Capacity retention data
The published output of the specific model at low outdoor temperatures, which varies widely between units sharing the same nominal rating. This is the document that drives the sizing decision.
Heat Pump Installation Designed Around the Balance Point
How many hours a season the expensive backup heat runs is decided at design time, not by the weather, and it is the difference between a system that pays for itself and one that does not.
- Balance point chosen, not inherited
- Backup heat staged and locked out
- Thermostat matched to the equipment
- Airflow verified in both modes
Almost every disappointing heat pump is a correctly manufactured machine that was configured badly. The equipment arrives capable of heating a building efficiently across most of the season. What determines whether it actually does is a set of decisions made before and during installation: what capacity was selected, where the balance point falls, how the backup heat is staged, and whether the thermostat understands what it is controlling.
Get those wrong and the outcome is predictable. The system falls back on electric resistance heat far more often than it should, the winter bill arrives, and the household concludes that heat pumps are unsuited to the climate. The equipment was never the problem.
Get them right and the compressor carries the building for the overwhelming majority of the heating season, with backup engaging only during genuinely severe conditions and during defrost cycles. The difference between those two outcomes is design work rather than hardware, and it costs nothing extra in materials.
This page describes how that design work is done. Heat pump installations have been part of the schedule in Kaloko, HI since 2006, across straight electric backup arrangements and dual fuel systems paired with an existing gas appliance.
Sizing a heat pump is two calculations, not one
A cooling system is sized to the cooling load and that is broadly the end of it. A heat pump has to satisfy a cooling load in summer and a heating load in winter, and in most buildings those two numbers are not the same. Size purely to the heating load and the system is oversized for summer, cycling short and controlling humidity poorly. Size purely to the cooling load and the heating capacity may fall short far earlier in the season than anyone wants.
The resolution is to size primarily to the cooling load, then examine what heating capacity that selection retains as outdoor temperature falls, and place the balance point accordingly. Variable capacity equipment makes this considerably easier because it can modulate down for summer while retaining more output at low ambient than a single stage unit of the same nominal rating.
The capacity retention curve for the specific model is the document that matters here. Nominal tonnage is a summer rating; what the unit delivers at low outdoor temperatures is a separate published figure and it varies widely between models with the same badge on the front.
Placing the balance point deliberately
Plot the building's heat loss against outdoor temperature as a straight line, then plot the equipment's heating output across the same range. Where the two cross is the balance point. Above it, the compressor alone is sufficient. Below it, the shortfall has to come from somewhere else.
Because the number of hours a location spends below any given temperature is known, the balance point translates directly into an estimate of how much of the season runs on backup. Moving it down by a few degrees, whether by selecting equipment with better low ambient retention or by improving the envelope, can remove a large share of those hours.
That is the calculation that should sit behind any heat pump proposal, and it is the one most frequently absent from them.
| Arrangement | How it behaves | Suits |
|---|---|---|
| Single stage electric strip | All available resistance capacity engages at once when called | Rarely the best choice, though it is the cheapest to install |
| Staged electric strip | Brings on only as much resistance capacity as the shortfall requires | Most all electric installations, with outdoor lockout configured |
| Dual fuel with existing furnace | Heat pump runs above a switchover temperature, furnace below it | Buildings that already have a sound gas appliance |
| Emergency heat mode | Bypasses the compressor entirely, resistance heat only | A fault condition setting, not a cold weather setting |
Backup heat, staging and lockout
Resistance heat is expensive to run, so the design goal is to use as little of it as possible while never leaving the building short. Three settings control that and all three get configured deliberately.
Staging determines how much resistance capacity engages at once. A system with several stages can bring on a fraction of the available backup to cover a small shortfall, rather than energizing everything the moment the room falls a degree behind.
Outdoor lockout prevents backup engaging above a set temperature entirely. Without it, a thermostat recovering from a setback will happily call resistance heat on a mild afternoon when the compressor would have handled it comfortably in twenty minutes. This one setting accounts for a large share of unexpected winter bills.
Intelligent recovery, where the thermostat supports it, learns how long the compressor needs to reach setpoint and starts early enough that backup is not required. Combined with a modest setback rather than an aggressive one, it keeps the system on the compressor through the daily cycle.
Outdoor lockout
A temperature above which backup heat simply cannot engage. The single most effective setting for keeping running costs where they were promised.
Defrost and auxiliary interaction
Backup engages during defrost so occupants do not feel cold air. Verified during commissioning by forcing a cycle and watching what energizes.
Elevated mounting
The outdoor unit is set on a raised stand or wall bracket so defrost water drains away rather than pooling and refreezing around the base of the cabinet.
Airflow matters more than on a furnace
A furnace produces hot supply air, so a modest volume carries a lot of heat. A heat pump produces warm supply air, so it needs to move considerably more of it to deliver the same heating effect. Distribution that was adequate behind a furnace can become the limiting factor behind a heat pump.
The practical consequence shows up in two ways. Rooms furthest from the air handler underperform, because the marginal airflow they were receiving is now carrying less heat per unit volume. And the blower works harder against a restricted return, which is both noisier and less efficient.
Static pressure is measured during the survey for that reason. Where the return is the constraint, which it usually is, adding capacity is modest work that changes the performance of the entire installation.
Emergency heat is a fault setting, not a cold weather setting. Switching a thermostat to emergency heat locks the compressor out entirely and runs the building on resistance heat alone, which is the most expensive way to stay warm.
Where the outdoor unit goes, and why it matters here
Siting an air conditioner is mostly about clearance and noise. Siting a heat pump adds a winter consideration that does not apply to cooling only equipment: the unit produces defrost water repeatedly through the heating season, and that water has to get away from the cabinet.
A unit set flat on a pad at ground level will, over a cold week, freeze its own defrost water into a mound underneath itself and eventually into the base of the coil. The remedy is elevation, either a raised stand or a wall bracket, with enough height to clear typical snow accumulation and a clear path for meltwater to drain.
Discharge direction matters too. In heating mode the unit blows cold air out, and aiming that at a patio, a path or a neighboring window is a nuisance that is difficult to correct later. So is placing it under a roof valley that will deposit ice on it every time there is a thaw.
None of these are expensive to get right on installation day and all of them are awkward and costly to correct afterwards, which is why they are settled during the survey rather than decided by whoever is carrying the cabinet.
Commissioning a machine that has to work in both seasons
Set and verify airflow
Blower speed configured for the measured duct, and airflow confirmed rather than assumed from a tap setting.
Weigh in and verify charge
The specified charge weighed in after evacuation, then verified by the correct method for the metering device fitted.
Prove cooling mode
Temperature split across the indoor coil, suction and liquid readings, and amp draws against nameplate.
Prove heating mode
Reversing valve confirmed to shift, temperature rise measured across the indoor coil, and compressor operation observed under heating conditions.
Force a defrost cycle
Initiation, outdoor fan stopping, backup engaging and termination all observed, so the circuit is known to work before the first frost rather than after it.
Verify backup staging
Each stage of resistance heat brought on individually and confirmed by amp draw, with the sequencers observed operating in order.
Configure the thermostat
Equipment type, staging, balance point, outdoor lockout and recovery behavior all set for this installation rather than left at defaults.
Record and hand over
Every reading written down, the difference between normal heat and emergency heat explained, and the filter and service access shown.
- Capacity retention at low ambient used in the sizing, not nominal tonnage
- Balance point stated in the proposal with the reasoning behind it
- Backup staged and locked out above a set outdoor temperature
- Defrost cycle forced and observed before handover
- Thermostat configured for the equipment rather than left at defaults
Before specifying a heat pump
What determines whether a heat pump performs as promised
Most of these questions are about configuration rather than hardware, which is where the outcome is actually decided.
It is the outdoor temperature at which the heat pump's output exactly equals the building's heat loss. Above it, the compressor alone carries the house. Below it, something has to make up the difference, and on most installations that something is electric resistance heat.
Where that point falls is the single most consequential design decision on the job, because it determines how many hours per season the expensive backup runs. A system sized and configured to place the balance point low will spend the overwhelming majority of the heating season on the compressor. One that places it high will fall back on resistance heat routinely, and the owner will conclude that heat pumps do not work here.
Modern equipment retains far more of its rated capacity at low ambient temperatures than earlier generations did, and variable capacity models hold output considerably better than single stage units. What does not change is the physics: as outdoor temperature falls, there is less heat available to move and the system works harder to move it.
So the honest answer is that a correctly specified system will carry the building unaided for most of the season and will need help during the coldest stretches. The design work is about making that help rare rather than pretending it is unnecessary.
Electric resistance elements in the air handler are the most common, staged so only as much capacity as needed engages. Where a gas appliance already exists, a dual fuel arrangement is often better: the heat pump handles mild and moderate conditions and the furnace takes over below a switchover temperature.
Dual fuel gives the lowest running cost in most cases because it avoids resistance heat almost entirely, but it requires both appliances and a thermostat capable of managing the handover. Which arrangement suits depends on what is already in the building and what the electrical service will support.
Frequently it needs checking and sometimes upgrading. A heat pump with electric backup can draw substantially more than the cooling equipment it replaces, because resistance elements are large loads and several stages may energize together.
The panel is assessed during the survey: available capacity, the existing load, and whether a dedicated circuit of adequate rating can be run. Discovering an inadequate service on installation day is disruptive and entirely avoidable.
It matters more here than with a furnace. A heat pump delivers a large volume of moderately warm air rather than a small volume of hot air, so it needs the distribution to move more air for the same heating effect. A duct system that was marginal behind a furnace becomes a genuine constraint behind a heat pump.
Static pressure is measured across the existing air handler during the survey and compared against what the proposed equipment expects. Where the return is undersized, which is the usual finding, adding return capacity is generally modest work and it changes the performance of the whole installation.
Because it is. Supply air from a furnace is hot; supply air from a heat pump is warm, delivered over much longer run times. Air below skin temperature registers as cool even while it is heating the room, which is why people new to heat pumps often assume something is wrong.
Longer, gentler run times actually produce more even temperatures between rooms than the blast and stop cycle of an oversized furnace. It is a different sensation rather than a worse one, but it is worth knowing about before the first cold morning.
One that understands heat pump staging. It needs to control the compressor and the backup independently, know about the reversing valve and which way it energizes, and ideally support an outdoor sensor so backup can be locked out above a set temperature.
Fitting an ordinary thermostat to a heat pump is a reliable way to produce enormous winter bills, because the control will happily bring on resistance heat whenever the room is a couple of degrees below setpoint. The thermostat is not an accessory on this equipment; it is part of the design.
It depends on the relative price of electricity and gas locally and on how the system is configured, so no honest answer applies everywhere. What is consistent is that a well configured heat pump running on its compressor is efficient, and the same heat pump leaning on resistance heat is not.
This is why the configuration matters more than the equipment. Two identical systems in identical buildings can produce very different bills purely on the basis of balance point, backup staging and thermostat setup.
Airflow measured and blower speed set for the duct present. Charge weighed in and verified by the appropriate method for the metering device. Temperature rise checked in heating mode and temperature split in cooling mode, since this equipment has to be correct in both.
Then the parts unique to a heat pump: a forced defrost cycle observed from initiation to termination, the reversing valve confirmed to shift in both directions, backup heat staging verified by amp draw, and the thermostat configuration checked against the equipment and the chosen balance point. Readings written down and left with the homeowner.
Work that shapes a heat pump installation
Distribution and controls affect this equipment more than they affect a furnace, so these pages are closely connected.
Ask where the balance point lands before you compare prices
Two proposals for the same building can place the balance point several degrees apart, and that difference will show up on every winter bill for the life of the equipment. It is a better basis for comparison than the badge on the cabinet.
Surveys and installations across Kaloko, HI, booked through the same dispatch desk that handles emergency work. If you already have a proposal from another contractor, bring it along: the capacity retention figures and the stated balance point are the two things worth comparing line by line.
Where the building already has a sound gas appliance, ask about a dual fuel arrangement specifically. It removes electric resistance heat from the picture almost entirely and it is frequently the lowest running cost option available, but it only works if both appliances and the control are set up to hand over cleanly.