How Heat Pumps Work: A Clear Look at the Refrigeration Cycle, Reversing Valve, and Real-World Costs

You’ve probably heard that a heat pump can heat and cool a house using less energy than a furnace. That sounds great on paper, but the moment you try to understand why, you run into terms like “reversing valve” and “HSPF.” It gets confusing fast. Let’s fix that.

This article walks through the actual mechanics of how heat pumps work — the refrigerant cycle, the part that reverses it, and what efficiency ratings really mean. You’ll also get a practical cost comparison against electric resistance heat, plus a look at cold-weather performance and maintenance. By the end, you’ll know enough to talk intelligently with an HVAC contractor and avoid getting sold something you don’t need.

If you’re planning a system, a complete package like the 3 Ton 15 SEER2 heat pump system from 1HVAC includes the condenser, air handler, and a basic thermostat. That’s useful because it removes the guesswork of matching components — the air handler even ships with a factory-installed TXV kit, which is a detail you’d otherwise have to specify separately.

What Is a Heat Pump? (Simple Definition)

A heat pump is a device that moves thermal energy from one place to another. In winter, it pulls heat from outside air (or the ground) and moves it inside. In summer, it reverses that flow and moves heat from inside to outside. It does not burn fuel to create heat. It just relocates it.

That distinction matters because moving heat takes far less power than creating it. An electric resistance heater converts every watt of electricity into about 3.4 BTUs of heat. A heat pump can move 3 to 4 times that amount of heat per watt, depending on conditions. That’s why the efficiency numbers look so different.

How a Heat Pump Moves Heat (Not Creates It)

The secret is refrigerant. This fluid boils at a very low temperature, so it can absorb heat from cold air and release it when compressed and condensed. The system has four main components: the compressor, the condenser coil, the expansion valve, and the evaporator coil. Each one does a specific job in a continuous loop.

The Refrigeration Cycle Step by Step

1. Evaporator (indoor coil in heating mode): Cold liquid refrigerant passes through the indoor coil. Warm indoor air blows across it, and the refrigerant absorbs heat, boiling into a low-pressure gas. Even at 30°F outside, the refrigerant’s boiling point is far lower, so it can still absorb heat from air that feels cold to you.
2. Compressor: The gas gets sucked into the compressor, which squeezes it into a high-pressure, high-temperature gas. This is where the magic happens — compressing a gas raises its temperature significantly. The gas leaves the compressor at roughly 150–200°F.
3. Condenser (outdoor coil in heating mode): The hot gas flows through the outdoor coil. A fan blows outside air across it, and the gas releases its heat, condensing back into a liquid. The heat that was absorbed from your indoor air is now dumped outside, which is why the outdoor unit blows cold air in cooling mode.
4. Expansion valve: The high-pressure liquid passes through a tiny orifice, which drops its pressure suddenly. That pressure drop makes the refrigerant much colder, and it’s ready to absorb heat again back in the evaporator.

That’s the whole cycle. It runs continuously, and the only thing that changes between heating and cooling is the direction of refrigerant flow.

The Reversing Valve: How Heat Pumps Switch Between Heating and Cooling

The reversing valve is the component that makes a heat pump different from a plain air conditioner. It’s a four-way valve located near the compressor. When it shifts, it swaps which coil acts as the evaporator and which acts as the condenser.

In cooling mode, the indoor coil is the evaporator (absorbing heat from inside) and the outdoor coil is the condenser (releasing heat outside). In heating mode, the valve slides to a different position, sending hot discharge gas from the compressor to the indoor coil instead. The indoor coil becomes the condenser, and the outdoor coil becomes the evaporator.

That shift happens in a fraction of a second. You hear a soft “thunk” when the valve slides. The thermostat sends a 24-volt signal to a solenoid on the valve, which moves a pilot valve that uses system pressure to slide the main valve. It’s simple and reliable, but it’s also the part most likely to fail on an older system. If you hear a hissing sound or the unit blows lukewarm air in both modes, the reversing valve is a prime suspect.

Heat Pump Efficiency Explained: COP, SEER, and HSPF

Efficiency ratings for heat pumps are not one number. You’ll see three: COP, SEER, and HSPF. Each measures something different, and knowing the difference stops you from comparing apples to oranges.

Rating	What It Measures	Typical Values	What It Tells You
COP (Coefficient of Performance)	Instantaneous heating efficiency at a specific temperature	2.5–4.5 at 47°F	How many units of heat you get per unit of electricity right now
SEER (Seasonal Energy Efficiency Ratio)	Cooling efficiency over a whole season	14–25+	How efficient the system is at air conditioning
HSPF (Heating Seasonal Performance Factor)	Heating efficiency over a whole season	8–13+	How efficient the system is at heating, averaged over the season

COP is the raw physics number. At 47°F, a good air-source heat pump has a COP around 3.5. That means for every watt of electricity, you get 3.5 watts of heat. At 17°F, that number drops to around 2.0–2.5 on a standard unit. Cold-climate models do better — some maintain a COP above 2.0 even at -13°F.

SEER and HSPF are seasonal averages. They smooth out the variations across the year, so they’re more useful for comparing systems than COP. A higher SEER means cheaper cooling; a higher HSPF means cheaper heating. The 15 SEER2 rating on that 1HVAC system is a solid mid-range number — not the cheapest, not the most expensive, but a reasonable balance for most climates.

Do Heat Pumps Work in Cold Climates?

Yes, but with caveats. The old rule was that heat pumps stop being useful below 30°F. That rule is outdated. Modern cold-climate heat pumps are designed to maintain decent capacity and efficiency down to -15°F or lower. But you need to understand the difference between capacity and efficiency, because they don’t drop at the same rate.

Capacity Loss vs. Efficiency Loss

Capacity is the total amount of heat the unit can deliver. Efficiency is how well it converts electricity into that heat. As outdoor temperature drops, both drop, but capacity drops faster. A typical unit rated at 36,000 BTU at 47°F might only deliver 24,000 BTU at 17°F. That’s a 33% capacity loss.

Efficiency loss is smaller. The same unit might have a COP of 3.5 at 47°F and 2.5 at 17°F. That’s still 2.5 times more efficient than electric resistance heat. The problem is that if the unit can’t deliver enough total heat to keep your house warm, you need backup — regardless of how efficient it is.

This is why sizing matters so much. A contractor who just matches the old furnace’s BTU rating without considering cold-weather capacity is setting you up for disappointment. You need a unit that can still deliver enough heat at your local design temperature, which is the coldest it typically gets in your area.

When Backup Heat Kicks In

Most heat pump systems include backup heat. It’s usually electric resistance strips inside the air handler, but it can also be a gas furnace in a hybrid system. The thermostat decides when to engage it based on outdoor temperature and how far the indoor temperature has fallen from the setpoint.

Backup heat is expensive to run. Electric resistance strips have a COP of exactly 1.0 — no multiplier. If you’re relying on them for weeks at a time, your power bill will hurt. A good cold-climate heat pump minimizes backup usage because it maintains capacity at lower temperatures. A standard unit might need backup below 25°F; a cold-climate model might not need it until -5°F.

Types of Heat Pumps: Air-Source, Ground-Source, and Ductless Mini-Splits

Air-source heat pumps are the most common. They use outside air as the heat source. They’re relatively cheap to install, work in most climates, and the technology has improved dramatically in the last decade. The main downside is noise — the outdoor unit has a fan and compressor that run constantly in extreme weather.

Ground-source (geothermal) heat pumps use buried loops of pipe filled with water or antifreeze. The ground temperature stays around 50–60°F year-round, so the system doesn’t have to work as hard. Efficiency is higher — COP of 4–5 is common — and the outdoor equipment lasts longer because it’s protected underground. The catch is installation cost. Digging trenches or drilling wells adds thousands of dollars to the upfront price. You’ll save money over 10–15 years, but only if you plan to stay in the house that long. For a deeper look at how these systems exchange heat, check out geothermal heat exchanger basics.

Ductless mini-splits are air-source heat pumps without ductwork. A small outdoor unit connects to one or more indoor wall-mounted units via refrigerant lines. They’re ideal for homes without ducts, additions, or garages. They also avoid duct losses, which can be 20–30% in unconditioned attics. The trade-off is aesthetics — some people don’t like the wall units — and the cost of multiple indoor heads if you need to condition several rooms.

Heat Pump vs. Furnace vs. Electric Resistance: Real Cost Comparison

Let’s put real numbers on this. Assume you need 40,000 BTU of heat per hour to keep your house at 70°F when it’s 20°F outside. Here’s what that costs with different systems, using an electricity price of $0.15/kWh and a gas price of $1.50/therm.

System	Efficiency	Energy Used for 40,000 BTU	Cost per Hour
Electric resistance (baseboard or strips)	COP 1.0	11.7 kWh	$1.76
Standard heat pump (47°F)	COP 3.5	3.3 kWh	$0.50
Cold-climate heat pump (17°F)	COP 2.5	4.7 kWh	$0.71
Gas furnace (80% AFUE)	80% efficient	0.50 therm	$0.75
Gas furnace (95% AFUE)	95% efficient	0.42 therm	$0.63

At 47°F, the heat pump crushes everything on operating cost. At 17°F, it’s still cheaper than electric resistance by a wide margin, and roughly comparable to a gas furnace. The crossover point depends on your local electricity and gas prices. If you have cheap gas, a high-efficiency furnace might win on operating cost alone. If you have solar panels or a time-of-use electricity plan, the heat pump probably wins year-round.

One thing people forget: the heat pump also cools your house. A furnace doesn’t. If you compare the cost of a furnace plus a separate air conditioner against a heat pump that does both jobs, the heat pump’s upfront cost looks much better. You’re paying for one system instead of two.

For more on how heat pumps stack up against water heaters specifically, see water heater vs heat pump differences.

Installation and Maintenance Tips for Maximum Performance

Installation quality matters more for heat pumps than for almost any other home appliance. A poorly installed system can easily lose 20–30% of its rated efficiency. Here’s what to watch for.

Refrigerant charge: The system must have exactly the right amount of refrigerant. Too little and the evaporator starves, reducing capacity. Too much and the compressor works harder, wasting electricity. A technician should verify the charge using superheat and subcooling measurements, not just pressure gauges.

Airflow: The air handler’s blower must move the right amount of air across the coils. Too little airflow drops efficiency and can freeze the indoor coil in cooling mode. Too much airflow blows water off the coil and reduces dehumidification. The ECM blower motor on the 1HVAC air handler is a plus here — it maintains consistent airflow despite filter loading.

Placement and noise: The outdoor unit makes noise — typically 55–65 decibels, about as loud as a normal conversation. Put it away from bedroom windows and property lines. Keep it at least 18 inches from the house wall and clear of shrubs and leaves. A unit tucked into a corner will recirculate its own cold discharge air in heating mode, which kills efficiency. And don’t cover it in winter. The defrost cycle needs to melt ice; a cover traps the cold air and makes it worse.

Maintenance checklist: Do these things and your system will hold its efficiency for years.

• Change or clean the indoor filter every 1–3 months. A dirty filter is the #1 cause of airflow problems.
• Rinse the outdoor coil with a garden hose once a year. Grass clippings and cottonwood seeds block airflow fast.
• Clear a 2-foot radius around the outdoor unit. No weeds, no stacked boxes, no dog houses.
• Check the condensate drain line for clogs twice a year. A clogged drain backs water up into the air handler.
• Schedule a professional checkup every 2 years. They’ll measure refrigerant charge, check the reversing valve operation, and test the defrost cycle.

Defrost cycles are normal in winter. The outdoor coil gets below freezing, frost builds up, and the system shifts into a brief cooling cycle to heat the coil and melt the ice. You’ll see steam rising from the unit. That’s fine. It lasts 5–10 minutes and happens a few times a day in humid conditions. The one thing to watch is a unit that defrosts constantly — that points to a faulty defrost control board or a low refrigerant charge.

Frequently Asked Questions

How long does a heat pump last?

Most air-source heat pumps last 15–20 years with regular maintenance. Ground-source systems last longer — the buried loop can last 50+ years, and the indoor equipment lasts 20–25 years because it’s not exposed to weather. The compressor is usually the first thing to fail, often due to a refrigerant leak or electrical issue.

Can a heat pump really heat a house in below-zero temperatures?

Yes, if you buy a cold-climate model. These units have variable-speed compressors, larger coils, and enhanced defrost cycles. They’re rated to deliver full capacity at -5°F and some at -15°F. A standard heat pump will struggle below 25°F and need backup heat more often. Check the spec sheet for the “capacity at 5°F” number — that tells you the real cold-weather performance.

Why does my heat pump blow cold air in heating mode?

It shouldn’t — not for long. During the defrost cycle, the outdoor fan stops and the system reverses to cool the outdoor coil. That sends cold air indoors for 5–10 minutes. Most thermostats activate backup heat to temper that airflow. If you feel cold air for more than 15 minutes, the reversing valve may be stuck, or the backup heat isn’t engaging. Either way, call a technician.

Are heat pumps noisy?

Modern units are much quieter than older ones. A good inverter heat pump runs at 40–50 decibels at full speed, which is quieter than a window air conditioner. The indoor unit is nearly silent. The biggest noise complaints come from poor installation — a unit mounted too close to a wall or a loose panel that vibrates. Placement matters more than the unit itself.

Do heat pumps work with existing ductwork?

Usually yes, if the ducts are sized correctly. Heat pumps move more air at a lower temperature than furnaces, so the ducts need to be large enough to handle the airflow without excessive static pressure. A 3-ton system needs about 1,200 CFM. If your ducts were designed for a furnace, they might be undersized. A contractor should perform a Manual D calculation to verify duct capacity before installing a heat pump. If the ducts are too small, you’ll hear noisy airflow and see reduced efficiency.

What You Actually Need to Remember

• A heat pump moves heat instead of creating it, which is why it can be 2–4 times more efficient than electric resistance heat.
• The reversing valve is the component that switches the system between heating and cooling — it’s the defining feature of a heat pump.
• Capacity and efficiency are different. In cold weather, capacity drops faster than efficiency, so size the system for your local design temperature, not just the annual average.
• COP tells you instantaneous efficiency; SEER and HSPF tell you seasonal performance. Compare HSPF for heating, SEER for cooling.
• Cold-climate heat pumps exist and work well, but they cost more upfront. Standard units need backup heat below about 25°F.
• Operating cost depends on local electricity and gas prices. In most areas, a heat pump beats electric resistance and is roughly competitive with gas furnaces.
• Installation quality and maintenance matter more than the brand. Dirty coils, low refrigerant, and undersized ducts will kill any heat pump’s performance.