Building Heat Loss & Boiler Sizing Calculator

Heat loss splits into what escapes through the fabric and what leaves with the air. Which one dominates decides whether insulation or ventilation control is the better investment.

A screening estimate, not a compliance calculation. Regulated projects need a full heat loss assessment to the applicable standard.

Fabric or ventilation — find out which dominates

The split decides where money should go. In a modern insulated building most of the loss leaves with the air, and adding insulation barely moves the number. In an old uninsulated shed the reverse is true. Calculating both separately is the point of the exercise.

Warm-up is a real allowance, not padding

A building heated continuously only needs to replace steady-state loss. One that is set back overnight or shut down at weekends has to drag the whole thermal mass back up before occupancy, and that peak is well above the steady-state figure. Sizing without it produces plant that cannot make Monday morning.

Oversizing has its own cost

A boiler far larger than the load cycles on and off, which wastes fuel, wears the burner and holds a less stable temperature. Modulating plant tolerates oversizing better than on/off plant, but the honest answer is to size on a real calculation rather than a rule of thumb per square metre.

Link to or cite this tool

If this calculator is useful in a specification, tender document or forum answer, please link to it rather than copying the figures — the assumptions behind it are stated above and may be updated.

https://www.hvacrnexus.com/tools/heating-load-calculator/

Working out the fan side

Once the refrigeration duty is fixed, the evaporator fans and any condenser fans are a separate calculation. Longwell publishes a set of fan-side tools — airflow, static pressure, fan laws and motor sizing — at longwellfans.com.

Equipment this calculation points to

Questions about this calculation

How do I size a boiler for a building?
Calculate steady-state heat loss — fabric plus ventilation — at your design outdoor temperature, then add a warm-up allowance appropriate to how the building is operated. Rules of thumb per square metre ignore the fabric and air change rate, which are exactly the variables that matter.
Why separate fabric and ventilation loss?
Because they point to different remedies. If ventilation dominates, draught sealing, door control and heat recovery pay back; if fabric dominates, insulation and glazing do. Treating heat loss as a single number hides which lever is worth pulling.
What warm-up allowance should I use?
Roughly 20% for a building on daily setback and 40% for one shut down at weekends, as a starting point. Buildings with heavy thermal mass need more; lightweight, continuously heated buildings need almost none.

Send us the duty and we will quote matched equipment

Give us the figure this calculator produced, your room temperature and your location — we will come back with condensing unit and evaporator options, price and lead time, usually within one working day.