You need to consider that some walls may contain water pipes. Not having those warmed up sufficiently could lead to a bursted pipe, which could lead to significant damage.
Adding to what others have mentioned, you probably don’t use the same type/source of heat for the full house. A typical space heater is resistive heat, which is simply turning electrical energy into heat energy (at 100% efficiency). Your central heat could be a Gas-powered furnace (converting chemical energy to heat energy at ~90% efficiency), heat pump (converting electrical energy to heat at 300+% efficiency), or a few other options.
In my area, gas is much cheaper per joule than electricity. it ends up being the same price to heat the entire place with gas as it would be to heat just a room with an electric space heater. If I were to use a kerosene space heater, or a heat pump, or whatever else, then the numbers would change.
Yes, basic math says it uses less energy to heat a smaller area than a larger one. But that’s assuming the same type, cost per unit, and efficiency. This is unlikely unless you have the worst option on both (resistive electric heat)
Lovely reply; this is not the simplest answer but it’s the truth.
Buying gas is way cheaper than buying electricity; however, if your electric grid contains any significant portion of carbon-zero producers (hydro, nuclear, solar, wind), using electric space heater will emit less CO2 than gas furnace, for the same amount of heating. If you care at all about CO2 that is. This is the main reason why burner heaters have been so difficult to replace - we thought we could price our way out, let market forces pull us gently into a renewable future without having to make personal sacrifices, but gas is just so much cheaper it’s tough to give up.
An electric heat pump will just about break even on cost with gas, while giving even greater CO2 savings. Annoyingly, a gas-powered heat pump will still be cheaper.
Please consider moisture (=mold) depending on humidity levels where you are. For example if you have one warm room the air will cool on the uninsulated interior walls to the next room, cooler air can not contain the same amount of humidity so this will condensate on (or in!) your walls. This is a good way to grow mold and get all kinds of expensive health issues.
Several years ago, I had a very lucrative, outdoor job over the winter. Between acclimating to the cold and long underwear, I would be roasting in “room temperatures”. I lived alone, and was only home weekends and about 6 hours a night.
I turned my thermostat down to 50F, put an oil-filled radiator-style space heater on a timer in my bathroom, and a heated mattress cover on my bed.
Yes, you can save an ass load of money on heating bills.
If you have plumbing in exterior walls, you need to be careful. I had to leave the cabinets under my kitchen sink open to get some heat back to the pipes.
You may not use the space, but you likely have water lines that can’t handle freezing temps, and other electronics/appliances that assume living space (non condensing humidity being important)
Now, setting your thermostat lower and only heating the room you’re in? Maybe. Whole house systems are damn efficient, so if you’re moving around the house with the heater, that’s a no go, but if you hang out in one closed door room, it’s an experiment you might want to try.
Expect the bathroom to be colder than you’d like if you’ve spent the day in your bedroom/office.
All these people talking about mould and freezing pipes, and so on, and I as an Aussie am deciding I’m totally out of my depth. An observation; We get only a few days of snow in winter every year. Our pipes don’t freeze, our houses don’t get mould from this. If you aren’t in a cold enough to freeze the pipes type place, perhaps do what Aussies do and heat only one room in the house, it’s fine.
