Backup Heat Sizing and Lockout Configuration

Purpose

This standing instruction guarantees that an electric backup heat bank is sized from the gap between the house's design load and the heat pump's published capacity at design temperature, that the lockout is set from the calculated balance point rather than a factory default, and that both gates are proved by watching the sequencer.

Two failures cost real money and neither shows up until the bill does. A bank sized to the whole-house load buys a service upgrade the house did not need, for heat it is locked out of anyway. A lockout left at a factory value in a climate that does not match lets resistance heat carry hours the compressor was already carrying, at several times the input for the same delivered heat, and nobody notices because the house is warm. A third failure is quieter still: a lockout set correctly and overridden by a recovery behavior in the same control, so the display reads right and the strips run anyway.

Safety actions that gate the work

• Open and lock the air handler disconnect AND the separate strip-heat breakers, and prove every conductor dead, proved on a known live source before and after per NFPA 70E-2021, 120.5, with work practices at 29 CFR 1910.333(b)(2), written for qualified persons under 1910.332. A strip bank is the highest-current thing in a residential air handler and its line-voltage terminals sit inches from the low-voltage board.
• Elements stay hot after the circuit opens. Do not put a hand into the element bank until it has cooled; proving dead says nothing about temperature.
• Never energize a bank whose limit, sequencer or blower interlock you have disturbed until the airflow interlock is confirmed. Strips energized without the blower open their limit repeatedly, and a jumpered limit sets the cabinet alight. If a limit opened during this work, establish why before treating it as the obstacle.
• Branch-circuit sizing is the electrician's determination, against the NEC edition your jurisdiction has adopted, whose Article 424 treats fixed electric space heating as a continuous load and sizes conductors and overcurrent protection at not less than 125 percent of the load. Where nobody on site holds that license, the tech states the kilowatts and the electrician sizes the circuit.

Scope

Covers backup heat sizing and lockout configuration on residential and light-commercial air-source heat pumps with electric resistance backup, from the load calculation through the proof at the sequencer.

Does not cover gas or propane backup, where there is no strip bank and the setting is a fuel-price changeover owned by the dual fuel SOP. Does not cover the load calculation itself, which this procedure consumes rather than performs, or the full thermostat installer menu, which the thermostat SOP owns.

Roles and handoffs

Procedure

1. Start from a load calculation and a named design temperature, and refuse to proceed without both. Acceptance: a whole-house design heating load in Btu/h from a Manual J calculation, dated, and the site's 99 percent winter design dry bulb named from the design condition table that calculation used. Wrong looks like sizing off square footage or off the old furnace's input rating, which was itself oversized. Stop rule: no load calculation means no lockout number, so book the load calc rather than guessing. Hazard: none at this step, it is a decision made at a desk.

2. Build the load line. Divide the design load by the difference between 65 F and the design temperature, giving a slope in Btu/h per degree F. Acceptance: the slope written with both inputs. The 65 F is the outdoor temperature at which a typical house with normal internal gains needs no heat, and it is an approximation that shifts on a very tight or a very leaky house. Wrong looks like using the thermostat setpoint as the no-load point, which steepens the line and sizes the bank large. Hazard: none, desk step.

3. Read the heat pump's capacity at three outdoor temperatures from the extended capacity table, at the entering air temperature the house actually runs rather than at the table's rating condition. Acceptance: three rows recorded with the entering air condition beside them. Wrong looks like using the nameplate tonnage, which is a cooling rating at one mild outdoor condition and tells you nothing about capacity at design. Hazard: none, desk step.

4. Solve for the balance point and print the check. Find the outdoor temperature where the load line and the capacity line cross, interpolating linearly between the two capacity rows that bracket it. Acceptance: a temperature, plus both sides evaluated at that temperature and shown to be equal. Wrong looks like reading a crossing off a sketch, where a 5 F error enters and stays. Hazard: none, desk step.

5. Size the bank from the deficit at design, then state what that bank does not cover. Deficit is the design load minus the capacity at design temperature; divide by 3,412 Btu per kilowatt-hour to get kilowatts and round up to the next standard bank. Acceptance: the deficit, the kilowatts, the bank chosen, and the outdoor temperature below which emergency heat alone will not hold the house. Wrong looks like sizing to the whole load to be safe, which buys a service upgrade for capacity that is locked out most of the winter. Stop rule: a deficit landing between two standard sizes goes to the manager with the shortfall stated. Hazard: none, desk step, but the number leaving this step is what the electrician sizes a circuit against.

6. Set the lockout, and set the second gate the control uses. Enter the lockout at the balance point plus a 3 to 5 F shop margin, which covers coil degradation, wind and the load line's own error. Then set the auxiliary droop, the temperature below setpoint at which the control is allowed to call backup, so a recovery behavior cannot authorize strips above the lockout. Acceptance: both values entered and read back from the control. Wrong looks like setting only the lockout and assuming it is absolute; on many controls it is not. Hazard: energized work at the low-voltage board with line voltage in the same cabinet, so the disconnect and the strip breakers stay open while your hands are inside.

7. Prove both gates at the sequencer, put the electrical back, and watch it come back. Acceptance: above the lockout with a large setpoint jump, no auxiliary stage energizes at the sequencer; below the lockout with the same jump, auxiliary energizes; and in emergency heat the compressor contactor stays open while the bank carries the load. Wrong looks like trusting the display's aux indicator, which shows what the control commanded rather than what pulled in. Stop rule: auxiliary energizing above the lockout sends you back to step 6 before anything else. Hazard: this is the step that puts the strip circuits back with the customer in the house, so stand to the hinge side when you close the breakers, confirm the blower runs before the elements energize, and watch the bank drop out at the end of the call rather than assuming it did.

The record this produces

A balance point worksheet filed to the equipment:

• Design load, design temperature, and the date of the load calculation
• The load line slope with both of its inputs
• The three capacity rows and the entering air condition they were read at
• The balance point, with both sides evaluated at it
• The deficit, the kilowatts, the bank installed, and the emergency-heat-alone limit
• Lockout as entered, auxiliary droop as entered, both read back
• The three step 7 observations at the sequencer
• The electrician's sign-off naming the kilowatts and the circuit

The next tech reads the lockout line before touching a high-bill complaint, which turns a diagnostic into a comparison. The office reads the emergency-heat-alone limit when a customer calls on a cold night saying the house will not hold, because that number is the honest answer and it belongs in the call before a truck rolls.

Worked pass: two-story house, 5 F design temperature, single-stage heat pump with electric backup

Step 1: Manual J load of 42,000 Btu/h at a 99 percent winter design dry bulb of 5 F, dated, both taken from the calculation rather than from the old equipment.

Step 2: load line slope is 42,000 divided by 65 minus 5, which leaves 60 F, giving 700 Btu/h per degree F.

Step 3: extended capacity table at the entering air this house runs gives 36,000 Btu/h at 47 F, 30,000 at 17 F, and 24,000 at 5 F.

Step 4: between the 17 F and 47 F rows the capacity line rises 36,000 minus 30,000, which leaves 6,000, over 30 F, so 200 Btu/h per degree F. Setting the two lines equal gives a crossing at 21 F. Check printed both ways: load at 21 F is 700 times 65 minus 21, which leaves 44 F, giving 30,800 Btu/h; capacity at 21 F is 30,000 plus 200 times 4 F above the 17 F row, giving 30,800 Btu/h. Equal, so the balance point is 21 F.

Step 5: deficit at design is 42,000 minus 24,000, which leaves 18,000 Btu/h. Divided by 3,412 that is 5.3 kW. The next standard bank up is 7.5 kW; a 5 kW bank would deliver 17,060 Btu/h and leave the house 940 Btu/h short at design, about 2 percent of the 42,000 load. The 7.5 kW bank delivers 7.5 times 3,412, which is 25,590 Btu/h, so on emergency heat alone the house holds to 65 minus 25,590 divided by 700, which leaves 28 F outdoor, and drifts below that. Recorded, and told to the customer at handover.

Step 6: lockout entered at the balance point plus a 4 F margin, so 25 F, and read back. Auxiliary droop entered at the control's minimum authorized value and read back.

Step 7 FAILS. Simulated outdoor at 32 F, well above the 25 F lockout, setpoint jumped 6 F. The first auxiliary stage pulls in at the sequencer. Stop rule taken: back to step 6 before the other two proofs are run. The control's recovery behavior authorizes backup on a large setpoint rise regardless of outdoor lockout, and the droop had been left at a value that permitted it. Droop widened so the control cannot call backup on a jump of that size, then retested at the same simulated 32 F with the same 6 F jump: no auxiliary stage energizes. Simulated at 18 F with the same jump: auxiliary energizes as intended. Emergency heat selected: compressor contactor stays open, both stages carry the load.

Step 7 restoration: strip breakers closed from the hinge side, blower confirmed running before the elements energized, and the bank watched dropping out at the end of the call.

Checking the pass against the sections above: step 4's acceptance names both sides evaluated at the crossing, and the run prints 30,800 Btu/h twice, once from each line. Step 5's acceptance names four values and the run prints the deficit, the kilowatts, the bank and the 28 F emergency-heat limit, each with its arithmetic shown. Step 7's acceptance has three parts, and the failing run breaks the first, which is why the stop rule fires before the other two are attempted rather than the step being scored on what came after the fix.

References

• Equipment manufacturer's extended capacity table, the only source for capacity at design temperature; a nameplate tonnage is a cooling rating and cannot be substituted
• ACCA Manual J for the load calculation and the design condition tables it draws from, and Manual S for equipment selection against that load
• The NEC edition your jurisdiction has adopted, Article 424, which treats fixed electric space heating as a continuous load for branch circuit and overcurrent sizing; the determination belongs to a licensed electrician
• 29 CFR 1910.333(b)(2) and NFPA 70E-2021, 120.5, for de-energizing and proving dead at the strip circuits
• See related: dual fuel changeover setup and verification, thermostat replacement and configuration handover, cold weather performance complaint visit