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ASHP guide

Air source heat pump calculator: inputs before output

Calculator outputs are only as good as the evidence behind them. Survey quality decides whether the numbers hold up on site.

MCS says compliant heat-load work for heat pumps is room by room. That is why missing room data, weak U-values, or vague emitter notes can make a neat-looking output unusable. · 3 min read

Guide

Inputs that make ASHP outputs usable

What strong looks like

Capture standards

  • Room-by-room dimensions and exposure captured with supporting context
  • Emitter condition and layout documented before sizing decisions
  • Plant location, access routes, and constraints photographed clearly
  • Assumptions recorded so design teams know what is confirmed vs pending
Where time is lost

Where teams lose time

  • Missing evidence around emitters and routing
  • Assumptions buried in ad-hoc notes
  • No consistent move from survey to design
Why it helps

Why teams use Vertex

  • Clearer reports for office and design
  • Faster quoting with less chasing
  • Install teams start with clearer site context
  • Portal visibility across teams and roles
Related guides
What is an air source heat pump? ASHP survey reports ASHP efficiency guide ASHP survey reports Heat loss calculations
Official reference points

What the MCS heat-load guidance expects the inputs to cover

Room by room

MCS says room-by-room heat-load calculations are important for sizing low-carbon heating properly. A whole-house shortcut is not the same standard.

Core variables

The MCS documentation points back to building fabric, room dimensions, air-change rates, target internal temperatures, and winter design temperatures as key inputs.

Practical implication

If any of those inputs are guessed badly, the calculator still gives a number. It is just a weaker number. That is why survey quality matters more than the interface.

Sources checked on 17 April 2026: MCS Heat Load Calculator, MCS heat-loss guidance, and MCS room schema. The third card is an inference from those sources plus normal survey practice.