Electric boilers: slow loads with real storage

Typical connected load

Electric boilers span a wide range. Small residential units serving hydronic baseboard or radiant floor start around 10 to 15 kW. Mid-size units run 24 to 36 kW. Commercial and multifamily boilers reach well past 100 kW and are usually built in staged element banks.

Like electric furnaces, boilers are staged. Elements come on in sequence as the aquastat calls for more output, which means the connected load is rarely the running load.

Why it spikes demand

Boilers run long, steady cycles rather than short bursts. That sounds better for demand and often is not, because a long cycle at 30 kW guarantees overlap with everything else in the building. A furnace that runs six minutes might miss your water heater's reheat. A boiler that runs forty minutes will not.

In commercial and multifamily settings the classic spike is morning startup. The building is on setback overnight, the schedule releases at 5 or 6 a.m., and every element comes on at once to bring a large volume of water up to temperature. That single event routinely sets the monthly peak for churches, schools and small offices.

Thermal storage and deferrability

Boilers are the best-storing hydronic load, and it is not close. Water has high specific heat, and a boiler system holds a meaningful volume of it, plus whatever is sitting in the loop, the baseboard, or the slab.

Radiant floor systems are better still. A heated concrete slab is a thermal battery measured in hours. The slab's temperature moves so slowly that holding the boiler for fifteen or twenty minutes produces no measurable change at the floor surface.

Deferrability is good to excellent, depending on the emitter type. Radiant slab: excellent. Fan coil: good. Bare baseboard in a poorly insulated room: moderate.

What an interruption feels like

In a radiant system, nothing. The floor stays the temperature it was. In a baseboard or fan coil system, the room temperature drifts down slowly, on the order of a degree over a fairly long hold, because the water in the loop keeps giving up heat after the elements stop.

The failure mode people worry about is the boiler short-cycling or locking out. Properly configured, that does not happen. The controller interrupts power to element stages rather than to the boiler's control circuit, so the boiler's own safety and sequencing logic stays intact.

Where it sits in the priority order

With the other electric heating loads, below air conditioning and above water heating. In buildings where the boiler also produces domestic hot water through an indirect tank, that function should be evaluated separately, because domestic hot water is more time-sensitive than space heat.

No minimum on or off times, same as any resistive load, which means the algorithm can work with fine granularity.

The honest caveat

Boilers are almost never a simple job. There is a burner or element control sequence, an aquastat, sometimes an outdoor reset control, sometimes zone valves and circulator pumps with their own logic, and sometimes a domestic hot water priority function that overrides everything else. Inserting demand control into that stack requires understanding what the existing controls are doing first.

We do not quote boiler control from a photo. It needs a site inspection, and in commercial buildings it usually needs a conversation with whoever maintains the equipment. If a proposal for boiler control shows up without that step, be skeptical of it.

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