The boiler itself was a strange hybrid. It had a standard burner, but also a modulating valve connected to a weather compensator. Mira programmed the controller: above 7°C outside, the air-source heat pump (hidden behind a yew hedge) ran silently. Below 7°C, when the heat pump’s efficiency crashed, the biofuel boiler kicked in with a soft, clean whoosh —burning fuel that smelled faintly of chips.
They worked for three weeks. The old single-skinned steel tank in the garden was exhumed—leaking, rusty, a monument to a careless age. In its place, Arthur installed a gleaming, double-skinned, polyethylene tank with a sensor in the interstitial gap, exactly as BS 5410-3 demanded (Clause 7.4.2.3). If the inner skin wept biofuel, the outer skin would catch it, and a red light would flash on a panel in Mrs. Hillingdon’s kitchen. bs 5410-3
Arthur tightened the last flue connection. The flue liner was special—stainless steel, grade 316L, resistant to the acidic condensate of bio-liquids. He’d ignored that once, on a test rig. The flue had corroded through in a month. The boiler itself was a strange hybrid
“Clause 9.3.1,” Mira read aloud, holding the standard in the rain. “‘The system shall automatically switch between energy sources without user intervention, prioritizing renewable electric heat where economically and environmentally beneficial.’” Below 7°C, when the heat pump’s efficiency crashed,