Dual Fuel Changeover Setup and Verification

Purpose

This standing instruction guarantees that a dual fuel changeover temperature is derived from the site's own two fuel prices and the equipment's published performance, entered in the control, and proved by watching that only one heat source runs on each side of it.

Dual fuel is sold as the best of both and delivered as neither when the changeover is left at whatever the control shipped with. Set too warm and the furnace carries hours the compressor would have carried more cheaply; set too cold and the compressor grinds at low output on nights the furnace would have won. The failure that costs equipment rather than money is different: a control configured to run backup alongside the compressor, correct for electric strips and wrong for a furnace, pushes furnace-heated air across the indoor coil while that coil is acting as the condenser.

Safety actions that gate the work

• Wear a personal carbon monoxide monitor, switched on, before the furnace fires at any point in this procedure, and keep it on you for every cycle. The house alarm is sited for sleeping occupants and is not your instrument. This procedure cycles a furnace far more than a normal day does.
• If anyone smells gas, everyone leaves the building immediately. No switches touched, no lights, nothing plugged or unplugged, no phone used inside. Call the gas utility from outside and do not go back in.
• Open and lock the equipment disconnect and prove dead before landing any control wire, 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.
• Do not jumper, defeat or bypass a rollout switch, high limit or pressure switch to get a test run. If one opens during this work, establish why before treating it as the obstacle.
• Confirm the vent connector is intact and the combustion air opening clear before the first cycle, because you are about to run this appliance harder in an hour than a normal morning does.

Scope

Covers changeover setup and verification on residential and light-commercial dual fuel systems: an air-source heat pump paired with a gas or propane furnace, controlled either by a dual-fuel-capable thermostat or by an outdoor-sensor interlock on the furnace board.

Does not cover electric resistance backup, where there is no fuel comparison to make and the balance point is set by the backup heat sizing SOP, or the full thermostat installer menu, which the thermostat SOP owns. Combustion setup and flue-gas analysis belong to the furnace maintenance SOP.

Roles and handoffs

Procedure

1. Confirm the system is genuinely dual fuel and identify which control owns the changeover. Acceptance: heat pump and furnace models recorded from nameplates, and the control scheme named as thermostat-managed or furnace-board-managed with an outdoor sensor. Wrong looks like assuming the thermostat owns it on a system whose furnace board has its own sensor, so you set one value and a second control overrides it. Stop rule: two controls both holding a changeover is a wiring decision, not a setting, so escalate before touching either. Hazard: none at this step, it is nameplate reading with the equipment idle.

2. Take the two winter unit rates off the customer's own bills and reduce them to one ratio. Divide the price of a kilowatt-hour by the price of a therm, same currency for both, which leaves a plain number with no units. Acceptance: the ratio written down with the two bill dates it came from. Wrong looks like an annual average or a published state figure; the changeover is a winter setting and winter rates apply. Stop rule: no bills, no arithmetic, so book the return rather than guessing. Hazard: none, this is desk work done before the truck moves.

3. Compute the break-even coefficient of performance. Break-even COP equals 29.31 times the furnace AFUE as a decimal, times the ratio from step 2. The 29.31 is 100,000 Btu in a therm divided by 3,412 Btu in a kilowatt-hour. Acceptance: a break-even COP written with all three inputs beside it. Wrong looks like folding fixed monthly or demand charges into the rates; those do not change with the changeover, so including them shifts the answer without cause. Note which way you rounded, because rounding the break-even COP up moves the changeover warmer and hands the marginal hours to the furnace. Hazard: none, desk step.

4. Find the outdoor temperature where this heat pump's published COP equals the break-even value. Read the manufacturer's extended capacity table at the entering air temperature the house runs, and interpolate linearly between the two rows that bracket the value. Acceptance: an outdoor temperature written with the two table rows it sits between and a note that the interpolation is linear and therefore approximate. Wrong looks like using the nameplate COP, which is published at one mild outdoor condition and says nothing about the temperature you want. Hazard: none, desk step.

5. Enter the changeover and the second lockout, then verify the outdoor sensor the control reads. Compressor locked out below the changeover, furnace locked out above it. Acceptance: both values read back from the control, and the control's outdoor reading against a reference thermometer in full shade, agreeing within a couple of degrees. Wrong looks like a sensor in afternoon sun, over a dryer vent or on a wall that stores heat, which fires the changeover at the wrong weather all winter. Stop rule: a sensor disagreeing with the reference gets relocated before the values mean anything. Hazard: reaching behind a running outdoor unit, so open, lock and tag the disconnect first under 29 CFR 1910.333(b)(2)(iii), confirm the fan has stopped rather than coasting, and stay out of the fan plane.

6. Prove each side of the changeover by watching the equipment, not the display. Move the changeover value temporarily to force each side in turn. Acceptance: above the changeover the compressor runs and the furnace does not fire; below it the furnace fires and the compressor contactor stays open; and at no point do both run together. Wrong looks like both running at once, which is what a control set to electric backup rather than dual fuel does. Stop rule: if both run, kill the call immediately and go back to step 1. Hazard: the personal CO monitor is on your body from the first ignition, and the command creates its own, because furnace supply air crossing the indoor coil while that coil is condensing drives head pressure up, which is why simultaneous operation is stopped rather than observed. The compressor side is only forced where the actual outdoor temperature is at or above the manufacturer's minimum operating ambient for that model: forcing a compressor below it is a test the equipment is not rated to survive, so where the weather does not permit it, the compressor side is proved on a later visit rather than by moving the value further.

7. Put every temporary value back, and prove the fuel side still works after you finish moving settings. Acceptance: design changeover and lockout re-entered and read back, one full furnace cycle watched from call to flame proof to blower off with no delayed ignition or rollout, and the personal monitor reading nothing in the occupied space during and after the cycle. Wrong looks like leaving a test value in the control, which is the most common way a commissioning visit becomes a January callback. Stop rule: any sustained CO reading stops the visit, the appliance is shut down and the combustion path goes to the furnace SOP. Hazard: this puts fuel back with the customer in the house, so the monitor stays on until you leave, not until the last setting is entered.

The record this produces

A changeover worksheet filed to the equipment:

• Both nameplates, and which control owns the changeover
• The two winter unit rates, their bill dates, and the ratio derived from them
• The furnace AFUE used and the break-even COP, with all three inputs shown
• The two table rows interpolated between and the resulting outdoor temperature
• Changeover and lockout values as entered and as read back
• Control outdoor reading against the reference thermometer, and where the sensor sits
• Both directions of the step 6 proof, and whether simultaneous operation was ever seen
• The step 7 furnace cycle observations and the CO monitor result

When rates move, the office re-runs step 3 against this sheet rather than starting over, and the visit becomes a setting change instead of a diagnosis.

Worked pass: single-stage heat pump over a 95 AFUE gas furnace, thermostat-managed

Step 1: nameplates recorded, control is a dual-fuel-capable thermostat with its own outdoor sensor, furnace board carries no second sensor. One owner.

Step 2: two winter bills. On this customer's winter rate a kilowatt-hour costs one tenth of what a therm costs, so the ratio is 0.10, recorded with both bill dates.

Step 3: AFUE 0.95 off the furnace nameplate. Break-even COP is 29.31 times 0.95, which gives 27.84, times 0.10, which leaves 2.78. Rounded to 2.8, direction noted: rounding up raises the required COP, moves the changeover warmer, and gives the marginal hours to the furnace.

Step 4: the extended capacity table for this model at 70 F entering air reads COP 3.1 at 47 F outdoor and 2.6 at 35 F outdoor, both read from that table. The slope is 3.1 minus 2.6, which leaves 0.5, over 47 minus 35, which leaves 12 F, so 0.042 COP per degree F. To reach 2.8 from 3.1 the COP must drop 0.3, and 0.3 divided by 0.042 gives 7.2 F. The crossing sits at 47 minus 7.2, which leaves 39.8 F. Recorded as 40 F, interpolated linearly between the 47 F and 35 F rows.

Step 5: compressor lockout entered at 40 F, furnace lockout above it, both read back. Control outdoor reading 38 F against a shaded reference at 37 F. Sensor is on the north wall, clear of the dryer vent.

Step 6 FAILS. Above the changeover the compressor runs alone, correctly. Below it, the furnace fires and the compressor contactor stays pulled in, with both running. Stop rule taken immediately: call killed at 40 seconds, not observed further. Back to step 1. The thermostat's system type reads heat pump with electric backup rather than dual fuel, a configuration designed to run backup alongside the compressor, correct for strips and wrong for a furnace. Changed to dual fuel and both sides retested: above 40 F only the compressor runs, below 40 F only the furnace fires and the contactor stays open.

Step 7: design values re-entered and read back at 40 F. One full furnace cycle watched from call to flame proof to blower off, ignition clean, no rollout. Personal monitor reads nothing in the occupied space during and after the cycle.

Checking the pass against the sections above: step 3 accepts a break-even COP with all three inputs beside it, and the run prints 29.31, 0.95 and 0.10 with both multiplications. Step 4 accepts an outdoor temperature with the two table rows it sits between, and the run prints both rows, the slope with its subtraction, and the interpolation. Step 6's acceptance has three parts, compressor alone above, furnace alone below, and never both; the failing run breaks the third while the first passed, which is why the stop rule fires rather than the step being scored on the parts that passed.

References

• The heat pump manufacturer's extended capacity and performance table, the only source for COP at a given outdoor and entering air temperature; nameplate ratings are published at one mild condition and cannot be used here
• Furnace nameplate AFUE, and the furnace installation instructions for any board-level changeover or outdoor sensor input
• Thermostat installer documentation for the difference between a dual fuel system type and a heat pump with electric backup, the setting this procedure most often finds wrong
• 29 CFR 1910.333(b)(2) and NFPA 70E-2021, 120.5, for de-energizing and proving dead
• See related: backup heat sizing and lockout configuration, thermostat replacement and configuration handover, furnace annual maintenance