What if I actually WANTED to use a solar (skylight) tube to help heat a room? (WANTED because some folks have reported unwanted heat coming down the shaft to be a problem in the summer.) It struck me that a Fresnel lens on the roof --- that could be manually adjusted from the kitchen two stories below --- could be used to aim light and heat down the shaft. If the shaft were a mirroring tube (as someone suggested already, of stainless steel) inside an unused chimney, the heat and light coming down would hit the hearth below and possibly warm those stones which would then radiate --- if they were the right kind of stones. The stainless steel tube and the chimney itself would be a safeguard against fire.
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You can want whatever you want to want, but that won't make it practical or efficient or cost-efficent (which comes back to practical.)
There are at least 2 things that might be called a "solar tube" that come immediately to mind, and they are quite dissimilar. One is an "evacuated tube solar collector" and other other is a tubular skylight. I rather suspect you are referring to a tublar skylight, which is not remotely practical for heating. Evacuated tube solar collectors are quite practical for heating, when the sun shines - they heat water or antifreeze which is pumped around a heating system and which can be used for heat or hot water in the usual manner.
Fresnel lenses have cost and performance tradeoffs relative to simply using more collector area, making them an unusual component of solar heating apparatus. They also suffer from needing to be aimed at the sun; when a system that is going to require a tracking system to point it at the sun is built, it's usually a better tradeoff to use parabolic mirrors rather than fresnel lenses, but some (generally unsuccessful, at least commercially) systems using fresnel lenses have been built.
If you really wanted to use solar skylight type tubes for solar heating, pretty much the only way I can think that might work would be to use a heliostat system to concentrate multiple suns worth of radiation onto a mirror that would reflect down the tube, (which has some inherent engineering problems, and is unlikely to be cost effective in any case.) It would have a "way to turn it off" in that you could direct the concentrator mirrors away from the target. If you happen to have enough money and ambition to pursue this path, I'd suggest using stainless-steel insulated chimney for the pipe, rather than the more usual material, as the walls of the pipe might well heat up. Or do something a bit more practical with the same money...
Heliostats do offer an interesting option in a more practical vein, that of potentially being able to direct sun into windows on all sides of the house at all times of the day (so long as the mirrors can be placed in unshaded positions, and the sun shines) and heliostats have become theoretically simpler to achieve with the advent of low-cost computing rather than having to depend on complex clockworks. But they are still somewhat specialized, low production volume and thus relatively costly (or entirely home-made) items.