How does a heat pump work? It moves heat instead of making it. A refrigerant circulates between an outdoor coil and an indoor coil, absorbing heat where it evaporates and releasing it where it condenses. In winter it pulls heat from outdoor air into the house; in summer a reversing valve flips the flow and it pulls heat out. A ducted system delivers that heat through your existing ducts.
The Seattle Heat Pump Company plans ducted heat pump installations across Seattle and King County, carried out by licensed and insured contractors. This guide explains the refrigeration cycle, walks through each heat pump component in a ducted system, and shows why defrost and airflow, more than the brand on the cabinet, decide how well one performs.
Key takeaways
- A heat pump moves heat using a refrigerant cycle; that is why it delivers more heat than the electricity it uses.
- The reversing valve lets the same equipment heat in winter and cool in summer.
- In Seattle's damp weather near freezing, regular defrost cycles are normal.
- A ducted heat pump needs more airflow than a gas furnace to deliver the same heat.
The refrigeration cycle
The cycle has four stages, and it is the same one inside a refrigerator or an air conditioner. It relies on a simple physical fact: when a liquid evaporates it absorbs heat, and when a vapor condenses it gives heat off. Pressure controls the temperature at which each happens.
- Evaporation. Cold, low-pressure liquid refrigerant enters a coil. Because it is colder than the air passing over the coil, it absorbs heat from that air and boils into a vapor.
- Compression. The compressor squeezes the vapor. Raising its pressure raises its temperature well above the temperature of the air it will heat.
- Condensation. The hot, high-pressure vapor flows through the second coil. Air passing over that coil is cooler than the refrigerant, so the refrigerant gives off its heat and condenses back into a liquid.
- Expansion. The liquid passes through an expansion device, which drops its pressure suddenly. Its temperature falls, and it returns to the first coil to start again.
In heating mode, the outdoor coil is the evaporator and the indoor coil is the condenser. In cooling mode, they swap. Either way, the energy the compressor uses moves heat that already exists, which is why a heat pump can deliver several units of heat for each unit of electricity. That ratio is the coefficient of performance, or COP. Electric resistance heat has a COP of 1, and the backup heat guide explains why that gap matters for your bill.
The components of a ducted heat pump
The outdoor unit
The outdoor unit, sometimes still called the condenser from its air conditioning roots, holds the outdoor coil, a large fan, the compressor, the reversing valve and the controls. Its coil is a grid of thin aluminum fins around copper or aluminum tubing, designed to put as much surface as possible in contact with outdoor air. The fan pulls air across that coil. Because the coil has to breathe, the unit needs clear space around it, which is one reason siting matters. In Seattle, siting also has to meet the city’s noise limits at the property line; the noise ordinance guide covers that.
The compressor
The compressor is the pump in heat pump. It is the part that does the work of raising the refrigerant’s pressure and temperature, and it uses most of the system’s electricity. Older and entry-level units use a single-speed compressor that is either fully on or off. Variable-speed, or inverter, compressors adjust their speed continuously to match the load, which in Seattle’s long mild heating season means long, quiet, efficient runs at low output. The variable-speed versus single-stage guide compares them.
The reversing valve
The reversing valve is what makes a heat pump different from an air conditioner. It is a four-way valve in the outdoor unit that redirects the compressor’s hot discharge gas either to the outdoor coil, for cooling, or to the indoor coil, for heating. The thermostat controls it through a low-voltage signal. When the valve shifts, you may hear a short whoosh as pressures equalize. That sound is normal.
The expansion device
The expansion device meters refrigerant into whichever coil is acting as the evaporator, and creates the pressure drop that makes the refrigerant cold. Simple systems use a fixed orifice. Most current systems use a thermostatic or electronic expansion valve that adjusts the flow as conditions change, which helps the system hold its efficiency across Seattle’s wide range of mild temperatures. A heat pump needs metering in both directions, so it may have a device at each coil or one that works both ways.
The indoor coil and air handler
Inside, the indoor coil, also called the evaporator coil because it evaporates refrigerant in cooling mode, sits in the airstream of a blower. That combination takes one of two forms. In a dual fuel system, the coil sits on top of a gas furnace and uses the furnace’s blower. In an all-electric system, an air handler holds the coil, a blower, often electric backup heat strips, and the filter. Either way, the blower pushes air across the coil and into your ducts. The outdoor unit and indoor coil are a matched pair, rated together; the air handler replacement page covers the indoor choice.
The line set
Two insulated copper lines, the line set, connect the outdoor and indoor units. One carries refrigerant vapor, the other liquid. Their size, length, routing and insulation are set by the manufacturer’s instructions. They are brazed and pressure tested on site, then evacuated to a deep vacuum before refrigerant is released. The installation process guide describes those steps.
The condensate drain
In cooling mode, the indoor coil is colder than the dew point of the indoor air, so water condenses on it, which is how a heat pump dehumidifies. That water drips into a pan and leaves through a condensate drain line with a trap. A blocked drain overflows, which is why installations include an overflow switch or a secondary pan.
Controls and the thermostat
The thermostat tells the system when to heat, cool, run backup heat or stay idle, and on many systems it also controls the reversing valve and the backup heat lockouts. A thermostat set up for a furnace rather than a heat pump is one of the most common reasons a good system runs poorly.
Find out which setup fits your houseThe assessment looks at your furnace, ducts and panel to decide between a coil on the furnace and a new air handler.
Get an EstimateWhy airflow matters more than people expect
A gas furnace heats air to well above body temperature, so it can deliver a lot of heat with modest airflow. A heat pump’s supply air is warm rather than hot. To deliver the same heat, it has to move more air. Ducts that were fine for a furnace can be too small or too restrictive for a heat pump, especially on the return side.
When airflow falls short, the effects run through the whole cycle: the indoor coil cannot reject heat fast enough in heating mode, pressures climb, efficiency falls, and the system leans on backup heat. That is why duct measurement belongs before equipment selection. See heat pumps with existing ductwork.
Defrost: what Seattle’s weather does to the outdoor coil
When the outdoor coil is the evaporator in winter, it runs colder than the outdoor air. In damp air near freezing, which describes many Seattle winter days, moisture in the air freezes onto the fins. Frost blocks airflow through the coil and cuts its ability to absorb heat.
The system handles it with a defrost cycle. When sensors or a timer decide the coil has frosted, the reversing valve briefly switches to cooling mode, sending hot refrigerant to the outdoor coil. The outdoor fan stops, the frost melts, and the meltwater drains away, often with a cloud of steam. Many systems bring on a stage of backup heat during defrost so the air coming out of the registers does not turn cool. Then the valve switches back and heating resumes.
Regular defrost is normal in Seattle, more frequent in wet weather around 30°F to 40°F than on dry, colder days. A coil that stays encased in ice is not normal and can point to an airflow obstruction, a defrost control fault or a refrigerant problem. The Seattle winter guide and the blog post on defrost go further.
Where this leads
Knowing how a heat pump works makes the rest of the decisions easier to follow: why sizing to a load calculation matters, why ducts get measured, why backup heat needs controls. The Seattle heat pump guide puts those decisions in order, the maintenance guide covers keeping each component in good shape, and the ducted heat pump installation page shows how a project is planned.