This was mentioned in another thread but I think it deserves its own. There is a long standing tradition that forced air furnaces “dry out” the house in winter. But I would think any form of heat would lower the humidity. As air warms up the relative humidity drops.
A quick check online didn’t offer much data. One site claimed that since a furnace uses flame that it “burns out” the water in the air. Har! To begin with, the combustion air is not circulated into the house - it goes up the chimney. Now older furnaces used a lot of combustion air and it has to come from somewhere. That air does come from outside the house so there is a influx of air that gets raised 30, 40 or more degrees F. This does lead to lower humidity both from the raising of the temperature plus the removal of any moist air from cooking, showers, etc.
But modern high efficiency furnaces use an air intake as well as an exhaust. How does a house with a high efficiency, forced air furnace compare with a house using baseboard heat?
My wag is that forced hit air causes pressure differences inside the house, areas of high and low pressure. This would seem to cause more outside air to leak in and inside air to leak out. Add any leaky venting anywhere except inside living spaces and the system makes the entire house negative pressure. Many newer ones will also have a fresh air intake to the furnace to make up for any pressure differences at the furnace itself (this is different then the air intake for combustion). In general they may tend to suck in dry outside air and blowing out moister inside air.
As you point out there should be nothing inherent in the heating the air that would do this in a ‘dryer’ way then any other method.
Old-fashioned steam radiators often have leak points somewhere, which increase humidity. This may be why the conventional wisdom has been that radiators don’t reduce humidity the way forced air does, and perhaps people think this was true of all types of radiant or convection heat, like hot-water baseboards.
My WAG is that it’s an old wives’ tale that may have had some element of truth in some special cases in the past – or more likely not – but is certainly not true today, where houses are well insulated and “leakage” is not an issue.
As for flame “burning out” the water, combustion in fact does the opposite – it adds water to the combustion gases (burning hydrocarbons primarily creates CO2, some amount of CO, and H2O) but this is irrelevant because, as noted, the gases always get exhausted up the chimney or out a vent in newer furnaces. In cold weather you can see vast clouds of vapor coming out of the vent in the side of my house. I don’t buy the “steam leakage” explanation either; such leakage would be hugely problematic (it would cause condensation and mold), and individual single-family houses that don’t use forced air typically use hot water rather than steam heat anyway.
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.
It seems to me that it gets dry in the house because it’s (usually) dry outside when it’s cold. Especially when it’s very cold.
In my house during a wet spell in winter (cold temp/high humidity), my indoor humidity is high enough that the HVAC system will not call for added humidity at all. Only when it gets very dry outside does the HVAC humidifier turn on.
I’m sure my house is not as “tight” as some, but it’s by no means leaky. It was built in 2005 and it was constructed well.
Heating a space will lower the relative humidity. If the furnace is in the house and does not have a source of fresh air from out side (not normal set up) then it would be pulling in outside air to the living space. But almost all have a source of outside air. The air coming from the vents will be higher than space air and therefore “dryer”, until it mixes with space air. But if you are sitting the draft of the vent you may notice this “dry air”. If a forced air system is causing negative air pressure in spaces then there is a problem with the system. If your forced air system ducts are getting dusty that is the results of either using the wrong filter or not changing the filter when it gets dirty.
An old in the wall space heater did use space air and exhausted to the out side. Bring in moisture with outside air. Also the mixing of the air in warm air in the living space was not very efficient.
Steam radiators are different. If a steam leak is not attended there can be a real danger to pipes bursting. Leaks should not be a source of water in the air from a steam system. But single pipe steam systems have to have a vent on the radiator to vent out any trapped air. And two pipe system should also have a vent on the radiator. This can be a source of moisture as the vent vents off moist air from the radiator.
I have lived with space heaters, oil stoves, wood stoves, and forced air systems. I will take forced air systems any day. After leaving the stat down or off over night or when away it is no problem to reheat. I can turn my stat up from 60 degrees to 68 degrees in the morning and in a very short period of time the house is up to temp in all rooms. The other methods takes quite a period of time. The exception is a good wood stove that is banked.
As far as hydronic heat systems. When they are new they are great. But it all depends on the water in the system and does the home owner or maintenance person know proper water chemistry? There were a lot of Ikler houses built in San Jose. Most of the hydronic heating systems have failed because of no water treatment.
While I am sure those types did exist a modern wall furnace uses a coaxial vent duct to bring outside air into the furnace for combustion and exhausts the hot air. No inside air is used. I currently have two such units in my garages. I used another one in a previous garage and that goes back to the 1970s.
When I lived in a house with radiators, you would put pans of water out on top of the radiators - that would add humidity to the rooms. A forced air furnace doesn’t have that “feature” I really haven’t noticed a difference in the various houses I’ve lived in - when moisture leave the air outside, it leaves the air in the house too - and everything is dry - regardless of the way you heat. Having water around (humidifiers, a fish tank, leaving an inch in the bathtub, running a kettle on the stove, or parking pans of water on top of radiators - even a lot of plants) is the only way to put water back in the air and make it so your face doesn’t fall off.
Our furnace has a built in humidifier with a connection to a water line. I would expect that to be standard now, especially in a climate like Edmonton’s where it can be -30 C. At a temperature like that there’s hardly any moisture in the outside air.
The dustiness of the ducts is immaterial in the long run. After a bit of time the amount of dust deposited in the ducts equals the amount of dust the ducts spew out.
And most important: prior to that the ducts are acting as dust trap. Taking dust out of the air!
While theoretically having your ducts cleaned periodically helps reset the clock on these dust traps, the problem is that this area of business is loaded with people who overcharge and do a crappy job.
Other than that forced air systems are just moving dust from point A to point B. They don’t cause dust.
My Mom just puts a 9x12 baking dish full of distilled water in front of her hot air register in her bedroom (it comes out of the wall, most registers are in the floor). I guess it works as I buy her a lot of distilled water.
I live in at high altitude in the Rockies. VERY dry air. We used to cast iron water containers on the wood stove. Don’t mess with it any more. We do have a new heated concrete floor for our main entry way. All the snow from our boots gets knocked off on that heated floor and I think helps with humidity.
Ours doesn’t (Minneapolis) - but its twenty years old now. We do put humidifiers throughout the house. But the idea that you bring moisture in from outside at this time of year is laughable to those of us who live where moisture in the air freezes in Winter.
Mine which is high efficiency 3 yrs old propane furnace does have a outside air inlet that blends with the recirculated air. This is separate from the air intake for combustion.
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. Not as bad as using a wood burning stove. Even burning “clean” wood with a good stove, enough particulate matter would get in the house to cause me respiratory problems. Radiators, both the old big clunkers and more modern baseboard heaters, can dry the air, but they don’t generate or spread dust. As someone pointed out, it’s easy to put a pan of water on the radiator, or in front of a heater. Or just use humidifiers in the room with with piano.
Oh, and you use distilled water to keep the minerals from building up and destroying the pan. That can happen really fast in places where there is a lot of lime/calcium in the water supply.
My Vermont Castings stove that is my Wife’s and my primary heat source uses a coaxial stove pipe. One pipe in another. The inner pipe brings in air for combustion, the outer vents exhaust (and keeps the outer pipe hot leaving more heat in the house).
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.