I have half custody of my children. As a result, half the time, three bedrooms go unused in the house.
I also have electric baseboard heat. Each room is heated independently.
So - in winter, when the children are gone, I turn off the heat in their rooms and close their bedroom doors. This means about 40% of the downstairs is unheated.
Is this better than leaving their heat on?
Model the downstairs as a simple box. The outer walls are insulated. Inside, partition off about 40% and that’s the unheated part separated from the warm part by a leaky, uninsulated wall.
My view is that I’m being more energy efficient even though I have this large heat sink next to my warm zone. Is there anything to back up this general feeling I have?
It is definitely going to be more efficient. Any extra load on your other heaters is going to be far outweighed by the fact that the bedroom heaters are off.
Whether it’s as comfortable to live in is another story. If your heaters are at their design limit, they won’t be able to increase their output to account for the cold rooms. You can also have cold spots near the rooms that don’t exist when the rooms are warm.
I expect your electric bill would back it up, if you’re able to distinguish the time periods in question.
But sure, what you’re doing is a savings relative to running all the heaters–it’s just not as much of a percentage savings as the percentage of heating elements you’re cutting off. You do lose some heat into the unheated interior area, but it’s not really a heat sink; you’re not throwing open the windows of the unheated rooms. The slightly-warmed, basically still air within those rooms (and within the interior walls) is itself an insulating layer between the full warmth and the full cold.
You are definitely saving energy. Just make sure that there aren’t any water pipes that are going through the unheated section that might freeze and burst if the temperature gets too low. Water pipes in an outside wall will be at a lower temperature than the room inside the wall (assuming the temperature outside is colder, of course).
To further reduce the airflow between the closed rooms and the heated part of the house, get a few of these, if you haven’t already, and put them over the cracks between the doors and the floor.
How much energy you save would be the result of a multitude of complicated factors that would be difficult to model, and best answered by experiment. The bottom line, though, is that you’re still definitely and unambiguously saving energy, and probably a significant amount of it.
As for “having a large heat sink next to your warm zone”, if you heat the whole house, then you have an absolutely huge heat sink next to your warm zone. Basically, you’re just making the “inside” a little smaller, and the “outside” a little bigger.
That, and adding 8-10’ of “still air” insulation. We do something similar with a suite of rooms that are set up for my parents. When they are with us, it’s heated to about 68°. When they are not here, we keep it at 52º. The gas bill we just got suggested that you can save 2-3% off your heating bill by turning down the thermostat 1°F. I bet we save $30-40/month in the winter, by not heating that space past 52°.