Practical guide · stated assumptions and checkable examples
Heat pump sizing: a floor-area estimate is not a design load
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Source and worked-example review; not professional advice or approval. Examples are synthetic unless explicitly identified otherwise. Follow the cited authority for current eligibility and legal requirements.
Two homes with the same floor area can have different heating loads. A simple planning worksheet can organize assumptions and show sensitivity, but it cannot establish a safe final equipment selection from floor area alone.
Know what the design process considers
ACCA Manual J is a residential load-calculation method covering building and design-condition inputs rather than a single watts-per-square-metre rule. Envelope areas, glazing, infiltration, orientation and design temperatures affect the answer. The AskCalculators screening model is not a Manual J implementation or a replacement for a room-by-room design.
References: ACCA — Manual J residential load calculation
Make the screening arithmetic explicit
Suppose an analyst enters 140 m² of conditioned floor area and an illustrative 60 W/m² screening allowance. The resulting 8,400 W, or 8.4 kW, is a scenario, not a climate-specific requirement. Entering 40 W/m² instead produces 5.6 kW. That large change is a reason to obtain better building evidence, not permission to choose the larger unit automatically.
Calculation: Screening thermal load = conditioned area × entered thermal-load allowance
Thermal output is not electrical input
With an illustrative operating coefficient of performance of 3, producing 8.4 kW of heat would require 2.8 kW of electrical input at that condition. A stated coefficient at one test point is not an annual value and does not establish cold-weather capacity. Keep kW of heat, kW of electricity and kWh of annual consumption in separate fields. The unit conversions are mathematical, not additional engineering validation.
References: NIST — Guide for the Use of the International System of Units
Match the load to equipment performance
ACCA Manual S addresses equipment selection using manufacturer performance information and design conditions. A nominal product class should not be assumed to deliver its labelled capacity at every outdoor temperature or required supply temperature. Defrost, backup heat, distribution and controls need attention in the actual design. A capacity label alone cannot answer these questions.
References: ACCA — Technical manuals, including Manual S equipment selection
Use the calculator as a questions list
Ask the designer which indoor and outdoor temperatures were used, how infiltration was assessed, what envelope improvements are planned and whether distribution work is included. Compare install-cost scenarios only after recording the scope: equipment, electrical supply, emitters or ducts, commissioning, removal and any backup system. Entered low and high costs are not local contractor quotations.
Questions about this guide
Is a bigger heat pump always safer?
The worksheet cannot decide that. Equipment selection requires an actual load calculation and performance data; increasing a screening number is not a design method.
Does selecting my country provide my climate design temperature?
No. Country selection alone does not establish a site-specific design condition. Verify location and building inputs explicitly.