[{"data":1,"prerenderedAt":155},["ShallowReactive",2],{"resource-/resources/the-heat-a-city-cannot-see":3},{"id":4,"title":5,"audience":6,"author":6,"body":7,"category":138,"date":139,"description":140,"draft":141,"extension":142,"image":6,"imageAlt":6,"meta":143,"navigation":144,"ogImage":17,"path":145,"pdfPath":6,"resourceType":6,"seo":146,"series":6,"stem":147,"tags":148,"visibility":153,"__hash__":154},"resources/resources/the-heat-a-city-cannot-see.md","The Heat a City Cannot See",null,{"type":8,"value":9,"toc":129},"minimark",[10,18,30,38,41,46,49,52,55,58,61,65,68,74,77,81,97,100,104],[11,12,13],"p",{},[14,15],"img",{"alt":16,"src":17},"Choropleth of Nottingham's twenty wards shaded by the ten metre bare pipe scenario, from 3.2 to 9.3 GWh a year per ward.","/images/resources/articles/heat-city-cannot-see-1600x900.png",[11,19,20],{},[21,22,23,24,29],"em",{},"This map is live. ",[25,26,28],"a",{"href":27},"/views/city","Open the city view"," to choose the\nscenario and read every ward yourself.",[11,31,32,33,37],{},"Last week we published ",[25,34,36],{"href":35},"/views/pipework","a small calculator",": what\ndoes the pipe between a heat pump and the house cost, one home at a\ntime. The honest answer\nwas modest, twenty-six pounds a year on the default settings, and one\nreader after another had the same reaction we did. That cannot be the\nwhole story.",[11,39,40],{},"It is not. To see the rest of it, you need a whole city.",[42,43,45],"h2",{"id":44},"the-envelope","The envelope",[11,47,48],{},"We took the 107,336 homes of Nottingham that our stock model can\nprice, every home compiled from the public record and simulated\nthrough the national calculation for a full year, hour by hour. Then\nwe asked one question as a scenario envelope: if each of these homes\nhad an external heat pump run of a stated length and insulation\nstate, what would that pipe shed in a year, using each home's own\nsimulated heating hours rather than anyone's assumption?",[11,50,51],{},"The pipe cases are explicit scenarios, stated in full in the\nmethod note. Nothing here claims to know any individual home's\npipework. What the envelope shows is the size of the thing the\nofficial figures cannot show.",[11,53,54],{},"At ten metres of bare pipe per home, a long siting, the city's loss\nwould be 142 GWh a year, nearly twelve percent of everything these\nhomes spend on space heating. At five metres, the more typical\nsiting, 71 GWh, six percent. Insulated\nto 25 mm throughout, the ten metre case falls from 142 GWh to 23.\nThe swing between bare and done properly, across one city,\nis 119 GWh a year: ten percent of the city's entire space heating,\nroughly the space heating of ten thousand homes.",[11,56,57],{},"The map above is not uniform, and its unevenness is information. Hyson\nGreen & Arboretum alone would shed 9.3 GWh a year in the bare\nscenario; the quietest ward a third of that. Density does some of\nthat work, but so do heating hours: the median home in Castle ward\nheats for 2,732 hours a year, in Berridge for 4,834. A city is not\none number, and neither is a street.",[11,59,60],{},"None of it appears in any compliance figure, for any home, anywhere.\nThe national calculation models no pipework loss on emitter circuits\nat all; where pipework is in scope, the insulation is declared and\nnever verified. A city can shed the heating of a town through a\ncomponent no instrument of policy can see.",[42,62,64],{"id":63},"the-correction-we-owed-our-own-calculator","The correction we owed our own calculator",[11,66,67],{},"The calculator we published assumes a flat 1,800-hour heating\nseason, the standard textbook figure. The city's records say\notherwise. Across 107,336 simulated homes, the median heating year\nis 3,592 hours. One home in ten heats for more than 5,175.",[11,69,70],{},[14,71],{"alt":72,"src":73},"Histogram of active heating hours across 107,336 homes, with the 1,800-hour assumption marked far left of the real median at 3,592.","/images/resources/articles/heat-city-hours-1600x900.png",[11,75,76],{},"The assumption fits about one home in forty: 2.6 percent of the\ncity heats for fewer than 1,800 hours. The standard assumption\nunderstates the median home's pipework loss by half. Our own page carried that assumption, stated openly in its\nmethod note, and the city run has now measured its conservatism: the\nmedian ten metre bare run is not 629 kWh a year but 1,255, and for\nthe tail that heats longest, nearer 1,808. We have updated nothing quietly.\nThe page said what it assumed; the city said what is real; both\nstatements stand, and the gap between them is exactly why a record\nkept per home beats a flat assumption.",[42,78,80],{"id":79},"where-this-leaves-you","Where this leaves you",[11,82,83,84,87,88,92,93,96],{},"If you look after housing stock, the question is not whether your\nhomes lose heat this way. It is that nobody currently records\nwhether they do. The envelope above is a scenario, deliberately so,\nand the record that produced it can read any real stock the same\nway, from public data, before anyone visits. The\n",[25,85,86],{"href":35},"view for one home"," prices one pipe; the\n",[25,89,91],{"href":90},"/views/refit-crossings","crossing queue"," shows one estate; and this\nessay's city run is now ",[25,94,95],{"href":27},"a view of its own",": the same\nrecord at the scale of a city, live, ward by ward.",[98,99],"hr",{},[42,101,103],{"id":102},"about-the-numbers","About the numbers",[105,106,107,111,114,117,120,123,126],"ul",{},[108,109,110],"li",{},"Nottingham: 107,336 homes eligible for analysis, from 107,894 public\nenergy certificates, each simulated through the national\ncalculation, full year, hourly. Active heating hours are each\nhome's simulated hours with nonzero space heat demand:\nP10 2,474, P50 3,592, P90 5,175.",[108,112,113],{},"The pipe cases are a scenario envelope over real homes, with the pipe\nassumptions stated: run lengths of 5 m and 10 m; bare 28 mm copper,\n13 mm elastomeric (k=0.037), 25 mm phenolic (k=0.026); water at\n45°C flow against 5°C ambient; the published pipe loss\nformula of the national calculation's technical reference.",[108,115,116],{},"Cohort annual space heat 1,195 GWh. Scenario cells: 5 m bare\n71.2 GWh; 5 m at 13 mm 22.0; 5 m at 25 mm 11.5; 10 m bare 142.4;\n10 m at 13 mm 44.0; 10 m at 25 mm 23.0. The swing from bare to 25 mm is\n5.0% of cohort space heat at 5 m, 10.0% at 10 m.",[108,118,119],{},"The run lengths are stated scenarios, not a survey. No national\nstatistic of external run lengths exists; installer guidance keeps\nruns as short as practicable, so 5 m is the more typical siting\nand 10 m the long one. The formula is linear in length: city wide,\nabout 14.2 GWh a year per metre of bare run.",[108,121,122],{},"Ward boundaries are the ONS Open Geography wards (Open Government\nLicence); homes were joined to wards through the ONS Postcode\nDirectory, May 2026 release, with all 107,336 homes matched. No\nward reported holds fewer than ten homes.",[108,124,125],{},"Every figure was recomputed twice by independent routes (272 independent\nchecks, counts exact, heat within 0.001 kWh) and the formula\nimplementation was checked against our published calculator\nto within 0.000001 kWh. Aggregates only; no figure for a single\ndwelling leaves the analysis.",[108,127,128],{},"Illustrative scenario envelope, not a forecast, and not a claim\nabout any individual home.",{"title":130,"searchDepth":131,"depth":131,"links":132},"",3,[133,135,136,137],{"id":44,"depth":134,"text":45},2,{"id":63,"depth":134,"text":64},{"id":79,"depth":134,"text":80},{"id":102,"depth":134,"text":103},"articles","2026-08-07","We ran a scenario envelope over the 107,336 homes of Nottingham that our stock model can price: at ten metres of bare heat pump pipework per home, a long siting, the loss would approach twelve percent of everything the city spends on space heating. None of it appears in any official figure, and the standard assumption understates it for the median home by half.",false,"md",{},true,"/resources/the-heat-a-city-cannot-see",{"title":5,"description":140},"resources/the-heat-a-city-cannot-see",[149,150,151,152],"heat pumps","pipework","housing and place","national calculation","public","VGCZ2f0ODT-CD6K4c1rENtWOHXZrPf5NpxAPWziI7_k",1786099864233]