Your First Winter With a New Heat Pump

Why this matters

A heat pump heats your house in a different way from the furnace or boiler you may be used to, and the first cold month is when most new owners call their installer about something that is working exactly as designed. The steam cloud, the pause, the air that feels warm rather than hot, the long run times: all normal. The calls that matter are the few things that are not normal, and the habit that quietly costs the most is one most people bring with them from their old system, the deep nighttime setback.

This card is about telling those apart, so you can leave the normal things alone and catch the real ones early.

How it heats, in one paragraph

A heat pump does not burn anything. It moves heat. Even cold outdoor air holds some heat, and the outdoor unit pulls it out of the air with a refrigerant loop and pumps it indoors, the way an air conditioner pumps heat out of the house in summer, run in reverse. Moving heat takes much less electricity than making it from scratch, which is the whole point. The catch is that as the outdoor air gets colder there is less heat to collect, so the system has to work longer to deliver the same warmth. Most systems also have a backup, called auxiliary heat: electric heating coils in the indoor unit (or, in a dual-fuel system, a gas furnace) that help when the heat pump alone cannot keep up.

What is normal

A cloud of steam and a pause. Frost builds up on the outdoor coil in cold, damp weather. Every so often the system runs a defrost cycle: it briefly reverses to send warm refrigerant through the outdoor coil and melt the frost. During defrost the outdoor fan usually stops, you may hear a whoosh or a clunk as the valve switches, and the unit can give off a large cloud of steam that looks alarming and is just melting frost. A defrost typically lasts a few minutes. Water will drip and run out from under the unit while it happens.

Air that feels warm, not hot. A furnace blows air hot enough that it feels hot on your hand. A heat pump delivers a larger volume of air at a lower temperature, so the air at a register can feel only warm, and on a hand that is damp it can even feel cool. The house still reaches the temperature you set. Judge it by the thermostat and how the rooms feel after an hour, not by the register.

Long run times, sometimes nearly all day. On a very cold day a heat pump may run for hours without stopping. That is how it is meant to work: steady, lower output rather than short hot blasts. Long running on its own is not a fault.

"Aux" or "Auxiliary" on the thermostat. You will see this come on at times: during a defrost (the backup heat runs briefly so cold air is not blown into the house), on the coldest days, and when the system is catching up after the thermostat has been turned up. Short and occasional is normal.

A light coat of frost on the outdoor coil. Thin frost that builds and then clears after a defrost is the system doing its job.

What is not normal

Ice that does not clear. A thick sheet or block of ice that stays on the coil, the top or the fan, after the unit has had time to run a defrost, means defrost is not working, airflow is blocked, or water is refreezing on the unit. Call your installer.

A unit buried in snow or sitting in ice. The outdoor unit needs air to flow freely through its sides and out the top. Snow drifted against it, or meltwater that has refrozen into a slab underneath it, starves it of air and can damage the fan.

"Emergency heat" lit when you did not choose it, or aux heat running constantly on a mild day. Constant backup heat at, say, 40 F outside usually means the heat pump itself has stopped heating and the backup is carrying the house. You will feel it on the electric bill before you feel it in the rooms.

Grinding, screeching or loud banging beyond the brief defrost switch-over sound, or a fan that does not turn when the unit is running and not in defrost.

A smell of burning that keeps on. The first time the electric backup heat runs in a season it can give off a faint dusty smell for a few minutes. A burning smell that persists, or any smell of hot electrical insulation, means turn the system off at the thermostat and call. If you see smoke or sparking, leave the house and call 911.

Where the outdoor unit needs room

            roof edge (eave)
  =========================+
                           |  drip line: no roof runoff
     gutter / no icicles   |  or icicle melt landing
     over the unit         v  on the unit
                     +-----------+
  clear 2 ft or more | fan top   |  clear above: nothing
  on the sides  ---> | OUTDOOR   |  stacked on or over it
  (check manual)     |   UNIT    |
                     +-----------+
                     |  stand /  |  stand holds it above
  snow line - - - - -|  riser    |- -typical snow depth
                     +-----------+
  ground   -> meltwater runs off and away, not pooling

The figures that matter for your unit (side clearance, how high the stand should be for your area's snow) are in the installation manual and were your installer's choice; two feet on the sides is a common figure, not a universal one. What you can do yourself:

• Brush snow off the top and away from the sides with a soft broom after a storm. Keep a path of clear ground around it.
• Watch the eave above it. If water dripping from the roof lands on the unit and freezes, that is a gutter or flashing job, and worth raising with your installer, because a unit encased by roof melt every night cannot defrost its way out.
• Never chip or pry ice off the coil with a shovel, a screwdriver or anything hard. The fins bend, and a puncture in a refrigerant line turns a service call into a major repair. Leave stubborn ice to the pro.
• Leave the area under and around the unit clear so defrost water can drain away. If it pools and refreezes under the unit, tell your installer.

The setback habit, worked through

With a furnace, many people drop the thermostat several degrees at night and bring it back up in the morning, and it saves fuel. With a heat pump that habit can cost money instead, and here is why.

When you raise the setting by several degrees on a cold morning, the thermostat sees the house is far below target and calls in the auxiliary heat to close the gap quickly. If your backup is electric coils, those coils turn electricity into heat one for one. The heat pump, on the other hand, moves more heat than it uses in electricity. How much more is called its coefficient of performance (COP), and it falls as the outdoor air gets colder.

Here is an illustrative case; your system's real numbers are on its specification sheet. Say that at 30 F outside your heat pump runs at a COP of 2.5, meaning each unit of electricity it uses delivers 2.5 units of heat into the house, while the electric backup coils run at a COP of 1.

Say you set the thermostat back from 68 F to 62 F overnight, and bringing the house back up 6 degrees in the morning takes 12 kWh of heat.

The same morning warm-up uses 12 / 4.8 = 2.5 times as much electricity when the backup coils do it. Whatever the house saved by sitting at 62 F overnight can be used up, and then some, by a short burst of the most expensive heat in the system. Whether it is depends on how cold the night was and how long the coils run, which is why the check below is to watch the thermostat rather than trust a rule.

What to do instead. Start with a small setback, 1 or 2 degrees, or none at all, and let the heat pump hold a steady temperature. Then watch: if your thermostat shows aux heat for long stretches every morning, the setback is too deep for it.

What changes this.

• A thermostat set up for a heat pump. Many current thermostats have a heat pump setting that brings the house back up slowly, ahead of schedule, and holds the backup off unless it is truly needed. If your installer configured it this way, a moderate setback can work. Ask them which way yours is set.
• A dual-fuel system. If your backup is a gas furnace rather than electric coils, the cost comparison is different, because the furnace takes over below an outdoor temperature your installer set, and a setback behaves much more like it did with your old furnace.
• A cold-climate heat pump. Some newer units hold a usable COP to much lower outdoor temperatures, so the backup runs less often. The manual's capacity table at your coldest design temperature tells you where yours stands.

The emergency heat setting

Most heat pump thermostats have an emergency heat or "EM heat" position. It shuts the heat pump off completely and heats the house on the backup alone. Use it only when the heat pump has stopped working, the outdoor unit is damaged or buried in ice, or your installer tells you to, and then call for service. Leaving it on by accident is one of the most common reasons for a shocking first winter bill, because the whole house is being heated at a COP of 1.

Checking your first winter is going well

Read the electric bill against the weather. Winter bills go up with a heat pump, because heating moved from gas or oil to electricity. Compare one month against the next and against how cold it was. A sudden jump with no cold snap to explain it is worth a look. The utility's online usage portal often shows daily use, which makes a bad day easy to spot.

Look at the outdoor unit after a cold, wet night. Frost that clears after a defrost is fine. Ice that has grown since yesterday is not.

Notice how often aux comes on. Occasional, short and on the coldest days is normal. Every few minutes on a mild day is a call.

Change the filter on schedule. A clogged filter cuts airflow, and a heat pump depends on airflow even more than a furnace does. See related: How to Change Your Furnace Filter.

When to call

Call your installer or a licensed HVAC technician for ice that does not clear, a fan that will not turn, aux or emergency heat running constantly, loud mechanical noise, water leaking inside from the indoor unit, or rooms that cannot reach the setting. A new system is usually under the installer's workmanship warranty and the manufacturer's parts warranty, so call the installer first and have the install date ready. See related: Your House Records File and What Goes In It.

Do not open the outdoor or indoor unit's panels, reset anything inside them, or turn the system's power off and on repeatedly to "clear" it. If you have shut the system off at the thermostat for safety, leave it off until the technician tells you otherwise.

References

• U.S. Department of Energy, heat pump systems and thermostat setback guidance for heat pumps
• The heat pump's installation manual and specification sheet (clearances, defrost behavior, capacity and COP at outdoor temperature)
• The thermostat's installation and user guide (heat pump configuration, emergency heat)
• See related: What Your Thermostat Settings Actually Do; Normal and Not-Normal Sounds from Your Heating and Cooling; When to Stop and Call a Licensed Pro Around the House