Basic airflow/heat question

[DavidP]

The air conditioners are net removing heat out of the interior air mass, making it colder and less dense relative to the outside so it wants to fall into the basement and force its way out down there... in turn sucking warm air in at the upper floors.

Making it even more complicated - the attic itself might be warmer than outside so there would be a positive stack effect there, fighting against the negative stack effect in the conditioned space. That's where good soffit venting comes in.

I have noticed that, when the window AC units upstairs are running full blast in hot weather, I can feel cold air coming down the front hall stairwell. So I get that much. But it doesn't make it to the basement, I don't think.

There is no question that the attic is warmer than the outside much of the time during the summer. I don't have soffit vents and I don't think I can, since my gutters are built into the edges of the roof at about the same spot where soffit vents are usually located. Also a slate roof with no ridge venting. Did they just not know about soffit vents back then or did they just accept a hot attic? I do have a window in each gable (one on each side of the house, total of four), which I always leave open from May through September.

[DavidP]

If you get the right combination of vents, you won't need a fan system.
Winter being still too hot upstairs... that baffles me, and also makes me think you are spending a ton on fuel oil. <snip>
All of the old houses I have lived in have had gas burning furnaces of some sort in them, and all have had the option of turning the fan on during the summer. If you have return ducts to the furnace, this will circulate the air through the house, hopefully equalizing the temperature throughout.

Part of the problem is that the center of the attic is very high while the windows are scarcely above floor level. (See photo below.) Hence the fan because there is no way for the hot air to get out naturally by rising. The fan does not remove the hot air from the peak, but at least keeps things moving somewhat closer to the attic floor. See my comment to previous post about slate roof etc. I don't think I spend an unusually high amount on fuel, as old houses around here go. That isn't saying a great deal, of course; I would like to spend less.

Things must be different in Canada. I have lived in many old houses and none of them had forced air heating (and cooling). We're talking big old fashioned cast iron radiators here, nary a duct or blower in sight. One of the amusing idiosyncracies about my house is that they put a huge radiator into the smallest bedroom upstairs. OK, it's a north room, but still. I hardly ever turn that one on.

[phil]

a lot of old houses ( like mine) originally had a heater in the basement near the middle, then they had vents to the main floor hall in the center of the house. all the rooms off that hall had doors except the living room, so you would control the heat by opening doors to the hall. I don't' think there ever was an air return it depended upon the house not being well sealed.
fast forward to the invent of central heat, they then installed ducts in the basement to distribute the heat and air returns to bring the cooler air back , the heat vents were now a the circumference near windows and such and the air return near the center of the house.

so the old system brought heat up the middle of the house and by windows it was drafty, now they make it more comfortable by blowing heat out near the windows and returning it down the middle, they kind of reversed the air distribution.

My house had the attic opened up early on but I don't think it was living space when the house was built.

I suspect the way the forced air system was designed was an afterthought and the ducting may have been better laid out in some houses than others.

what some do is install heaters in the attic that are designed to blow the heat down, and that gains real estate in the basement which they are using more often for suites and living space.. I think most have the heaters in the basement as that is just what was common and it is easier to blow heat upward than downward..

[Don M]

Our old farm house had a coal fired furness in the cellar in the center of the house as Phil described. It was a gravity heater with out a blower; the heated air rose up thru the big grate in the hallway floor, spread out on the first floor & some heat rose up the open stairway to the second floor & spread out there too. There were some openings in the ceiling in rooms on the first floor that also allowed some heat to rise up into rooms on the second floor. It was a very inefficient system but better than just burning wood in the fireplaces.

[DavidP]

so the old system brought heat up the middle of the house and by windows it was drafty, now they make it more comfortable by blowing heat out near the windows and returning it down the middle, they kind of reversed the air distribution.

Phil, that's a great summary of how things have developed! My current house has always had hot-water radiators, but my aunt and uncle's farmhouse in Vt (1850) had exactly what you and Don describe--a wood-burning furnace in the basement topped by a big grate in the living room, and a few small holes plus the stairwell to get heat into upstairs rooms. Our ancestors were tough. I remember when my uncle had a forced air system installed in the 1960s (registers under the windows, as you said) to supplement the wood furnace, which they continued to use.

[JRC]

My house, built in 1902, originally had a coal-fired gravity furnace. But, it was ducted to each room. I know this, because it still has the heavy cast-iron registers, and even the woodwork frames the openings.

I'm not sure how the return air worked, though, because the return grilles seem to have been added later. (I'm guessing when the forced air furnace was installed, in the early 50s) I've seen houses with old/original return grilles, and they were usually wood, and set into the floor, near an outside wall. My return grilles are metal, and the openings appear to have been cut in later.

[Don M]

In a gravity system return air wasn't needed; I think that only became useful once a blower was added in a forced hot air system.