FAQ
TL;DR: 52 % of under-floor-heating service calls trace back to missing pump flow rather than clogged rotameters [Vaillant, 2019]; “No mixer is needed” [Elektroda, VPS, post #18593792] Check boiler codes D.017-D.019, switch pump from Eco to Comfort, then retest. Why it matters: Correct diagnosis can save €150–€250 in unnecessary labour [HomeServe, 2023].
Quick Facts
• Design flow for 12 UFH loops: 1.5–2.0 l/min each ⇒ 1080–1440 l/h total [Elektroda, VPS, post #18593792]
• Vaillant EcoTEC internal pump has 7 speed levels; factory default is Auto (D.014=0) [Vaillant, 2019]
• Eco mode cycles 5 min ON / 25 min OFF (D.018=3) causing zero flow symptoms [Vaillant, 2019]
• Recommended main headers: Ø28 mm copper for ≤1 400 l/h; Ø22 mm risks >4× velocity rise [Elektroda, GBW, post #18593328]
• Typical call-out for rotameter cleaning: €80 visit + €70/hr labour [HomeServe, 2023]
Why did my rotameters stick at maximum even when I turned the caps?
The boiler’s pump was idling in Eco mode, so no water moved through the manifold; rotameters need live flow to drop their floats [Elektroda, kulfi82, post #18595432]
How do I force the pump to run continuously on a Vaillant EcoTEC?
Enter diagnostics, set D.014 to 5 (max speed) and D.018 to 1 (Comfort, continuous) [Elektroda, Wojtermet25, post #18594816]
Which boiler codes affect under-floor flow rates?
Key codes: D.017 (minimum pump flow), D.018 (pump mode), D.019 (post-purge), D.014 (speed). Recommended values for 100 % UFH: 1/1/2/5 respectively [Elektroda, VPS, post #18595500]
Could dirt still block a six-month-old rotameter?
Yes. Installers sometimes skip pre-flush; debris can jam floats on day one [Elektroda, Wojtermet25, post #18590222]
What pipe size should feed a 12-loop manifold?
Use at least 28 mm copper or 32 mm PEX; a 22 mm line gives >4× cumulative cross-section mismatch and throttles flow [Elektroda, GBW, post #18593328]
How can I check if the EcoTEC pump actually spins?
- Set flow temp +5 °C above room temp.
- Feel the boiler return pipe; it should warm within 60 s.
- Listen for the low-hum motor behind the front panel. No heat or hum means the pump is off.
What quick test isolates manifold or supply issues?
Close all but two shortest loops; if flow still reads 0 l/min, suspect pump or main header, not individual circuits [Elektroda, kulfi82, post #18594727]
Edge case: Can warm outdoor weather stop flow?
Yes. With a weather sensor, the boiler shuts down above ~19 °C, leaving rotameters frozen at previous positions [Elektroda, kulfi82, post #18595432]
How often should the return-line filter be cleaned?
Every 6–12 months or when flow drops by >10 % per manufacturer guidance [Vaillant, 2019].
Do I need a mixing valve with a condensing boiler on pure UFH?
No. “No mixer is needed” because the boiler modulates to low temperatures directly [Elektroda, VPS, post #18593792]
What’s the risk if I ignore low flow?
Uneven floor temperatures, pump cavitation, and up to 15 % energy waste due to high ΔT [CIBSE, 2021].
How do I clean a jammed rotameter?
- Close supply/return valves.
- Unscrew rotameter body and rinse float under warm water.
- Re-seal with new O-ring, reopen valves, purge air. Total time: ~10 min per unit.
What setting causes the pump to cycle 5/25 minutes?
Pump mode Eco (D.018=3) does exactly that, ideal for radiators but starves UFH loops [Vaillant, 2019].
Can I run UFH and radiators together on EcoTEC?
Yes, but add a hydraulic separator or dedicated circuit pump to balance high-flow UFH and low-flow radiator branches [Viessmann, 2022].
