Lesson Heating and IAQ · Electric heat
Electric heat: principles
Electric heat turns electrical energy into heat in resistance elements—duct heaters, furnaces, and heat-pump strips. There is no flame, but there is high current, staging, and limit protection. If you understand kW → Btu/h, sequencers/contactors, and airflow across elements, you stop treating electric heat like a mystery box.
Energy conversion
Electrical power in the elements becomes heat in the airstream. Rough math:
Btu/h ≈ kW × 3413
A 10 kW package is about 34,000 Btu/h if all stages run and air carries the heat away.
Where you find it
- Electric furnaces
- Packaged units / heat pumps with auxiliary / emergency strips
- Duct heaters and VAV reheat
- Baseboard (less common for HVAC techs, same physics)
Staging
Elements energize in stages so current inrush and duct temperature stay manageable. Sequencers or boards bring stages on with time delay; blower often proves first or alongside.
Limits and safeties
High-limit switches open if airflow is lost. Never bypass limits to “get heat.” Fix filters, blowers, and closed dampers.
Airflow is mandatory
No air → red-hot elements → tripped limits → warped racks. Electric heat diagnosis always includes CFM path.
Thermostat modes
On heat pumps, understand aux vs emergency heat. Emergency often locks out the pump and runs strips only—expensive; educate the customer.
Duct heater vs furnace kits
Slide-in heater kits on heat pumps and furnaces differ from standalone duct heaters with their own airflow proving. Some require a certain minimum CFM listed on the door chart—installing a high-MERV filter that tanks airflow will trip limits even when “the heat is fine electrically.”
Element construction
Open wire, coiled, and finned elements appear in different packages. Broken ceramics and sagging coils short to ground and trip breakers. Visual inspection through the access door catches what amp clamps alone may not explain after a trip.
Staging vs comfort
Bringing all stages instantly can overshoot small zones. Sequencers exist partly for electrical service limits and partly for smoother delivery. On emergency heat, expect higher bills—explain before the customer sees the statement.
Interlocks
Many kits prove blower rotation or sail switches before elements energize. A belt slipped on an older blower looks like “no heat” with a healthy thermostat call.
Practice on the job
Before you leave the topic in your head, force a slow walkthrough on the next real call:
- Name the component or condition you are standing in front of without looking at the ticket.
- Say out loud what “good” looks like for airflow, temperatures, and safety.
- Write one measurement you will take even if the customer wants a parts swap now.
- Tie the symptom to a cause you can prove—not a habit charge or a jumper.
- Recheck after the fix under the same ambient and load conditions you logged at the start.
That five-step habit turns book knowledge into field judgment. If you cannot explain the “why” to a helper on their first month, you do not own the lesson yet.
Tie-backs that prevent callbacks
Callbacks usually come from skipping the boring proofs: clean air paths, correct control sequence, solid liquid or safe vapor where the design expects it, and safeties that still work. When pressures look confusing, return to the physical story—heat in, heat out, metering, and compression—then re-measure. When a heating or IAQ complaint shows up, return to air, fuel/electric safety, and moisture before you upsell gadgets.
Keep OEM charts and nameplates above folklore. Typical numbers in training are compasses; the plate and the chart for today’s indoor/outdoor conditions are the map. Photograph settings you change so the next visit does not undo a careful repair.
Field case
Situation. “No heat” on electric furnace. Blower runs; no stage lights. Filter collapsed against coil, limit open.
How to think it.
- Principles: strips need air; limit did its job.
- Replace filter; verify static; limits reset; stages amp correctly.
- No sequencer replacement needed.
Takeaway: airflow belongs in principles, not only diagnosis day two.
In the field
Symptom
No heat, tripped limit, high bill with little comfort
Where to look
kW stages, sequencers, limits, filter/blower
Likely causes
- Open limit, failed sequencer, no call, no air
What to measure
- Volts at elements, amps per stage, rise across heater
What not to do
- Jumper limits
Checklist
- I convert kW to approximate Btu/h
- I explain staged electric heat
- I treat airflow as required for element life
- I know aux vs emergency on heat pumps
- I respect high limits