Years ago I was inside an older home that was originally built with smallish holes in the masonry near a fireplace, and then other openings in a nearby wall that were I think larger … or possibly smaller. (Sorry, my memory of the details is hazy, hence my q here.) It was explained to me that the size difference creates a slight vacuum or draft, and this helps circulate warmth from the fireplace to other areas by pulling the warm air away.
Does this ring any bells? I want to say the air movement started at smaller holes & was pulled towards the larger ones, and I have the vaguest sense that the principle was somehow related to the way an airplane wing is shaped to create an uplift in air pressure.
I’m not sure I’m explaining this properly, or even coherently, but I hope someone can make sense of my post enough to identify the name of that particular architectural detail - assuming it does exist. They added a decorative embellishment, but clearly had a practical function as their primary reason for existence and weren’t just there to look pretty.
On a related note, are there diagrams or basic design principles I could study, for promoting passive heating (or cooling) in general, using something along the same lines? (E.g. smaller holes * here * and larger holes * there * to help move air with different temperatures around a bit.)
Sounds similar to the Roman hypocaust, but I don’t think they used changes in the size of the heat pipes to promote the flow of heat. I think they just assumed hot air would generally flow upwards.
Not to be to cynical, but it probably didn’t work, at least not very well.
Warm air rises. Making different size/shaped holes is unlikely to enhance that principle very much. Also, if the original designers were on the ball, they wouldn’t have used a traditional fireplace at all, since they are horrendously inefficient, and would have used a Franklin stove, modern versions of which are very efficient.
My parents had a home with a lower level wood stove in the family room. Their bedroom was directly above that, and there were two vents that passed through the ceiling to the room above. It worked really well; they would use the stove in the winter and the heat rising from it kept them warm all night.
I’m pretty sure there’s nothing about having different hole sizes that draws air in one direction or the other - and I’m a scientist who has been concentrating on buoyant advection and heat transfer this last couple of months. What helps is having lots of space for air heated by the fire to rise to someplace you want to heat. In the special case of a fireplace, there is a large air flow from the room into the fireplace and up the chimney, which the fireplace heats more effectively than it heats anything else. So, it’s very important how the replacement air gets to the front of the fireplace. Many fireplaces likely radiate significant heat to people near them, but keep rinsing the entire room with cold outdoor air drawn from leaks all over the exterior and sweeping toward the fireplace.
If you want a principle to employ, see if you can deliberately bring in outdoor air, heat it using the fire, deposit it in the periphery of the house, and let that heated air be what is swept through the house by the draft of the fire. I’m picturing a heat exchanger above the fire that is fed with cold outdoor air, which is heated and then piped to far corners of the indoor space. All this would be driven by the draft of the fire, aided by the fact that the house is otherwise quite well sealed.
The house I grew up in, built before Franklin stoves were common (though not before they were invented), had fireplaces designed so that they actually got significant heat into the house, which most modern fireplaces don’t (though the house had additional heating systems by the time I grew up there.) That’s partly because of the bulk of surrounding brickwork and partly because of the angle of the back of the fireplace; but one of those fireplaces also had openings near the bottom of the masonry surrounding the fireplace and another pair of openings in that masonry higher up in the room, which I was told were joined by an air channel within the masonry so as to get more warm air out into the room. This didn’t have to do with size of holes, but with the general principle that warm air rises compared to cold air. I don’t know what this type of design is called.
The house I now live in has a grate set into the floor above the wood stove, which I installed when the stove was put in, about thirty years ago. (The house originally had wood stoves, but in different locations, and the original chimneys were gone, though one of them still had a remnant sticking out of the roof when I got here.) Hot air comes up out of the grate into my bedroom above; this works rather too well, since the chimney also comes through my bedroom and heats it, and I often have the grate shut off except in the coldest weather. The layout of the house is also such that, presuming doors are left open, heat tends to circulate up the staircase and back down a grate in another bedroom (which was here when I got here), keeping most of the house warmer than it would be otherwise.
Some modern stove installations have an air inlet duct coming under the floor to supply air to the stove from outside; this means that the cold air coming in doesn’t create a cold draft all through the house. I think it’s mostly done because some modern houses are so tight that there may not be sufficient air flow without the duct, though. But if you’re trying to set up an installation from scratch, you might look into that.
Heat transter towers often have the reverse configuration: wide at the bottom, narrow at the top. If you have something else, it suggests that they wanted small holes at the bottom for some other reason.