The Number That Decides a Heat Pump Project Is the Balance Point

Sizing a heat pump is the intersection of two lines. The first is the building heat loss, which rises steadily as it gets colder outside and comes from a room by room calculation. The second is the equipment heating capacity, which falls as it gets colder, because the outdoor coil has less heat available to extract. Plot both against outdoor temperature and they cross at one point. That crossing is the balance point, and it is the single most informative number in the whole project.

Getting the equipment curve right requires the manufacturer extended performance data rather than the number on the brochure. Published heating capacity is commonly stated at a mild outdoor condition, and capacity at the temperature your building actually sees on a cold night can be considerably lower. Inverter driven cold climate equipment holds its output far better than older single speed units, which is why the arithmetic has genuinely changed in recent years, but the only way to know for a specific model is to read its tables.

Where the balance point lands drives the rest of the design. A high balance point means backup heat carries a large share of the season, which is expensive if that backup is electric resistance. A low balance point means the heat pump does almost everything, but usually implies larger or more capable equipment, and oversizing for winter can compromise summer dehumidification on a system that also cools. Inverter equipment with a wide turndown range eases that tension considerably, which is one of the strongest practical arguments for it.

The backup strategy then has to be an actual decision. Electric resistance is simple, cheap to install and expensive to run. An existing gas furnace retained as a dual fuel arrangement is often the lowest running cost option where gas is already present, with a switchover point set on economics rather than on capacity. Doing nothing and accepting a cold building below a certain temperature is occasionally reasonable in a very well insulated property. What is not acceptable is installing resistance strips, leaving the thermostat at factory defaults, and discovering the consequences in February.

One practical note on outdoor unit siting, since it affects heating performance in a way it never does on a cooling only system. A heat pump running in winter produces condensate that freezes, so the unit needs to stand clear of the ground on a raised platform with drainage beneath it, away from roof drip lines and out of the path of snow shed. A unit sitting in a tray of ice loses coil surface, defrosts more often and works harder for less, and the remedy costs almost nothing at installation and considerably more afterwards.

Commissioning covers the usual refrigerant side work plus everything specific to the reversal: defrost operation observed through a full cycle, reversing valve shift confirmed in both directions, auxiliary staging tested to prove it energises only at the intended conditions, and the thermostat configuration written down. That last item matters more than it sounds, because a control configuration nobody recorded is one the next technician will reset to default.