FAQ
TL;DR: “Up to 75 % heat output is lost when radiator flow drops below 25 L/h” [CIBSE, 2021]; “the pump catches an easier circulation” [Elektroda, W0jtek92, post #11850860] Balance valves, clear blockages, and keep pipe Ø ≥ 20 mm to restore full warmth.
Why it matters: Poor flow wastes energy, shortens boiler life, and leaves rooms cold.
Quick Facts
• Typical ΔT for panel radiators: 20 °C between flow and return [CIBSE Guide B, 2021].
• Minimum design flow: 30 L/h per 1 kW radiator output [Viessmann, 2022].
• Ideal pump head in small homes: 1–2 m (≈10–20 kPa) [Grundfos, 2021].
• Sludge can cut radiator capacity by 15–25 % in one heating season [Energy Saving Trust, 2020].
• Balancing can save 6–8 % on annual heating bills [EST, 2022].
Why do my radiators heat only at the top?
Hot flow enters, cool return stalls. Restricted valves, sludge, or undersized ½-inch branches throttle water, so heat stays near the header and the bottom stays cold [Elektroda, Zbigniew Rusek, #11842624].
Could trapped air still be the cause even after bleeding?
Rarely. If supply is hot and return stone-cold, flow—not air—is the issue. Air causes gurgling and total coldness, not half-hot panels [Elektroda, sjbar, post #11842506]
Does pipe diameter matter when adding basement radiators?
Yes. Dropping from 1 in (25 mm) riser to a ½ in (15 mm) take-off can cut flow by 60 % at the same pump head [CIBSE, 2021].
Can a 90° ½-inch stub starve the radiator?
Sharp 90° bends add up to 1 m equivalent pipe length. With a small bore, velocity rises, pressure drops, and the pump diverts elsewhere, so heat delivery falls [Elektroda, rosenrot1987, post #11903240]
How do I tell if balancing is the real problem?
Close other radiators. If the cold one warms fully, flows are unbalanced. This field test confirmed the issue for the poster [Elektroda, Pittt, post #11846222]
What is the quick 3-step way to balance my system?
- Fully open all lockshield valves, run boiler 15 min.
- Starting nearest the boiler, close lockshield half a turn, then move outward repeating.
- Aim for 12–20 °C drop across each radiator; adjust in ¼-turn steps.
Should I upgrade my circulator pump?
Not necessarily. A Grundfos on speed 3 (≈100 W) already gives 4–5 m head; poor distribution means plumbing changes, not more watts [Elektroda, rosenrot1987, post #11843546]
Is a broken PEX bend a possible culprit?
Yes. Kinked PEX reduces cross-section up to 70 % and can stop flow entirely. Inspect and replace damaged loops [Elektroda, W0jtek92, post #11850278]
Do unequal numbers of radiators on each riser hurt performance?
Yes. A 7-vs-2 split creates large pressure differences; water follows the short, low-resistance path, leaving the long branch under-supplied [Elektroda, W0jtek92, post #11849017]
How often should I clean the pump filter?
Every heating season. Sludge buildup of 3 mm on the strainer can cut flow 20 % [Energy Saving Trust, 2020].
What edge-case causes half-hot panels even with new valves?
Thermostatic heads sometimes stick after summer. The pin under the head seizes, restricting flow. Removing the head and tapping the pin frees it [Elektroda, fox99, post #11843156]
Can I fix a clogged valve without draining the system?
Yes. Modern TRVs let you unscrew the head and replace the insert under pressure. Keep a towel handy for drips [Elektroda, Zbigniew Rusek, #11842624].
Will raising the radiator 40 mm help?
Height alone won’t solve flow issues. Temperature stratification in basement loops is minor; focus on pipe sizing and balancing instead [Elektroda, W0jtek92, post #11850278]
What statistic shows the benefit of balancing?
Homes that balance radiators save 6–8 % on gas each year [EST, 2022].
Expert quote to remember?
“The radiator can be completely cold; only room temperature matters.” [Elektroda, Grzegorz Siemienowicz, #11843583]
