Inverter Driven Compressor: Why a Heat Pump Modulates

How replacing an on-off switch with a speed dial changes a heat pump’s comfort, its winter capacity and the way it has to be sized for a New York house.

What an Inverter Driven Compressor Actually Does

An older air conditioner or heat pump has a compressor with two states. It
is running at full output, or it is off. The thermostat closes, the machine
starts at one hundred per cent, it overshoots the setpoint, it stops, and the
room drifts back until it starts again. Everything about how that equipment
feels and what it costs to run follows from those two states.

An inverter driven compressor replaces the switch with a dial. The motor
speed is controlled continuously, so the machine can produce a small fraction
of its rated output and hold it indefinitely, matching what the building is
actually losing at that moment instead of overshooting and stopping. In a New
York winter, where the demand on a heating system varies enormously between
October and January, that turns out to matter more than any other single
specification on the sheet.

This inverter is not the one on your roof

The word does double duty on a site like this one, and the two devices are
genuinely different jobs.

A solar inverter takes direct current from an array and produces alternating
current synchronised with the grid, so it can be exported and used. Its whole
purpose is to match an external reference.

A compressor inverter — a variable-frequency drive — does the opposite. It
takes the alternating current from your panel, rectifies it to direct current,
and then synthesises new alternating current at whatever frequency it chooses.
Because the speed of the motor follows the frequency it is fed, controlling
that frequency controls the compressor’s speed, and therefore the machine’s
output. Nothing leaves the house. Both are power electronics; only one talks to
the utility.

What modulation changes

Overspeed is the property that makes cold-climate heating work

Every air source heat pump has the same fundamental problem: its capacity
falls as the outdoor temperature falls, because there is less heat in the
outdoor air to move and the machine has to work harder to move it. The demand
curve and the capacity curve run in opposite directions, and the gap between
them opens exactly when the house needs the most.

An inverter compressor narrows that gap by running faster than its nominal
speed when conditions demand it, trading efficiency for output. That is the
engineering behind cold-climate ratings, and it changes which number on a
specification sheet actually matters.

The nominal tonnage is not the number to read. Ask for the
rated heating capacity at a low outdoor temperature representative of your part
of the state, and the coefficient of performance at that point. A unit that
sounds generous at a mild rating point may have very little left at the design
condition, and two units with the same nominal size can behave completely
differently in January. This is also why
what you change and do not change on the controls in
extreme cold is worth understanding: the equipment is already doing
something clever, and interfering with it usually makes matters worse.

Defrost is unchanged by modulation

Modulation does not remove the need to melt frost off the outdoor coil.
Below a certain combination of temperature and humidity the coil accumulates
ice, and the machine reverses to clear it. Inverter equipment often manages
this more intelligently — sensing when it is needed rather than running on a
timer — but the cycle still happens, still uses energy, and is still normal.
Owners who mistake it for a fault frequently call for service on
something the machine is supposed to do.

What it changes about sizing

The intuitive conclusion — that a modulating unit can be oversized safely
because it will just run slower — is wrong, and it is the most common way these
systems are ruined. Every inverter compressor has a minimum speed as well as a
maximum, and the ratio between them is its turndown range. Oversize the
equipment and mild weather falls below that minimum, at which point the machine
is back to cycling on and off exactly like the single-stage unit it
replaced.

So the specification to read is the range, not the maximum: what
this unit can deliver at the coldest design condition, and how little it can
deliver in October. Both ends have to fit the building, which means the
building’s heat loss has to be calculated rather than estimated from the size
of the old furnace. That load calculation is the input
the whole selection depends on, and it is the step most often skipped. If solar
is part of the same plan, the annual consumption the heat pump adds is what
array sizing has to account for.

What it changes about installation and service

Modulating equipment is less tolerant of installation error than the
equipment it replaced, because it runs closer to its design points for longer
and relies on electronic control of the refrigerant flow. Three things become
disproportionately important.

• Refrigerant charge. An electronic expansion valve and a
  variable-speed compressor operate over a wide range of conditions, and a charge
  that is out by a modest amount degrades performance across all of it rather
  than at one operating point. The charge should be
  weighed in rather than judged by pressures, with the
  line set length accounted for.
• Line set length and rise. Manufacturers publish limits for
  the distance and vertical separation between the indoor and outdoor sections,
  and exceeding them is not a soft failure — it affects oil return to the
  compressor over the life of the machine.
• Airflow. The indoor blower on this equipment usually
  modulates too. When airflow degrades, the control system compensates by working
  harder rather than by producing an obvious symptom, so the loss shows up on the
  bill rather than as a complaint —
  which is where quiet losses hide.

What it does not fix

An inverter compressor is a better way of delivering heat into a building.
It is not a way of making the building need less. A leaky envelope, undersized
or leaking ductwork, and rooms the distribution never reached properly are all
unchanged by it, and a modulating machine asked to overcome them will simply
run at high speed for longer.

It also asks for a different habit from its owner. Deep thermostat setbacks
are counterproductive on modulating equipment: recovering from a large setback
forces the compressor to its highest, least efficient speed, or brings in
resistance backup heat, which can cost more than was saved overnight. Steady
setpoints suit this equipment, which is genuinely different advice from what
worked with a furnace.

The questions worth asking a contractor

• What is the rated heating capacity and efficiency at a low outdoor design
  temperature for my location — not at the mild rating point?
• What is the minimum output, and how does that compare with my house’s heat
  loss in mild weather?
• What heat loss calculation was this selection based on, and can I see
  it?
• How will the charge be set, and how is the line set length accounted
  for?
• Are the controls proprietary, and what happens to the thermostat and the
  diagnostics if I want a different one later?
• What does the machine sound like at maximum speed, and where is the outdoor
  unit going relative to the property line?

Put simply, an inverter driven compressor is a compressor that can be told
how hard to work rather than only whether to work. That single change is what
lets a heat pump match a mild autumn day and still reach into a January cold
snap — and it is also why the equipment rewards a careful load calculation and
punishes a lazy one.

Frequently Asked Questions

What is an inverter driven compressor?

It is a compressor whose motor speed is controlled continuously by a variable-frequency drive, rather than simply switched on and off. Because the motor follows the frequency it is supplied with, the drive can run the compressor anywhere between a low minimum and a high maximum, so the machine’s output can match what the building needs at that moment instead of overshooting and stopping. Almost every difference in comfort and running cost follows from that.

Is this the same as the inverter for my solar panels?

No, although both are power electronics. A solar inverter converts direct current from the array into alternating current synchronised with the grid so it can be exported. A compressor inverter takes household alternating current, rectifies it, and synthesises new alternating current at whatever frequency it needs to set the motor’s speed. Nothing leaves the house, and it has no role in interconnection or metering.

Does an inverter heat pump work in an upstate New York winter?

That is largely what the technology is for. Capacity falls as it gets colder for any air source heat pump, and an inverter compressor narrows the gap by running above its nominal speed when conditions demand it. What determines whether a particular unit suits your location is the rated capacity and efficiency at a low outdoor design temperature, not the nominal tonnage. Ask for that figure specifically when comparing equipment.

Can I oversize a modulating unit safely?

No, and assuming otherwise is the most common way these systems disappoint. Every inverter compressor has a minimum speed as well as a maximum. Oversize the equipment and mild weather falls below that minimum, at which point it cycles on and off exactly like the single-stage unit it replaced, losing the comfort and efficiency you paid for. The specification that matters is the range between minimum and maximum output against your building’s actual heat loss.

Why does my inverter heat pump run all the time?

Because that is the intended behaviour. Long runs at low speed are how the equipment matches the building’s heat loss without overshooting, and continuous low-speed operation is generally more efficient than repeated starting and stopping. It also removes more moisture in summer. A machine that instead runs at high speed continuously in mild weather is a different situation and worth investigating, starting with airflow and refrigerant charge.

Should I set the thermostat back at night?

Deep setbacks generally work against modulating equipment. Recovering from a large setback drives the compressor to its highest and least efficient speed, or brings in electric resistance backup, which can cost more than the overnight saving. Steady setpoints suit this equipment, which is genuinely different from the advice that worked with a furnace. Small adjustments are fine; large scheduled swings usually are not.

Does inverter equipment need different installation care?

Yes. It runs closer to its design points for far longer and relies on electronic control of refrigerant flow, so it is less forgiving than what it replaced. Refrigerant charge should be weighed in with the line set length accounted for, published line set length and vertical rise limits should be respected, and indoor airflow matters more, because the controls quietly compensate for a restriction instead of producing an obvious symptom.