New Heat Pump Not Heating Well? Check Your Ducts First

Key Takeaways

1. Why Heat Pumps Are More Sensitive to Duct Problems

A gas furnace heats air to a high temperature and pushes it through the ducts. Even if some heat is lost on the way, the air at the vent still feels warm. A heat pump works differently. It moves heat from outdoors to indoors, and the air coming off its coil is cooler. Heat pump supply air is often cited at around 90–100°F, depending on outdoor temperature and equipment, compared with noticeably hotter air from a typical gas furnace.

That smaller margin is the reason ducts matter so much. Air at 95°F already feels mild against your skin. If some of it leaks into the attic, picks up chill in an uninsulated crawl space run, or trickles out at a weak register, the room never feels warm. This feeling of cool air from the vents is often called cold blow.

2. Signs Your Ducts Are Holding the Heat Pump Back

• Air at some vents feels cool or barely warm while the system is running
• One or two rooms never reach the thermostat setting
• The heat pump runs for long stretches, or the backup heat strip comes on often
• Weak airflow at registers far from the air handler
• Ducts run through an attic, crawl space or garage
• The problems started after a furnace was swapped for a heat pump on the same old ducts

If several of these sound familiar, the ducts deserve a closer look before you call for heat pump repair.

3. How Leakage and Low Airflow Create Cold Blow

CFM means cubic feet per minute, the volume of air moving through a duct or register.

4. What to Test Before Blaming the Heat Pump

Three measurements tell you most of what you need to know.

Duct leakage. A calibrated fan pressurizes the ducts to 25 Pascals and reports leakage as CFM25. Over 400 usually means several rooms are starved.

Airflow at each register. A flow hood shows how much air each room actually receives. This is where cold rooms usually explain themselves.

Static pressure. This is the resistance the blower pushes against, measured in inches of water column. High static pressure points to restrictive ducts, a clogged filter or an undersized return, all of which cut airflow across the heat pump coil.

5. Step-by-Step: How We Check a Heat Pump’s Ducts

Our Typical Heat Pump Duct Check Process

1. Listen first. We ask which rooms feel cold, when the problem started and whether the ducts were reused from an older system.
2. Measure airflow. We read every supply register with a flow hood and compare rooms.
3. Test leakage. We seal the registers temporarily and measure total leakage at 25 Pa.
4. Check pressures if needed. For persistent problems, the full diagnostic adds static pressure readings and a room-by-room Manual J.
5. Explain the results. You get a written report showing whether ducts, airflow or equipment sizing is the main issue.

6. What Sealing Typically Changes

When ducts are the problem, sealing them sends more of the heat pump’s air into your rooms. Across 38 Bay Area systems MaxSeal sealed with Aeroseal, average leakage fell from 400 to 54 CFM25 and register airflow rose 47% on average (median 38%). Every system improved. Systems that started leakier gained more: those starting at 450–650 CFM25 averaged a 64% airflow gain.

On one system (job H-04), leakage dropped from 485 to 53 CFM25 and delivered airflow went from 238 to 670 CFM, a 182% increase. Results vary, and an already-strong system gains less. Job M-17 started at 970 CFM delivered and gained 10%.

7. When It’s the Equipment or the Sizing

Tight ducts don’t fix everything. If your leakage and airflow numbers look good and rooms are still cold, look at these:

• Sizing. A heat pump sized by rule of thumb may be too small for your coldest nights, or too large to run steadily. A Manual J load calculation estimates the heating each room needs based on its walls, windows, insulation and air leakage.
• Refrigerant charge and settings. The Department of Energy lists incorrect refrigerant charge as a common heat pump problem. This is work for your HVAC contractor.
• Thermostat and defrost behavior. Short bursts of cooler air during defrost are normal. Constant cool air is not.

MaxSeal measures and seals ducts. We don’t install or repair heat pumps, so we’ll tell you plainly when the numbers point to the equipment.

8. Heat Pumps With Ducts in Crawl Spaces and Attics

Many Bay Area homes have a heat pump air handler in the attic or garage and ducts running under the floor. The Department of Energy notes that sealing matters most when ducts are in an unconditioned area such as an attic or vented crawl space. A disconnected run under the house can dump warm air straight into the crawl space while the room above stays cold. If your heat pump installation reused the old ducts, ask whether they were tested at the time. If your heat pump is in a crawl space, see our guide to ducts in the crawl space.