Electric Heat Strips and Sequencers
Supplemental electric heat is the backup that carries a heat pump through its coldest hours and its defrost cycles. Get the wiring, staging, and sequencer logic right and you deliver comfort without cooking the customer's bill — or their air handler.
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In This Guide
Why Heat Pumps Need Supplemental Heat
A heat pump moves heat instead of making it, and its capacity falls as the outdoor temperature drops — right when the building's heat loss is climbing. The point where the two lines cross is the balance point, typically somewhere around 30–40°F for an air-source system. Below the balance point the compressor alone can't keep up, and that is exactly what the electric heat strips are there to cover.
Strips also carry the load during defrost. When the outdoor coil ices and the unit reverses into a cooling cycle to melt it, the indoor coil would otherwise blow cold air into the space. The control board energizes auxiliary heat during defrost so the supply air stays reasonable. That is why you'll see the strips cycle on a mild but damp day even though it's well above the balance point.
Auxiliary vs. Emergency Heat
Auxiliary (AUX) runs the strips alongside the compressor when the heat pump falls behind. Emergency (EM) heat locks the compressor out entirely and runs on strips only — a customer selects it when the outdoor unit is down. EM heat is expensive resistance heat with no compressor efficiency, so it should never be the everyday mode.
How Sequencers Stage the Elements
A resistance heat kit can pull serious current — a single 5 kW element draws roughly 21 amps at 240V. If every element in a 15 or 20 kW kit slammed on at once, you'd get a nasty inrush and a hard thump on the service. The sequencer solves that by bringing elements on one at a time with a built-in time delay.
A classic sequencer is a bimetal device: 24V is applied to a small internal heater coil, that coil warms a bimetal strip, and after roughly 30 to 90 seconds the strip warps enough to close a set of line-voltage contacts. Multi-pole sequencers stack several contact sets on one coil, each closing at a slightly different time so the elements stage in. On shutdown they open in reverse as the coil cools, so the last element in is the first out.
Newer air handlers often replace the sequencer with a relay pack or the control board driving individual heat relays or contactors, sometimes with an anti-cycle delay built into firmware. The staging goal is identical — spread the inrush and let the blower prove airflow before the elements go glowing.
Field note
The blower must run whenever strips are energized. Most kits interlock the fan through the sequencer or a fan relay so the elements can't glow into dead air. If you ever bypass a sequencer to "test" the strips, confirm the blower is running first — resistance elements with no airflow will trip the limit or open a fusible link in seconds.
Wiring: W2, AUX, and Emergency Heat
On the low-voltage side, supplemental heat is called through the W2 (or AUX) terminal at the thermostat and air handler. The reversing valve stays on the O/B terminal, the compressor contactor on Y, and the strips answer to W2. A typical heat-pump thermostat energizes W2 on a second-stage demand or when the indoor temperature droops a set number of degrees below setpoint.
The high-voltage side is straightforward: two legs of 240V feed each element through the sequencer or heat relay contacts, protected by a fusible link and a thermal limit in series with the element. Larger kits split into multiple circuits, each with its own pull-out fuse block, because a single 20 kW kit exceeds what one 60A circuit can carry.
| Terminal | Signal | Function |
|---|---|---|
| Y | 24V | Compressor contactor (heat pump stage 1) |
| O/B | 24V | Reversing valve (O energizes in cool, B in heat) |
| W2 / AUX | 24V | Supplemental electric heat call |
| E | 24V | Emergency heat (strips only, compressor locked out) |
| L | 24V in | Fault/lockout signal from outdoor board to stat |
Many systems add an outdoor thermostat (sometimes two) in series with the W2 signal to the strips. This lockout keeps a stage of resistance heat from energizing until the outdoor air falls below a set point — say 35°F for the first bank and 20°F for the second. It's an energy-saving control, and it's also a classic "no aux heat" culprit: if that outdoor stat is set too low or has failed open, the strips will never come on during a normal call.
Best practice
On a new install, set outdoor thermostat lockouts just below the calculated balance point, not at some arbitrary round number. Locking out the first stage too high wastes strip energy on mild days; locking it out too low leaves the customer cold at the exact hours they need help.
Sizing: kW, Amps, and MCA/MOCP
Heat kits are rated in kilowatts, but you wire and protect them in amps. Two relationships do all the work:
Core equations
Amps = (kW × 1000) / Volts
BTU/hr = kW × 3412
A resistance element is essentially 100% efficient at converting watts to heat, so 1 kW always equals 3,412 BTU/hr of output — no derating for elements.
Because the elements are a fixed resistance, they draw the same current whether they're the primary heat or defrost backup. Size the conductors from the nameplate MCA (Minimum Circuit Ampacity) and set the breaker or fuse from the MOCP (Maximum Overcurrent Protection) — never guess from the kW alone. The nameplate already accounts for the 125% continuous-load factor on the heat kit.
| Heat Kit | Amps @ 240V | Output (BTU/hr) | Typical Protection |
|---|---|---|---|
| 5 kW | 20.8 A | 17,060 | 30 A |
| 10 kW | 41.7 A | 34,120 | 60 A |
| 15 kW | 62.5 A | 51,180 | Two circuits (e.g. 60 + 30 A) |
| 20 kW | 83.3 A | 68,240 | Two circuits (e.g. 60 + 60 A) |
Note that these amp figures assume a full 240V. On a 208V commercial supply the same element draws less current but also delivers proportionally less heat — a 5 kW element at 208V puts out closer to 3.75 kW. Always read the nameplate rating at the actual supply voltage.
Worked Example: kW to Amps
Problem
A 10 kW heat kit is installed in a 240V air handler with two 5 kW elements, each on its own sequencer contact. You're checking that the strips are pulling full output. What should each element and the total draw read, and what output are you delivering?
Step 1: Amps per 5 kW element
A = (5 × 1000) / 240 = 20.8 A per element
Step 2: Total kit current, both elements staged in
A = (10 × 1000) / 240 = 41.7 A total
Step 3: Convert to heat output
BTU/hr = 10 × 3412 = 34,120 BTU/hr
Step 4: Sanity-check the temperature rise
Delta-T = BTU / (1.08 × CFM) = 34,120 / (1.08 × 1200) = 26°F rise
What good looks like
Clamp each element leg: you want roughly 20–21 A per 5 kW stage and about 42 A total with both staged in. With ~1,200 CFM across the coil that's a 26°F strip-only rise, which lands right in the reasonable window for electric heat. Read much less and you've got an open element or a contact that never closed.
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Step-by-Step Troubleshooting
"Heat pump not putting out enough heat" and "no heat on emergency" are two of the most common winter calls, and the fault is very often in the strip circuit rather than the compressor. Work the signal from the thermostat down to the elements:
1. Confirm the W2/AUX call. Put the stat in emergency or force a second-stage demand and meter W2 to C. No 24V here means the problem is at the thermostat, its configuration, or the outdoor thermostat lockout — not the heat kit.
2. Verify high-voltage supply. Meter 240V at the heat kit line terminals and check the pull-out fuses. A blown strip fuse or tripped breaker is common after an element shorts to the cabinet.
3. Prove the sequencer coil. With W2 energized, measure 24V across the sequencer's control coil. Present but no heat after 90 seconds = the bimetal contacts aren't closing. Absent = broken low-voltage path or open safety.
4. Check each load contact. After the delay, meter across each set of sequencer or relay contacts. A closed contact reads near 0V across it; a stuck-open contact reads full line voltage and leaves its element cold.
5. Test the safeties. With power off, ohm the fusible link, thermal limit, and any secondary limits. Open = replace, and find out why it opened — usually low airflow, not a random failure.
6. Clamp each element. Compare measured amps to the kW math. A leg reading near zero with the contact closed means an open element. All legs low but present can mean a 208V supply feeding a 240V kit.
Common failure: sequencer never times in
If 24V is on the coil but the strips stay cold past two minutes and the coil isn't warming the bimetal, the sequencer is done — replace it with the correct pole count and coil rating. Don't "temporarily" jumper the load contacts closed and leave it; that defeats the staged inrush protection and the fan interlock.
Safety and Common Callbacks
- Always verify airflow first. Most opened fusible links and cycling limits trace back to a dirty filter, collapsed flex, or a failed blower — fix the airflow or the new limit will open too.
- Respect MCA/MOCP. Wire and protect from the nameplate, and confirm the existing feeder and breaker actually match the installed heat kit. Undersized supply on a swapped-in bigger kit is a real fire risk.
- Lock out and verify zero energy. Heat kits often have two separate high-voltage circuits. Pulling one disconnect can leave a second bank live — meter every leg before you touch elements.
- Watch the strip-on-cooling trap. A miswired O/B or a jumper left between W2 and G can leave strips energizing at the wrong time. If a customer reports high bills, check whether aux heat is running when it shouldn't.
- Set staging to protect the compressor. Aux heat should assist the heat pump, not replace it on mild days. A stat configured to jump straight to strips wastes efficiency and drives comfort complaints.
When in doubt, walk the signal and confirm each stage does what the sequence of operation says it should. The strip circuit is simple, but its interlocks — outdoor thermostat, fusible link, limit, fan relay — give it plenty of places to hide a fault.
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