# Forced air furnaces drying out the air **URL:** \ **Category:** In My Humble Opinion\ **Created:** [January 23, 2019, 4:39pm UTC](https://boards.straightdope.com/t/forced-air-furnaces-drying-out-the-air/828279 "2019-01-23T16:39:05Z")\ **Posts on this page:** 14\ **Page:** 2
**Author:** ![Hermitian](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hermitian/32/470_2.png) [@Hermitian](https://boards.straightdope.com/u/Hermitian)\ **Post date:** [January 28, 2019, 2:26pm UTC](https://boards.straightdope.com/t/forced-air-furnaces-drying-out-the-air/828279/21 "2019-01-28T14:26:24Z")
> [@Tapiotar](#): > > This. Every time I’ve lived in a place with forced air heat, the deposition of dust throughout the place was rapid and astonishing. I have had dry eye and sinus problems galore. I loathe forced air heat. The forced air heat system is the same system used for air conditioning. I assume one is not worse at spreading dust than the other. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [January 28, 2019, 4:00pm UTC](https://boards.straightdope.com/t/forced-air-furnaces-drying-out-the-air/828279/22 "2019-01-28T16:00:22Z")
> [@guestchaz](#): > > I don’t have this problem these days, but as a kid, our house had a coal furnace. In the winter, yep the air dries and inside the house (which being built in the 1930s was far from air tight) the warmed air was even more relatively dry. Enough to cause nose bleeds. The furnace had a water reservoir expressly for the purpose of humidifying the air. This confirms my suspicions is it’s not the method of heat that causes the dryness, it’s the leakiness of the house, so greater exchange of outside air, forced hot air causes pressure differences, some modern ones bring in some outside air, leaky ducts in unheated spaces cause outside air to enter somewhere. Same for leaky older homes. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [January 28, 2019, 4:02pm UTC](https://boards.straightdope.com/t/forced-air-furnaces-drying-out-the-air/828279/23 "2019-01-28T16:02:35Z")
> [@Hermitian](#): > > The forced air heat system is the same system used for air conditioning. I assume one is not worse at spreading dust than the other. One difference that I can see is that the evaporator which the air is blown through to cool it is moist from condensation and should help trap dust and drain it with the condensate. To be to some degree self cleaning. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [January 28, 2019, 4:30pm UTC](https://boards.straightdope.com/t/forced-air-furnaces-drying-out-the-air/828279/24 "2019-01-28T16:30:47Z")
> [@wolfpup](#): > > One thing about forced air that I’ll agree with is that if the ducts get dusty, they’re terrific at spreading dust over the house. Hot-water systems don’t do that. Conversely I suppose a forced air system that moves a lot of air and has a really good specialized electronic filter could actually be used to clean up the air, but in my experience, dust-wise, forced air typically does more harm than good. The cleanest part of my house with respect to dust accumulation is the finished part of the basement, where the furnace vents are shut off. Still, I appreciate forced air because it’s effective at both heating and cooling. Most people who own homes with forced-air heating get the cheapest filters they can buy, which are basically good enough to trap lint. If you care about indoor air quality, look at the [MERV (Minimum Efficiency Reporting Value) rating](https://en.wikipedia.org/wiki/Minimum_efficiency_reporting_value) for the filter you’re buying, and try to get something that’s an 11 or 12. ---
**Author:** ![mixdenny](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mixdenny/32/2962_2.png) [@mixdenny](https://boards.straightdope.com/u/mixdenny)\ **Post date:** [January 28, 2019, 8:49pm UTC](https://boards.straightdope.com/t/forced-air-furnaces-drying-out-the-air/828279/25 "2019-01-28T20:49:50Z")
I always buy the second highest rated 3M filters as the top ones add odor control which is not needed and is short lived. We certainly don’t get dust blowing all over the place. But then I don’t recall being dusty back when the filters were more basic. Where do people see all this dust? Dust is all around. The ceiling fans on my front porch accumulate dust on the blades probably 4 times faster then the ones in the house. As for pressure differentials - well, that is needed to make it work. There is a significantly higher pressure in the supply ducts. The return ducts have a negative pressure but it should be much less of a reduced pressure if the ducts are designed properly. As rule of thumb you want at least 1 1/2 times the area of the supply duct for one area for the returns. This minimizes losses due to friction. Those two pressures are far less in the rooms due to their large area. I don’t think any room is at any particularly high or low pressure\*\*\* if the design is good.\*\*\* Many poorer designs depend on a door being open. I installed my own furnace and ducts although I was building on what was already there so there are compromises. The upstairs bedroom needs the door cracked for full flow but I have the supply adjusted to overcome that as we keep it closed. There was just no easy way to get a good return up there. The 3/4" crack under the door works OK but the room is slightly pressurized. You can feel it blow on you if you crack the door open and put your face there. Dennis ---
**Author:** ![Doctor\_Jackson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/doctor_jackson/32/32_2.png) [@Doctor\_Jackson](https://boards.straightdope.com/u/Doctor_Jackson)\ **Post date:** [January 28, 2019, 10:06pm UTC](https://boards.straightdope.com/t/forced-air-furnaces-drying-out-the-air/828279/26 "2019-01-28T22:06:50Z")
The outside air where I live is substantially drier in the winter, so that is one factor. Currently the outside humidity is 40% (less than half the summer average) and the indoor humidity, with forced air heat running, is 34%. That’s about a 15% differential, which is pretty substantial. How much of that is due to the type of heating system, I have no idea. I have never in my 55 years lived in a place that did not have forced air heat. Radiators and coal are/were uncommon in homes. ---
**Author:** ![Chimera](https://avatars.discourse-cdn.com/v4/letter/c/8e8cbc/32.png) [@Chimera](https://boards.straightdope.com/u/Chimera)\ **Post date:** [January 28, 2019, 11:11pm UTC](https://boards.straightdope.com/t/forced-air-furnaces-drying-out-the-air/828279/27 "2019-01-28T23:11:07Z")
Cold air can hold much less water than warm air. If you take the same volume of air and increase its temperature by 50 degrees, it can hold a lot more water, but unless you’re adding water (the water reserve), it now holds a much smaller percentage of its maximum. Translation: The drop in humidity is directly caused by the increase in temperature. You had a cup of water in a barrel. Now you have a cup of water in a vat. ---
**Author:** ![california\_jobcase](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@california\_jobcase](https://boards.straightdope.com/u/california_jobcase)\ **Post date:** [March 5, 2019, 3:41pm UTC](https://boards.straightdope.com/t/forced-air-furnaces-drying-out-the-air/828279/29 "2019-03-05T15:41:06Z")
I think we may be missing the real reason here. As far as humidity goes, there’s absolute humidity, which is the actual amount of water in a certain volume of air, and relative humidity, which is the amount of water in the air compared to how much it can hold at that temperature. When do people run furnaces? When it’s cold out. When it’s cold out, the absolute humidity of the air drops a great deal. Where does the air in a house come from? Outside, where it’s cold. If it’s cold, the amount of water in the air is quite low. Heat that air up, and you have warm, dry air, with a relative humidity that is lower than when it was outside. I know houses these days are tighter than they used to be, but there is air exchange every time an exterior door is used. Keep in mind, the idea that the heat is drying the air is an old one, from when buildings were more drafty. Overall, it’s the weather that causes dry air inside in the winter. ---
**Author:** ![CookingWithGas](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cookingwithgas/32/485_2.png) [@CookingWithGas](https://boards.straightdope.com/u/CookingWithGas)\ **Post date:** [March 5, 2019, 5:47pm UTC](https://boards.straightdope.com/t/forced-air-furnaces-drying-out-the-air/828279/30 "2019-03-05T17:47:07Z")
> [@Chimera](#): > > Cold air can hold much less water than warm air. > [@california\_jobcase](#): > > Heat that air up, and you have warm, dry air, with a relative humidity that is lower than when it was outside. I think we all know this. The question is: Does heating air using a furnace with forced air cause the inside air to be any drier than air heating to the same temperature using a radiator system/baseboard heat, and if so, why? ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [March 5, 2019, 6:31pm UTC](https://boards.straightdope.com/t/forced-air-furnaces-drying-out-the-air/828279/31 "2019-03-05T18:31:41Z")
> [@CookingWithGas](#): > > The question is: Does heating air using a furnace with forced air cause the inside air to be any drier than air heating to the same temperature using a radiator system/baseboard heat, and if so, why? The summary: An older forced-air furnace that takes its combustion air from your living space will require that makeup air be admitted to your living space from the great outdoors through myriad cracks and crevices all over you home. If the absolute humidity of the great outdoors is lower than the absolute humidity in your living space (and that’s probably the case since you’ve been humidifying your living space, even if only by the moisture of your breath and sweat), then operating such a furnace will reduce your indoor humidity. A newer forced-air furnace that takes its combustion air from the great outdoors should not cause any change to the humidity in your living space. Baseboard heat and radiator systems should not cause any changes to the humidity in your living space either, unless their working fluid is leaking - in which case you have bigger problems to worry about than dry air. ---
**Author:** ![california\_jobcase](https://avatars.discourse-cdn.com/v4/letter/c/9d8465/32.png) [@california\_jobcase](https://boards.straightdope.com/u/california_jobcase)\ **Post date:** [March 5, 2019, 8:50pm UTC](https://boards.straightdope.com/t/forced-air-furnaces-drying-out-the-air/828279/32 "2019-03-05T20:50:49Z")
> [@CookingWithGas](#): > > I think we all know this. The question is: Does heating air using a furnace with forced air cause the inside air to be any drier than air heating to the same temperature using a radiator system/baseboard heat, and if so, why? If it’s a modern, properly working system, then no. I grew up in a house with electric radiant heat in the ceiling, and the air was just as dry in the winter as it was in my grandparents’ forced air houses. ---
**Author:** ![SingleMalt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/singlemalt/32/3961_2.png) [@SingleMalt](https://boards.straightdope.com/u/SingleMalt)\ **Post date:** [March 5, 2019, 9:04pm UTC](https://boards.straightdope.com/t/forced-air-furnaces-drying-out-the-air/828279/33 "2019-03-05T21:04:03Z")
Generally speaking, all that’s required for the drop in humidity is heating up the air. At 0°C (32°F), air with a 70% Relative Humidity will contain 3.41g H2O for every cubic meter. If you take that same amount of water, 3.41g, in one cubic meter of air at 25°C, (77°F), the RH drops to 14.8%. So, from comfortable moisture content to very dry just by heating the air. No other change necessary, and it doesn’t matter what mechanism is heating the air. ---
**Author:** ![kanicbird](https://avatars.discourse-cdn.com/v4/letter/k/5f8ce5/32.png) [@kanicbird](https://boards.straightdope.com/u/kanicbird)\ **Post date:** [March 5, 2019, 10:22pm UTC](https://boards.straightdope.com/t/forced-air-furnaces-drying-out-the-air/828279/34 "2019-03-05T22:22:37Z")
> [@Machine\_Elf](#): > > The summary: > > An older forced-air furnace that takes its combustion air from your living space will require that makeup air be admitted to your living space from the great outdoors through myriad cracks and crevices all over you home… Older boilers also takes combustion air from inside the living space, so again no difference that would blame forced hot air. ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [March 6, 2019, 12:24am UTC](https://boards.straightdope.com/t/forced-air-furnaces-drying-out-the-air/828279/35 "2019-03-06T00:24:20Z")
Centrally-heated American houses were _always_ known to be dry, compared to European buildings with no heating or exposed to fire-exhaust products. It’s the reason 19th century American furniture and pianos had to be constructed differently than European pianos and furniture. [Previous page](https://boards.straightdope.com/t/forced-air-furnaces-drying-out-the-air/828279.md?page=1)