Being a cheapskate, I did this when I lived in Chicago winters. My concern was sometimes I would forget to drain the water. I was creating a big thermal mass which my heater would have to work to maintain at equilibrium.

This also has the benefit of potentially raising the indoor humidity, which is otherwise going to be quite dry.

> My concern was sometimes I would forget to drain the water. I was creating a big thermal mass which my heater would have to work to maintain at equilibrium.

Is that a bad thing? I honestly ask. I'm twisting and turning the idea in my head and can argue in both direction.

I guess it can be a bad thing if you live by letting the interior go to cold frequently and then try to heat it back to room temp. For example if you would turn off the heating for the day, and then try to heat it back up from near freezing to room temp. That would be bad in itself, and the the extra mass would make it worse.

But if it is just maintained near the room temperature point then it is not that clear to me if it is bad. Would be happy to hear either way.

The more I think on it, the less certain I am one way or another.

From a real-world standpoint, I would lower the thermostat say 15 degrees F during work hours, so re-heating that water when I return seems like it would be a definite negative.

It might help to think of the tub of water as a heat battery/capacitor: the energy you put in is the energy you get out. So, yes, it will slowly absorb energy when you are heating the room. However, it will also release an equal amount of energy as the room cools down. On that front I'd say it equals out.

Technically the tub of water will change how fast the room changes temperature. Because the difference between the room temperature and outside temperature affects the rate that energy escapes the room, this could affect how much energy the room emits to the outside. For example, imagine that you had so much water that the room only went down to 65 degrees F throughout the day; that means that you put enough energy into the bath battery to heat the room to 65 degrees F throughout the day.

The other way that you would be consuming extra energy is by evaporating the water. At <70 degrees F and little air circulation, I don't imagine that evaporation is consuming a lot of energy. A bath tub of room temperature water doesn't seem like it would be very effective as a swamp cooler.

My guess is that a bathtub of room temperature water wouldn't measurable affect your heating bill.

If you drain the water when it’s at its coldest—right after you get back from work, say—that larger temperature swing might actually work in your favor.

But in some areas / seasons, the added humidity is a negative.

Added humidity is definitely a concern. Perhaps we could fix that by covering the water with plastic to prevent evaporation, but I never look into it.

Probably not in Chicago in the winter though.

Water is a necessary evil regarding buildings, in general.

But it can be managed, to mitigate mold and decay.