Electrical – Increasing the capacity of an electric baseboard thermostat

You are missing something.

The schematic you drew is a fairly ordinary wiring arrangement where multiple thermostats and heaters are daisy-chained off of one electrical circuit.

In your drawing, the circuit feeding into the box might be shown at the bottom. The thermostat controlled circuit leaves on the top right to connect to the local heater, and the feed through to the next thermostat and heater in the chain leaves on the top left.

The next thermostat and heater in the chain may be in a different room entirely and have nothing in common with the local thermostat except it is fed from the same branch circuit.

First, are you really sure they are still powered? Heaters have a fair amount of "thermal inertia" - they take awhile to warm up, and they also give off heat for some minutes after they're shut off. Obviously, the sure way to tell is to pop the service covers off and test with preferably non-contact testers, ideally a clamp-on ammeter. Although at some point you'll want to measure actual voltage.

One way to measure is your house's electric meter, as it has a spinning disc or similar digital readout that will show rate of power consumption. A stopwatch and a bit of research will tell you exactly what your whole house is drawing. To measure the heater, shut off all other large loads.

Here's a little secret about resistive heaters that I use intentionally sometimes. If you apply 120V to a 240V heater, you get 25% of the heat. So, if you're still getting (roughly 1/4) of the heat an hour after you "shut the thermostat off", there's a good chance a heater is seeing 120V instead of 240V. The very serious question is WHY? Because whatever's doing that could burn down your house.