Electric furnaces: big, blunt and very controllable

Typical connected load

Electric furnaces are the largest single load in most buildings that have one. Residential units commonly run 15 to 25 kW. Light commercial units run higher. The heating element assembly is built in stages, typically 5 kW each, energized in sequence by a sequencer relay as the thermostat calls for more heat.

For comparison, the entire rest of a typical home, lights and refrigerator and electronics and everything else, runs around 1 to 2 kW. A 20 kW furnace at full output is ten times your base load.

Why it spikes demand

Resistance heat converts electricity to heat at exactly one-to-one. There is no efficiency multiplier and no way to get more heat out than you put watts in. When the house needs heat, the furnace draws its full nameplate.

The staging happens fast. Most sequencers bring stages online over a period of seconds to a couple of minutes, so a call for heat becomes full-rated draw well inside a demand interval. If that call happens during your on-peak window, and it will, your peak is set by the furnace and nothing else in the building matters.

Recovery from setback makes this worse for the same reason it does with heat pumps. A large gap between room temperature and setpoint means every stage comes on and stays on.

Thermal storage and deferrability

The furnace itself stores almost nothing. Elements go cold within seconds of losing power.

The building, though, stores plenty, and this is where electric furnaces become manageable. A house at 70 degrees does not become uncomfortable in the ten minutes it takes to defer a stage. Deferrability is good, and it improves dramatically when you stop treating the furnace as one load.

The right approach breaks a large heating load into blocks. Bill Brayden describes commercial load management as Tetris: break every load into pieces no larger than about 5 kW, then fit them together. A 25 kW furnace with stages of 10, 10 and 5 kW is best handled as five 5 kW blocks rather than one 25 kW block. Controlling one enormous block causes oscillation. The controller sees a 10 kW jump, sheds hard, the load returns, and the cycle repeats. Small blocks let the system hold the house at a steady demand level with the furnace running at partial output continuously instead of slamming on and off.

What an interruption feels like

With staged control, almost nothing. The furnace still runs. It just runs at two stages instead of four for part of the cycle, so the house comes up to temperature a few minutes slower. Airflow from the registers is slightly cooler, which some people notice and most do not.

With all-or-nothing control on a single large block, you would feel it: full heat, then no heat, then full heat. That is why block sizing matters.

Where it sits in the priority order

Between air conditioning and water heating. Bill's standard guidance is dryer first, then all air conditioning grouped together, then electric heat, then water heater, spa and pool pump at the bottom.

Like all resistive loads, electric furnace stages get no minimum on-time and no minimum off-time. There is no compressor and no mechanical wear concern, so the demand algorithm is left completely unrestricted. That freedom is what lets it hold demand flat.

The honest caveat

An electric furnace in a genuinely cold climate has hours where it needs to run at full output and there is no clever way around it. If it is 5 degrees outside and the house is at 66, deferring heat makes the house colder. Demand control in that situation is about shaving the peak, not eliminating the load, and the demand limit has to be set with enough headroom that the furnace can do its job.

The other honest point: your kWh does not go down. The furnace still delivers the same heat and still consumes the same energy over the day. What changes is the shape of the curve.

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