How to Confirm Backup Heat Runtime From the Thermostat and a Clamp

Why this matters

On a heat pump, the backup is electric resistance strip heat, and resistance heat makes one unit of heat from one unit of electricity. The heat pump itself, in the mild part of winter, typically moves two to three units of heat per unit of electricity. So an hour of strips doing the heating can cost two or three times what the compressor would have spent for the same comfort. When a heat pump customer's winter bill jumps, strip runtime is the first suspect, and it is also the most frequently accused without evidence.

This procedure gathers the evidence: what the thermostat says it asked for, what the strip circuit actually drew, and what the weather was doing at the time. It ends with a number in kWh you can put in front of the customer. It does not diagnose why the strips ran; for that, see Packaged Heat Pump Auxiliary Heat Verification and the aux heat decision trees in the References.

Who does which part

Steps 1, 2 and 4 are reading and arithmetic. Anyone at the shop can do them, including office staff on the phone with the customer reading the thermostat app. Step 3 is a current measurement on an energized 240 V circuit inside an air handler or at its disconnect. It is done only by an HVAC tech qualified for energized diagnostic work under the shop's electrical safety program, following the work practices in 29 CFR 1910.333 and wearing the voltage-rated gloves and arc-rated protection NFPA 70E (current edition) calls for at that task. If nobody on the visit is qualified, stop after Step 2 and schedule the measurement.

Step 1: Pull the thermostat's runtime history

Most connected and many programmable thermostats keep a daily runtime report split by stage: compressor heating, auxiliary heat, and sometimes emergency heat as its own line. Get at least one full billing period and, if the thermostat keeps it, the same weeks of the prior winter.

Record, per day: compressor heat hours, aux heat hours, emergency heat hours if listed, and the day's outdoor low and average. Many thermostats log outdoor temperature from a sensor or a weather feed; if not, pull the nearest public weather station.

Why the runtime report and not the customer's memory: customers notice the "aux" icon on cold mornings and remember it as "all the time", or never look at the screen at all. The report is a day-by-day count.

What it cannot tell you: the report records the thermostat's call for aux heat, not whether strips energized. An outdoor lockout control, a failed sequencer or a tripped strip breaker can block a call that the report still logs. And strips that run without a call, from a welded sequencer or relay, or strips that the air handler brings on during defrost to temper the air, may not appear in the report at all. That gap is why Step 3 exists.

Step 2: Read the thermostat's mode and backup settings

On the thermostat itself, not the app's summary screen, record:

• System mode: Heat, Auto, Emergency Heat (sometimes "Em Heat" or "Aux Only"). In emergency heat, most thermostats lock out the compressor entirely and heat only on strips.
• Aux heat lockout or balance-point setting: the outdoor temperature above which the thermostat will not call for aux heat, if the thermostat has an outdoor reading.
• Compressor lockout: the outdoor temperature below which the compressor is not allowed to run.
• Staging or droop settings: how far below setpoint, or how long, before aux is called.
• Setback schedule: a deep night setback on a heat pump often produces a morning recovery that calls aux to catch up.

What breaks if skipped: you clamp the strips, see them running, and blame the equipment, when the thermostat was simply set to Emergency Heat or a replacement thermostat shipped with aux lockout disabled. The mode check takes a minute and resolves a large share of these calls on its own. See related: The Bill That Doubled From Emergency Heat.

Step 3: Clamp the strip heat circuit while the system runs

With the qualifications and protection named above in place:

1. Identify the strip heat supply. Many air handlers have a separate circuit and breaker for the heater kit, often two circuits on larger kits; the nameplate on the heater lists kW at a stated voltage.
2. Open only the electrical compartment needed to reach one conductor of the strip circuit. On many air handlers the blower door carries an interlock that cuts power when removed; the electrical access panel is usually separate. Do not defeat an interlock to take this reading.
3. With the thermostat calling for heat, clamp one conductor of the strip circuit (one per circuit, on a kit with two) and read amps. Read the voltage at the heater supply terminals with your meter leads, hands behind the guards.
4. Ask the thermostat for aux (raise the setpoint a few degrees above room temperature, which most staging logic answers with aux after its delay) and watch the current step up as each element energizes. Then return the setpoint to where it was and watch the current fall to zero. Do not use the clamp step to force a call on a system the customer reports tripping a breaker or smelling hot; that is a repair visit, not a measurement.
5. Close the compartment before restoring normal operation, and confirm the system returns to its normal mode.

Convert to watts. Strip heaters are resistive, so power is volts times amps with no power factor correction: watts = V x A.

Mind the voltage. A heater's output is rated at a stated voltage and falls with the square of the supply voltage. A 10 kW heater rated at 240 V delivers 10 x (208 / 240) squared = 10 x 0.751 = 7.5 kW on a 208 V supply. Use the measured volts and measured amps, never the nameplate kW alone.

Why clamp at all, when the thermostat already reported runtime: this is the step that turns "the thermostat called for aux" into "the strips drew current". It also catches the opposite: current through the strips with no call at all, which points to a stuck sequencer or relay and becomes a repair ticket.

Step 4: Put runtime, power and weather on one line

Multiply logged aux hours by measured kW to get kWh per day, and set each day beside its outdoor temperature. The question you are answering: does the aux runtime track cold weather, the way backup heat should, or does it show up on mild days too?

Worked example

A customer with a 3-ton heat pump says January cost far more than last winter. The thermostat was replaced in November by another contractor.

Step 1, runtime report, January billing period (31 days): compressor heat averaged 9.1 hours a day; aux heat averaged 4.2 hours a day. Last January on the old thermostat, aux averaged under an hour a day in comparable weather.

Step 2, settings: mode is Heat, not Emergency. The aux lockout field reads "None", meaning the thermostat will call aux at any outdoor temperature. The night setback is 6 F, from 70 to 64, ending at 6 AM.

Step 3, clamp: heater nameplate 10 kW at 240 V. Measured supply 236 V. With aux called, one strip conductor read 40.8 A. That is 236 x 40.8 = 9,629 W, call it 9.63 kW. As a check against the nameplate: the element resistance at its rating is 240 squared / 10,000 = 5.76 ohms, so at 236 V it should draw 236 / 5.76 = 41.0 A. Measured 40.8 A is within normal element tolerance, so all elements are energizing. When the setpoint was returned, current fell to zero within the sequencer's delay: no stuck relay.

Step 4, the line:

• Energy: 4.2 h x 9.63 kW = 40.4 kWh per day on strips.
• Billing period: 40.4 x 31 = 1,254 kWh, recomputed as 4.2 x 31 = 130.2 hours x 9.63 kW = 1,254 kWh.
• Weather: on the 11 days with an outdoor low above 40 F, the report still logged 2.5 to 3.5 hours of aux, nearly all of it in the 6 to 8 AM window. Strips running hard on 40-degree mornings, when this heat pump has ample capacity, is the pattern of a setback recovery with no aux lockout, not of a system short on capacity.

What goes to the customer: on this January bill, about 1,254 kWh was strip heat, measured by runtime and current. Much of it ran on mild mornings when the heat pump could have carried the house. The likely contributors are the missing aux lockout and the 6 F setback recovery. Changing those settings is the next step; what the next bill will be depends on weather and rate, so no figure is promised.

Where the answer flips

• Runtime log shows aux, clamp shows no current. The call is being blocked: outdoor lockout control, failed sequencer, open limit, or a tripped strip breaker. The strips are not the bill, and the house may be short on heat on cold days. Route to the repair decision trees.
• Clamp shows current, runtime log shows no aux. Strips are running without a call, or during every defrost. A sequencer or relay that stays closed is a repair; strips during defrost are normal design on many systems, but their share of the bill can be estimated from defrost frequency.
• Very cold climate, low outdoor temperatures all period. Long aux runtime below the house's balance point is the system doing its job. The bill conversation then moves to rate, weather and equipment sizing, not to a fault. See related: How to Weather-Normalize Two Winters of Bills.

References

• See related: Packaged Heat Pump Auxiliary Heat Verification
• See related: Heat Pump Aux Runs Above Balance Point: Decision Tree
• See related: No Heat But Electric Bill High: Aux Strip Lock-On Cross System Decision Tree
• 29 CFR 1910.333, selection and use of work practices for energized parts
• NFPA 70E, Standard for Electrical Safety in the Workplace, current edition, as adopted by the employer's safety program