How to Assess Heat Pump Readiness on a Furnace Call

Why this matters

The best time to learn whether a house can take a heat pump is on a day when nothing is urgent. The worst time is a January no-heat call with a cracked heat exchanger, when the customer has to decide by tonight and the only thing you can install by tonight is another furnace. Most furnaces that get replaced in an emergency were serviced by someone a year or two earlier, and that visit was the chance to find out that the panel has room, the ducts are tight enough, and there is a sensible spot for an outdoor unit.

Fifteen minutes at the end of an ordinary furnace call produces a readiness sheet: not a sale, not a design, just the facts that decide whether a heat pump is a straightforward swap, a swap with conditions, or a project. This card covers doing it in that time. The full electrification survey is owned by The Electrification Site Survey SOP; this is the version that fits inside a service visit.

Before you start the clock

You are on a furnace call, so you are already wearing a personal CO monitor; keep it on, because two of these checks happen beside the running furnace. If it has read 35 ppm or more in living space at any point, that is the call, not this one: shut the furnace off at its gas valve, ventilate, and deal with the combustion problem first.

Ask the customer one question before you begin: "Have you thought about what you'd replace this with when the time comes?" If the answer is no, say you are noting a few things for their file that will save time later. The check is useful whether or not they ever buy one.

The steps, by the clock

Minutes 0 to 3: the service and the panel

Read the main breaker's rating and the panel's bus rating from the panel label (inside the door, without removing the dead front), count open spaces, and list the big existing loads you can see from the breaker handles: range, dryer, water heater, AC, EV charger, well pump, hot tub. Note whether the meter and main are combined outside.

Why first: the service is usually the constraint on a conversion, and it is the one fact that turns a two-day job into a six-week wait on the utility. What a skip costs: a quote that assumes a service upgrade the house did not need, or misses one it did. Do not decide the service question from this; flag whether a measured load check is worth offering. See related: Measuring Existing Load Before You Sell a Service Upgrade.

Minutes 3 to 5: the existing air conditioner

At the condenser, read the data plate: manufacture date (or decode it from the serial), nominal tonnage, refrigerant, and the electrical data (MCA and MOCP). Look at the pad, the coil fins and the line set insulation.

Why: an AC under about eight years old changes the conversation, because a heat pump added later may mean pairing with an existing coil or line set, and an R-22 system means everything outdoors and the coil are replaced. The existing MCA and breaker also tell you what circuit is already in place for an outdoor unit. What a skip costs: proposing a full system to someone whose condenser is three years old, or a coil reuse on a refrigerant it cannot serve.

Minutes 5 to 9: the ducts

Measure total external static pressure (TESP): drill or use existing test ports in the supply plenum and in the return just before the blower, run the blower at its cooling speed, and add the two readings as magnitudes. Compare to the furnace's rated external static, read from its data plate or installation manual, often 0.50 in. w.c. for residential equipment. While the blower runs, look at the visible duct runs for disconnected joints, crushed flex and a missing or undersized return.

Hazard at this step: never drill the plenum section that houses the AC evaporator coil, since one bit through the coil releases refrigerant and destroys the coil; drill above it on the supply side, and check for wiring behind the sheet metal before you drill. Plug every hole you make with a test port plug.

Why: heat pumps want more airflow per unit of capacity than a furnace in heating, commonly around 350 to 450 cfm per ton, and they deliver cooler supply air. A duct system already fighting high static on a furnace will be louder and less comfortable on a heat pump. What a skip costs: the customer who hates the cool air, which usually started at the return grille. See related: Ductwork When a Heat Pump Replaces a Furnace.

Minutes 9 to 11: where the outdoor unit goes

Walk outside to the existing AC pad, or the likely spot if there is none. Check four things: clearance to walls and other equipment against what manufacturers typically ask; whether snow drifts or roof runoff hit that spot (a heat pump in heating needs to sit above the snow line and drain its defrost melt somewhere that will not ice a walkway); distance to bedroom windows and to the neighbor's property line, because local noise or setback rules sometimes apply; and the line set route to the indoor equipment.

Minutes 11 to 12: condensate

Find where the existing AC coil drains (floor drain, condensate pump, or outdoors) and whether the air handler location would need a secondary drain pan and float switch, which the IRC requires for equipment where overflow would damage the building, as adopted (Section M1411). Note it for an attic air handler especially.

Minutes 12 to 14: the envelope, at a glance

From the attic hatch, estimate insulation depth in inches. Note window type (single, double, storm), obvious air leakage, and the furnace's input against the house's floor area. A furnace whose input in Btu/h is more than about 40 times the conditioned square footage hints that it was oversized, which matters because heat pump sizing should come from a load calculation, not from the old furnace. See related: Heat Pump Sizing for Cold Climate Reference.

Minutes 14 to 15: what stays on gas

List the remaining gas appliances and their venting: water heater (natural draft into a shared chimney?), range, dryer, fireplace. Removing the furnace changes the water heater's venting. See related: What Removing a Gas Appliance Does to the Ones Left.

Worked example: the readiness sheet

A spring maintenance call on a 1986 two-story, 1,800 sq ft of conditioned floor area. The furnace is an 80 percent unit, 88,000 Btu/h input, installed in 2009. The tech finishes the maintenance and does the readiness check.

HEAT PUMP READINESS        1986 2-story, 1,800 sq ft
SERVICE  150A main, 150A bus, 4 open spaces
         loads: elec range, elec dryer, AC 30A
         -> measured load check worth offering
AC       2013, 3 ton, R-410A, MCA 17.4 MOCP 30
         13 yrs old, coil corroded at bottom
DUCTS    TESP 0.38 supply + 0.41 return = 0.79
         rated 0.50 in. w.c.  -> HIGH
         one 14x8 return grille for whole house
OUTDOOR  existing pad, north side, 3 ft clear
         under eave drip line, 9 ft to bedroom
COND     AC coil to floor drain, basement
ENVELOPE attic ~6 in loose fill, double pane
         88,000 / 1,800 = 49 Btu/h per sq ft
GAS LEFT water heater, natural draft, shares
         masonry chimney with furnace
VERDICT  ready WITH CONDITIONS: return air,
         load check, water heater venting plan

Reading it. The service is not obviously short: a 150 A service with an electric range and dryer already on it could go either way, which is exactly when a measured load check is worth offering rather than guessing. The AC is 13 years old with a corroded coil, so a heat pump would replace the whole outdoor side and the coil anyway, and the existing 30 A outdoor circuit is a likely reuse depending on the new unit's MOCP.

The ducts are the real finding. A TESP of 0.79 in. w.c. against a rated 0.50 is 0.29 in. w.c. high, and a single return grille for a two-story house explains most of it. On a heat pump moving 1,050 to 1,350 cfm for 3 tons (350 to 450 cfm per ton), that return would be worse. Adding a second return is a defined, quotable piece of work.

The outdoor spot works with one change: the pad sits under the eave drip line, so a new unit would move out of it or get a gutter fix above it, since roof water landing on a heat pump in winter freezes on the coil.

The furnace's 49 Btu/h per square foot is above the rough 40 the step uses as a hint, so its 88,000 says little about what a heat pump should be. The load calculation decides that.

The verdict is "ready with conditions", and the conditions have names: return air, a load check, and a plan for the water heater that will be orphaned on the chimney.

What changes the verdict

No AC at all. Then there is no pad, no outdoor circuit and no line set, and no history of how the ducts behave in cooling. The outdoor location and electrical checks carry more weight, and the line set route becomes a quote item.

A boiler instead of a furnace. No ducts. The readiness questions become ductless heads or an air-to-water heat pump, and this sheet does not cover them. Note "hydronic, survey required" and stop.

An attic air handler. Condensate and secondary drain protection move up the list, and the attic's insulation depth becomes a duct-loss question as well as an envelope one.

A 100 A service with a range, dryer and water heater all electric. The service line moves from "worth a check" to "likely constraint". Note it and point the customer to the load measurement and the alternatives before anyone says upgrade. See related: Alternatives to a Service Upgrade.

Where the sheet goes

File the sheet with the customer's equipment record, not in the tech's truck, so the next person who answers this customer's no-heat call at 6 p.m. in January sees "ready with conditions" and the three conditions, and can offer a real choice instead of a furnace by default.

References

• Furnace and condensing unit installation manuals for rated external static pressure and electrical data
• International Residential Code Section M1411, as adopted, for condensate and secondary drainage
• See related: The Electrification Site Survey SOP; Measuring Existing Load Before You Sell a Service Upgrade; Ductwork When a Heat Pump Replaces a Furnace; Heat Pump Sizing for Cold Climate Reference