Electrical – Why won’t the heat shut off in one zone

I hope the 90K is the output or it's truly undersized! Even so, it's quite common to oversize furnaces beyond the calculated heat loss by 20-30%. It's also possible the heat loss calculation is low, it's really just an estimate, and part of why the furnaces are oversized. Even if it is undersized, it would not significantly alter your energy costs. Costs are dependent mainly on the home's overall insulation and the furnace efficiency, the capacity and running time is not a big factor.

Changing ductwork will better distribute the heat, but if more heat is leaking out than the furnace can generate, it will not help. While a larger furnace will better heat your home, it will not help your energy costs. If your costs are out of line for your type and size of house in your area, then more heat is being lost than is typical. In the long run, you are better off preventing heat loss than generating more heat.

There are energy consultants in most areas that have the proper tools and equipment to advise you where most of the heat is being lost and what could be done to most effectively reduce heat loss.

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.