Forced-Air Shop Heat Versus Radiant Heating

      A few thoughts on the effect different heating system types have on comfort for people, and conditions for wood or machinery. April 10, 2008

Question
We are looking at a new building that is heated with overhead gas powered infra-fed radiant heaters. My current shop has a large single fan-forced gas heater. What are the differences between these two heat sources? How long, for example, does it take to warm a space with the radiant heaters? Are they comfortable to stand under? Do either of these heat sources have any impact on building humidity? I have heard that my current fan-forced gas heater adds moisture to the air. I don't know where I heard that. It might be just the movie in my own head.

Forum Responses
(Cabinetmaking Forum)
From contributor J:
Water vapor is a byproduct of natural gas combustion, but that should be vented outside; your current furnace shouldn't add moisture to the air unless it's equipped with a humidifier.

I used to work in a shop that had overhead gas-fired radiant heat. I remember it as being like standing in direct sun on a chilly morning; the parts of you that have a direct line of sight to the heater feel warm, while the parts of you that are "shaded" feel cool, until the air finally warms up. If you leave the shop cold overnight, then such a system might provide some sensation of warmth very quickly, but it would be awhile before the air temperature got to where it was supposed to be. To the extent that you keep the shop reasonably warm all the time (to facilitate finish curing, glue drying, wood stability and such), I'm not sure there'd be much benefit to this over forced air.



From the original questioner:
I thought that radiant heaters only warmed objects. Do they also ultimately warm the air? Is this like a radiant heat floor, but from the top instead of the bottom? My primary concern is what kind of effects this type of heating would have on lumber moisture content or stability.


From contributor R:
I'm of the opinion that radiant heating is more pleasant, as it doesn't blow at you. The thing about the infrared that I would be concerned about is the wood dust that might land on it. Have you asked an HVAC guy about this?

I don't know how a forced air heater could add moisture to the air unless it brings in exterior air. They usually are re-circulating air within the building, so humidity would be constant.



From contributor J:
For the most part they do heat objects, which in turn heat the air. This indirectness is good in a sense; I'd rather stand around in 50-degree air than have to have my hands on 50-degree cast iron machinery.


From the original questioner:
My current heater does re-circulate air within the building. It has an exhaust duct for the fumes associated with the heater. I'm not concerned about the impact of my current fan-forced heater on lumber stability, but I am curious whether radiant heat has any issues associated with lumber stability.


From contributor R:
Humidity within your building is a constant unless it is affected by moisture getting in from outside, or through artificially added means such as humidifiers, or if you have a dehumidifier or other ventilation that removes the air from the building. The radiant heat should not be a concern. Heated air (regardless of the type of heater) should not affect the humidity unless one of these factors comes into play. Also, an unheated building does not, by itself, increase or decrease humidity.


From contributor J:
I think contributor R is leaving out an important point. Absolute humidity may be fairly constant in the building, but relative humidity depends on temperature. Where it's warm, RH will be lower; where it's cool, RH will be higher. Notice how moisture condenses on the insides of your windows in cold weather; the cold glass creates a microclimate where the RH rises above 100%, even though the situation is much different in the rest of the room.

Two potential kinds of damage come to mind. If lumber were directly exposed to intense radiant heat, it might cup towards the side facing the heater. Hopefully you can keep things moving through the shop briskly enough that this never becomes an issue.

The other potential issue would be if you allowed the building to cool off significantly at night (assuming you don't have three shifts going). Sticks inside a big pile might never get fully warmed, so they might take on extra moisture.



From contributor O:
Our shop is heated with forced air heaters, and the humidity in the winter is a factor. It is actually the lack of humidity. We keep an eye on the humidity in the shop, and can always tell when it is getting low. It has not been a huge problem, but you notice the top sheet on every lift lifting up on its ends. The easy cure for sheet goods is to just keep a cover sheet on any units not being processed. We also do not keep a large quantity of wip sitting around. As it's finished, it goes in the truck and is delivered.

We also have a couple of shop built humidifiers that we sometimes use. With two of them we can control the humidity in 10,000 sf. If I had the choice and the ceiling height, I would go with the radiant, as you just don't get that drying effect.



From contributor A:
The radiant units do heat objects, including you. That is why you get a cold/hot side. Go to a big box store like Homie Dopie. They often have the radiant units at the checkout line doors. It's like the Sahara under them. How many radiant units does the new building have and how many square feet?


From contributor I:
A shop I used to work in had overhead infrared tubes in the finishing room. It made things comfortable but you couldn't put freshly sprayed parts under the heaters or the CV would boil off and cause bubbles. If you set them off to the side a bit it worked okay. In my shop I have an ultra high efficiency boiler and some of the old style cast iron radiators and they do an excellent job of heating the shop and my budget was cut in half from what I paid when we had a forced air unit.


From contributor S:
I currently have radiant tube heaters in my shop and I don't think I will ever go back to forced air again. If you climb up on a ladder in a shop with forced air you will find it is a lot hotter up by the ceiling than down on the floor, sometimes 30-40 degrees. With the radiant tubes you only have a heat difference of 2-3. And when you open the doors, you don't lose all your heat.


The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor B:
I've worked in non-wood shops that have had both. Radiant makes for a building that is workable fairly soon, but areas under benches, inside cabinets, and drawers take a long time to get warm. I always wanted heavy boots and thick socks in the radiant heated building. The shop with the overhead gas heaters always had dust control problems. It would create dust in one corner, and it would make it to the other corner minutes later.

The nicest shop for heat that I've been to had hot water in the floor. Dust was less of a problem, and feet were toasty so you could wear sneakers which made walking around and over all the stuff less clumsy.

For example, the paint booth was just a corner with heavy plastic to the floor, a fairly small exhaust fan, and one of those hang-from-the-ceiling dust collector boxes to provide make up air from the room to the cabinet. The exhaust was at the bottom of the room. The net effect was that filtered, somewhat warmer, air from the room filled the paint booth and was pushed to the floor and exited outside when the booth was in use, and crept out under the plastic when the exhaust fan was off. The shop had a couple of big slow ceiling fans to reduce air layering, but with the floor heat it didn't matter much.

The downside was that you didn't change the temperature of the shop in a hurry. It took about six hours for the slab to heat up. The original slab was not insulated, so the shot was heating eight inches of concrete and a couple feet of earth.

This became a problem in spring and fall when the temperature swings outside were substantial. The slab basically keeps the inside warmer than the outside. A sudden spring warm spell has you working with the main door open. Similarly a cold snap meant the shop was chilly for a day while the heat brought the slab to a higher temperature.



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