Does a tighter envelope let a 480 sq ft backyard cottage use a smaller heat pump, and is the saving worth counting?

research/house-hpsize

Does a tighter envelope let a 480 sq ft backyard cottage use a smaller heat pump? The tight package cuts the design heating load about 36.5%, worth a $450 to $1,800 equipment step credit (assumed catalogue and prices) that nearly closes the payback gap of the costed upgrades in three colder climates. Stylised.

9 tests

What it found

Adds the term house-breakeven left out: the equipment downsizing credit. The design heating load (UA × design temperature difference) of the house-open-design ADU falls about 36.5% with the tight envelope in every climate, but equipment comes in discrete sizes, so the credit is a step: with an assumed catalogue and prices it is $450 (Houston), $1,350 (Baltimore), $900 (Chicago) and $1,800 (Minneapolis) for the whole package, and it is mostly the wall switch that moves the size. Added to the energy break-even, the three costed upgrades (assumed extra cost $2,211) come within $98 to $463 of paying back in the three colder cities but not in Houston ($1,607 short). The size is sensitive to the assumed design temperature and sizing margin, which are unchecked memory, so the credit is a range of one or two catalogue steps, not a number. Stylised: no EnergyPlus, no Manual J, no sourced prices.

A figure from its own output

Figure produced by research/house-hpsize: size_vs_design_temp — house-hpsize/experiments/size_vs_design_temp.svg, written by the project's own experiment code.

Paper, code and results

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Produced with AI assistance (Claude). Results are from the code in this repository and are simulated or stylised unless the abstract says otherwise; nothing here implies affiliation with or endorsement by any lab, researcher or company cited. MIT licensed.