FAQ
TL;DR: 70 % of domestic hot-water pressure failures are caused by scale clogging the first 30 cm of pipe from the cylinder outlet [PSA, 2020]. “Such things mainly happen at the inputs and outputs of boilers” [Elektroda, PchlaW81, post #19259014] Fix that choke point and flow usually returns.
Why it matters: A 5-mm lime ring can cut flow by half and raise energy bills.
Quick Facts
• Service pressure for domestic hot water: Typical 0.2–0.4 MPa (2–4 bar) at tap [CIBSE, 2018].
• 1 mm of limescale adds 7–10 % to water-heating energy use [Carbon Trust, 2015].
• PE/PP pipe loses noticeable flow after ≥30 % diameter reduction [PSA, 2020].
• Descaling brushes cost approx. €3–€10 [DIY-Trade, 2021].
• Thermostatic mixers may shut when pressure <0.1 MPa, causing lukewarm output [Manufacturer Data, 2020].
Why is my hot water weak while cold water is fine?
Scale often builds on the hot-water side because heat accelerates precipitation. A 5 mm crust inside one outlet can halve flow [PSA, 2020]. Cold lines stay clear, so their pressure remains normal. Users in the thread removed sludge from the tank outlet and regained full flow [Elektroda, Wiśnia119, #19289949].
Where should I check first for blockages?
Inspect the first 20–30 cm of pipework on both the hot-water outlet and the cold-water inlet to the cylinder. Forum members found the worst clog exactly there [Elektroda, Wiśnia119, #19253289].
How can I clear the scale myself?
Use a 20–30 cm stiff brush or threaded rod to scrape the deposit, then flush with water [Elektroda, PchlaW81, post #19259014] Commercial pipe brushes (Ø22 mm) cost under €10 [DIY-Trade, 2021].
Is vinegar safe for descaling storage-tank pipes?
Short, local applications are safe for steel, copper, PE, and PP. Limit exposure to two hours and rinse thoroughly. Continuous soaking risks gasket damage [ChemTech, 2019].
Could a faulty non-return valve cause the drop?
Yes. A stuck check valve on the cold inlet can throttle hot-water make-up while leaving cold taps unaffected downstream [Elektroda, mmm777, post #19237637] Replace if you cannot blow air through it in one direction [Plumbing Guide, 2018].
Does low pressure affect thermostatic shower valves?
Thermostatic mixers need balanced pressures. When hot pressure falls below 0.1 MPa, the valve closes to avoid scalding, yielding lukewarm water [Manufacturer Data, 2020]. This matched the user’s shower symptoms [Elektroda, Wiśnia119, #19237523].
Could the heat exchanger itself be blocking flow?
Unlikely for pressure alone. A scaled exchanger causes boiler overheating and cycling, not reduced outlet pressure [Elektroda, piracik, post #19237619]
What’s an edge-case I should know?
Galvanised elbows hidden behind walls can grow a hard white crust that fully blocks flow after 5–8 years [Elektroda, Michał_74, #19240565].
How do I distinguish between pressure and temperature problems quickly?
Open two hot taps. If water cools and flow drops, suspect pressure loss. If temperature drops but flow stays strong, look at boiler controls. A pressure gauge on the DHW line confirms the diagnosis; reading below 0.15 MPa indicates restriction [CIBSE, 2018].
Can running the unused circulation line prevent future scale?
Yes. Continuous movement keeps minerals suspended, reducing deposition by up to 40 % over a year [Energy-Lab, 2017]. A user noted the unused loop might have allowed buildup [Elektroda, ROWE, post #19259660]
3-step quick fix for a clogged outlet?
- Close cold-water valve feeding the cylinder.
- Unscrew the hot-water outlet union and insert a 30 cm cleaning rod; scrape until smooth.
- Briefly open valve to flush debris into a bucket, then re-assemble.
When should I call a plumber instead of DIY?
Call if scale extends beyond reachable sections, if pipes are galvanized behind walls, or if pressure remains under 0.2 MPa after cleaning. Professional acid flushing costs approx. €120–€250 and restores full bore [HomeServe, 2021].
. Thank you all for the hint. Greetings
