidify produces additional heat.

So my take on your question is that shop heaters dry the place out and I am not speaking of transitioning from a cold shop to a warm one by turning the heat on. Mitch is right that excess moisture present in the materials of your shop (walls, floors, benches, etc.) will start coming out into the room - hence the condensation.

Depending on the heat source and there is another thread right now discussing this, moving air is drying too. In my shop the sequence seems to be that the furnace comes on, the temp rises, air is moved around, and the RH drops slightly triggering the humidifier. The furnace turns off, air stops moving, the humidifier reaches it's upper setting (digital humidistat) and turns off.

Also dehumidifiers don't work well below 70F and the coils can freeze up and when that happens they won't remove a drop of RH. So there seems to be a sweet spot and balance of sorts with temperature and RH where everything works in concert but outside the required range, what ever it is for where you are, the devices that we use to heat, humidify, and dehumidify may be working against each other.
Author:  jfmckenna [ Thu Oct 30, 2008 12:29 pm ]
Post subject:  Re: Do shop heaters raise relative humidity?....
Mike Mahar wrote:
Normally raising the air temperature will cause the relative humidity to drop. I'll sometime turn on my shop heater if the dehumidifier isn't keeping up. My shop heater is part of my home hot water heating system so it doesn't add anything to the air except heat.

Do propane heaters emit water vapor? An electric heater won't emit anything and running one should cause the relative humidity to drop.


I must say I am very confused from these responses too. I have oil heat and when it's on I need to add water to the air. I think maybe where I live in Virginia when it gets cold out it tends to dry up. Hot air will hold more moisture indeed but when you heat up already cold dry air RH drops.
Author:  Mike Collins [ Thu Oct 30, 2008 2:12 pm ]
Post subject:  Re: Do shop heaters raise relative humidity?....
The only shop heaters i know of that raise humidity are the non-vent propane ones!
I think the moisture is a by-product of the combustion!
All forced hot air and direct vent heaters lower humidity !

I have a direct vent propane furnace & a quartz heater (electric)
i have a tough time keeping the humidity above 40% in the winter!
I run humidifiers (one for each room ) in my shop .
I like making guitars at 40 % they never have a problem with cracks !

Mike
Author:  Steve Walden [ Thu Oct 30, 2008 3:15 pm ]
Post subject:  Re: Do shop heaters raise relative humidity?....
Here are a couple of sources that talk about relative humidity and temperature:
http://www.sp.uconn.edu/~mdarre/NE-127/NewFiles/psychrometric_inset.html
http://en.wikipedia.org/wiki/Humidity

I am trying to convert a .bmp image to .jpg that shows a simplified psychrometric chart. What happens is even with a humidifier your buildings are not air/water tight. Your shop will try to equalize with the exterior temperature and humidity.

In the winter, when we run our heaters, we take air from the outside at 20* F at 50% relative humidity (RH) and heat it up to 70* F. The absolute amount of water in the air stays the same but the ability of the air to hold water rises dramatically. At 70*F the RH will now be in the teens.

Mike Collins is correct that if the products of combustion remain in the heated space you are putting moisture into the air. However, most of us heat with other than a catalytic propane or nat gas heater. Hence, no more water is introduced and we have to humidify artificially.

RH in air is not complicated and once you understand the relationship you will always get it. An analogy is a balloon blown halfway up with air, which represents your shop volume at a cold temperature. As the air is heated in the balloon it expands. The mass of air and water has not changed but the volume has and the air can 'hold' more water vapor.

When you see fog outside on a cool morning the air in that area is holding the most it can at that temerature. It is at the Dew Point or 100% RH. As the sun rises and the air heats up during the day the air's capacity to hold water vapor increases and the fog (condensed water) evaporates the RH drops even though the absolute amount of water vapor present has not changed.

A sling psycrometer works on this evaporative principal. The wet bulb temperature give the dew point of the air sampled. Working with a psychrometric chart and the ambient temperature you can calculate or read the RH from the chart.

Wood and air are always seeking equilibrium in terms of water vapor. It is based on the RH of the air in the space where the wood is kept. If, like where I live the average RH is over 65% then my zoot will be too wet to build with. Hence, I have to run the dehumidifier.

We have had this discussion before and please excuse my wordiness. I will try to change the bitmap to a jpeg and continue this if there is a need.

Thanks,
Author:  TonyKarol [ Thu Oct 30, 2008 5:57 pm ]
Post subject:  Re: Do shop heaters raise relative humidity?....
L:ike Michael Payne says in another post - you learn somehting every day - Non venting propane heater ... raises RH as the combustion products stay in the room - sounds interesting, but somewhat scary to me .... I like breathing fresh air, well oK, somewhat polluted greater Toronto fresh air ...
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