Heat Pump Suitability Assessment Before a Quote

Purpose

This standing instruction guarantees that every constraint capable of stopping a heat pump installation is found and written down before the proposal is built, not after the equipment is on the truck.

The load calculation is the part everybody remembers and it is almost never what kills the job. What kills it is the electrical service that cannot carry the compressor and the backup bank together, and the duct system that carried a furnace's small volume of very hot air and cannot carry the larger volume of mild air a heat pump needs. Both are discoverable in an hour on the survey, and both cost several times more to discover on install day, with the customer out of heat and the crew standing still.

Safety actions that gate the work

• Do not open a panel dead front to count breakers. The existing-demand method below reads utility data and a panel directory, and neither requires opening anything. Where a dead front genuinely has to come off, that is energized work for a qualified person under 29 CFR 1910.332, controlled by 29 CFR 1910.333(b)(2), with proving per NFPA 70E-2021, 120.5; on most surveys the right answer is to send the electrician.
• An attic in summer runs far hotter than the yard. Work it early, carry water, set a check-in interval, and come down at the first headache or nausea rather than finishing the measurement.
• Walk only on the framing, never on the ceiling, and lay a board before you kneel.
• A crawlspace is not automatically a permit-required confined space, but it can be one. Where access is restricted and the atmosphere unknown, that entry is not part of this survey; it goes to a crew trained and equipped for it under 29 CFR 1910.146.

Scope

Covers the pre-quote survey for a residential or light-commercial air-source heat pump, ducted or ductless, whether replacing a furnace, replacing a heat pump, or going in new.

Does not cover the proposal or the pricing conversation, the handover after install, or the load calculation software itself, which this procedure consumes. The electrician's service determination is triggered here and waited for, not performed. Backup sizing and lockout are named here as scope and set by the backup heat SOP after install.

Roles and handoffs

Procedure

1. Start from a load calculation at a named design condition. Acceptance: a room-by-room Manual J result, dated, with the site's 99 percent winter design dry bulb named from the design condition table it used. Wrong looks like sizing from the outgoing furnace's input rating, which was oversized when it was installed and has been oversized ever since. Stop rule: no load calculation means no proposal; the survey is not complete. Hazard: none at this step, it is a decision made at a desk from measured room dimensions.

2. Select against the load at design and record the deficit, not just the tonnage. Read the candidate equipment's capacity at the design temperature from its extended capacity table and subtract it from the design load. Acceptance: the capacity at design, the deficit in Btu/h, and that deficit converted to kilowatts at 3,412 Btu per kilowatt-hour and rounded up to the next standard bank. Wrong looks like matching the old unit's tonnage, which selects on cooling and leaves the heating side to chance. Hazard: none, desk step, but the kilowatt figure leaving here is the input to step 4.

3. Measure the existing airflow and compare it against what the selected equipment needs. A heat pump delivers a far lower supply temperature than a furnace, so it needs more air to move the same heat, and a duct system sized for a furnace is routinely short. Acceptance: total external static measured at the equipment, a CFM read from the blower table row for that static and tap, and the selected equipment's required airflow beside it, with any gap stated as a percentage. Wrong looks like assuming ducts that carried a furnace will carry this. Stop rule: a gap either gets duct modification scoped and priced now, or the proposal does not go out. Hazard: attic or crawlspace access for the measurement, so the heat and framing cautions above apply before the hatch opens.

4. Establish whether the electrical service carries the addition, and send it to an electrician. Acceptance: the existing maximum demand, taken from the 12 months of utility demand data the adopted NEC's Article 220 permits for an existing dwelling at 220.87; plus the heat pump's minimum circuit ampacity from its nameplate, which already carries its own continuous-load factor and is not multiplied again; plus the backup bank at 125 percent, because Article 424 treats fixed electric space heating as a continuous load. Sum them against the service rating. On a heat pump the compressor and the bank run together, so they are not a noncoincident pair and cannot be taken as the larger of the two. Wrong looks like adding the bank at its nameplate current with no factor. Stop rule: a sum over the service rating holds the proposal until a licensed electrician has run and signed the calculation. Hazard: none if you do it this way, which is the point of doing it this way.

5. Site the outdoor unit against snow, drip and clearance before you promise a location. Acceptance: mounting height above the snow line as the customer describes it for their own property, the clearance beneath the unit the manufacturer states for defrost melt, the manufacturer's clearances on every side, and no position under a gutterless eave or a valley that dumps onto the coil. Wrong looks like a pad on grade in a snow climate, which buries the coil and freezes the base pan solid. Hazard: walking the property in winter, so watch footing at the proposed location rather than while looking up at the eave.

6. Check the two worst rooms at the register, not on the drawing. Acceptance: for the two rooms with the highest load per square foot, the branch's deliverable airflow against that room's calculated load, with the shortfall stated. Wrong looks like a whole-house sale that leaves the bonus room over the garage exactly as cold as it was, which is the room the job gets judged by. Stop rule: a room short of its load gets scoped as a branch modification or a supplementary head, or the customer is told in writing it will not improve. Hazard: none at the register; where a branch is traced into an attic, step 3's cautions apply again.

7. Write the constraints and get them signed before the proposal is built. Acceptance: a constraint sheet listing every finding from steps 3 to 6 with its resolution marked included, excluded or deferred, signed and dated by the customer. Wrong looks like carrying constraints in the advisor's head and mentioning them on install day. Stop rule: an unsigned constraint sheet means the proposal does not go out. Hazard: none, it is a conversation at a table, and it is the step that protects the crew standing in that house in three weeks.

The record this produces

A constraint sheet and a survey worksheet, both attached to the opportunity and carried to the job file if it sells:

• Design load, design temperature, and the load calculation date
• Selected equipment, its capacity at design, the deficit in Btu/h, and the bank in kilowatts
• Measured external static, CFM read, required CFM, and the gap as a percentage
• Existing maximum demand and its source, the MCA, the bank at 125 percent, the sum, the service rating
• The electrician's written statement where step 4 triggered one
• Outdoor siting: height above snow, clearance beneath, and the eave and drip check
• The two worst rooms, their loads, and what was scoped for each
• The signed constraint sheet, each item marked included, excluded or deferred

The install crew reads the constraint sheet before they load the truck, which is the whole point. The sales manager reads the step 4 line across a season, because a run of jobs where the service was marginal and nobody noticed until install day is a survey habit, not bad luck.

Worked pass: 1970s single-story, replacing a gas furnace and an old condenser, 10 F design temperature

Step 1: room-by-room Manual J returns a whole-house design heating load of 33,000 Btu/h at a 99 percent winter design dry bulb of 10 F, dated, with the design condition named from the table the software used.

Step 2: the candidate nominal 3 ton heat pump's extended capacity table reads 21,600 Btu/h at 10 F. Deficit is 33,000 minus 21,600, which leaves 11,400 Btu/h. Divided by 3,412 that is 3.3 kW, so the next standard bank up is 5 kW.

Step 3: total external static measures 0.78 in w.c. and the blower table reads 940 CFM at that static on the installed tap. The selected equipment needs 1,200 CFM at its design airflow. The gap is 1,200 minus 940, which leaves 260 CFM, so the existing system delivers 940 divided by 1,200, which is 78 percent of what is needed. Duct modification scoped and priced as its own line rather than absorbed.

Step 4 FAILS. The utility's 12 months of demand data give an existing maximum demand of 68 A, taken under the existing-dwelling method the adopted NEC permits at 220.87. The nameplate MCA is 24 A, used as published rather than multiplied again. The 5 kW bank at 240 V draws 5,000 divided by 240, which is 20.8 A, and at 125 percent that is 26.0 A. The sum is 68 plus 24, which gives 92, plus 26.0, which leaves 118 A, against a 100 A service, so it is over by 18 A. Stop rule taken: the proposal is held. A licensed electrician runs and signs the calculation, confirms the shortfall, and three options go to the customer: a service upgrade, a smaller bank with a lower lockout and a stated emergency-heat limit, or gas backup, which moves the job to the dual fuel path. The customer chooses the upgrade, scoped and priced as its own line, and the 118 A sum then sits inside the upgraded service.

Step 5: outdoor unit set on stands above the height the customer reports for a normal winter's snow at that corner, with the manufacturer's clearance beneath and on all sides, and the location moved off the north eave, which has no gutter.

Step 6: the two highest-load rooms are the west bedroom and the bonus room. The bedroom's branch is adequate; the bonus room's is short of its calculated load, and a branch modification is scoped.

Step 7: constraint sheet listing the duct modification, the service upgrade and the bonus room branch, each marked included, signed and dated before the proposal was written.

Checking the pass against the sections above: step 4's acceptance names four quantities and a comparison, and the run prints the demand with its source, the MCA as published, the bank at 125 percent with its division shown, the sum with both additions, and the service rating. Step 2's kilowatt figure is the input step 4 uses, and the 5 kW carries the same value in both. Step 3 asks for the gap as a percentage, and the run prints 78 percent with the division rather than calling the ducts undersized.

References

• ACCA Manual J for the load calculation and design conditions, and Manual S for selecting against that load rather than against the outgoing unit
• Equipment manufacturer's extended capacity table, required airflow, and outdoor unit clearance and mounting height requirements
• The NEC edition your jurisdiction has adopted: Article 220 including the existing-dwelling maximum demand method at 220.87, and Article 424 for fixed electric space heating as a continuous load. The determination belongs to a licensed electrician
• 29 CFR 1910.333(b)(2) with NFPA 70E-2021, 120.5, if a dead front must come off, and 29 CFR 1910.146 for any crawlspace that meets the permit-required definition
• See related: backup heat sizing and lockout configuration, dual fuel changeover setup and verification, duct leakage inspection and report, heat pump customer education at handover