Installation is required of (BROEN) Ballomax (full flow ball valves) on the ***water pipes*** in the rooms facing east due to insufficient specification by the adviser

sources in Danish which render "stistrenge" as synonymous with "vandrør":

Orientering til beboerne FSB bolig
www.bellahoej.net/.../Beboerinformation -18_... -
PDF/Adobe Acrobat - Quick View
Den gennemgribende renovering af vores badeværelser, faldstammer, ***stistrenge (vandrør)***, samt opsætning af fjern aflæste vandmålere, er nu afsluttet – og det ...

Afdelingsbestyrelsens årsberetning
www.bellahoej.net/.../Årsberatning 2010-2011... -
PDF/Adobe Acrobat - Quick View
27. sep 2011 – Den gennemgribende renovering af vores badeværelser, faldstammer, ***stistrenge (vandrør)***, herunder opsætning af fjernaflæste vandmålere er ...


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Note added at 15 hrs (2012-04-01 13:27:20 GMT)
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"stistrenge" also translates as "path strings" in a totally unrelated context:

Danish translations for squeeze package. # Copyright (C) 2008 The ...
https://translations.xfce.org/projects/p/squeeze/c/master/ra...
16. jan 2012 – src/archive_store.c:251 ../src/notebook.c:182 msgid "Show the full ***path strings for each entry" msgstr "Vis den fulde stistreng*** for hvert element" ...

there is an additional term in Danish with similar spelling "stigstrenge" which translates as "risers":

Instructions ASV-M - Danfoss.com
heating.danfoss.com/.../VIA1L39M_ASV-M.pdf -
PDF/Adobe Acrobat - Quick View
af differenstryk i ***stigstrenge med ... differential pressure in risers*** where the radiator valves have .... der Anlage einen Schmutzfilter z.B. Danfoss. Typ FV ...

there are a number of advertisements for renovated real estate properties which mention the term "stistrenge":

Slangerupgade 24, 1. tv., 2200 København N - Andelsbolig på 62 ...
www.danbolig.dk/?propertyid=HB5169...087
Denne andelsforening har gennemgået en række forbedringer: Tegltag, faldstammer, ***stistrenge***, nydeligt renoverede opgange samt smuk anlagt gård.

Eddagården 3, 4. th - 2200 København N - Andelsbolig til salg på 59 ...
www.boligsiden.dk/salg/323712537/
Herudover er der termovinduer (1988), som er renoveret/malet i nyere tid. Der er for nyligt indlagt fjernvarme, skiftet ***stistrenge*** og radiatorerne er flyttet hen under ...

Rejsegilde på beboerhuset i Lønstrupgård
unikventeliste.dk/pdf/.../tale_hl_loenstrupgaard.pd... - PDF/Adobe Acrobat - Quick View
renovering af kloakker og ***stistrenge***, isolering af gavle, sammenlægning af hybler samt facaderenovering. Landsbyggefonden støtter renoveringssagen med ca.

deducing from the context of these three advertisements, the translation for "stistrenge" in your specific context would be "water pipes" instead of "path strings" or "risers". "Path strings" would not make any sense at all and neither would "risers" as radiators are not being renovated but only moved while sewer connections, tile roofs, downpipes and "water pipes" are being renovated.

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Note added at 19 hrs (2012-04-01 17:38:31 GMT)
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extrapolating a bit further, there is a certain visual analogy between "water pipes" (renovation of real estate property) and "path strings" (information technology) which might explain "stistrenge" as being the common term for both in Danish

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Note added at 2 days5 hrs (2012-04-03 03:35:09 GMT)
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Christine Andersen certainly is right in that this could also refer to highrise firefighting "standpipe risers" (either "dry" or "wet" risers)

as an additional reference on this subject, there is an interesting explanatory source:

High Rise Fire-fighting : Dry Risers
www.highrisefirefighting.co.uk/dr.html
The dry riser is the opposite of a "wet riser" or "wet standpipe" system where the pipes are kept full of water for manual or automatic fire fighting operations.