Ductwork When a Heat Pump Replaces a Furnace

Why this matters

The heat pump is sized right, the line set is tight, the charge is dead on, and the customer still says the house is drafty, the system is loud, and the back bedroom never warms up. More often than not the equipment is fine and the ducts are not. A duct system that served a furnace for twenty years was built for a smaller quantity of hotter air. A heat pump needs a larger quantity of cooler air to deliver the same heat, and pushing that extra air through the same ducts raises static pressure, raises blower power and noise, and still may not get the air where it needs to go.

This card covers why the airflow requirement changes, how to measure whether the existing system can carry it, and which parts of the system usually need work. Duct design itself, friction rate, equivalent length and sizing runs, belongs to the Duct Design Fundamentals (Manual D) Reference.

Same heat, cooler air, more of it

The heat an air system delivers depends on how much air moves and how much warmer it is than the room. At standard air density (0.075 lb per cubic foot, sea level and around 70 F):

Btu/h = 1.08 x cfm x temperature rise (F)

The 1.08 assumes standard air; at high altitude the air is thinner and the constant is smaller, so the same cfm delivers less heat.

A furnace runs a large temperature rise. A 60,000 Btu/h output furnace with a 55 F rise needs 60,000 divided by (1.08 times 55), about 1,010 cfm. The supply air leaves at around 125 F with a 70 F return.

A heat pump runs a much smaller rise, because its condensing temperature is limited. It is also sized by airflow per ton of capacity, commonly around 400 cfm per ton at full speed (manufacturers specify a range, often 350 to 450). A 3-ton heat pump moves about 1,200 cfm. At 47 F outdoors it delivers 36,000 Btu/h, so the rise is 36,000 divided by (1.08 times 1,200), about 27.8 F. Supply air leaves at about 98 F. At 17 F outdoors, with 30,000 Btu/h delivered, the rise falls to about 23 F and supply air to about 93 F.

So the heat pump moves about 20 percent more air than the furnace (1,200 against 1,010) to deliver heat at a much lower temperature. And the old system's cooling airflow is often lower still: if the furnace carried a 2.5-ton air conditioner, the ducts were only ever asked for about 1,000 cfm in summer too.

Static pressure: what the extra air costs

Air moving through ducts loses pressure to friction and fittings. The blower makes up that loss. The sum of the pressure the blower sees outside its own cabinet is the total external static pressure (TESP), measured in inches of water column (in. w.c.). Every air handler is rated to deliver its airflow up to a stated TESP, commonly 0.5 in. w.c. for residential equipment; the fan table in the installation manual shows how much air it actually moves at each static.

For a fixed duct system, with turbulent flow, pressure drop rises with the square of airflow. That is the fan law, and it is why a modest airflow increase produces a large pressure increase: 20 percent more air through the same ducts means 1.2 squared, about 1.44 times the static pressure.

What happens when static exceeds the rating depends on the blower:

• A PSC blower (fixed-speed motor) simply moves less air. The heat pump runs with low airflow, which raises head pressure in heating, lowers suction in cooling, and can freeze the coil.
• An ECM blower (variable-speed, constant-airflow motor) ramps up power to hold the programd airflow until it hits its limit. The customer hears it as roar and whistle at the registers. Blower watt draw climbs steeply, and past its limit it too falls short of target airflow.

Either way the heat pump does not get the air it was sized and charged for.

Measuring the existing system before you commit

Measure TESP with a manometer at two test ports: one in the return between the last return fitting and the equipment, and one in the supply between the equipment and the first supply fitting. The sum of the magnitudes is TESP.

What counts as "external" depends on what is inside the rated cabinet. This is the step people get wrong when the equipment changes:

• A filter inside the air handler cabinet is internal on some equipment and external on others; a filter in a return grille or a separate filter rack is external.
• A cased evaporator coil sitting on a furnace is external to the furnace. In a heat pump air handler, the coil is inside the rated cabinet and is internal.

So a furnace's measured TESP includes the coil's drop, and a heat pump air handler's does not. When you predict how the new air handler will see the old ducts, take the coil out of the old figure before you scale it.

Measure at a known airflow. The furnace's fan table at the measured static tells you roughly what it was moving; a flow hood or a duct traverse is better if you have one.

Worked example: 2.5-ton AC and furnace to a 3-ton heat pump

The existing system: a 60,000 Btu/h output furnace with a cased coil on top, a 1-inch filter in a slot at the return, one central return. At the furnace's heating speed the fan table puts airflow at about 1,000 cfm. The readings:

• Return tap, upstream of the filter: minus 0.28 in. w.c.
• Across the filter alone: 0.15 in. w.c. of that 0.28.
• Supply tap between furnace and coil: plus 0.37 in. w.c.
• Supply tap above the coil, at the start of the trunk: plus 0.22 in. w.c.

Furnace TESP: 0.28 plus 0.37, which is 0.65 in. w.c. The coil's drop is 0.37 minus 0.22, which is 0.15 in. w.c.

The new 3-ton air handler has its coil inside the cabinet, so the coil comes off the external side. Re-based to the new equipment, external static at 1,000 cfm is the return side, 0.28, plus the supply duct alone, 0.22, which is 0.50 in. w.c.

Scale to the heat pump's 1,200 cfm: 1,200 divided by 1,000 is 1.2, squared is 1.44, and 0.50 times 1.44 is 0.72 in. w.c. Scaling the filter's share by the square is conservative, because a media filter's drop rises closer to linearly with flow, so treat 0.72 as an upper estimate. The air handler's fan table shows that at 0.7 in. w.c. it delivers only about 1,050 cfm on its top tap, well short of 1,200.

The return side is carrying 0.28 of the 0.50, more than half, and the filter alone is more than half of the return side. That is where the work goes:

• Replace the 1-inch filter slot with a 4-inch media filter cabinet sized for the airflow. Its published drop at 1,000 cfm is 0.06 in. w.c., against the old filter's 0.15.
• Add a second return from the bedroom wing, which also fixes the closed-door problem in those rooms. Estimate the return duct and grille share falling from 0.13 to about 0.05 at 1,000 cfm, and confirm by measurement afterward.

New return side at 1,000 cfm: 0.06 plus 0.05, which is 0.11. Add the supply duct's 0.22 and TESP at 1,000 cfm is 0.33. Scaled to 1,200 cfm: 0.33 times 1.44 is 0.475 in. w.c., inside the 0.5 rating with little to spare. After the work, the installer measured 0.46 in. w.c. at 1,200 cfm on the new air handler, which confirms the estimate came out slightly high, as expected from the filter scaling.

The supply side was left alone because at 0.22 it was not the problem. Had it been the larger share, the fix would have been different: an undersized trunk, crushed flex, or too few supply runs.

Where existing duct systems usually need work

Returns. The single most common shortfall. Older houses often have one central return sized for a furnace's lower airflow, and bedrooms with doors that close and no path back. A room that cannot return its air pressurizes, its supply falls off, and it never reaches temperature. Second returns, transfer grilles or jumper ducts fix it. Filter face velocity matters too: a filter grille sized for 1,000 cfm may need to grow by a fifth or more.

Supply registers and throw. Air at 95 F is only a few degrees above skin temperature, so it feels cool when it blows across a person. That is the most common comfort complaint after a conversion. Registers that throw air across a seating area need redirecting so the air mixes before it reaches people. See related: The Customer Who Hated the Warm Air.

Ducts in unconditioned space. Leaks and missing insulation in an attic or crawlspace cost more on a heat pump, because the system has less capacity margin than an oversized furnace had, so a fixed duct loss takes a larger share of what the equipment can deliver. Sealing and insulating those runs is often worth as much as the equipment upgrade. See related: Duct Leakage Inspection and Report.

Rooms the ducts never served. An addition or a bonus room on the end of a long run may never have been heated well by the furnace either. Rather than rebuilding the trunk, a ductless head for that room is often the cleaner fix. See related: Ductless Heads in a House With Ducts.

What changes the answer

A variable-speed heat pump. It runs at low speed and low airflow most of the year, so static is only at its worst at full output. That softens the noise complaint, but the system still has to deliver full airflow on the design day and on the hottest afternoon, so size the duct work for maximum airflow, not average.

A smaller heat pump. Choosing a smaller unit to fit the ducts trades a duct problem for a capacity problem. Fix the ducts or add capacity elsewhere rather than undersizing the equipment.

Equipment with a higher static rating. Some air handlers are rated for higher TESP, and the fan table shows their airflow there. That helps, at a cost in blower power and noise, and it does not help a room that has no return path at all.

How to verify you got this right

After the work, measure TESP at the new air handler at full airflow and record it against the fan table's rating for that speed setting. Confirm actual airflow at full speed, by flow hood, traverse or the air handler's own reported airflow if it publishes one, rather than assuming the setting delivered it. Measure supply and return temperatures and confirm the rise is in the range the manufacturer specifies. And walk the house with the doors closed: every bedroom should hold temperature within a couple of degrees of the thermostat.

References

• ACCA Manual D (residential duct design), in the edition your jurisdiction or program requires.
• Air handler installation manual and fan performance table for the equipment installed (rated TESP, airflow by speed tap).
• Filter manufacturer's published pressure drop by airflow and face velocity.
• See related: Duct Design Fundamentals (Manual D) Reference; ACCA Manual D Duct Design Reference; Sizing a Heat Pump for a Cold Climate.
• See related: Ductless Heads in a House With Ducts; The Customer Who Hated the Warm Air.